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to ultradeep-buried volcaniclastic sandstone at depths of 5000–8000 m is an important hydrocarbon reservoir in the central Junggar Basin, northwestern China. This study investigates diagenetic evolution and deciphers the formation mechanisms of anomalously high reservoir quality in deep-buried environments. Based on petrographic and geochemical evidences, extensive alterations of volcanic detritus lead to complicated diagenetic pathways. During eogenesis, grain-coating chlorite, first-stage laumontite (60–70 °C) and subordinate quartz were precipitated through hydration of volcanic glass. More complicated diagenetic products occurred during mesogenesis and contained (1) dissolution of volcanic rock fragments and precipitation of second-stage laumontite (110–120 °C); (2) dissolution of laumontite cements at elevated partial pressures of CO 2 (PCO 2) followed by calcite cementation (δ13C PDB from −16.51‰ to −7.30‰ and δ18O PDB from −13.67‰ to −9.23‰); and (3) quartz cements and albitization. Integration of depositional lithofacies, grain-coating chlorite as well as laumontite alteration, exerts significant impacts on formation of anomalously high reservoir quality in the deeply buried volcaniclastic sandstones. Medium-to coarse-grained sandstones are relatively well-sorted and could maintain high reservoir porosity owing to the development of grain-coating chlorite. Mechanic compaction is commonly effective during eogenetic regimes when the sandstones are lack of ample mineral cementation. The origin of anomalously high porosity zone largely depends on the types and abundances of mineral alteration in the deep buried regimes. Moderate abundances of grain-coating chlorite and laumontite cementation directly determine the formation of anomalously high primary porosity zones and secondary zones, respectively. • Anomalously high reservoir quality is developed in deep-buried volcaniclastic sandstones. • Diagenetic evolution pathway is strongly affected by alterations of volcanic detritus. • Formation mechanisms contain: 1) depositional lithofacies; 2) grain-coating chlorite; 3) laumontite cementation. • Different types and abundances of mineral alterations lead to different origins of anomalously high porosity zones. [ABSTRACT FROM AUTHOR]

This study investigates the importance of multiple organic–inorganic interactions on heterogeneous carbonate cementation patterns in Silurian deeply buried sandstones, central Tarim Basin, north‐western China, to evaluate their effects on reservoir quality. Petrographic observations and mineral geochemistry identify two stages of carbonate cementation: (i) eogenetic, poikilotopic blocky calcite precipitated at 20 to 50°C (δ13CVPDB from −0.3‰ to +6.8‰; δ18OVPDB between −6.6‰ and −2.9‰) and dolomite precipitated at 40 to 62°C (δ13CVPDB from +1.9‰ to +5.3‰; δ18OVPDB between −6.2‰ and −2.1‰); and (ii) mesogenetic, isolated pore‐filling ankerite formed at 70 to 120°C (δ13CVPDB from −14.4‰ to −6.6‰; δ18OVPDB between −11.6‰ and −7.8‰). The eogenetic carbonates are predominantly distributed as carbonate‐cemented beds or concretions along sandstone–mudstone contacts and were derived mainly from microbial methanogenesis of organic matter and dissolution of carbonate minerals in adjacent calcareous mudstones. The mesogenetic ankerite cements occur predominantly as carbonate‐cemented beds, concretions and patches, and are more concentrated in the central sections of sandstone bodies. It is inferred that ankerite cements were sourced from dissolution of eogenetic calcite and dolomite via in situ generation of organic CO2 related to thermal decarboxylation of organic acids within sandstones. Kinetic modelling results coupled with petrographic observations illustrate that mesogenetic dissolution of eogenetic calcite and dolomite cements did not enhance total reservoir porosity due to re‐precipitation of ankerite cements on a very local scale within sandstone bodies. The development of extensively carbonate‐cemented geometries (beds, concretions and patches) created by different generations of carbonate cements derived from external sources, led to reservoir heterogeneity and significant destruction of sandstone reservoir quality especially during diagenesis. The results of this study demonstrate the importance of multi‐stage, intra‐formational mass transfer induced by concentration gradients during organic–inorganic interactions in mediating a variety of spatial patterns of carbonate cementation under different diagenetic regimes. Better insights of these organic–inorganic interactions in a coupled sandstone–mudstone system could improve predictive models of heterogeneous diagenetic alterations in deeply buried sandstones of similar origin. [ABSTRACT FROM AUTHOR]

The diagenetic evolution of sandstone is very complicated under the conditions of high temperatures and pressures in deep-water, deep-buried regimes, which have great influence on reservoir quality. This study investigates the typical reservoir target of Neogene deep-water, submarine-fan sandstones under high-temperature, high-pressure regimes in the Qiongdongnan Basin, South China Sea. Utilizing a thin section, scanning electron microscope (SEM), mineral geochemistry combined with burial history evolution, complex diagenetic events, and main controlling factors of the sandstone in the Neogene Meishan Formation were determined. The results show that the evolution of sandstone reservoirs is initially controlled by depositional framework compositions and subsequently modified by eogenetic and mesogenetic alterations during progressive burial. Eogenetic alterations mainly include the following: (1) mechanical compaction; (2) dissolution of feldspar; (3) low-Fe calcite cementation. Mesogenetic events were identified as the following: (1) dissolution of feldspar; (2) ferroan calcite and ankerite formation; (3) precipitation of quartz and clay mineral. Mechanical compaction is greatly influenced by the original depositional framework composition, and sandstone samples enriched in high contents of detrital clay matrix always experienced extensive mechanical compaction. Different phases of carbonate cement during different diagenetic regimes lead to continuous destruction on reservoir porosity. The dissolution of unstable feldspar minerals during eogenetic and mesogenetic environments leads to the development of secondary porosities and would enhance the quality of the reservoir. Overpressure formation is pervasively developed owing to early disequilibrium compaction and subsequent natural gas charging. Only well-sorted sandstones with low contents of detrital clay matrix could resist early mechanical compaction, lead to ample residual original porosities, and then undergo extensive mineral dissolution to generate sufficient secondary porosities. Subsequently, these porosities would be effectively protected by overpressure formation. Poor-sorted sandstones with high contents of detrital clay matrix would experience strong mechanical compaction and extensive destruction of original porosities. Thus, these sandstones are difficult to have significant dissolution and are unable to be effectively protected by overpressure formation. Therefore, the interplay between the original framework composition and the corresponding diagenetic pathways coupled with overpressure formation would result in strong reservoir heterogeneity for the deep-buried sandstones during progressive burial. [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]

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

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]

This study explores CO 2 -water-rock interactions in Eocene hydrocarbon sandstone reservoirs in eastern Bohai Bay Basin, China, to document mineral alterations and their implications for CO 2 geological sequestration. Petrographic, geochemical and reactive transport modeling evidences reveal that various inorganic mineral alterations occur during organic CO 2 intrusion from adjacent source rocks into sandstone reservoirs including: (1) pervasive dissolution of calcite and chlorite minerals that are re-precipitated as secondary, more stable dolomite and ankerite cements; (2) precipitation of low-temperature calcite cements (27–49 °C) and relatively high-temperature dolomite (88–111 °C) and ankerite (103–136 °C) cements based on their δ18O PDB values (−11.91‰ to −7.86‰ for calcite; −15.16‰ to −12.48‰ for dolomite and −17.71‰ to −14.32‰ for ankerite); (3) Carbon sources for dolomite and ankerite cements (δ13C PDB from −7.59‰ to −3.58‰) are a mixture of carbon derived from early-formed calcite precursors (δ13C PDB from–0.26‰ to +3.75‰) with significant proportions of organic carbon (δ13C PDB from −25‰ to −10‰); (4) plagioclase dissolution spatially associated with precipitation of quartz and kaolinite. Reactive transport modeling results illustrate that mineral dissolution is coupled temporally and spatially with precipitation of secondary mineral phases. It is inferred that advective and diffusive mass transport of dissolved species in the extremely low pore-fluids is greatly restricted and mineral kinetics exerts more influences on CO 2 –water–rock interactions in the deep burial environments. The overall mineral alterations in the CO 2 -mediated geochemical system are the consequences of complicated interconnected reactions in the simulated model. Kinetically slow dissolution reactions of aluminosilicate minerals (e.g. chlorite) are regarded as the rate-limiting step for precipitation of more stable carbonate phases and CO 2 sequestration. Therefore, the occurrences, abundances and distribution patterns of the aluminosilicate minerals coupled with their kinetic reaction rates are likely to affect the long-term fate of the safe CO 2 storage in the deeply buried siliciclastic reservoirs. [ABSTRACT FROM AUTHOR]

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

This study deciphers the origin and distribution of multiphase carbonate cementations and assesses their implications for reservoir quality of the Eocene delta hydrocarbon sandstones in the Bohai Bay Basin. Petrographic observation and stable isotopic analysis identified two phases of carbonate cementations: (1) eodiagenetic phase including non-ferroan calcite cement precipitated at 25–42 °C (δ13C VPDB : 1.0‰ to +5.7‰ and δ18O VPDB : 10.7‰ to −7.5‰); and (2) mesodiagenetic phase which includes ferroan calcite cement (δ13C VPDB : 11.8‰ to −5.7‰ and δ18O VPDB : 17.0‰ to −12.6‰) and ankerite cement (δ13C VPDB : 13.7‰ to −5.9‰ and δ18O VPDB : 17.1‰ to −12.4‰) precipitated at 90–134 °C. The eodiagenetic non-ferroan calcite cement that is mainly sourced from microbial methanogenesis of organic matter in adjoining mudrocks, extensively cemented at or near sandstone-mudstone contact. Mesodiagenetic ferroan carbonate cements are derived from predominantly thermal decarboxylation of organic acids and subordinate dissolution of eodiagenetic calcite cements in sandstones. These ferroan carbonates are preferentially concentrated at distances of 0.5–1.0 m from sandstone-mudstone contacts. Kinetic modelling reproduces evolution pathways of different diagenetic minerals in the Es4s sandstones (the upper part of the fourth member of the Eocene Shahejie Formation). More importantly, results of modelling reveal that the dissolution of non-ferroan calcite cements would not enhance total reservoir quality due to the precipitation of more stable ferroan calcite and ankerite cements. Therefore, the occurrence of multi-phase carbonate cementations at different sections of sandstones leads to significant destruction of porosity and reservoir heterogeneity during progressive diagenesis. • The fan-delta sandstones in Es4s interval are important hydrocarbon reservoirs. • The eodiagenetic non-ferroan calcite and the mesodiagenetic ferroan carbonate cements are identified. • Different generations of carbonates are mainly sourced from the alteration of organic matter in the adjacent mudstone. • Kinetic modeling reproduces evolution pathways of different diagenetic minerals. • The multi-phased carbonate cementations lead to significant destruction in the reservoir quality. [ABSTRACT FROM AUTHOR]