Search

With ongoing shale oil and gas exploration and development across the world, the sedimentology of deep‐water fine‐grained sedimentary rocks remains an important focus for research. In deep‐lacustrine settings, in particular, mud‐grade sediments are distributed widely. However, the very fine particle size of mud‐grade sediments limits the clear identification of micro‐sedimentary structure; the mud‐grade sediments are in most facies models believed to be deposited via suspension settling in deep‐lacustrine settings. In this study, thin‐section observations, advanced mineral identification and characterization system analyses, scanning electron microscopy observations and three‐dimensional reconstruction analysis by a focused ion beam‐electron beam dual‐beam system were conducted on deep‐lacustrine mud‐grade sediments of the Chang 73 sub‐member of Triassic Yanchang Formation in the Ordos Basin. Through this multi‐proxy approach, evidence of bedload transport of the mud‐grade sediments in the deep‐lacustrine settings is provided. During the deposition of deep‐lacustrine sediments, a significant amount of volcanic material within the mud‐grade sediments provided large quantities of high‐valence metal cations into the lake waters, which promoted the formation of flocs through electrostatic forces. Subsequently, these flocs are transported as bedload and deposited on the substrate, forming numerous floccule ripples in the deep‐lacustrine settings. When floccule ripples form, the flocs continuously pass over the ripple crests under the action of underflow and accumulate downward along the sloping surface of the floccule ripples, developing multiple foresets. Lateral migration and frontal progradation of foresets results in multiple stacked and laterally‐connected foresets, ultimately controlling the accumulation and preservation sites of these deep‐lacustrine mud‐grade sediments. The discovery of floccule ripples and foresets confirms a dynamic deep‐lacustrine setting, in which the mud‐grade particles can be transported as bedload under the action of underflow and then deposited. A new understanding of the sedimentary process of deep‐lacustrine mud‐grade particles provides a theoretical basis for clarifying the distribution of mineral composition and organic matter content in mud‐grade sediments. It further supports the prediction of favourable shale oil/gas reservoirs, promoting the efficient exploration and development of shale oil and gas resources, globally. [ABSTRACT FROM AUTHOR]

Hyperpycnal flow deposits, one of the most important deep‐water gravity‐flow deposits in lacustrine basins, have become the research focus in recent years. However, the sedimentary characteristics and depositional model of hyperpycnal flow deposits in lacustrine basins remain unclear due to the differences of depositional settings between lacustrine and marine environments. Hyperpycnal flow deposits observed in the middle of the third member of the Shahejie Formation (Es3z) in the Bonan Sag, Bohai Bay Basin, Eastern China, provide a rare case study to reveal the characteristics and depositional model in lacustrine basins. For the first time, detailed core analysis, high‐resolution 3D seismic data, petrology and grain size analysis were used to unravel the characteristics and depositional model of hyperpycnal flow deposits in this study. Twelve lithofacies, six bed types and four bedsets (corresponding to feeder channel, distributary channel, levee and lobe) were recognized from detailed facies analysis. Plant fragment, an important identification mark for hyperpycnal flow deposits, can be classified into three types: completely broken plant fragments, partially broken plant fragments and complete leaves. The proximal part of the deposit develops a small amount of scattered and completely broken plant fragments in massive or spaced stratified pebbly sandstone and massive sandstone due to strong erosion of sustained high‐density turbidity current. The medial part of the deposit is dominated by laminated partially broken plant fragments in planar laminated or rippled sandstone due to suspended settling of sustained high‐density turbidity current and quasi‐steady low‐density turbidity current. Layered partially broken plant fragments with some complete leaves are common in the upper part mud rich division of hybrid event bed and laminated siltstone in the distal part of the deposit. The distribution pattern of hyperpycnites is controlled comprehensively by palaeogeomorphy and sediment supply. Palaeogullies determine the provenance direction of hyperpycnal flow. The formation of synsedimentary faults and troughs control the transport routing patterns. Local micro‐palaeogeomorphy and depression areas further restrict the distribution of sand bodies. During the early stage of deposition with insufficient sediment supply, sediments are transported to the deep basin along confined faulted troughs forming elongated sandy bodies. During the late stage of deposition with sufficient sediment supply, sediments are transported to the deep basin without confinement accumulating fan‐shaped sandy bodies. This study offers insight for enhancing the recognition criteria of hyperpycnites, as well as their depositional model in lacustrine basins. [ABSTRACT FROM AUTHOR]

Contrary to widely used traditional sequence‐stratigraphic models, highstand sublacustrine fans developed in the deep‐water environment in the middle of the third member of the Eocene Shahejie Formation (Es3z, 42–40.5 Ma), Dongying Sag. Using four cores and 3D seismic data, we describe the sequence stratigraphy frameworks, sediment characteristics, depositional elements, and depositional model of the Liangjialou highstand sublacustrine fan. The Es3z is a complete third‐order sequence stratigraphic, bounded by T4 at the top and T6 at the base. Between T4 and T6, there is the maximum flooding surface (MFS) distributed from proximal to distal of the system in the third‐order stratigraphic sequence. Therefore, the Liangjialou Fan at the top of the Es3z, between MFS and T4, is a typical highstand sublacustrine fan as determined from seismic data. This highstand sublacustrine fan is characterised by gravel to fine‐grained sand, abundant amalgamation surfaces, and crude stratifications common in coarse‐grained structureless‐ to normal‐ graded sandstones. Climbing ripples and thin laminae rich in plant debris are common in inverse‐then normal‐graded sandstones. Channel‐levee, channel‐lobe transition zone, and lobes developed in this highstand sublacustrine fan, with clear internal progradation stacking patterns caused by hyperpycnal flow. The sudden increase in sediment supply in an accommodation decrease situation and the narrow shelf width are the primary controlling factors for forming highstand sublacustrine fans. In the late stage of the highstand systems tract, gravity‐flow deposits caused by hyperpycnal flow are developed from another new point source to form this highstand sublacustrine fan. Hyperpycnal flow in lacustrine basins can transport coarse‐grained sediments with the help of strong flood energy and steep slopes. Gravity flow deposits in lacustrine basins are not confined by sequence stratigraphy frameworks, and many additional areas of sand caused by hyperpycnal flow could be found in lacustrine basins. [ABSTRACT FROM AUTHOR]

Clastic rock reservoirs in petroliferous basins are generally rich in feldspars. Feldspar dissolution has developed widely in clastic reservoirs, and the resulting secondary pores are crucial in deeply buried reservoirs. Based on a study of the diagenesis of clastic reservoirs in the Bohai Bay Basin, Tarim Basin, and Pearl River Mouth Basin and physical and numerical simulation experiments of fluid-rock interactions, this paper proposed a successive formation model of secondary pores via feldspar dissolution in deeply buried clastic reservoirs, considering the global research progresses in feldspar dissolution in clastic rocks. Feldspar dissolution can occur from shallow open systems to deep-ultra deep closed systems in petroliferous basins, resulting in the successive formation of secondary pores at different diagenetic stages. The successive mechanism includes three aspects. The first aspect is the succession of corrosive fluids that dissolve minerals. Meteoric freshwater dominates at the Earth's surface and the early diagenetic A stage. Subsequently, organic acids and CO2 formed via kerogen maturation dominate at the early diagenetic B stage to the middle diagenetic stage. CO2 and organic acids formed via hydrocarbon oxidation in hydrocarbon reservoirs dominate at the middle diagenetic B stage to the late diagenetic stage. The second aspect is the successive formation processes of secondary pores via feldspar dissolution. Large-scale feldspar secondary pores identified in deep reservoirs include secondary pores formed at shallow-medium depths that are subsequently preserved into deep layers, as well as secondary pores formed at deep depths. Existing secondary pores in deeply buried reservoirs are the superposition of successively feldspar dissolution caused by different acids at different stages. The third aspect is a successive change in the feldspar alteration pathways and porosity enhancement/preservation effect. Open to semi-open diagenetic systems are developed from the Earth's surface to the early diagenetic stage, and feldspar dissolution forms enhanced secondary pores. Nearly closed to closed diagenetic systems develop in the middle to late diagenetic stages, and feldspar dissolution forms redistributional secondary pores. The associated cementation causes compression resistance of the rock, which is favorable for the preservation of secondary pores in deep layers. These new insights extend the formation window of secondary pores in petroliferous basins from the traditional acid-oil generation window to a high-temperature gas generation window after hydrocarbon charging. The proposed model explains the genesis of deep-ultra deep high-quality reservoirs with low-permeability, medium-porosity and dominating feldspar secondary pores, which is significant for hydrocarbon exploration in deep to ultra-deep layers. [ABSTRACT FROM AUTHOR]

Crystalline dolomite is a common carbonate mineral in lacustrine fine-grained rocks, but it has a confusing origin that has been a focus of much debate in recent years. In our research, the fine-grained rocks rich in crystalline dolomite were sampled from the upper fourth member (UMbr 4) and lower third member (LMbr 3) of the Shahejie Formation in the Jiyang Depression, and analyzed by a variety of analytical techniques, including field emission scanning electron microscopy (FESEM), cathodoluminescence (CL), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), micro-area carbon, oxygen, strontium isotopes and electron probe analyses. The results show: ①The crystalline dolomites in the lacustrine fine-grained rocks are primarily composed of ferroan-poor dolomite (FeO < 1%), type I ferroan dolomite (FeO ≈ 5%) and type II ferroan dolomite (FeO ≈ 16%); ②The δ18O of ferroan-poor dolomites in mudstone is between −0.89 and 0.32‰, the precipitation temperature is less than 60 °C, and their formation is related to the microbial action during the early diagenetic stage A, while the ferroan-poor dolomites in dolostone are primarily formed by the recrystallization of micritic dolomite; ③The δ18O of type I ferroan dolomites ranges from −3.74 to −3.89‰, the precipitation temperature ranges from 68.4 to 69.4 °C, and they are primarily formed by the microbial action and clay mineral transformation during the early diagenetic stage B; ④The δ18O of type II ferroan dolomites is between −7.78 and −8.33‰, the precipitation temperature is more than 90 °C, and their formation is related to the dissolution of early carbonate and clay mineral transformation during the middle diagenetic stage; ⑤The origin mechanism of crystalline dolomite indicates the diagenetic evolution of fine-grained rocks and the migration path and storage of shale oil. • The degree of Fe enrichment in crystalline dolomite increases with the burial diagenetic evolution. • Fe-poor crystalline dolomite is primarily originated by the microbial action during the early diagenetic stage. • Fe-rich crystalline dolomite is primarily originated by multi-component evolution during the middle diagenetic stage. • The origin mechanism of crystalline dolomite indicates the migration and enrichment of shale oil. [ABSTRACT FROM AUTHOR]

The precipitation of authigenic quartz plays a significant role to reduce the reservoir characteristics and enhance the stiffness of the rock. The Es1 sandstone of Shahejie Formation is acting as a significant hydrocarbon producing rock in the Nanpu Sag. Various methods like thin section petrography, cathodoluminescence (CL), scanning electron microscope (SEM, with EDS), and electron microprobe analysis has been used to reveal the origin of quartz cement as well as to evaluate the effect of quartz cement on reservoir quality. The studied sandstone is classified as immature to mature feldspathic litharenite and lithic arkose and consists of quartz, feldspar, rock fragments and micas. Petrographic studies and SEM analysis shows that the authigenic quartz is acting a significant cement that reduces the reservoir quality. Whereas clay minerals (kaolinite and mixed layer illite to smectite) are dominant in the Es1 sandstone, that can reduce the reservoir quality. SEM, CL and thin section analysis reveal that there are two stages of quartz cement in the studied samples; that are pore filling authigenic cement and quartz overgrowth cement. Fluid inclusion homogenization temperatures shows that stages of quartz cement were developed with continuous process from 70°C to 130°C. Quartz cements were generally originated from I/S reaction, feldspar dissolution, conversion of rock fragments and pressure solution. Feldspar dissolution (K‐feldspar) and kaolinite to illite reaction is an insignificant silica source for the silica cement which is internally precipitated in a close system with diffusion transporting mechanism. Overall, quartz cement significantly enhance the rock strengthen and brittleness effectively as well as it reduce the overall reservoir quality. [ABSTRACT FROM AUTHOR]

Sandy-conglomerate reservoir has gradually become a major target of oil and gas exploration. Complex diagenetic process and diagenetic fluid play a significant role in affecting reservoir heterogeneity. Carbonate cements form at various stages of the diagenesis process and record various geological fluid information. Recently, one-billion-ton sandy conglomerate oil field was exposed in Triassic Baikouquan Formation, Mahu sag, Junggar Basin. Therefore, an integrated study applying casting thin sections, cathodeluminescence, fluorescence, carbon and oxygen stable isotopes, electronic probe microanalysis and aqueous fluid inclusions measurements was performed in order to identify the types of carbonate mineral and its representative diagenetic environment and discuss the influences of different CO2 injections on reservoir quality. The main findings are as follows: The reservoir is mainly composed of 70.33% conglomerate and 16.06% coarse-grained sandstone. They are characterized by low compositional maturity and abundant lithic debris. Four types carbonate cements are identified according to the petrological and geochemical characteristics, including two types of Mn-rich calcite, ferroan calcite, siderite and dawsonite. They display an unusual broad spectrum of δ13C values (−54.99‰ to +8.8‰), suggesting both organic and inorganic CO2 injections. The δ13C values of siderite are close to 0, and its formation is related to meteoric water. The δ13C values of ferroan calcite and the occurrence of dawsonite indicate the trace of inorganic mantle-derived magmatic fluids. The δ13C values and trace elements of Mn-rich calcite record the information of hydrocarbon-bearing fluids. The fluid inclusions measurement data and reservoir properties and oil-test data show that the oil content of reservoir is not only affected by the formation time of different cements, but also by the relative content of dissolution and cementation. For these reservoirs altered by carbonate cements, it does not cause poor oil-bearing due to blockage of secondary minerals. [ABSTRACT FROM AUTHOR]

The Shahejie Formation is a fundamental rock unit for hydrocarbon exploration and production in the Nanpu Sag, Bohai Bay Basin. Methodology including thin sectioning, core observations, fluorescence, scanning electron microscopy, cathodoluminescence, fluid inclusion, laser scanning confocal microscopy and geochemical analysis (C, O isotopes) were all utilized to investigate the reservoir characteristics and origin of the Es1 Sandstone. Thin section study showed that the reservoir rock consisted of feldspathic litharenite and lithic arkose. The reservoir pores are categorized as intergranular pores, fracture pores, dissolution pores and intergranular cement dissolution pores. The studied sandstone had good porosity (0.05%–35%) and permeability (0.006–7000 mD). The Es1 reservoir is classified as a fractured reservoir, a primary intergranular pore‐associated reservoir and a dissolution reservoir. Deposition, diagenesis and tectogenesis are the main factors that played important roles in the development of the reservoir. Sedimentation is the foundation and assumption for reservoir development, but the effective reservoir is primarily controlled by primary pores, fractures, lithofacies, tectonic elements and dissolution pores. Moreover, compaction, fracture filling and cementation were the primary sources of reservoir densification. The reservoir was progressively formed through the influence of different geological and diagenetic events. The present study provides significant information and references for hydrocarbon exploration and development in the Nanpu Sag. [ABSTRACT FROM AUTHOR]

The internal fault architecture is crucial in assessing the significance of faults in fluid migration. The development of overlapping zones between segments and subsidiary structures is characteristic of a strike–slip faults. However, their internal architectures and roles in fluid migration are still poorly understood. The Tarim Basin's recently identified strike–slip faults imply that the petroleum resource is hosted in caves that were formed by subsequent dissolution after the formation of the fault zones in carbonate rocks, indicating that the internal fault architecture may be closely linked to the accumulation of petroleum. We investigated the architecture of the strike–slip fault zone using field, geochemical, seismic and well‐logging data. The results revealed that the strike–slip faults contain flower‐like structures in their vertical profiles and an en échelon and 'X' conjugate pattern in their horizontal slices. The fault core may become more complex because of the flower structure as fault breccia, slip surfaces, hydrothermal veins, dissolved pores and caves develop, and the damage zone contains multiple stages of fractures with high dip angles. Compared with 'X' pattern conjugate faults, NE‐trending strike–slip faults have a more developed and connected fault zone. The fault core acts as a fast conduit for fluid transport and experiences significant elemental losses, and the elemental variations in the damage zone may relate in long‐term and relatively lower‐level fluid–rock interactions. Three fault zone architecture models were created, namely, a releasing bend, a restraining bend and a single segment, and their controlling impacts on fluid migration were addressed accordingly. Our findings imply that fluid migration and accumulation are more favourable at the releasing bend than at the restraining bend and single segment. [ABSTRACT FROM AUTHOR]

This paper discusses the effect of the source rock–reservoir assemblage and the assemblage–induced variability in hydrocarbon expulsion and the subsequent molecular composition as well as molecular thermal maturity within a hybrid shale system. The characterization work was conducted on lithofacies and microlamina scales using core samples from the Chang 7 3 sub-member of the Triassic Yanchang Formation in the Ordos Basin, China. Samples were collected from a narrow interval with a depth range of less than 15 m, and the main characterization work was performed by Rock-Eval pyrolysis and GC–MS analysis. The results show that chemical fractionation of preferential expulsion and migration of the saturated fraction exists in the source rock–reservoir assemblages at both the lithofacies and lamina scales. However, the molecular composition behaves differently at the lithofacies and lamina scale's source rock–reservoir assemblages, in which ∑C 21 − /∑C 22+ is higher in lamina scale reservoir but lower in lithofacies scale reservoir. It is assumed that the low-molecular weight n -alkanes also follow molecular fractionation. The lithofacies reservoir has a lower ∑C 21 − /∑C 22+ because of the strong storage capacity of the laminated micro-reservoir within shale, which prevents the newly generated lighter oil from being charged into the lithofacies reservoir. The variation trends of thermal maturity indices Ts/hopane, the relative pregnane content, and TA(I)/TA(I + II) ratios, which have the same chemical basis with ∑C 21 − /∑C 22+ , carry the same maturity signature as ∑C 21 − /∑C 22+. The above profile of the molecular composition and molecular-derived thermal maturity parameters indicate that within the short interval of a shale system where no differences in thermal maturity are expected, chain scission reactions and their derived thermal maturity indicators are very sensitive to source rock and reservoir. In addition, within a shale system, oil is more easily to expel out from the organic-rich lithofacies that are interbedded with organic-lean lithofacies. Oil expulsion may promote both chain cracking of oil and subsequently kerogen decomposition. This may provide geological evidence to explain why the frequent-stacking assemblage of source rock and reservoir lithofacies in a hybrid shale system is an ideal target for shale oil exploration. [ABSTRACT FROM AUTHOR]

The Es1 sandstone is a fruitful hydrocarbon zone of Eocene Shahejie Formation. Therefore, the present study comprises the subsurface facies analysis established on the description of seven wells. Methods include core observation, thin section petrography, scanning electron microscope (SEM), cathodoluminescence (CL), and X-ray diffraction to evaluate the microfacies, lithofacies, and rock characteristics. Two microfacies of lithofacies have been established as sandstone microfacies and mudstone microfacies on the basis of grain size. Among them, sandstone is further divided into four sub-microfacies on the basis of microscopic studies, rock fragments and mineral composition, which are arkose sandstone, lithic arkose, feldspathic litharenite, and litharenite. These microfacies consisting quartz, feldspar as abundant detrital grains followed by rock fragments and micas, whereas calcite, ferro-calcite, quartz overgrowth, and clay are the main cement. The Es1 sandstone consists of good primary porosity and permeability as well as good secondary porosity caused by fracturing and dissolution of unstable minerals that enhance the reservoir quality. Whereas cementation and compaction reduce the reservoir characteristics. Studied formation divided into five lithofacies including conglomerate sandstone lithofacies, medium to coarse-grain sandstone lithofacies, fine sandstone lithofacies, siltstone lithofacies, and mudstone lithofacies. Core observation, sedimentary structures, facies sequences of the well log, and sedimentary composition reveal that Es1 deposited into the lacustrine delta fan depositional environment. Rock grain size indicates that Es1 contains mixtures of grains including pebbles, sand-size as well as minor silt and clay size. Grain size varies from conglomerate to clay size. The Es1 sandstone was deposited in braided river (conglomerates sandstone), channel bar, lacustrine, fluvial channel (massive bedding, cross-bedding, and graded bedding sandstone), deltaic, and flood-plain (siltstone and mudstone) sedimentary environment. It is concluded that higher transport energies in channels deposited coarse-grain sediments in shallowest part of the lake setting. However, the energy decreases towards greater depth which deposited mudstone in central part of the lake. [ABSTRACT FROM AUTHOR]

The Permian Fengcheng Formation is a set of complex sedimentary strata in the Mahu Sag, northwestern Junggar Basin, where various types of carbonate minerals occur. Coarse crystalline calcite assemblages and other carbonate minerals, such as dolomite, shortite and sodium carbonates, are also common in the Fengcheng Formation. Their dissolution by meteoric freshwater leaching at tectonic high positions, which was controlled by an unconformity, developed uncommonly high-quality secondary porosity and contributed to unconventional oil and gas exploration. Based on core analyses, thin section studies under optical and cathodoluminescence (CL) microscopes, geological interpretations of seismic profiles, and carbon and oxygen isotope composition analyses from coarse crystalline calcite assemblages and comparisons to other carbonate minerals, this study focuses on the origin of coarse crystalline calcite assemblages and the mechanism of unconformity-related dissolution to form hydrocarbon reservoir spaces. Coarse crystalline calcite assemblages are the products of multiple stages of recrystallization of other carbonate minerals in nearby host rocks. Hydrolysis of feldspar and volcanic sediments provided additional Ca2+ for calcite precipitation. The dissolution and reprecipitation of carbonate minerals were associated with organic maturation and hydrocarbon expulsion and migration. The crystallization process was slow, which allowed the crystal assemblages to grow to large sizes. Organic carbon was involved in the formation of the coarse crystalline calcite assemblages, which resulted in much more negative carbon isotope compositions than those of other carbonate minerals via diluting the carbon isotope compositions of preexisting carbonate minerals. Later tectonic uplift caused a regional unconformity and formed dissolution reservoirs in tectonically high positions. Furthermore, faults and fractures provided paths for the solutes to transfer to lower positions and reprecipitate when they became saturated. [ABSTRACT FROM AUTHOR]

The characteristics, heterogeneous distribution and origins of carbonate cements within beach-bar sandstones in the Boxing Sag and implications on the mass transfer from mudstone to sandstone during diagenetic process was investigated systematically by an integrated analysis invoking petrography, carbon and oxygen stable isotopes and porosity-permeability, with specific references to the contrasts among the sharp (S)-change lithology (S-mudstone-S-sandstone interbeds), the gradual (G) -change lithology (G-mudstone-G-sandstone interbeds) and the center of sandstones. There are distinct differences in the occurrence, content, distribution, carbon and oxygen isotopic compositions and precipitation temperatures of the carbonate cements among the S-sandstone, G-sandstone and center of sandstone. Material sources of carbonate cements in the S-sandstones are mainly derived from the diagenetic alterations of the nearby S-mudstones through mass transfer with the participation of organic CO 2 , while in the G-sandstones, the material sources are mainly internal and originated from the calcareous matrix. Dissolution of feldspars and conversion of clay minerals provided the main materials for the precipitation of carbonate cements in the center of sandstones. Mass transfer of carbonate cements mainly occurred between the S-mudstones and the S-sandstones with limited transmission distance, which controlled the distribution of carbonate cements in the S-mudstones. There was almost no mass transfer between the G-mudstones and G-sandstones because of the small concentration gradient, instead, the depositional components and recrystallization controlled the carbonate distribution of the G-sandstones. • Material sources of carbonates in S-sandstones were mainly from nearby S-mudstones. • Material sources of carbonates in G-sandstones were mainly from in situ recrystallization. • Mass transfer of carbonate cements mainly occurred through sharp contact between mudstone and sandstone. [ABSTRACT FROM AUTHOR]

The study of pore characteristics is of great importance in reservoir evaluation, especially in deeply buried sandstone. It controls the storage mechanism and reservoir fluid properties of the permeable horizons. The first member of Eocene Shahejie Formation (Es1) sandstone is classified as feldspathic litharenite and lithic arkose. The present research investigates the pore characteristics and reservoir features of the deeply buried sandstone reservoir of Es1 member of Shahejie Formation. The techniques including thin-section petrography, mercury injection capillary pressure (MICP), scanning electron microscopy and laser scanning confocal microscope images were used to demarcate the pores including primary intergranular pores and secondary intergranular, intragranular, dissolution and fracture pores. Mercury injection test and routine core analysis were led to demarcate the pore network characteristics of the studied reservoir. Pore size and pore throat size distribution are acquired from mercury injection test. Porosity values range from 0.5% to 30%, and permeability ranges 0.006–7000 mD. Pore radii of coarse-grained sandstone and fine-grained sandstone range from 0.2 to > 4 µm and 1 nm to 1.60 µm, respectively, by MICP analysis. The mineral composition also plays an important role in protecting the pores with pressure from failure. Fractured sandstone and coarse-grained sandstone consist of large and interconnected pores that enhance the reservoir porosity and permeability, whereas fine-grained sandstone and siltstone consist of numerous pores but not well interconnected, and so they consist of high porosity with low permeability. [ABSTRACT FROM AUTHOR]

• Carbonate cements and chlorite can alter some pore spaces from water-wet to oil-wet. • High oil saturation occurred in tight reservoirs with moderate carbonates or chlorite. • The high quality reservoirs mainly located in between with sandstone-mudstone interfaces and central parts of the sand bodies. Oil accumulation, being difficult and complicated, is an important issue in petroleum exploration researches. Authigenic minerals, such as carbonate cements and chlorite, can in certain reservoirs alter the wettability of some pore spaces from water-wet to oil-wet. Generally, these oil-wet pore spaces are favorable for oil accumulation. The alteration of reservoir wettability induced by authigenic minerals and the impacts on oil accumulation in tight sandstone reservoirs were investigated using a suite of mineralogical and geochemical characterization techniques, including thin section observation, SEM-EDS, XRD, QEMSEM, CL, quantitative grain fluorescence (QGF), fluorescence spectral analysis, contact angle measurement and sealed coring oil saturation testing on the fourth member of the Lower Cretaceous Quantou Formation (K 1 q 4) in the southern Songliao Basin, China. The study shows that the tight sandstone reservoirs are compositionally immature with detrital grains distributed homogeneously. Quartz, carbonates and clay minerals that show heterogeneous distribution characteristics are the major authigenic minerals in some parts of the intergranular pores. The detrital mineral assemblage suggests that the reservoir rocks at deposition and before diagenesis had characterized by strong water-wet properties. With the development of authigenic minerals, carbonate cements and authigenic chlorite tend to alter the wettability of some parts of the existing pore spaces from water-wet to oil-wet. In the K 1 q 4 sandstone reservoirs, oil prefers to accumulate in the cemented residual pore spaces around carbonate cements and chlorite. Reservoirs containing about 4–5% carbonate cements are suggested to be more preferable to oil accumulation. These reservoirs are mainly located between sandstone-mudstone interfaces and central parts of the sand bodies. Chlorites have mainly two effects: on one hand, chlorite alters the wettability of existing pore spaces and provides preferential accumulation sites for oil in the tight sandstone reservoirs, on the other hand it can reduce the adhesion of oil through forming "clay-oil flocs" and promote further migration. Consequently, the reservoirs with moderate amount of carbonate cements or chlorite always show relative high oil saturation in the K 1 q 4 tight sandstones. [ABSTRACT FROM AUTHOR]

Net pay cut-off values are key parameters in the evaluation of both conventional and unconventional reservoirs. However, there is no widely accepted method for determining net pay cut-off values. The low permeability turbidite reservoirs in the middle part of the third member of the Shahejie Formation (Es3z) in the Dongying Sag are taken as an example; the net pay permeability and porosity cut-off values are determined according to detailed core descriptions, thin section analyses, porosity and permeability test, mercury injection analyses, and well test data. Taking economic benefits into consideration, the concept of a net pay permeability cut-off value refers to the minimum permeability of a clastic reservoir that can produce hydrocarbons and meet the minimum economic benefits under the availability of existing technologies. The proposed methodology of this work is based on balancing the cost and income, and considers related parameters such as the cost of well drilling, investment in surface construction, operational cost, oil price, and oil production. The net pay permeability cut-off values of reservoirs with different thicknesses of development intervals are calculated based on the constraint of the oil price at $50/bbl. Then, the net pay porosity cut-off values are calculated following the pore-throat structure analyses. The proposed method and the net pay permeability and porosity cut-off values for the low permeability reservoirs in this article are acceptable since fit for purpose principle and data available principle have been considered. Meanwhile, this method enables the dynamic evaluation of reservoirs at any burial depth. • The concept of net-pay permeability cutoff in low permeability reservoirs. • The determination method of net-pay permeability cutoff values. • The determination method of net-pay porosity cutoff values. [ABSTRACT FROM AUTHOR]

As important metal sulfides in the geochemical cycle of sulfur, the characteristics and formation processes of pyrites can provide useful clues regarding their environment. Based on previous findings, shale pyrites were divided into three major classes (euhedral pyrites, framboidal pyrites (framboids) and metasomatic pyrites) and six sub-classes in this study. At the microscopic scale, each type of pyrite is associated with a different formation process. Framboids are formed by burst nucleation in environments with a homogeneous distribution of nutrients while euhedral pyrites are usually formed on pre-existing sites (such as =FeS on the minerals surface) in the heterogeneous system. Metasomatic pyrites formed by the replacement of other ions in accountable material by iron ions and hydrogen sulfide ions in hydrothermal events. The morphology and isotope value of pyrite provide information to track the origins of their nutrient and characteristics of sulfur and iron pools. In addition, the trace element content of pyrite can serve as a proxy for paleo-ocean trace element abundance, indicating changes in atmospheric oxygen content. Additionally, pyrite can also serves as an indicator of shale gas reservoirs. [ABSTRACT FROM AUTHOR]

Facies identification is a fundamental issue on evaluation and prediction for siliciclastic sediments. The purpose of this paper is to identify sedimentary-diagenetic facies and investigate the influence of sedimentary-diagenetic facies on reservoir quality in coarse-grained deposits from the Eocene Es4 interval, Dongying Depression, Bohai Bay Basin. Based on detailed core observation, thin section petrography combined with conventional wireline logs and core measurement analysis, 12 types of sedimentary- diagenetic facies were divided according to their lithofacies, diagenetic products and related contributions to reservoir quality. The identification and spatial distribution of these sedimentary-diagenetic facies can be determined based on the Bayes discriminant analysis and related cross-plot analysis. The variations of porosity and permeability with burial depth were precisely determined with ±2% for porosity and ±1 × 10–3 μm2 for permeability. The spatial distribution of reservoir porosity and permeability can be predicted quantitatively: high zones of porosity and permeability mainly distributed in the central sections of thick-bedded sandbody, ranging from 6% to greater than 12% for reservoir porosity and from 1 × 10–3 μm2 to greater than 6 × 10–3 μm2; relatively low zones of porosity (less than 6%) and permeability (less than 1 × 10–3 μm2) mainly occur in the bottom and the margin of the thick-bedded sandbody. [ABSTRACT FROM AUTHOR]

Abstract The feldspar group is one of the most common types of minerals in the earth's crust. Feldspar alteration (including the whole processes of feldspar dissolution, transfer of released solutes, and secondary mineral precipitation) is ubiquitous and important in fields including resources and environmental sciences. This paper provides a critical review of feldspar alteration and its geological significance in shallow aquifers to deep hydrocarbon reservoirs, as assessed from peer-reviewed paper in the literature. A variety of mechanisms such as the surface reaction-controlled dissolution mechanism, the preferential leaching-diffusion controlled mechanism, the diffusion-precipitation controlled dissolution mechanism and the interfacial dissolution-reprecipitation mechanism have been proposed to be responsible for the dissolution of feldspars. Feldspar dissolution rates can be affected by the crystal structure, Al/Si ordering, temperature, pH, surface area, organic acids, chemical affinity, and precipitation of secondary minerals. Five main dissolution rate laws have been used to describe feldspar dissolution rates, including the linear transition state theory (L-TST) rate law, non-linear TST rate law, parallel rate law, stepwave model rate law, and partial equilibrium law. The rate inconsistency between laboratory experiments and field observations is interpreted with hypotheses that include the armoring effects of the coating secondary minerals on feldspar surfaces, the possible effects of leached layers, the approach to saturation with respect to feldspars, the inhibition by absorbed Al3+ on the feldspar surface, and the inhibition by simultaneous slow clay precipitation rates. The inorganic-original (meteoric water and deep hot water) and organic-original (kerogen and hydrocarbon degradation) hydrogen ion (H+) in a fluid can probably act as a significant catalyzer of fast dissolution of feldspars in shallow aquifers and deep hydrocarbon reservoirs. Various mineral assemblages including extensively leached feldspars with a wide range of associated amounts of clay minerals and quartz cements can be identified in subsurface reservoirs under different geological conditions. Feldspar dissolution can generate enhanced secondary porosities and rock permeability in open geochemical systems at shallow depth or at a moderate-deep depth where faults develop widely. While in closed geochemical systems at moderate-deep depth, feldspar dissolution is likely to generate redistributional secondary porosities and to decrease rock permeability. Authigenic clay minerals formed following feldspar dissolution alter rock wettability and affect the charging and entrapment of hydrocarbons in reservoir. Feldspar alteration may promote hydrocarbon degradation by promoting bioactivity or by consuming low molecular weight organic acids and CO 2 produced via oil degradation. Further work should be conducted to study hydrocarbon-water-feldspar interactions in deeply buried hydrocarbon reservoirs. Feldspar alteration may promote CO 2 sequestration by consumption of H+, generation of HCO 3 −, and pH buffering of formation water. K-feldspar alteration may also promote illitization in interbedded mudstones by supplying K+. [ABSTRACT FROM AUTHOR]

Abstract Subaqueous sediment gravity-flow (SSGF) deposits have recently been recognized as a major reservoir plays in lacustrine basins. Research on their genesis and depositional models are crucial to understand gravity flow evolution and the distribution of their deposits. This study analysed SSGF deposits in the third member of the Shahejie Formation in the Jiyang Depression, Eastern China, based on 3-D seismic data, well-log data, and core observations. The types and origins of SSGF, the distribution patterns and depositional models of SSGF deposits have been investigated. The dominant types of SSGF include: muddy debris flows, sandy debris flows, high density turbidity currents, surge-like low density turbidity currents, and quasi-steady low density turbidity currents, which are mainly caused by river floods and sediment failures. The overall distribution of SSGF deposits is controlled by the sediment supplies, basin structure, and sequence stratigraphic framework. Paleotopography, flood energy, triggering mechanism, and the slope angle of the delta front are also important. SSGF deposits associated with river floods can be further subdivided into elongated and fan-shaped deposits. Elongated sandstones are composed predominantly of bedload-dominated and suspension load-dominated hyperpycnal flow deposits. The fan-shaped sandstones can be subdivided into an inner fan, a middle fan, and an outer fan. Inner fan deposits are dominated by gully-filling debrites. Middle fan deposits are composed predominantly of channel-fill deposits, while outer fan deposits consist of thin bedded and laterally extensive lobe intervals. SSGF deposits triggered by sediment failures can be further subdivided according to their proximity to slumping: proximal, intermediate and distal parts. The proximal deposits are composed predominantly of slide and slump deposits. The intermediate deposits are dominated by sandy debrite tongues, while the distal deposits are composed predominantly of thin-bedded, surge-like, low-density turbidites. This study offers some insight into the types, origin of gravity-flow, as well as the distribution of their deposits in a lacustrine rift basin. Highlights • Types of subaqueous sediment gravity-flow in a lacustrine rift basin. • Origin of subaqueous sediment gravity-flow deposits in a lacustrine rift basin. • Controlling factors of distribution pattern of subaqueous sediment gravity-flow deposits in a lacustrine rift basin. • Depositional model of subaqueous sediment gravity-flow deposits in a lacustrine rift basin. [ABSTRACT FROM AUTHOR]

Abstract Organic-inorganic interactions after oil charging are critical for determining the ongoing evolution of hydrocarbons and rock quality in water-wet siliciclastic reservoirs. It is the conceptual approach of this study to simulate and decipher these interactions by using quantitative analyses of the interrelated changes of minerals, water, hydrocarbons, gases, and organic acids in heated oil-water-rock systems. The experimental results show that organic-inorganic interactions occur between the organic oil and inorganic feldspar in the presence of water. Water promotes the oil degradation by an extra supply of H+ and OH− ions. In the oil-water-rock systems, mutual exchanges of H+ and OH− ions among minerals, water, and hydrocarbons probably result in the mutual interactions between oil degradation and mineral alteration, with water serving as a matrix for the ion exchange. In the oil-water-feldspar system, feldspar alteration does not cease in the oil zone in the presence of some water and, inversely, oil degradation can even accelerate the alterations of the aluminosilicate minerals. The mineral alterations from feldspar to boehmite, illite, and muscovite promote the oil degradation in the oil-deionized water-feldspar systems with the mutual ion exchange. Due to a possible competition between the Brønsted acid sites in the minerals and the halide anions or the direct replacement of K-feldspar by albite, the mineral alterations retard the oil degradation in the oil-NaCl water-feldspar systems. The experimental results also indicate that oil degradation via free radical cross-linking and free radical thermal cracking is extensive in anhydrous oil systems and may lead to blocking of pores and throats by the generated pyrobitumen in the oil zone of a reservoir. In contrast, in the oil-water transition zone of a reservoir, oil degradation via thermal cracking and oxidative decomposition may dominate the oil degradation and may lead to the generation of secondary pores with leaching of minerals by the generated CO 2 and organic acids. This study demonstrates that organic-inorganic interactions are complex and proceed in diverse pathways in different hydrogeochemical systems. Thus, further quantitative investigations of the reaction pathways and reaction kinetics of coupled mineral alteration and oil degradation in oil-water-rock systems are warrented. Meanwhile, the incorporation of such organic-inorganic reactions into geochemical models will improve the prediction of the evolution of organic and inorganic species in petroleum reservoirs. [ABSTRACT FROM AUTHOR]

This article reviews the applications of light stable isotope, including carbon, oxygen and hydrogen, in the studies on origin and formation temperature of authigenic carbonate, quartz and clay minerals. Theoretical knowledge and analytical methods for major light stable isotopes are introduced in detail. Negative and positive δ13C values indicate significant differences on the origin of carbonate cements. The δ18O value is an effective palaeotemperature scale for authigenic minerals formation. Various fractionation equations between δ18O and temperature are proposed for carbonate cements, quartz cements and clay minerals, whose merit and demerit, applicable conditions are clarified clearly. Clumped isotope analysis can reconstruct the temperature of carbonate precipitation with no requirement on the δ18O of initial waters, which makes temperature calculation of carbonate cements formation more convenient and accurate. Hydrogen and oxygen isotopes mainly reflect the origin of diagenetic fluid for clay mineral formation, providing reliable evidence for diagenetic environment analysis. This work aims at helping researchers for better understanding the applications of light stable isotope in sandstone diagenesis. [ABSTRACT FROM AUTHOR]

Abstract The Lower Permian Fengcheng Formation in the Mahu Sag of northwestern China is well-known for its alkaline lacustrine deposits. However, previous research is scarce due to this formation's notable burial depth (up to 5000 m) and limited oil and gas exploration. Using XRD, XRF, micro-XRF, and C-O isotope analyses, we determined: (1) the stratigraphic trends in mineral compositions, (2) the cyclic depositional processes, and (3) paleoclimate variations. The petrologic and geochemical data elucidate depositional and diagenetic processes in the mixed siliciclastic, volcaniclastic and chemical strata of the Fengcheng Formation. Sodium carbonate minerals in the studied core are dominated by wegscheiderite (Na 2 CO 3 ·3NaHCO 3) and trona (Na 2 CO 3 ·NaHCO 3 ·2H 2 O). Nahcolite (NaHCO 3) and a Mg-Na carbonate mineral, northupite (Na 2 CO 3 ·MgCO 3 ·NaCl), also occur, although their abundance is obviously lower than that of wegscheiderite and trona. These substances grew as bottom-growth crystals from lakes overlying tuffs. The volumetrically important Ca-Na carbonate mineral, shortite (Na 2 CO 3 ·2CaCO 3), and some northupite precipitated diagenetically from shallow groundwater brines in unlithified, dark-colored siliciclastic sediment and volcanic ash. Shortite mainly precipitated from the Ca-Na-CO 3 -rich shallow groundwater, whereas northupite precipitated from the Mg-Na-CO 3 -Cl-rich shallow groundwater. Sodium carbonates and tuffs, fine-grained tuffaceous rocks, and siliciclastic rocks formed repetitive centimeter- and meter-scale cycles that are shown as dark-colored lithological layers during lake expansion and contraction stages or during volcanic eruptions. The brine composition, which controlled the precipitation of sodium carbonates, was influenced strongly by volcanism and volcanism-related hydrothermal activity. Changes in climate and volcanic eruptions controlled the deposition of alternating beds of sodium carbonates and dark-colored lithologies. Changes in climate caused the expansion and contraction stages in the lakes. During relatively humid climate stages, diluted water entered the basin and resupplied the lake water with calcium and magnesium during flooding events. Sandy to silty beds were deposited by turbidity currents during this stage. During arid climate stages, bedded sodium carbonates grew in the lakes, and their co-occurrence suggests high atmospheric pCO 2 and high temperatures. The ashes that are provided by volcanic eruptions during arid climate stages can also stop the precipitation of sodium carbonates. Highlights • Sodium carbonates successions consist of alternating light-colored sodium carbonates layer and dark-colored tuff/tuffaceous layer. • The sodium carbonate minerals dominated by wegscheiderite and trona. Shortite and northupite occurred as transitional carbonate minerals. • Climates evolutions and periodic volcano eruptions joint to control the formation of depositional cycles in the sodium carbonates successions. [ABSTRACT FROM AUTHOR]

Abstract The Permian Fengcheng Formation in the Mahu Sag in the Junggar Basin represents deposition in a rare alkaline saline lake, with hydrologically closed conditions, alkaline saline brines, and intense volcanic activity. The Fengcheng Formation contains large volumes of high-quality source rocks, which form not only in the depocenter but also in the wide, shallow lake margins. The source rocks analyzed in this study are different from those in most other ancient lakes in the following aspects: genesis and occurrence of organic matter, kerogen type, lithological composition, and hydrocarbon migration pathway. This study showed that the Fengcheng Formation has high hydrocarbon generation potential and that combinations of multiple paths enhanced the hydrocarbon expulsion and migration. Benthic microbial mats and algae that bloomed in the alkaline saline lake represent the dominant types of organic matter. Due to their specific composition, the evaporites may have catalyzed hydrocarbon generation via organic-inorganic interactions, while intergranular seams in bedded/laminated evaporites provided paths for hydrocarbon migration. Stylolites, with high contents of brittle minerals and low clay content, also allowed favorable paths for hydrocarbon expulsion and migration. Combining hydrocarbon expulsion fractures in the organic-rich laminae, fractures and microfractures caused by compressive tectonic stress, pathway networks formed that enhanced the efficiencies of both hydrocarbon expulsion and migration. Highlights • Benthic microbial mats and algae are dominant organic matters in a Permian alkaline saline lake. • The combinations of hydrocarbon expulsion microfractures, tectonic microfractures, stylolites enhanced the efficiencies of hydrocarbon expulsion and migration. • A Permian alkaline saline lake formed with hydrological closed conditions and intense volcanic activity. [ABSTRACT FROM AUTHOR]

Abstract: The origin of the fourth member of the Eocene Shahejie Formation in the northern steep slopes of the Minfeng Sub‐sag, Dongying Sag, China, was investigated by integrating core studies and flume tank depositional simulations. A non‐channelized depositional model is proposed in this paper for nearshore subaqueous fans in steep fault‐controlled slopes of lacustrine rift basins. The deposits of nearshore subaqueous fans along the base of steep border‐fault slopes of rift basins are typically composed of deep‐water coarse‐grained sediment gravity‐flow deposits directly sourced from adjacent footwalls. Sedimentation processes of nearshore subaqueous fans respond to tectonic activities of boundary faults and to seasonal rainfall. During tectonically active stages, subaqueous debris flows triggered by episodic movements of border‐faults dominate the sedimentation. During tectonically quiescent stages, hyperpycnal flows generated by seasonal rainfall‐generated floods, normal discharges of mountain‐derived rivers and deep‐lacustrine suspension sedimentation are commonly present. The results of a series of flume tank depositional simulations show that the sediments deposited by subaqueous debris flows are wedge‐shaped and non‐channelized, whereas the sediments deposited by hyperpycnal flows generated by sporadic floods from seasonal rainfall are characterized by non‐channelized, coarse‐grained lobate depositional bodies which switch laterally because of compensation sedimentation of hyperpycanal flows. The hyperpycnal‐flow‐deposited non‐channelized lobate depositional bodies can be divided into a main body and lateral edges. The main body can be further subdivided into a proximal part, middle part and frontal part. Normal mountain‐derived river‐discharge‐deposited sediments are characterized by thin‐bedded, fine‐grained sandstones and siltstones with a limited distribution range. Normal mountain‐derived river‐discharge‐deposited sediments and deep‐lacustrine mudstones are commonly eroded in the area close to boundary faults. A nearshore subaqueous fan can be divided into three segments: inner fan, middle fan and outer fan. The inner fan is composed of debrites and the proximal part of the main body. The middle fan consists of the middle part of the main body and lateral edges, normal mountain‐derived river‐discharge‐deposited fine‐grained sediments and deep‐lacustrine mudstones. The outer fan comprises the frontal part of the main body, lateral edges, and deep‐lacustrine mudstones. Based on the non‐channelized depositional model for nearshore subaqueous fans, criteria for stratigraphic subdivision and correlation are discussed and applied. [ABSTRACT FROM AUTHOR]

• Extracted bitumen and solid bitumen were characterized simultaneously. • Characterization was carried out at lamina scale and lithofacies scale. • Biomarkers reveal greater thermal maturity in source rocks than reservoirs. • Raman spectroscopy reveals greater thermal maturity in reservoirs than source rocks. • Organic–inorganic interactions increase the aromaticity of solid bitumen. Organic solvent extracted bitumen (EB) and microscopically observed solid bitumen (SB) carry many geological implications in unconventional source-rock reservoirs. EB is a commonly used term in organic geochemistry, and SB is normally used in organic petrology. Although both EB and SB are secondary organic matter initially formed from kerogen degradation and partially describe the same components, they are defined by different physical and chemical criteria and have specific applications, and thus are rarely comparatively investigated. In this study, by taking the shale of the seventh member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, as a case study, we performed an integrated characterization on the two types of bitumen. The characterization was carried out on source rocks and reservoir assemblages at two scales: lamina-scale (organic-rich lamina and silty lamina) and lithofacies-scale (shale and sandstone). Programmed temperature pyrolysis (Rock-Eval 7 pyrolysis), gas chromatography–mass spectrometry (GC–MS), and Raman spectroscopy were used in this study. The samples are distributed within a 12 m interval and therefore should have experienced the same degree of thermal stress. However, maturity parameters derived from GC–MS for EB and Raman spectroscopy for SB exhibit a different inferred thermal maturity between source rocks and reservoirs at both lithofacies- and lamina-scales. The side-chain scission reactions related (non-)biomarker parameters such as Ts/C 30 H, Σ n C 21− /Σ n C 22+ alkanes, relative C 21 + C 22 sterane content and TA(I)/TA(I + II) suggest higher thermal maturity in source rocks (shale and organic-rich lamina) than the corresponding reservoirs (sandstone and silty lamina), while Raman-derived parameters RBS and G-FWHM indicate higher maturity in reservoirs than the corresponding source rocks. It is speculated that the EB measured by GC–MS comprises saturated and aromatic components corresponding to relatively mobile hydrocarbons. The maturation of the source rock exerts greater control over this component than that of the reservoir. In comparison, the SB measured using Raman spectroscopy mainly consists of solid residue left behind after migration and/or decomposition of a once-liquid oil phase that is less readily able to move. It is more intensely altered by the organic–inorganic interactions (mineral dissolution–precipitation processes) in the reservoir than that in the source rock, resulting in a consolidated SB with higher aromaticity. The storage ability of silty lamina in shale may complicate the data interpretation of geochemical differences in EB- and SB-derived parameters. On a practical note, when assessing thermal maturity, taking into the account the lithology or rock texture, which affects the organic–inorganic interactions, as well as specific components detected by different techniques may provide helpful clues to explain some contradictory results. [ABSTRACT FROM AUTHOR]

The diagenetic evolution of the lower Eocene red-bed high-quality reservoirs in the Dongying Depression was systematically investigated through an integrated petrographic, petrophysical, fluid evolution and thermal history analysis. The reservoirs experienced several phases of diagenesis, including compaction, carbonate cementation, gypsum and ankerite cementation, feldspar and carbonate dissolution, quartz overgrowth cementation, and clay-mineral cementation and transformation. An early alkaline diagenetic environment caused significant cementation of calcite and gypsum in the reservoir sandstone units near the sandstone-mudstone contacts. Acidic formation fluids caused widespread dissolution of the feldspar and carbonate cements, leading to synchronous occurrences of primary intergranular pores and acidic dissolution pores. From the middle parts of the reservoir sandstone units to the sandstone-mudstone contacts, dissolution wanes gradually with ferro-carbonate progressively diminishing. Migration of exogenetic thermal fluid along faults may have contributed to the gradual decrease in dissolution and the gradual increase in cementation from the lower part to the upper part of fault blocks. The alternating alkaline and acidic diagenetic environments and the distribution of diagenetic products caused the development of the high-quality red-bed reservoirs in the middle part of the thick-layered sandstone units in the lower part of fault blocks. Because of the spatial and temporal relationships between the diagenetic stages and their products, lithological traps of a diagenetic origin were developed in the red-bed reservoirs. [ABSTRACT FROM AUTHOR]

Lacustrine carbonate-rich shales are well developed within the Mesozoic-Cenozoic strata of the Bohai Bay Basin (BBB) of eastern China and across southeast Asia. Developing an understanding of the diagenesis of these shales is essential to research on mass balance, diagenetic fluid transport and exchange, and organic-inorganic interactions in black shales. This study investigates the origin and distribution of authigenic minerals and their diagenetic characteristics, processes, and pathways at the scale of lacustrine laminae within the Es4s-Es3x shale sequence of the BBB. The research presented in this study is based on thin sections, field emission scanning electron microscope (FESEM) and SEM-catholuminescence (CL) observations of well core samples combined with the use of X-ray diffraction (XRD), energy dispersive spectroscopy, electron microprobe analysis, and carbon and oxygen isotope analyses performed using a laser microprobe mass spectrometer. The dominant lithofacies within the Es4s-Es3x sequence are a laminated calcareous shale (LCS-1) and a laminated clay shale (LCS-2). The results of this study show that calcite recrystallization 1 is the overarching diagenetic process affecting the LCS-1, related to acid generation from organic matter (OM) thermal evolution. This evolutionary transition is the key factor driving the diagenesis of this lithofacies, while the transformation of clay minerals is the main diagenetic attribute of the LCS-2. Diagenetic differences occur within different laminae and at variable locations within the same lamina level, controlled by variations in mineral composition and the properties of laminae interfaces. The diagenetic fluid migration scale is vertical and responses (dissolution and replacement) are limited to individual laminae, between zero and 100 μm in width. In contrast, the dominant migration pathway for diagenetic fluid is lateral, along the abrupt interfaces between laminae boundaries, which leads to the vertical transmission of diagenetic responses. The recrystallization boundaries between calcite laminae act as the main migration pathways for the expulsion of hydrocarbons from these carbonate-rich lacustrine shales. However, because the interaction between diagenetic fluids and the shales themselves is limited to the scale of individual lamina, this system is normally closed. The occurrence of abnormal pressure fractures can open the diagenetic system, however, and cause interactions to occur throughout laminae; in particular, the closed-open (C–O) diagenetic process at this scale is critical to this shale interval. Multi-scale C–O systems are ubiquitous and episodic ranging from the scale of laminae to the whole basin. Observations show that such small-scale systems are often superimposed onto larger ones to constitute the complex diagenetic system seen within the BBB combining fluid transport, material and energy exchange, and solid-liquid and organic-inorganic interactions. [ABSTRACT FROM AUTHOR]

The diagenetic evolution and chemical changes of mudstones have a significant influence on the reservoir quality of the mudstones and their interbedded sandstones. To investigate, a variety of methods were applied to the mudstones in the middle of the third member of the Shahejie Formation (Es3z), which were formed in a lacustrine basin during 43.7–38.2 Ma. Optical microscopy and XRD analysis show that the mudstones are dominated by clay minerals (56.8%) followed by carbonate (18.2%), quartz (14.9%), feldspar (8.4%), pyrite (0.9%), anhydrite (0.8%), and total organic carbon (TOC: 2.4%). Cementation and replacement are the main diagenetic events in the mudstone of Es3z. Carbonates and clay minerals are the most common cements, but occasionally authigenic pyrite, albite, and quartz are also present. At the eogenetic stage, the diagenetic events comprised precipitation of pyrite, siderite, dolomite, and calcite besides transformation of K-feldspar into kaolinite. During the mesogenetic stage, the main diagenetic events included precipitation of fracture-filling calcite, ferroan calcite, ankerite, microcrystalline quartz, quartz overgrowth, and transformation of K-feldspar into illite and albite. The limited variation of the Nb/Ta and Zr/Hf ratios and the almost identical distribution pattern of the REE and trace elements indicate that the detrital material in the mudstones was sourced by a common provenance of felsic igneous rocks. The CIA values of Es3z mudstones vary from 50.59 to 65.05 and the PIA values vary from 50.86 to 75.58, which confirm a low-intensity weathering of the source area. Thus, the variation in the bulk-rock geochemical composition of the mudstones may not be primary, but instead a result of mass transfer with the interbedded sandstones during diagenesis. Changes ratios between aim elements (Al, Si, Ca and K) and immobile elements with burial depth in combination with mass balance calculation indicate that aluminum and silicon were retained by the precipitation of authigenic minerals in the mudstone. The mudstones have received progressively more potassium with increasing depths, whereas calcium has been expelled. The reservoir quality of the Es3z sandstones has mainly been affected by the diagenetically induced decrease in the carbonate content in the mudstones causing carbonate cementation in the sandstones. [ABSTRACT FROM AUTHOR]

The lacustrine deep-water gravity-flow sandstone reservoirs in the third member of the Shahejie Formation are the main exploration target for hydrocarbons in the Dongying Sag, Eastern China. Carbonate cementation is responsible for much of the porosity and permeability reduction in the lacustrine deep-water gravity-flow sandstone reservoirs. The sandstones are mainly lithic arkose with an average framework composition of Q 43 F 33 L 24 . The carbonate cements are dominated by calcite, ferroan calcite, ankerite and a small amount of dolomite. The calcite and ferroan calcite are mainly poikilotopic blocky crystals, while the dolomite and ankerite are mainly euhedral rhombohedra crystals filling intergranular and intragranular pores. The relatively positive δ 13 C values (−2‰ to +3.9‰) of the carbonate cements in the sandstone reflect a mainly inorganically sourced carbon. From 32 Ma to 25 Ma, the pore water was rich in bicarbonate and Ca 2+ due to carbonate dissolution in mudstone, and which were transported with the pore water from mudstone to sandstone via advection and precipitated calcite cementation in thinly bedded sandstones and some high permeability zones in the middle of medium-to-thick sandstone beds. From 12 Ma to present, abundant Ca 2+ , Fe 3+ , Fe 2+ , Mg 2+ and bicarbonate had been transported from mudstone to sandstone via diffusion to form tight ferroan calcite cementation in the upper and lower parts of the medium-to-thick bedded sandstones. Ankerite is mainly distributed in the reservoirs associated with oil migration or charge, because change of Fe 3+ to Fe 2+ from oil charge may supply sufficient Fe 2+ for ankerite precipitation. The center of sandstone beds (>0.6 m) is with potential of high-quality reservoirs in the research area. Carbonate cementation appears to be an important factor that controls the accumulation of oil in deep-water gravity-flow sandstone reservoirs in the study area. [ABSTRACT FROM AUTHOR]

Petrographic analysis combined with thin section observation, fluid inclusions, burial history analysis, and core porosity and permeability, were used to determine diagenetic evolution and its effect on reservoir-quality of beach-bar sandstone reservoirs of Eocene Es4s interval in the Boxing sag, Bohai Bay Basin. Diagenesis is mainly characterized by strong compaction, undercompaction, cementation of carbonate, quartz, pyrite and clay minerals, dissolution of detrital grains and carbonate cements, lime-mud matrix recrystallization and hydrocarbon filling. Paragenetic sequence is determined as 'early compaction / precipitation of calcite, dolomite and framboidal pyrite → dissolution of feldspar grains and early-formed carbonate cements/precipitation of quartz → hydrocarbon filling → precipitation of ferroan calcite and ankerite/lime-mud matrix recrystallization → dissolution of ferroan carbonate cements'. In the same diagenetic environment, diagenetic processes for different lithofacies significantly varied and subsequently resulted in great heterogeneity for reservoir quality during progressive burial. Lithofacies for both bar body and beach edge subfacies which are far from sandstone-mudstone contacts, are well sorted and have a strong ability to resist compaction. Abundant primary and secondary porosities are well developed because of modifications by acidic dissolution and hydrocarbon filling. The present physical properties are relatively well and numerous oil beds are developed through shallow to deep burial depth. Lithofacies for both bar body and beach edge subfacies which are adjacent to sandstone-mudstone contacts, are typically characterized by extensive cementation of calcite and dolomite. The present physical properties are bad and dry beds are extensively developed for sandstone reservoirs in both bar body and beach edge subfacies which are adjacent to sandstone-mudstone contacts. Lithofacies for both bar edge and beach mat subfacies were deposited in weak hydrodynamic conditions with high contents of limemud matrix. The reservoir porosity decreases continually due to strong compaction and weak dissolution. The present physical properties are extremely poor and dry beds are commonly developed through shallow to deep burial except for minor oil beds developed in the shallow burial. [ABSTRACT FROM AUTHOR]

• An optimal amount of low-permeable layers can increase the swept area of CO 2 plume. • An optimal amount of low-permeable layers can reduce the ratio of CO 2 in free phase. • Proximal-medial fluvial lithofacies associations are more suitable for CO 2 storage. Fluvial system deposits often form suitable reservoirs for CO 2 geological storage (CGS). These potential storage sites usually present heterogeneous fluvial architectures at multiple scales. This heterogeneity can exert varied effects on different aspects of CGS, resulting in significant storage efficiency variability. Here, we investigate the effects of variable fluvial lithofacies associations on CO 2 storage, using the Puig-reig anticline in the SE Pyrenees (Spain) as a reservoir analog. To test this, we employ a multidisciplinary approach that integrates field sedimentology, reservoir modeling, and numerical simulation of CO 2 injection to produce models akin to different fluvial lithofacies associations. The storage volume and injectivity of CO 2 are found to decrease in reservoirs with decreasing fractions and sizes of high-permeable facies from the proximal to the medial-distal lithofacies associations. The flow barriers created by low-permeable facies can hinder the vertical migration of the CO 2 plume and prevent it from reaching the reservoir top, hence reducing the direct contact between the CO 2 plume and the overlying caprock. Furthermore, an optimal amount of low-permeable layers (around 30% in this study) can increase the swept area of CO 2 and reduce the proportions of free CO 2 phase. These aspects can collectively increase the amount of permanently trapped CO 2 and reduce the leakage risks of the injected CO 2. Based on the characteristics of the resulting models (i.e., storage volume, injectivity, distribution and phases of CO 2), a multi-criteria decision-making method has been used to quantitatively rank the different lithofacies associations according to their suitability for CO 2 storage. In this analysis, the proximal-medial fluvial lithofacies associations are assessed to be the most suitable ones because they feature low proportions of the injected CO 2 reaching the reservoir top and in free phase while maintaining the high storage volume and injectivity. This study reveals that heterogeneous reservoir architectures have mixed effects on CO 2 storage, and that reservoirs featuring moderately heterogeneous architectures (i.e., fractions of low-permeable facies ranging from 30% to 40%) are beneficial to keeping the balance among different aspects of CO 2 storage. This provides new insights for the screening and selection of potential geological sites for CO 2 storage. [ABSTRACT FROM AUTHOR]

Various types of geofluids exist in deep and ultra-deep layers in petroliferous basins. The geofluids are much more active under high-temperature and high-pressure (HTHP) conditions, but their properties are unclear. We simulated the mixing of different fluids in CH4/C3H8/C6H14/C8H18-water systems and C6H14/C8H18-CO2-H2O systems at temperatures of 25°C to 425°C and pressures of 5 MPa to 105 MPa, using an in-situ micron quartz capillary tube thermal simulation system and molecular dynamics numerical simulation software. The mixing processes, patterns, and mechanisms of various fluids were analyzed at microscale under increasing temperature and pressure conditions. The results show that the miscibility of fluids in the different alkane-H2O and alkane-CO2-H2O systems is not instantaneous, but the miscibility degree between different fluid phases increases as the temperature and pressure rise during the experiments. The physical thermal experiments (PTEs) show that the mixing process can be divided into three stages: initial miscibility, segmented dynamic miscibility, and complete miscibility. The molecular dynamics numerical simulations (MDNSs) indicate that the mixing process of fluids in the alkane-H2O and alkane-CO2-H2O systems can be divided into seven and eight stages, respectively. The carbon number affects the miscibility of alkanes and water, and the temperature and pressure required to reach the same miscibility stage with water increase with the carbon number (C3H8, C6H14, CH4, C8H18). CO2 has a critical bridge role in the miscibility of alkanes and water, and its presence significantly reduces the temperatures required to reach the initial, dynamic, and complete miscibility of alkanes and water. The results are of great significance for analyzing and understanding the miscibility of geofluids in deep and ultra-deep HTHP systems. [ABSTRACT FROM AUTHOR]

Dissolution of feldspars and precipitation of secondary minerals (kaolinite, illite and quartz) are significant diagenetic processes in arkosic sandstones. We examined moderately buried sandstones in the Eocene Shahejie Formation from two sags in the Bohai Bay Basin, East China. Three different types of mineral assemblages (MA) were identified: extensively leached feldspars with a large amount of authigenic kaolinite and quartz cement (MA-1), extensively leached feldspars with a large amount of authigenic kaolinite and minor quartz cement (MA-2), and extensively leached feldspars with a small amount of both authigenic kaolinite and quartz cement (MA-3). Numerical simulations at the continuum scale using Geochemist’s Workbench 9.0 were conducted to decipher the origin of the different mineral assemblages. The physicochemical reactions including feldspar dissolution, transport of Al 3+ and SiO 2 (aq), and precipitation of kaolinite and quartz are coupled together in these simulations, with constraints of chemical reactions, kinetic law, dispersion, and advection. Modeling results suggest that a dissolution zone, a transitional zone, and a precipitation zone can be formed in a sandstone unit with suitable constraints of temperature, flow rate, fluid composition and mineral reaction rate. And MA-3, MA-2, and MA-1 assemblages develop in these three zones respectively. The higher SiO 2 (aq) concentration required for the saturation of quartz than for kaolinite and the low Al 3+ concentration needed for the saturation of kaolinite lead to the precipitation of only kaolinite in the transitional zone in a geochemical system with feldspar dissolution serving as the dominant source of SiO 2 (aq) and Al 3+ . Comparisons between modeling results and observations of natural sandstone diagenesis suggest that an MA-1 assemblage is likely to occur in buried sandstones at high temperatures (>70–80 °C) and low flow rates. An MA-2 assemblage may occur in moderately buried sandstones at moderate temperatures (40–70 °C), in deeply buried sandstones with faults and fractures serving as conduits of meteoric freshwater, or in shallow sandstones where meteoric water is not abundant. An MA-3 assemblage tends to occur in shallow sandstones at low temperatures (<40–50 °C) and high flow rates, or in buried sandstones where faults and fractures develop widely and serve as freshwater conduits. These proposals are valid in natural arkosic sandstones and of great significance in deciphering the diagenetic environments where the feldspar dissolution and secondary mineral precipitation have occurred. [ABSTRACT FROM AUTHOR]

An example of diagenesis and reservoir quality of buried sandstones with ancient incursion of meteoric freshwater is presented in this study. The interpretation is based on information including porosity and permeability, petrography, stable isotopic composition of authigenic minerals, homogenization temperatures (T h ) of aqueous fluid inclusions (AFIs), and pore water chemistry. These sandstones, closely beneath or far from the regional unconformity formed during the late Paleogene period, are located in the thick Shahejie Formation in the Gaoliu area of Nanpu Sag, Bohai Bay Basin, East China. Early-diagenetic calcite cements were leached to form intergranular secondary pores without precipitation of late-diagenetic calcite cements in most sandstones. Feldspars were leached to form abundant intragranular secondary pores, but with small amounts of concomitant secondary minerals including authigenic quartz and kaolinite. The mass imbalance between the amount of leached minerals and associated secondary minerals suggests that mineral leaching reactions occurred most likely in an open geochemical system, and diagenetic petrography textures suggest that advective flow dominated the transfer of solutes from leached feldspars and calcites. Low salinity and ion concentrations of present pore waters, and extensive water rock interactions suggest significant incursion of meteoric freshwater flux in the sandstones. Distances of the sandstones to the regional unconformity can reach up to 1800 m, while with significant uplift in the Gaoliu area, the burial depth of such sandstones (below sea level) can be less than 800–1000 m during the uplift and initial reburial stage. Significant uplift during the Oligocene period provided substantial hydraulic drive and widely developed faults served as favorable conduits for downward penetration of meteoric freshwater from the earth's surface (unconformity) to these sandstone beds. Extensive feldspar leaching has been occurring since the uplift period. Coupled high T h (95∼115 °C) of AFI and low δ 18 O (SMOW) values (+17∼+20‰) within the quartz overgrowths show that quartz cementation occurred in the presence of diagenetic modified meteoric freshwater with δ 18 O (SMOW) values of −7∼−2‰, indicating that authigenic quartz only have been formed during the late reburial stage when meteoric fresh water penetration slowed down. Secondary pores in thin sections and tested porosity suggest that meteoric freshwater leaching of feldspars and calcite minerals generated approximately 7–10% enhanced secondary porosity in these sandstones. Meteoric freshwater leaching reactions cannot be ignored in similar sandstones that located deep beneath the unconformity, with great uplift moving these sandstones above or close to sea level and with faults connecting the earth's surface with the sandstone beds. [ABSTRACT FROM AUTHOR]

Accurate prediction of reservoir porosity and permeability is essential for prospecting hydrocarbon reserves and petroleum production capacity. We propose an innovative reservoir porosity and permeability prediction method through identifying sedimentary-diagenetic facies, determining the porosity-permeability trends using core measurement data, extrapolating the spatial distribution of the sedimentary-diagenetic facies using log data through the Bayes discriminant analysis and predicting the reservoir porosity and permeability. The essence of the method was illustrated and its effectiveness was demonstrated using the Eocene beach-bar sandstones in the Dongying Depression, Bohai Bay Basin, eastern China. The Eocene beach bar sandstones are classified into fine sandstone, siltstone, and argillaceous siltstone based on grain sizes, sorting, and matrix contents. The major diagenetic processes that influence the porosity and permeability of the beach-bar sandstones are compaction, carbonate cementation and feldspar dissolution. Seven sedimentary-diagenetic facies were identified in the beach-bar sandstones based on the lithological types, and their corresponding diagenesis and influence extent on reservoir properties. The variation ranges of porosity and permeability (log K) of these sedimentary-diagenetic facies are typically less than 6% and 1.2, respectively. Both the porosity and permeability have well defined functional relationships with depth. The sedimentary-diagenetic facies can be identified effectively from logging data through the use of the Bayes discriminant analysis and corresponding cross-plots. The porosity and permeability (log K) of the well evaluated in the study were predicted with errors in the range of ±3% and ±0.6, respectively, based on the distribution and the fitting equation of the trend lines for the seven sedimentary-diagenetic facies. The predicted porosities and permeability of sedimentary-diagenetic facies match the measured porosities and permeability well. [ABSTRACT FROM AUTHOR]

This paper investigates the reservoir potential of deeply-buried Eocene sublacustrine fan sandstones in the Bohai Bay Basin, China by evaluating the link between depositional lithofacies that controlled primary sediment compositions, and diagenetic processes that involved dissolution, precipitation and transformation of minerals. This petrographic, mineralogical, and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones. Eogenetic alterations of the sandstones include: 1) mechanical compaction; and 2) partial to extensive non-ferroan carbonate and gypsum cementation. Typical mesogenetic alterations include: (1) dissolution of feldspar, non-ferroan carbonate cements, gypsum and anhydrite; (2) precipitation of quartz, kaolinite and ferroan carbonate cements; (3) transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite. This study demonstrates that: 1) depositional lithofacies critically influenced diagenesis, which resulted in good reservoir quality of the better-sorted, middle-fan, but poor reservoir quality in the inner- and outer-fan lithofacies; 2) formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality; and 3) oil emplacement during early mesodiagenesis (temperatures > 70 °C) protected reservoirs from cementation and compaction. [ABSTRACT FROM AUTHOR]

The diagenetic environment, diagenetic responses, diagenetic transformation model and formation mechanisms of high-quality reservoirs (beach-bar sandstones of the Paleogene fourth member) in the Dongying depression were studied through the analysis of fluid inclusions, thin section and burial evolution history. The diagenetic fluids of the beach-bar sandstone reservoirs evolved from early high salinity and weak alkalinity to low salinity and strong acidity, late high salinity and strong alkalinity and late low salinity and acidity, which were accompanied by two stages of oil and gas filling. The fluids at the margins of the sandbodies were continuously highly saline and strongly alkaline. The western (eastern) reservoirs experienced early open (closed), middle open, and late closed diagenetic environments during their burial history. The flow pattern was characterized by upwelling during the majority of the diagenesis (in the east, a non-circulating pattern transitioned into an upwelling current). Due to the evolution of the diagenetic fluids, the diagenetic sequence of the beach-bar reservoirs was as follows: early weak carbonate cementation; feldspar and carbonate cement dissolution and authigenic quartz cementation; late carbonate and anhydrite cementation, authigenic feldspar cementation, and late quartz dissolution; and late carbonate cementation, feldspar dissolution, and authigenic quartz cementation. The diagenetic strength during these stages varied or was absent altogether in different parts of the reservoirs. Due to the closeness of the diagenetic environment and the flow pattern of the diagenetic fluids, the diagenetic products are variably distributed in the sandstones interbedded with mudstones and in the fault blocks. The evolution of multiple alternating alkaline and acidic diagenetic environments controlled the distribution patterns of the reservoir diagenesis and reservoir space, and the reservoir quality index, RQI, increased gradually from the margins to the centers of the sandstones. The closeness of the diagenetic environment and the flow patterns of the diagenetic fluids controlled the differences in the reservoir properties among the fault blocks. With increasing distance from the oil-source faults, the RQI values in the west gradually decreased and in the east initially increased and then decreased. [ABSTRACT FROM AUTHOR]

Feldspar dissolution is a pervasive water-rock interaction in siliciclastic sediments and has critical impact on reservoir quality. The purpose of this paper is to evaluate the influence of feldspar dissolution on reservoir quality in tight gas sandstones from the Eocene Es4 interval, Dongying Depression. Based on petrographic, mineralogical, and geochemical analysis, distribution patterns of authigenic minerals associated with feldspar dissolution are recognized. Dissolution of feldspar in Eocene Es4 interval is related to organic acids and CO 2 expelled from adjacent source rocks in deep burial environment. Diffusion is inferred to be the predominant transport mechanism for dissolved solids during feldspar dissolution and result in dissolution by-products (e.g. quartz, kaolinite, illite and albite cements) precipitated in-situ or in adjacent pores. Mass balance calculations also demonstrate most of silica derived from K-feldspar dissolution was precipitated as quartz cements within tight sandstone reservoirs. The net porosities created by feldspar dissolution were insignificant but reservoir permeability was reduced significantly by the precipitation of authigenic quartz, kaolinite, albite, and especially by conversion of kaolinite to hair-like illite at higher temperatures. [ABSTRACT FROM AUTHOR]

Pore-throat size is a very crucial factor controlling the reservoir quality and oiliness of tight sandstones, which primarily affects rock-properties such as permeability and drainage capillary pressure. However, the wide range of size makes it difficult to understand their distribution characteristics as well as the specific controls on reservoir quality and oiliness. In order to better understand about pore-throat size distribution, petrographic, scanning electron microscopy (SEM), pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), quantitative grain fluorescence (QGF) and environmental scanning electron microscopy (ESEM) investigations under laboratory pressure conditions were performed on a suite of tight reservoir from the fourth member of the Lower Cretaceous Quantou Formation (K 1 q 4 ) in the southern Songliao Basin, China. The sandstones in this study showed different types of pore structures: intergranular pores, dissolution pores, pores within clay aggregates and even some pores related to micro fractures. The pore-throat sizes vary from nano- to micro-scale. The PMI technique views the pore-throat size ranging from 0.001 μm to 63 μm and revealed that the pore-throats with radius larger than 1.0 μm are rare and the pore-throat size distribution curves show evident fluctuations. RMI measurements indicated that the pore size distribution characteristics of the samples with different porosity and permeability values look similar. The throat size and pore throat radius ratio distribution curves had however significant differences. The overall pore-throat size distribution of the K 1 q 4 tight sandstones was obtained with the combination of the PMI and RMI methods. The permeability is mainly contributed by a small part of larger pore-throats (less than 30%) and the ratio of the smaller pore-throats in the samples increases with decreasing permeability. Although smaller pore-throats have negligible contribution on reservoir flow potential, they are very significant for the reservoir storage capacity. The pore-throats with average radius larger than 1.0 μm mainly exist in reservoirs with permeability higher than 0.1mD. When the permeability is lower than 0.1mD, the sandstones are mainly dominated by pore-throats with average radius from 0.1 μm to 1.0 μm. The ratio of different sized pore-throats controls the permeability of the tight sandstone reservoirs in different ways. We suggest that splitting or organizing key parameters defining permeability systematically into different classes or functions can enhance the ability of formulating predictive models about permeability in tight sandstone reservoirs. The PMI combined with QGF analyses indicate that oil emplacement mainly occurred in the pore-throats with radius larger than about 0.25–0.3 μm. This result is supported by the remnant oil micro-occurrence evidence observed by SEM and ESEM. [ABSTRACT FROM AUTHOR]

Eocene, sublacustrine-fan, sandstones that developed in a rift basin are important tight gas reservoirs in the Dongying Depression, Bohai Bay Basin, northeastern China. Two units of evaporites, developed at the top and bottom of the lower unit of the Es4 interval (Es4x), consist predominantly of anhydrite with subordinate gypsum. Evaporite and related diagenetic processes greatly influenced reservoir quality. δ 13 C values for micritic dolomite cements in Es4x are depleted (−7.45 to −2.57‰) due to microbial sulfate reduction (MSR) under shallow burial conditions and this interpretation is supported by large δ 34 S fractionation between anhydrite and framboidal pyrite. Precipitation temperatures for micritic dolomite are calculated as 57.5–72.8 °C. Anhydritization of gypsum probably occurred at 100.5–145.2 °C during progressive burial as evidenced by homogenization temperatures of aqueous inclusions within anhydrite cements. This process resulted in dehydration fractures within anhydrite cements that increased reservoir permeability by connecting isolated pores. Thermochemical sulfate reduction (TSR) probably resulted in dissolution of gypsum and anhydrite cements under relatively deep burial conditions. Ankerite cements are replaced by anhydrite cements and are enclosed by solid bitumen in Es4x. Ankerite cements likely were derived from TSR as reflected in negative δ 13 C values (−7.12 to −3.70‰) and high calculated temperatures (121.3–185.1 °C). Dissolution by-products (e.g. saddle dolomite, ankerite, nodular pyrite) related to TSR precipitated in adjacent pores. A lack of significant δ 34 S fractionation between parent sulfate and nodular pyrite indicates that TSR occurred in a relatively closed system. Therefore, dissolution of gypsum and anhydrite related to TSR contributed little to reservoir quality. Middle-fan lithofacies with better sorting, porosity and permeability than inner- and outer-fan lithofacies constitute high-quality reservoirs in Es4x. [ABSTRACT FROM AUTHOR]

Generally, researchers believe that authigenic quartz precipitates in acidic medium and authigenic albite precipitates in alkaline medium. However, the lacustrine carbonate-rich fine-grained sedimentary rocks in the upper fourth sub-member and the lower third sub-member (Es 4 s–Es 3 x) of the Shahejie Formation in the Jiyang Depression were observed and it was found that authigenic microcrystalline quartz and albite generally develop together. Moreover, authigenic quartz and albite also develop with microcrystalline Fe-rich dolomite and calcite in clay laminae, while in carbonate laminae, they primarily develop with recrystallized calcite. In this study, thin-section, cathodoluminescence, field-emission scanning electron microscopy (FESEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and inclusion analyses were conducted to study authigenic minerals. The results show that authigenic quartz and albite are characterized by rich Ge in mudstone, and the material components mainly originate from the transformation of clay minerals and the dissolution of terrigenous feldspar debris. In limestone, they are characterized by poor Ge, and the material components primarily originate from the transformation of clay minerals. The paragrowth and intergrowth of authigenic quartz and albite primarily depend on the degree of supersaturation of the corresponding ion concentration under high temperature, high or ultrahigh pressure. During the early diagenetic stage, authigenic quartz and albite initially formed in a weakly alkaline environment; however, on entering the middle diagenetic period, authigenic quartz, albite, and sparry calcite formed in a weakly acidic environment, and the formation rate reached its peak. Our results elucidate the migration direction and enrichment law of shale oil in calcareous fine-grained sedimentary rocks through the genetic mechanism of authigenic quartzs and albites. Shale oil is generated from organic matter in the clay lamina, following which it migrates, accumulates, and preserves to the adjacent sparry calcite lamina under the joint driving effect of organic matter hydrocarbon generation overpressure and clay mineral dehydration overpressure. • The paragrowth and intergrowth of authigenic quartz and albite mainly depend on the degree of ionic supersaturation. • Authigenic quartz and albite in mudstone primarily originate from clay minerals transformation and feldspar clasts dissolution. • Authigenic quartz and albite in limestone primarily originate from the transformation of clay minerals. • The origin mechanism of authigenic quartz and albite indicates the migration direction and enrichment mechanism of shale oil. [ABSTRACT FROM AUTHOR]

Lamina, a common lithologic unit in shale, is the smallest unit for organic–inorganic fluid–rock interactions. The effects of lamina-scale diagenetic processes are superimposed to exert on lithofacies-scale diagenetic sequences, which further impose on the entire diagenetic system of a shale reservoir. Organic acids from kerogen maturation are indispensable components in diagenetic reaction. However, previous studies mainly focused on low molecular weight (LMW) water soluble acids, few studies have paid attention to the role of high molecular weight (HMW) carboxylic acids dissolved in fluid–rock reactions. Herein, by taking the shale of the seventh member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, as a case study, we investigate the mutual impacts between mineral alteration and HMW organic acid degradation. The characterization is carried out at two scales: lamina-scale (organic-rich clay-rich lamina and organic-lean silty lamina) and lithofacies-scale (shale and sandstone). Integrated analytical techniques (microdrill technique, thin section observation, X-ray diffraction, advanced mineral identification and characterization system, Rock-Eval pyrolysis, Fourier transform ion cyclotron resonance mass spectrometry) are performed. Results show that, in reservoirs of both silty lamina and sandstone, consumption of LMW aqueous acids through mineral transformation reactions promotes the forward evolution of HMW carboxylic acids in the bitumen phase toward decarboxylation or dehydration. During diagenesis, short chain fatty acids (SCFAs) are more easily broken down into small acids and, thus, more susceptible to be consumed. The lamina-scale reservoirs (silty lamina) have the privilege to receive abundant aqueous organic acids to maintain the micro-geochemical environment and progressively boost organic–inorganic fluid–rock interactions. The resultant dissolution pores not only accumulate plenty of bitumen but also an appreciable amount of bitumen dissolved acids. Because of the strong storage capacity of silty lamina within lithofacies-scale source rock (shale), fluids are less readily to expel out to lithofacies-scale reservoir (sandstone). As a result, the sandstone may not receive enough fluids from shale to maintain a consistent diagenetic environment, resulting in the observed discontinuous diagenesis of quartz precipitation transition to quartz dissolution. [ABSTRACT FROM AUTHOR]

The compositions, distribution and its interaction with rocks of the evolving pore fluids controls the distribution of carbonate cements and reservoir storage spaces. The reservoir quality of the red-bed sandstone reservoirs in the Dongying Depression was investigated by an integrated and systematic analysis including carbonate cement petrology, mineralogy, carbon and oxygen isotope ratios and fluid inclusions. The investigation was also facilitated by probing the mineral origins, precipitation mechanisms, pore fluid evolution and distribution, and water-rock interaction of carbonate cements and their influences on reservoir quality. Diagenetic-evolving fluids in the interbedded mudstones are the main source for the precipitation of calcite cements that completely fill the intergranular volume (CFIV calcite) with heavier oxygen and carbon isotopes. The ferro-carbonate cements in the reservoir sandstone are enriched in lighter carbon and oxygen isotopes. In addition to the cations released by the conversion of clay minerals in reservoirs, products of organic acid decarboxylation and the associated feldspar dissolution process provide important sources for such carbonate cementation. The carbon isotopes of CO 2 and the oxygen isotopic composition of fluids equilibrated with the CFIV calcite, ferro-calcite, dolomite and ankerite cements indicate that the pore in the red-bed reservoirs experienced high salinity fluids, which evolved from the early-formed interbedded mudstones, through organic acid input and to organic acid decarboxylation. Pore fluids from nearby mudstones migrated from the edge to the centre of sandbodies, causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones. Pore fluids associated with organic CO 2 and acids and organic acid decarboxylation are mainly distributed in the internal portion of sandbodies, causing feldspar dissolution and precipitation of ferro-carbonate cements. The distribution of pore fluids caused the zonal distribution of carbonate cements in sandbodies during different periods. This may be advantageous to preserve the porosity of reservoirs as exemplified by the distribution of high-quality reservoirs in the red-bed sandbodies. [ABSTRACT FROM AUTHOR]

The Chepaizi–Xinguang area is a petroliferous region in the western Junggar basin that contains several Permian tight reservoirs and has a complex petroleum system and accumulation mechanism. The oil sources and accumulation mechanisms are investigated by an integrated analysis of biomarker assemblages in reservoir oil samples, microscopic analysis of rock samples, study of fluid inclusions, and simulation of the thermal evolution. The results show that the Permian oil in the Xinguang bulge is mainly generated from the Permian Fengcheng Formation with tricyclic terpane ratios C 19 TT/C 23 TT lower than 0.15, C 23 TT/C 21 TT higher than 1.0 and gammacerane/αβ C 30 hopane higher than 0.6. The Permian oil in the Chepaizi area is mostly derived from Permian Wuerhe Formation with C 19 TT/C 23 TT higher than 0.15, C 23 TT/C 21 TT lower than 1.0 and gammacerane / αβ C 30 hopane lower than 0.4. The Permian oils from the Xinguang bulge show higher maturity compared to the Permian oils from the Chepaizi area possibly as a result of later oil filling. The fluid inclusions and fluorescence record the main oil charging movements during the Late Triassic to Early Jurassic and the Late Jurassic to Early Cretaceous in this area, and most of the generated oils in late Permian are lost. The unconformities between Permian strata show higher porosity and more oil and gas enrichment comparatively and are the main pathways for lateral oil migration. Laumontite has had a considerable effect on reservoir properties. Laumontite dissolution creates pores that provide good oil reservoir space and favorable exploration targets. [ABSTRACT FROM AUTHOR]

The Lower Cretaceous Quantou Formation in the southern Songliao Basin is the typical tight oil sandstone in China. For effective exploration, appraisal and production from such a tight oil sandstone, the diagenesis and reservoir quality must be thoroughly studied first. The tight oil sandstone has been examined by a variety of methods, including core and thin section observation, XRD, SEM, CL, fluorescence, electron probing analysis, fluid inclusion and isotope testing and quantitative determination of reservoir properties. The sandstones are mostly lithic arkoses and feldspathic litharenites with fine to medium grain size and moderate to good sorting. The sandstones are dominated by feldspar, quartz, and volcanic rock fragments showing various stages of disintegration. The reservoir properties are quite poor, with low porosity (average 8.54%) and permeability (average 0.493 mD), small pore-throat radius (average 0.206 μm) and high displacement pressure (mostly higher than 1 MPa). The tight sandstone reservoirs have undergone significant diagenetic alterations such as compaction, feldspar dissolution, quartz cementation, carbonate cementation (mainly ferrocalcite and ankerite) and clay mineral alteration. As to the onset time, the oil emplacement was prior to the carbonate cementation but posterior to the quartz cementation and feldspar dissolution. The smectite to illite reaction and pressure solution at stylolites provide a most important silica sources for quartz cementation. Carbonate cements increase towards interbedded mudstones. Mechanical compaction has played a more important role than cementation in destroying the reservoir quality of the K 1 q 4 sandstone reservoirs. Mixed-layer illite/smectite and illite reduced the porosity and permeability significantly, while chlorite preserved the porosity and permeability since it tends to be oil wet so that later carbonate cementation can be inhibited to some extent. It is likely that the oil emplacement occurred later than the tight rock formation (with the porosity close to 10%). However, thicker sandstone bodies (more than 2 m) constitute potential hydrocarbon reservoirs. [ABSTRACT FROM AUTHOR]

The tight sandstones of the Cretaceous Quantou formation are the main exploration target for hydrocarbons in the southern Songliao basin. Authigenic quartz is a significant cementing material in these sandstones, significantly reducing porosity and permeability. For efficient predicting and extrapolating the petrophysical properties within these tight sandstones, the quartz cement and its origin need to be better understood. The tight sandstones have been examined by a variety of methods. The sandstones are mostly lithic arkoses and feldspathic litharenites, compositionally immature with an average framework composition of Q 43 F 26 L 31 , which are characterized by abundant volcanic rock fragments. Mixed-layer illite/smectite (I/S) ordered interstratified with R = 1 and R = 3 is the dominating clay mineral in the studied sandstone reservoirs. Two different types of quartz cementation modes, namely quartz grain overgrowth and pore-filling authigenic quartz, have been identified through petrographic observations, CL and SEM analysis. Homogenization temperatures of the aqueous fluid inclusions indicate that both quartz overgrowths and pore-filling authigenic quartz formed with a continuous process from about 70 °C to 130 °C. Sources for quartz cement produced are the conversion of volcanic fragments, smectite to illite reaction and pressure solution at micro stylolites. Potassium for the illitization of smectite has been sourced from K-feldspar dissolution and albitization. Silica sourced from K-feldspars dissolution and kaolinite to illite conversion is probably only minor amount and volumetrically insignificant. The internal supplied silica precipitate within a closed system where the transport mechanism is diffusion. The quartz cementation can destroy both porosity and permeability, but strengthen the rock framework and increase the rock brittleness effectively at the same time. [ABSTRACT FROM AUTHOR]

Understanding how subaqueous sediment gravity flows (SSGF) evolve in time and space, and how their deposits vary spatially, is a key research focus for gravity flow sedimentology. This study investigates depositional facies, depositional elements, and sediment transport processes of supercritical flows and hybrid event beds (HEBs) of the Lingshandao Formation in Lingshan Island, Eastern China. Three kinds of depositional elements were recognized: mass transport deposits (MTDs), channel-lobe transition zone (CLTZs), and lobe complexes. MTDs can be sub-divided into proximal and distal deposits. CLTZs are characterized by facies changes from massive coarse-grained pebbly sandstone to backset bedding sandstone in a down-slope direction, which are the deposits of supercritical turbidity currents. HEBs are common in both proximal and distal lobe settings. Tripartite structure in HEBs, which may be caused by up-slope substrate erosion, implies a relatively proximal origin compared with bipartite HEBs, perhaps caused by fluid fractionation in the research area. The succession is formed by a prograding lobe unit, followed by MTDs which are themselves overlain by deposits from a CLTZ. This vertical stacking pattern implies the potential longitudinal facies tract from CLTZ; to lobe complex deposits, is accompanied by emplacement of MTDs at the slope break. The flow types accompanied by sediment transport processes imply that erosion by supercritical turbidity currents associated with a hydraulic jump in the channel-lobe transition zone may be the main reason for mud-clasts and matrix addition to the flow. The large-scale addition of mud-clasts and suspended mud may dampen turbulence in proximal to medial lobe settings, and result in medium-to thick-bedded HEBs with common erosional features and tripartite structures. The remaining suspension flow (with abundant mud) may further transport down-dip and form medium-to thin-bedded HEBs with bipartite structures and rare mud-clasts. These findings may be applicable to other SSGF systems with supercritical flow deposits and hybrid event beds, emphasizing the downdip and lateral variation in depositional elements associated with gravity flow evolution. • CLTZs are characterised by bypass lags, backset bedding, cut-and-fill deposits, soft sediment deformation structures. • The erosion by supercritical turbidity currents caused mud matrix and clasts being added to the flow, and result in HEBs. • Vertical distribution represents the longitudinal sediment transport processes from CLTZ to lobe complex deposits. [ABSTRACT FROM AUTHOR]

A large quantity of drilling core, paleontology, geochemistry and geophysics data revealed several features of the Jiyang subbasin during the deposition of the Ek1-Es4x members: (1) the paleotopography of the gentle slope belt had an extremely low gradient; (2) the paleoclimate frequently alternated between dry and wet periods in a generally arid setting; (3) there was strong weathering around the periphery of the basin; (4) the lake was very shallow; (5) the lake level frequently rose and fell; and (6) the sedimentary environment of the gentle slope belt was an over-flooding lake. All of these factors provided favorable geological conditions for the development of an over-flooding lake delta. The lithologies of the continental over-flooding lake delta deposits are complex and diverse. The compositional maturity is moderate to low, and the grain size distribution curves and sedimentary structures indicate the presence of both gravity and traction currents. The sedimentary microfacies associations consist of a combination of ordered superposition of flood channels, distributary channels and sheet sands. The delta exhibits a weak foreset seismic reflection. The over-flooding lake delta deposits are laterally extensive. The sandstone content is high, and the individual sandstone beds are thin. The flood channel and distributary channel deposits exhibit evidence of bifurcation and lateral migration. The distribution of the sandbodies and the oxidation color of the mudstones provide evidence of cyclic deposition. The paleoclimate was the dominant factor controlling the development of the over-flooding lake delta. Due to the frequently alternating wet and dry paleoclimates, the over-flooding lake delta is characterized by the development of a broad upper plain and a lower delta plain. The upper delta plain is characterized by flood channel deposits, whereas the lower delta plain is represented by distributary channel deposits. The transition zone is characterized by the interaction of flood channels and distributary channels. Due to fault activity, the sandbodies of the over-flooding lake delta were juxtaposed against hydrocarbon source rocks, which was favorable for the development of lithologic reservoirs or structural-lithologic reservoirs. The lower delta plain deposits comprise the most favorable reservoirs. [ABSTRACT FROM AUTHOR]