Search

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]

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]

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]

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]

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]

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]