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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 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]

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 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]

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]

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]

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]

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]

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]

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]

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]

Carbonates are suggested to dissolve rapidly than feldspars by laboratory experiments. Petrography texture of selective dissolution of feldspars in the presence of carbonates, however, is widespread in buried sandstones and even shales, inspiring a revisit to the chemistry of burial secondary pores. Feldspar dissolution, precipitation of secondary minerals (quartz, clays), and carbonate cementation are common chemical reactions in the Eocene sandstones in the northern Dongying Sag. Petrography evidence demonstrates the selective dissolution of feldspars in the presence of carbonate minerals (both detrital and authigenic minerals) in these buried sandstones. The equilibrium constant of calcite leaching reactions is much smaller than that of K-feldspar leaching reactions. Numerical simulations of chemical reactions in K-feldspar-calcite–CO 2 –H 2 O systems utilizing the Geochemist's Workbench 9.0 (GWB) indicate that only a small amount of calcite was dissolved at the onset of simulation processes, while much K-feldspar was dissolved with precipitation of quartz, clays and some calcite for extended periods of time. Precipitation of secondary calcite could also promote feldspar dissolution. Simulation of reactions in a simplified sandstone system with constraints of present-day pore water and partical pressure of carbon dioxide (pCO 2 ) in the northern Dongying Sag indicates that the pore waters are close to equilibrium with calcite. Petrography evidence and modeling results share consistence in confirming that only feldspar could be dissolved extensively, with precipitation of quartz, clays and some carbonate minerals. The organic CO 2 -leaching theory (Schmidt and McDonald, 1979) regarding the dissolution of mainly carbonates and, to a lesser degree, feldspars to form secondary pores has been questioned due to supposed extensive internal consumption of organic CO 2 in source rocks by carbonate minerals. The CO 2 in sandstones and shales and the isotopic composition of CO 2 and carbonate cements in sandstones, however, suggest migration of organic CO 2 from mudstones (even with large amounts of carbonate) to sandstones. Calculations in this study indicate that, with suppression of carbonate dissolution in sedimentary rocks, many secondary pores can be generated in sandstones through selective dissolution of feldspar by organic CO 2 from thermal evolution of organic matter. Selective dissolution of feldspars, rather than carbonates, in sedimentary rocks is then likely a general mechanism to decipher the chemistry of secondary porosity. Other secondary porosity generation mechanisms were also reevaluated with numerical simulations. Results demonstrate that reverse weathering reactions of silicates may take place with increasing of temperatures, but cannot provide enough H + to dissolve carbonates at elevated temperatures. Additionally, cooling of hot fluids can dissolve only insignificant amounts of carbonate unless significant amounts of water are provided. [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]

The Permian Fengcheng Formation represents a series of deposits formed in an alkaline saline lake, characterized by high proportions of cherts, located in the Northwestern Junggar Basin in China. While discussions on silica-rich hot springs as sources of silica for brine and subsequent chert deposition have been extensively conducted for Quaternary and Mesozoic-Cenozoic alkaline saline lakes worldwide, investigations into Paleozoic alkaline saline lakes are scarce due to challenges associated with identifying evidences of hot spring environments. In this study, we present detailed SEM-EDS analyses and examine silica isotope compositions as well as oxygen isotope compositions to discuss hydrothermal-associated silica precipitation processes. Our findings suggest that migrating silica-rich hot springs along deep-rooted faults released silica into the brine, leading to chert deposition during cooling periods and induction of thermophilic microorganism degradation. Microorganism-associated siliceous precipitates primarily consist of opal-A spheres adhered by extracellular polymeric substances (EPS), while numerous microorganism spheres have undergone silicification. Opal-A spheres within these structures were partially transformed into well-crystallized quartz through multiple stages of dissolution-precipitation processes during prolonged burial history. Furthermore, our results indicate that deep-rooted fault-related hot springs can occur both in marginal and subaqueous areas within alkaline saline lakes, with chert formations observed in shallow-marginal saline lake settings as well as deeper regions. The findings of this study provide valuable insights into the paleoenvironmental conditions, which can be further explored in investigating the accumulation of organic matter in chert-rich successions formed within an alkaline saline lake. • Hot spring in an organic-rich alkaline saline lake. • Spherical microorganisms undergoing silicification through opaline silica deposition. • Silica occurrences transferred by dissolutions and reprecipitations. [ABSTRACT FROM AUTHOR]

Braided river sandstones in the Permian Upper Shihezi (US) Formation in the Huanghua Depression, Bohai Bay Basin, East China, are reservoirs with large accumulations of hydrocarbons. The Upper Paleozoic formations, including the Carboniferous and the Permian strata experienced multi-stage of subsidence and uplift in the past 320 Ma, leading to occurrence of subaerial exposure and deep burial of the Permian US sandstones. Due to complex tectonic evolution, the Upper Paleozoic formations in different tectonic zones of the Huanghua Depression experienced various burial-thermal histories, resulting in the development of long-term open (LTO), early-open to late-closed (EOLC), and long-term closed (LTC) geochemical systems in different Permian US sandstones, as well as different evolutionary patterns of diagenesis and reservoir quality. After extensive diagenetic alteration, the present sandstones are primarily quartzose, subfeldsarenite, and sublitharenite sandstones. Pores in shaly sandstones and fine-grained sandstones were destroyed extensively, only pebbly to medium-grained sandstones can be potential effective reservoirs. The Permian US sandstones with the LTO systems, which are currently overlain by the Paleogene strata and buried less than 2500 m, experienced two stages of meteoric freshwater leaching and are characterized by extensive feldspar dissolution and weak precipitation of authigenic kaolin and quartz. Intergranular and intragranular pores dominate the 10–20% porosity, and permeability ranges from 1mD to 800 mD. The Permian US sandstones with the EOLC systems, which are currently overlain by thick Jurassic and Cretaceous strata on a regional scale and buried deeper than 3000 m, experienced both meteoric freshwater leaching and deep burial leaching and are currently characterized by extensive feldspar dissolution, extensive kaolin precipitation, and weak-moderate quartz cementation. Intragranular secondary pores and kaolin intercrystal pores dominate the 5–15% porosity, and permeability ranges from 0.1 mD to 10 mD. The Permian US sandstones with the LTC systems, which are currently overlain by a thick Permian Shiqianfeng Formation on a regional scale and buried deeper than 3000 m, experienced no meteoric freshwater leaching but only burial dissolution; they are currently characterized by extensive feldspar dissolution and extensive precipitation of clays and quartz cements. Intercrystal pores in the authigenic clay and intragranular secondary pores dominate the 5–10% porosity, and permeability ranges from 0.01mD to 1mD. Since the unconformities, overlying strata, and tectonic evolution relevant to the Permian US sandstone unit can be analyzed using seismic data, the proposed genetic models can support the predrill evaluation of reservoir quality of the Permian sandstones. • Multi-stage subsidence and uplift led to development of different diagenetic systems in the different Permian US sandstones. • After extensive diagenesis, only pebbly to medium-grained sandstones can be potential effective reservoirs. • Sandstones in LTO systems have the highest poroperm, followed by sandstones in EOLC systems, and then in LTC systems. • Deeply buried sandstones on slopes and in sags serving as ancient freshwater recharge zones are potentially good reservoirs. [ABSTRACT FROM AUTHOR]

It has been shown that ankerite in clastic reservoirs can preserve an extended record of diagenesis and thus the fluid flow history in sedimentary basins. Ankerite cements with distinct zoning textures are well developed in gravity-flow sandstones associated with magmatic intrusions in the Linnan Sag, Eastern China. This example offering a rare opportunity to investigate the history of complex diagenetic fluid flow events in a lacustrine rift basin. An integrated study including thin section petrography, fluid inclusion micro-thermometry, carbon and oxygen isotope, SEM-EDS, XRD, CL, BSE-EPMA, LA-ICP-MS analyses was undertaken. The aim is to resolve the mineralogical, geochemical and isotopic composition of the sandstone and constitutional minerals. The sandstones are mainly of lithic arkoses (Q 43 F 34 L 23), with an average carbonate cements of approximately 8.2 wt %. The carbonate cements are dominated by zoned ankerite appearing as rhombohedral and poikilotopic blocky crystals. Calcite, ferroan calcite, dolomite and siderite cements are also present. Non-carbonate cements including barite, authigenic clay minerals, and quartz overgrowth are common in the sandstone. The δ13 C values of ferroan calcite range from −4.2‰ to +1.2‰ PDB. Ferroan calcite was re-precipitated at a temperature of 93.1 °C at the expense of the dissolution of eodiagenesis carbonate cement in the sandstone. Ankerite was precipitated at temperature ranges from 107.8 to 140.9 °C, accompanied with the formation of pore-filling barite. The δ13C values of ankerite range from −1.8‰ to +1.1‰, and δ18O values range from −14.8‰ to −10.5‰, indicating an involvement of hydrothermal fluids in the formation of the zoned ankerite. The hydrothermal fluids were probably related to the mafic magmatic activities and migrated into the sandstones along faults. These fluids are rich in iron and magnesium, and also contains abundant inorganic carbon dioxide and sulphate, episodic charging into the reservoir. The abundances of most of the elements, including magnesium, iron, REEs and trace elements in the zoned ankerite consistently decrease away from the magma intrusion centres. We therefore conclude that multiple magmatic activities were primarily responsible for the zoned ankerite precipitation in the research area. The intimate relationship between magmatic activity and carbonate cementation in sandstone reservoirs documented in this paper may have significant implications for studying fluid flow in lacustrine rift basins. • Carbonate cements are dominated by zoned aankerite exhibit as rhombohedral and poikilotopic blocky crystals. • Geochemical data induced the origin of a zoned ankerite effect of hydrothermal fluid. • The multiple period of magma extrusion and the associated hydrothermal fluid flow is the main reason for zoned ankerite precipitation. [ABSTRACT FROM AUTHOR]

The Huanghua Depression in the Bohai Bay Basin has experienced significant development of massive NNE-NE faults, influenced by the Eocene activation of the Lanliao Fault and its subsequent Oligocene dextral intensification. These faults serve as a conduit for connate fluid and magmatic hydrothermal fluid (MHF), thereby forming a complex fluid system. Gaining an understanding of these fluid activities is essential for research into mass balance and hydrocarbon migration within shales. This study explores the origin and distribution of analcime, aligns the fluid evolution stage with different analcime types, and constructs a comprehensive water-rock reaction sequence. The study identifies six types of analcime based on occurrence characteristics (occurring as laminae, lens, fracture filling, bioshell filling, vein marginal crystal and cement). The above six types of analcime is further classified into hydrothermal fluid origin analcime (HFOA: include analcime cement, vein marginal crystal, lens, fracture filling and bioshell filling) and connate fluid origin analcime (CFOA: analcime laminae) based on major elemental indicators (Si/Al and 10K/(10K + Na)). HFOA has lower ∑REE (rare earth element) and strong positive correlation between ∑REE and LILEs (large ionic lithophilic elements); while CFOA has higher ∑REE and weak positive correlation between ∑REE and LILEs. Different analcime types correlate with varying fluid properties and transport stages. HFOA forms during MHF upwelling. After the MHF entering and mixing with the lake water, thermal repulsions between the crystal particles made it move to form fine-grained sedimentary layer, the connate fluid trapped in pores directly precipitate to form CFOA, or form CFOA by modifying clay minerals and feldspars. Considering the tectonic context, we infer that the formation of analcime was primarily driven by high-frequency, low-intensity magmatism, governed by the activation of Lanliao deep fault. The study of analcime can aid in identifying and classifying fluid activity processes, offering a novel perspective for investigating sedimentation and diagenesis in continental shales. • Analcime indicate deep fluids activities in faulted-depressed lacustrine basin. • LILEs and REEs reflect different modes and stages of fluid activities in shales. • Analcime can be used to construct a complete water-rock reaction sequence. [ABSTRACT FROM AUTHOR]

Diagenesis plays a crucial role in shale oil reservoir formation. However, complex organic-inorganic interactions result in pore system formation and heterogeneity changes at the centimeter, or even micrometer scale, which controls the evolution of shale oil reservoirs. In this paper, an integrated approach including core observation, thin section observation, cathodoluminescence thin section observation, scanning electron microscope, μ-XRF analysis, electron microprobe analysis, AMICS analysis, LA-ICP-MS analysis, and isotope analysis was used to clarify the laminae combination carbonate-rich shale, diagenetic alterations in different types of shale laminae and the controls on shale oil reservoir formation. According to the lamina combination, the carbonate-rich shale in the Permian Lucaogou Formation can be divided into three types. They are shale consists of dolomitic lamina and terrigenous felsic lamina (Type A shale), shale consists of calcite-rich tufaceous lamina and terrigenous felsic lamina (Type B shale) and shale consists of calcareous lamina and tufaceous lamina (Type C shale). Dolomite cementation is the major diagenesis in terrigenous felsic lamina of Type A shale. The dolomite can be identified as the early stage forming from recrystallization of micrite dolomite, and the late stage precipitate from adjacent dolomitic lamina. In the terrigenous felsic lamina of Type B shale, however, the calcite cementation is the dominated diagenesis, which was sourced from adjacent calcite-rich tufaceous lamina. Also, some feldspar dissolutions occurred in terrigenous felsic lamina during organic matter evolution. For the Type C shale, the main diagenetic alteration is micrite calcite recrystallization in calcareous lamina. The dissolution and precipitation of carbonate minerals between different laminae of carbonate-rich shale reveal that the organic matter evolution significantly influence the diagenetic alterations of inorganic minerals. Particularly, the diagenetic mass transfer and redistribution at the micro-scale in a relatively diagenetic system were confirmed by organic-inorganic interactions in laminated shale. In this process, dissolution pores and recrystallization pores have provided considerable reservoir pores for shale oil. • The laminae combination and diagenetic alterations of carbonate-rich shale were identified. • The diagenetic mass transfer at the micro-scale during organic-inorganic interactions in laminated shale were clarified. • The pores induced by diagenetic mass transfer within a closed system is crucial for shale reservoir formation. [ABSTRACT FROM AUTHOR]

The Paleogene Shahejie formation of the Dongying Depression in eastern China is a unique region and can offer insights into the suppression of thermal maturity in a large set of successive lacustrine shales. This study focuses on the Paleogene Shahejie formation of N-38 Well in the Dongying Depression. The unusual distribution of thermal maturity indicators, such as vitrinite reflectance (VR O) and C 29 sterane thermal parameters was investigated through geochemical analysis. The research interval (2770–3376 m) was divided into three parts: the upper part of E s 3 2 (2770–2990 m), the lower part of E s 3 2 (2990–3190 m), and the E s 3 1 (3190–3376 m). The Es 3 1 mainly contained organic matter type I and some type II, which included mostly good and excellent source rocks. Meanwhile, the upper E s 3 2 mainly contained organic matter type III, which included mostly poor and moderate source rocks. The parameters G /C 30 H, C 35 H/C 31-35 H, C 31 M/C 31 H, C 29 -20S/(20S + 20R), C 31 -22S/(22S + 22R), and C 32 -22S/(22S + 22R) showed a different distribution corresponding to the different lake sedimentary environments. Various organic facies were developed at the different evolution stages of the lake basin, which showed a noticeable influence on the parameters. The thermal maturity indicator VR O and molecular thermal parameters, such as C 29 -20S/(20S + 20R), C 31 -22S/(22S + 22R), C 32 -22S/(22S + 22R), and C 31 M/C 31 H, presented abnormal distributions in the large set of lacustrine shales, especially in the Es 3 1. This observation suggested suppression of several thermal maturity indicators. VR O suppression positively correlated well with the corrected hydrogen index and alginite content in maceral composition. The high content of alginite group was considered to be the primary reason behind the evident negative VR O anomaly. • Variation of thermal maturity parameter in lacustrine shales was investigated. • Quantitative correlations of VR o suppression to organic compositions were examined. • High content of alginite suppressed the thermal maturation of organic matter. • Suppression of biomarker thermal parameters in lacustrine shale was analyzed. [ABSTRACT FROM AUTHOR]

Abstract Microtextures in lacustrine fine-grained sedimentary rocks generally include the basic features of quantity, size, shape and arrangement of particles, which are the internal factors that affect the mechanical properties of rocks. In this paper, based on mechanical experiments and a rock slice image analysis technique, fracability is systematically evaluated in terms of textures by using the Mesozoic and Cenozoic lacustrine fine-grained sedimentary rocks in eastern China. Analysis show that the fracability of fine-grained sedimentary rocks is improved with increasing particle quantity and size but is reduced with increasing particle sorting, roundness, vertical distribution uniformity and holistic distribution uniformity. Regarding composition, fracability is reduced with increases in clay minerals and total organic carbon (TOC) but is first reduced and then improved with increases in the content of feldspar or carbonate minerals, i.e., when the mineral content exceeds 30%, fracability is positively correlated with it, but it is negatively or weakly correlated for mineral contents below 30%. Fracability is improved with enhanced the diagenesis strength. Regarding structural aspect, the fine-grained sedimentary rocks with strong laminability and continuity have poor fracability. A comparative analysis also shows that the stronger control factors on fracability are vitrinite reflectance (Ro), particle quantity, size, sorting, roundness and holistic distribution uniformity, while the components (e.g., TOC, clay minerals, and felsic minerals) and laminar development degree (e.g., laminar continuity and laminar quantity) are relatively weak factors. Diagenesis plays an important role in fracability for the fine-grained sedimentary rocks in different target layers, but for the fine-grained sedimentary rocks in a target layer, the texture factors are the intrinsic factors controlling the fracability and should be considered as primary considerations in engineering fracturing operations. Highlights • The textures are characterized and quantified by image analysis technique. • Fracability is improved with increasing of particle quantity and size. • Fracability is weakened with increasing of particle sorting, the roundness, etc. • Fracability is also controlled by the composition, laminar structure and diagenesis. • The controlling factors on fracability are successively Ro, particle quantity, particle size, etc. [ABSTRACT FROM AUTHOR]

Abstract Lacustrine beach-bar sandstone reservoirs are economical important exploration targets in the Dongying Depression, Bohai Bay Basin. The factors that controls reservoir quality and hydrocarbon accumulation in lacustrine beach-bar sandstones were systematically investigated through an integrated petrographic, petrophysical, and diagenesis analysis. The reservoir properties of the beach-bar sandstones are mainly affected by compaction, carbonate cementation and dissolution. Significant negative relationships exist between both the porosity and permeability and the volume of carbonate cement. Carbonate cement is mainly distributed along the sandstone-mudstone contacts. Sandstones with thicknesses less than 1 m are generally tightly cemented to form zones with low petrophysical properties. The material source of the carbonate cement in the tightly cemented zone was highly likely from the interbedded mudstone. Content of carbonate cements in the center of the thick sandstones is low, whereas compaction is the dominant diagenetic process, with some dissolution. There is a positive correlation between the petrophysical properties of the beach-bar sandstones and the content of kaolinite. The petrophysical properties have an inverse correlation with the contents of illite and illite-smectite mixed-layer clays. Reservoir petrophysical property largely controls hydrocarbon accumulation in the beach-bar sandstones. Compared with non-oil bearing sandstones, oil-bearing sandstones have higher porosity and a larger pore-throat size. Sandstones with a porosity of less than 8.5% and/or low permeability of less than 1.02 × 10−3 μm2 are mostly incapable of accumulating oil. Highlights • The reservoir quality is controlled by compaction, carbonate cementation and dissolution. • Carbonate cementation mainly affects the reservoirs along the sandstone-mudstone contacts. • Compaction and dissolution mainly affect the reservoirs in the center of sandstones. • Reservoir petrophysical properties largely control hydrocarbon accumulation. • Lower limits of the porosity and permeability are required for hydrocarbon accumulation. [ABSTRACT FROM AUTHOR]

Understanding diagenetic fluid types and their effects on diagenesis is important to identify the main control factors of reservoir characteristics and predict reservoir sweet spots. Reservoir space destruction caused by strong cementation is notable in the Lower Jurassic Ahe Formation sandstones near the deep-rooted fault (Yiqikelike Fault) in the Kuqa depression. Different authigenic minerals and widely varying cementation contents indicate the complexity of diagenetic fluids. Therefore, we investigated strong cementation using petrographic and geochemical methods, including cathodoluminescence, scanning electron microscopy, laser ablation inductively coupled plasma mass spectrometry, and stable isotope analysis. The contents of authigenic quartz and calcite increased as they approached the deep-rooted fault, with maximum concentration of quartz and calcite of up to 8.73% and 8.92%, respectively. Abnormally high homogenization temperatures of the fluid inclusions were observed in the quartz overgrowth, one of which reached 156 °C. In calcite, the rare earth element Eu exhibited abnormal characteristics, and the carbon isotopes ranged from −8.00‰ to −2.00‰. Therefore, it is likely that deep fluids are involved in reservoir diagenesis near a deep-rooted fault. This study provides a good example of diagenetic evolution in deep sandstone reservoirs and the negative effects of the involvement of deep fluids in diagenetic processes. • The deep fluids were involved in cementation of diagenesis in Jurassic Ahe Formation sandstones near the deep-rooted fault. • The deep fluids provided material source for authigenic quartz and calcite. • The deep fluid is one reason for the difference in diagenesis between the areas near and far from the deep-rooted fault. [ABSTRACT FROM AUTHOR]

Understanding the flow processes that form gravity-flow deposits is vital for modeling and predicting sandstone bodies in the subsurface, which is of great significance for conventional and unconventional oil and gas exploration and development in lacustrine basins. This study analyses gravity-flow deposits of the Late Triassic Chang 7 oil member of the Yanchang Formation in the Ordos Basin, using a combination of well-log analysis, core observations, thin-section analysis, and laboratory measurements. The sedimentary facies, formation mechanisms, distribution patterns, and depositional models of gravity-flow deposits are investigated. Thirteen facies and eight bed-types are recognized in the gravity-flow deposits. Bed types represent deposits of sandy slides, sandy slumps, debrites, high-density turbidites, hybrid event beds, transitional flow deposits, surge-like low-density turbidites, and quasi-steady low-density turbidites. Gravity-flow deposits, particularly those caused by sediment failure, are composed of slides, slumps, debrites, high-density turbidites, hybrid event beds, transitional flow deposits, and surge-like low-density turbidites. These deposits form as isolated lenticular sand bodies, with retrogradational internal stacking patterns. In contrast, gravity-flow deposits caused by flooding rivers are composed of high-density turbidites, hybrid event beds, transitional flow deposits, and quasi-steady low-density turbidites. These deposits exhibit elongate morphologies with progradation stacking patterns internally. The collision between the North China Block and the South China Block, which occurred during the closing of the Qinling Ocean, lead to frequent volcanic eruptions and earthquakes. These activities promoted the development of gravity-flow deposits associated with sediment failure from delta fronts in both the northeastern and southwestern parts of the basin. Concurrently, extremely humid climatic conditions promoted increased fluvial drainage, leading to enhanced hyperpycnal flows into the deep-lacustrine basin depocenters in the Late Triassic. This study emphasized that detailed facies analysis and distribution pattern analysis are the fundamental way to identify the formation mechanisms of gravity flow deposits in lacustrine depression basins. • Thirteen facies and eight bed-types are recognized amongst the gravity-flow deposits in the Late Triassic Chang 7 oil member of the Yanchang Formation in the Ordos Basin, which are composed of sandy slide, sandy slump, muddy debrites, sandy debrites, hybrid event beds, transitional flow deposits, surge-like low-density turbidite, quasi-steady low-density turbidite. • Gravity-flow deposits in the late Triassic Chang 7 oil member of the Yanchang Formation in the Ordos Basin are due to both sediment failure and flooding river. • The collision between the north China block and the south China block during the closing of Qinling Ocean, and extremely humid environment in the Late Triassic promotes the development of gravity-flow deposits in the Ordos Basin. [ABSTRACT FROM AUTHOR]

The Ordovician carbonate fault-controlled reservoir in the Shunnan area is substantially different from the typical karst reservoir closely related to an unconformity. The development of fault-controlled reservoirs is controlled by the deep and large strike-slip fault in the Middle Caledonian period. The characteristics, diagenesis types, diagenetic fluid evolution characteristics, fluid-rock interactions, and reservoir response of fault-controlled reservoirs were investigated systematically by analyzing 3D seismic, logging, and drilling core data. Production data, reservoir petrology, and chemical filling geochemistry were also used. The formation and evolution of fault-controlled reservoirs are affected by the coupling of the structure, lithofacies, and fluid. The dolomitization in the quasi-syngenetic period in the early stage of Episode I in the Middle Caledonian provided the material basis for the development of fault-controlled reservoirs. The Mg-rich high-salinity thermal fluid in the late Caledonian and the Early Hercynian provided a constructive transformation for the development of fault-controlled reservoirs. In contrast, the transformation of reservoirs by the Si-rich magmatic thermal fluid in the late Hercynian was mainly destructive, and the action range of the deep thermal fluid was strongly controlled by strike-slip fault zones. The releasing bend and superimposed parts of different structural styles in the fault zone have high activity intensity. These are areas where dissolution and transformation of the fluid occur preferentially. Dolomite is often more conducive to the development of fault fracture zones and promotes the dissolution and transformation of the deep thermal fluid. Therefore, the strike-slip fault cutting through the basement causes the hydrothermal fluid to enter the fault, dissolving carbonate minerals and forming a hydrothermal-modified fault-controlled reservoir composed of fractures, dissolution pores, and dolomite intercrystalline pores. The results provide a new approach and direction for the study of different types of deep and ultra-deep carbonate reservoirs. • The fault zone controls the distribution of fault-controlled reservoirs. • The dolomitization and Mg-rich thermal fluid are factors affecting reservoirs. • Dolomite promotes fault development and thermal fluid dissolution. [ABSTRACT FROM AUTHOR]

The Eocene sandstones in the northern Dongying Sag, Bohai Bay Basin, China, are reservoirs for large accumulations of hydrocarbons. The sandstones are mainly lithic arkoses and feldspathic litharenites, texturally and compositionally immature. These sandstones have a wide range of porosity (0.4–37%) and permeability (0.004–6969 mD) and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level. The reduction in reservoir quality is a product of several digenetic processes; these include compaction, precipitation of dolomite and calcite in eodiagenetic stage; compaction, feldspar dissolution, precipitation of quartz cements and clays (kaolin and illite) and precipitation of ferrocalcite and ankerite in mesodiagenetic stage. Mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones. Carbonate cementation, together with compaction reduced the sandstones’ porosity and permeability significantly. In a sandstone bed, marginal sandstones with distance to sandstone/mudstone interface less than one meter always have lower porosity than central sandstones. As burial depth exceeds 4000 m, marginal sandstones have very low porosities (<5%), indicating that thin sandstone beds (<2 m) were totally destroyed by cementation and compaction, and only thick sandstone beds (>2 m) can be potential effective reservoirs. Feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity. Mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones, and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0.25%. However, although there is little or no net import of matter to the sandstones, the pore architecture changes dramatically. Primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases, occurring mainly between clay crystals. The overall result is that permeability is significantly degraded. [ABSTRACT FROM AUTHOR]

The diagenetic characteristics and differences between the Eocene red-bed sandstone deposited in an arid environment and gray-bed sandstone deposited in a relatively humid environment were investigated systematically by an integrated analysis of petrography, X-ray diffraction, carbon and oxygen stable isotopes and fluid inclusion microthermometry. Strong eodiagenetic calcite and gypsum/anhydrite cementation occurred at the boundary of the Eocene red-bed sandstone, while the centre of the red-bed sandstone was characterized by relatively strong quartz dissolution and weak mesodiagenetic feldspar dissolution and ferro-carbonate cementation. Eodiagenetic calcite cementation at the boundary of the gray-bed sandstone and quartz dissolution were relatively weak, while the carbonate cemented zone at the boundary of the gray-bed sandstone was strengthened by mesodiagenetic ferro-carbonate cement. Relatively strong feldspar dissolution and ferro-carbonate cementation occurred in the centre of gray-bed sandstone beds. By comparing the diagenetic differences between the red-bed sandstones and gray-bed sandstones, it is shown that sedimentary environment had an obvious influence on reservoir diagenetic evolution and reservoir properties through influencing the chemical properties of connate water, the transformation rate and degree of clay minerals in interbedded mudstone, and the properties of diagenetic fluids during burial process. • Eodiagenetic calcite cementation and quartz dissolution in the red-bed were stronger than that in the gray-bed sandstone. • Mesodiagenetic feldspar dissolution and carbonate cementation in the red-bed were weaker than that in the gray-bed sandstone. • Reservoir space in the red-bed is mainly primary pores, while that in the gray-bed is mainly secondary pores. • Sedimentary environment had obvious influence on the reservoir diagenetic evolution. [ABSTRACT FROM AUTHOR]

The influence of oil emplacement on calcite cementation from external sources is a topic of significant concern that is surrounded by controversy. Microscopic heterogeneity of oil-water distribution is probably the crux of the controversy. However, the influence of the heterogeneous distribution of oil and water on calcite cementation has not been directly observed and demonstrated. In this study, visual physical simulation experiments of multistage oil emplacement and calcite precipitation were conducted in glass micromodels. The processes of multistage oil emplacement and calcite precipitation occurred alternately. Characteristics of heterogeneous distribution of oil and water and the calcite precipitation processes were clearly observed and recorded in real-time under a microscope during the experiments. Three important factors, including heterogeneous distribution of the sizes of pore-throat spaces, the previously charged oil, and the presence of precipitated oil-wet calcites, jointly control the heterogeneous distribution of oil and water in the pore-throat spaces. With the supply of materials from external sources, relatively rapid material advection resulted in abundant calcites precipitation in the interconnected pore-throat spaces completely filled with water. With the gradual increase in the proportion of oil to water in pores and throats, a gradual decrease occurred in the diffusion rate and relatively low-content calcites precipitated in water-bearing position in pores and water films. Calcites scarcely precipitated in the pores completely filled with oil. Experimental results are of significance for the processes of carbonate precipitation after oil emplacement in clastic reservoirs. The reservoirs with high paleo-oil saturation after first-stage oil emplacement are conducive to formation of high-quality reservoirs. • Heterogeneity of oil-water distribution was observed in heterogeneous pore-throat spaces containing oil and oil-wet calcites. • Influence of heterogeneity of oil-water distribution on calcite precipitation was directly observed in physical experiments. • Difference of ion migration characteristics influenced by oil-water ratio in pores controls calcite precipitation. [ABSTRACT FROM AUTHOR]

The influence of oil-wet calcite cements on oil migration in clastic reservoirs is of significant interest in the field of petroleum exploration. However, the microscopic migration of oil in microscopic pore spaces with oil-wet calcite cements has not been directly observed. Microscopic observations of oil migration were conducted in a glass micromodel at normal temperature and pressure to investigate the influence mechanism of oil-wet calcite cements on oil migration in pore spaces. The calcites were precipitated as cements prior to the oil migration. A two-dimensional glass micromodel with two interconnected spaces with the same pore throat structures was used in the experimental study. A new method to create firmly adhering calcites was used to precipitate equal amounts of calcites in the two interconnected spaces. The oil migration was recorded in real-time under a microscope throughout the experiment. The experimental results show that oil migration occurs faster in spaces with than without oil-wet calcites. During the oil migration in the pore-throat spaces, the oil migrates towards the surfaces of the oil-wet calcites when the oil is in contact with the calcites. The presence of oil-wet calcites significantly reduces the oil breakthrough pressure. The electrostatic attraction between the carboxylic groups and calcites and the intermolecular forces between the carboxylic groups and organic molecules in the oil cause the oil migration towards the surfaces of the oil-wet calcites. The experimental results indicate that oil preferentially migrates into clastic reservoirs that possess appropriate amounts of oil-wet calcite cements precipitated prior to oil emplacement. • Experimental method that allows calcite to precipitate and adhere tightly in glass micromodel was developed. • Microscopic processes of the influence of oil-wet calcites on oil migration in pore spaces were observed in micromodel. • Mechanism of oil-wet carbonate minerals affecting oil migration and accumulation in pore spaces was discussed. [ABSTRACT FROM AUTHOR]

Porosity evolution and reservoir quality are strongly affected by mechanical compaction. However, the compaction-induced porosity reduction process in sandstone reservoirs during the postaccumulation period and its effect on gas saturation remain poorly understood. By studying the sandstones in the Bayan'aobao area, we were able to quantify the porosity evolution and gas saturation variations caused by mechanical compaction during the postaccumulation period through fluid inclusion analysis and mathematical modeling. Moreover, we verified our findings by compaction experiments and geological statistics. Fluid inclusion microthermometry was employed to determine the hydrocarbon accumulation periods, and the highest homogenization temperature of the two-phase aqueous fluid inclusions coexisting with natural gas fluid inclusions, i.e., ~115 °C (with a corresponding geological age of ~140 Ma), was regarded as the beginning of the postaccumulation stages. With these data, we constructed an inversion evolution model (based on the geologic statistical model of sandstone mechanical compaction) for the postaccumulation period and determined the gas saturation changes (based on compaction porosity evolution). The modeling results indicate that the sandstone porosity decreased greatly during postaccumulation subsidence and increased slightly during late-stage uplift, while the gas saturation increased with both the porosity decrease during the postaccumulation subsidence and the porosity increases during the later uplift. The modeling results of subsidence are concordant with the compaction experiments, which also show that the larger the initial porosity, the greater the porosity reduction. The uplift modeling results demonstrate that an erosion-induced decrease in burial depth allowed the sandstone porosity to increase, which is supported by some scholars' findings (e.g., Neuzil and Pollock, 1983; Jiang et al., 2004; Zhang, 2013) and by the positive correlation between sandstone porosities and restored geologic unit thicknesses in the study area. However, the inversion modeling indicates that the reason for the porosity rebound in litharenites is that the increase in elastic porosity slightly exceeds the decrease in viscoplastic porosity, which would have been different if the uplift-induced erosion rate had been distinctly lower. Gas production test data and restored geologic unit thicknesses indicate that uplift and erosion may be able to enrich sandstone gas reservoirs to a certain extent, consistent with the calculated gas saturation variation results, but a full understanding of gas saturation evolution awaits further studies. The inversion model can provide a powerful tool for quantitative analysis of the sandstone compaction process during the postaccumulation period, which could be used for reference in other similar regions. • We studied postaccumulation sandstone porosity evolution by mechanical compaction in quantitative inversion modeling. • Compaction-induced porosity reduction differences are revealed in compaction experiments and inversion modeling. • Uplift-induced sandstone porosity rebound is indicated by inversion modeling and geological statistics. • Porosity increase in response to late-stage uplift would likely promote the enrichment of gas reservoirs. [ABSTRACT FROM AUTHOR]

Authigenic quartz is a significant cause of reduced porosity and permeability in the cement of the Permian Upper Shihezi Formation (P2s) sandstones in the Gaoqing area, Dongying Sag, Bohai Bay Basin, China. The sandstones are mainly quartzarenite with minor amounts of clay minerals. Authigenic quartz is generally the most commonly occurring cement and occurs as quartz overgrowth, microfracture-healing quartz, pore-filling quartz, and microquartz in the sandstones. Three distinct, separate phases of authigenic quartz (I, II, and III) were distinguished by using cathodoluminescence microscopy (CL). In the scanned CL images, phase I quartz generally appears with light brown luminescence in the 450–670 nm range, phase II quartz is non-luminescent in the 420–480 nm range, and phase III quartz appears with dark brown luminescence in the 470–650 nm range. Phase I quartz, originating mainly from pressure solution, has a composition similar to that of detrital quartz grains and was formed at approximately 79.3–104 °C. In the phase II quartz, Li+, Na+, Mg2+, Al3+, and Ba2+ are enriched, and the homogenisation temperature of some aqueous inclusions is significantly higher than its maximum burial temperature of 130 °C. The δEu of phase II quartz is up to 1.42. The silica sources for phase II quartz are dominated by hydrothermal fluids; pressure solution, feldspar dissolution, and illitisation of smectite also provided a small amount of silica. In the phase III quartz, all of the aqueous inclusions are almost higher than its maximum burial temperature. Al3+, Mg2+, and Rb+ are enriched in most of this quartz, which was sourced mainly from hydrothermal fluids; pressure solution also provided a very small amount of silica. The hydrothermal fluids providing the silica source for phases II and III quartz migrated into the P2s sandstones along the adjacent Gaoqing Fault. • The siliceous sources are dominated by external hydrothermal fluid and internal pressure solution of quartz grains. • Hydrothermal fluid was proved by the homogenisation temperature and the chemical composition of mineral assemblages. • Three distinct, separate phases of authigenic quartz were distinguished. • Phase I quartz is mainly from pressure solution. Phase II and III quartz are dominated by deep hydrothermal fluids. [ABSTRACT FROM AUTHOR]

Abstract Mixed deposits are sediments consisting of external clastic (epiclastic or terrigenous), intrabasinal components and pyroclastic components. The mixture, comprising variable amounts of the three components, is defined as "mixed sedimentary rocks". The Permian Lucaogou Formation (P 2 l) in the Jimusar Sag of the Junggar Basin is a promising tight oil target in western China, the fine-grained mixed sedimentary rocks of which are rich in organic matter (OM) and two sweet spot intervals with relatively high porosity. However, the sediment composition, provenance and deposition environmental settings have not been studied in detail. In this study coupled chemostratigraphic-petrographic analysis were used to reconstruct their depositional environments. The results show that the fine-grained sedimentary rocks have three major sediment sources, external clastic input (terrigenous clastics), intrabasinal autochthonous to parautochthonous components (carbonates, siliceous skeletal debris and OM) and pyroclastic input. Main lithofacies include siltstone/fine sandstone, mudstone, dolomite and tuffite. The silt/sandstones were mainly sourced from rocks with calc-alkaline composition, while the tuffaceous sedimentary rocks were sourced from high-K calc-alkaline rocks. Elemental proxies suggest that the carbonate rocks were generally deposited under warm and arid conditions, whereas the fine-grained clastic sediments were deposited under relatively humid conditions. The muddy or silty tuffaceous mixed rocks were deposited under relatively reducing conditions compared with carbonates and sandstones. Variations of lithofacies and OM accumulation of different intervals reflect changing deposition environmental settings, and the frequently altered high TOC content rocks and good reservoirs are benefit for tight oil formation. The work may provide some useful insights and serve as a reference for studying other mixed fine-grained sedimentary rocks and tight oil plays in similar lacustrine basins elsewhere. Highlights • Lacustrine mixed fine-grained sedimentary rocks have three sources: extrabasinal, intrabasinal and pyroclastic component. • Coupled chemostratigraphic-petrographic analyses for depositional conditions of the mixed fine-grained sedimentary rocks. • The effect of sedimentary environments on lithofacies and OM accumulation variation. • High TOC content and high porosity reservoir interbedded layers are benefit for the development of tight oil. [ABSTRACT FROM AUTHOR]

Abstract In order to better predict the petrophysical properties of the tight sandstone reservoirs in the Upper Triassic Yanchang Formation, Ordos Basin, China, the nature of the diagenetic system involved was investigated using a suite of petrographic and geochemical techniques including thin section and X-Ray Diffraction analysis, scanning electron microscopy, MAPS mineralogy, Cathodeluminescence, electron probe microanalysis and fluid inclusion analysis on a set of selected tight sandstone samples. The sandstones investigated are texturally mature but compositionally immature with an average framework grain composition of Q 32 F 41 L 27. Authigenic quartz, calcite, and chlorite coatings are the major cements, while feldspars are partially dissolved. Two groups of quartz cement are present in the tight sandstone reservoirs. Smectite to illite conversion provided silica source for the first group of quartz overgrowth (Qo-I), while the silica source for the second quartz overgrowth (Qo-II) was mainly originated from feldspar dissolution. Calcite cements consist of two types, namely Ca-I and Ca-II. The Ca-I calcite cement contains no chlorite coatings and have higher concentrations of Fe2+ and Mg2+, whereas the Ca-II calcite cement developed over the chlorite coatings. Plagioclase dissolution appears to be always accompanied by calcite cementation, while K-feldspar albitization forms euhedral albite locally. The burial diagenesis processes were, in most cases, not episodic but occurred as slow adjustments in response to increased burial depths and temperatures. In all the studied diagenetic minerals, elemental distributions appear to be strictly constrained by the interaction of authigenic minerals involved, indicating that the chemical reactions during the diagenesis comply well with the general principle of mass balance. The diagenesis in the Upper Triassic Yanchang Formation tight sandstones was thus mainly developed in a closed geochemical system, where mineral dissolution and precipitation are approximately balanced, and diagenetic fluids were not affected by external sources significantly. Mechanical compaction has played a more important role in destroying the primary porosities of the Upper Triassic Yanchang Formation than the cementation. Highlights • There are two groups of quartz cement in the tight sandstone reservoirs. • Two types of calcite cement are present in the tight sandstone reservoirs. • Diagenesis of the tight sandstone reservoirs occurred in a closed geochemical system. [ABSTRACT FROM AUTHOR]

Abstract This research is focused on the characteristics and origin of major authigenic minerals and the influences on the reservoir quality of the Permian Wutonggou Formation sandstones in the eastern slope of Fukang Sag, Junggar Basin, Western China. Authigenic cements were observed and described from thin sections, scanning electron microscopy, fluorescence, and cathodoluminescence. The elemental concentrations of the authigenic minerals were measured by electron probe microanalysis. Additional characterizations were obtained from O and C stable isotope ratios and from the homogenization temperature of aqueous fluid inclusions. The results show that the sandstones in the Wutonggou Formation are dominated by volcanic lithic fragments. Carbonates and laumontites are the major authigenic minerals in the study area. Authigenic quartz and kaolinite can also be observed. Two types of carbonate cements are identified. The first type is calcite with yellow fluorescent hydrocarbon inclusions. The carbon and oxygen isotopes in the calcites are highly C-depleted and O-depleted. The homogenization temperatures of the aqueous fluid inclusions in the calcite cements are higher than the highest paleogeotemperatures during the burial history. Combined with the presence of authigenic fibrous illites and the illitization of kaolinites, it is determined that the precipitation of the calcites is related to the upwelling of deep fluids and was impacted by the CO 2 from the thermal decarboxylation of organic matters during the middle-late Jurassic. The second type is ferroan calcite with blue fluorescent hydrocarbon inclusions and higher contents of Fe, Mn, Cr, and Mg than in the calcites. The ferroan calcites in the Beisantai uplift are rich in 13C (δ13C (V-PDB) values are +22.10‰ and +22.16‰). The precipitation of these ferroan calcites was impacted by the CO 2 from crude oil biodegradation after the late Jurassic. Two types of laumontites are identified. The first type is partially dissolved and is characterized by a high Ca/Na ratio and low Fe contents. The second type is hardly dissolved, has hydrocarbon inclusions with yellow and blue fluorescence, and is characterized by a low Ca/Na ratio and high Fe contents. The first type of laumontite was precipitated prior to hydrocarbon charging and the second type was formed after hydrocarbon charging and meteoric water leaching. The hydration of the volcanic debris provided the source materials for both types of laumontite. The authigenic quartz and kaolinite were formed by the alteration of feldspars, laumontites, and volcanic materials. The evolution of the reservoir properties was impacted by various degrees of cementation and dissolution; as a result, the reservoir quality in the Beisantai uplift was better than in the Shaqiu area. Highlights • Origin and formation time of calcite cements is determined under the context of multi-stage tectonic subsidence and uplift. • The precipitation of the ferrocalcites in Beisantai uplift was impacted by biodegradation of crude oil, which was firstly found in this area. • Multistage laumontite are precipitated in the study area. • The diagenetic paragenetic sequence and evolution of the reservoir properties are established in study area which experienced a multi-stage uplift and subsidence process. [ABSTRACT FROM AUTHOR]