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The tight sandstones of the Upper Triassic Xujiahe Formation are important gas reservoirs in the western part of the Sichuan Basin, China. However, their diagenesis and the factors controlling diagenetic processes are poorly constrained. The petrological and geochemical nature of diagenetic minerals of the Xujiahe sandstone reservoirs was analysed from 105 core samples of three wells in the Xinchang Gas Field. The lower member, Xu‐2, sandstones have an average framework composition of Q65F16R19, and 47% of the total lithic fragments are volcanic rock fragments (VRFs). In contrast, the upper member, Xu‐4, sandstones have an average framework composition of Q53F0R47 with a near absence of feldspar and VRFs. The main phase of carbonate cementation in the Xu‐2 sandstones is mesogenetic ferroan dolomite, whereas in the Xu‐4 sandstones, it is intergranular poikilotopic calcite. A fair amount (0.97% in bulk rock and 29% in clay fraction) of authigenic kaolinites appear in the Xu‐4 sandstones, but they are completely absent in the Xu‐2 sandstones. The observed textural occurrence of kaolinites, in combination with the positive relationship between δ13C and δ18O in the calcite cements, suggests that meteoric water flushed the Xu‐4 sandstones, resulting in the dissolution of feldspar to a negligible amount. Quartz cementation is more extensive (1.4 vol%) in the Xu‐2 sandstones than in the Xu‐4 sandstones (0.4 vol%). The fluid inclusions within the quartz cements in the Xu‐2 sandstones have a higher homogenization temperature interval (84–217°C) than those in the Xu‐4 sandstones (81–147°C), indicating that quartz cementation continued to affect the Xu‐2 sandstones during deep burial but ceased relatively early in the Xu‐4 sandstones. High‐intensity chlorite authigenesis, feldspar overgrowth, and albitization occur only in the Xu‐2 sandstones, suggesting that the source of these sandstones was primarily from the alteration of VRFs during late‐stage diagenesis. These clear variations in diagenetic features between the Xu‐2 and Xu‐4 members were most likely controlled by the significant variations in their depositional environments (marine vs. non‐marine) and in the original framework compositions. [ABSTRACT FROM AUTHOR]

The Xiayan Rise (Junggar Basin, NW China) preserves a record of fluvial–deltaic–lacustrine, low‐accommodation deposits of the Lower Jurassic Sangonghe Formation. Sequence stratigraphy, depositional systems, and petroliferous plays are investigated with seismic sections, wireline logs, and cores. The results indicate that two third‐order sequences and eight fourth‐order sequences are recognized. The sequence architecture is controlled by episodic tectonic movement, sediment supply, and climate. In addition, several sedimentary microfacies are identified including braided delta deposits and shore‐shallow lacustrine deposits. During the depositional period of J1s22, basin tectonics result in a decrease in water depth and accommodation space. Subaqueous distributary channels are wide and flat which superimpose and connect with each other. During the depositional period of J1s21‐2, tectonic movement results in increasing water depth and accommodation space during a phase of warm and humid climate. During the depositional period of J1s21‐1, the accommodation space was limited but sediment supply continued which is easy to form forced regression. Low rates of tectonic subsidence and low accommodation space results in the distribution of isolated subaqueous distributary channels which resulted from lacustrine depth. Petroliferous plays comprising source rocks, reservoirs, and seals can be found in LST1 and LST2. [ABSTRACT FROM AUTHOR]

The Junggar Basin is a typical superimposed system characterized by multilateral steep thrusts and extrusions and one of the primary petroliferous basins in Western China. To date, geologists have been most interested in the basin's tectonic evolution and petroleum geology characteristics. Based on field outcrops, lithologic combinations, logging curves, and seismic reflection characteristics, we present a systematic study of the sequence stratigraphy pattern for the Jurassic central Junggar Basin. Herein, we identify the central Junggar area as having a "dual" characteristic sequence framework composed of a lowstand system tract (LST) and a transgressive system tract (TST), which is distinguishable from the traditional three system tract (LST, TST, and highstand system tract [HST]). In each third‐order sequence, the LST consists of conglomerate or pebbly sandstone deposits, displaying the characteristics of braided channel and fan delta. The TST consists of siltstone, argillaceous siltstone, and mudstone, displaying fine‐grained, shore‐shallow lacustrine deposits. It is considered that the combination of multiphasic thrust extrusion and subsequent, steady deposition is likely responsible for the dual‐system tract. The forming process of the dual‐system tract pattern could divide into four stages: the quiet stage of tectonic activity, the early stage of compression, the continuous stage of compression, and the rapid rebound stage. Additionally, two reservoir predictive models are proposed: the progradational predicting model on the edge of the depression and truncated and pinchout predicting model on the palaeo‐uplift. We conclude that the coarse clastic unit of the LST has bright prospects for oil and gas exploration within the superimposed system in Western China. [ABSTRACT FROM AUTHOR]

Deeply buried (3.5–4.5 km) fan deltaic and braided deltaic sandstones of the Eocene Shahejie Formation are important hydrocarbon reservoirs in the Raoyang Sag of the Bohai Bay Basin (North China). An integrated approach incorporating petrophysics, thin‐section petrography, scanning electron microscope (SEM), cathodoluminescence (CL), and well log analysis was applied to investigate diagenesis, diagenetic minerals, and their impacts on reservoir quality. The sandstones of the Sha‐3 (the third Member of the Shahejie Formation) are dominated by lithic arkoses and arkoses with a low‐moderate compositional maturity and a moderate textural maturity. The pore systems are dominated by secondary pores resulting from dissolution of framework grains, micropores among authigenic clays, and minor amounts of primary intergranular pores. Chemically unstable framework grains such as feldspar and lithic fragments experienced alteration and resulted in formation of authigenic kaolinite. Dissolution and alteration of feldspars and rock fragments and pressure solution of detrital quartz grains were the main sources for quartz cements. Mechanical compaction and cementation by calcite, dolomite, quartz, kaolinite, and mixed‐layer illite/smectite destroyed primary pores, whereas dissolution of framework grains generated secondary pores. Five types of diagenetic facies are identified based on framework mineralogy, texture, diagenetic minerals, and pore systems. These comprise the following: (a) quartz‐cemented sandstone, (b) tightly compacted sandstone, (c) carbonate‐cemented sandstone, (d) clay‐mineral‐cemented sandstone, and (e) clean dissolved sandstone. Diagenetic processes and evolution sequences for the five diagenetic facies were reconstructed based on textural relationships from thin section, SEM, and CL studies. The reservoir quality evolution of various diagenetic facies is predicted by considering variations in grain size, sorting, shape, and matrix content of sandstones. This work, which investigates the diagenetic sequence of sandstones, will provide insights into reservoir quality prediction for low‐permeability sandstones with similar tectono‐depositional settings. [ABSTRACT FROM AUTHOR]

Significant achievements have been reported for deep oil and gas exploration in the Bohai Bay Basin. Among them, the Nanpu Sag has achieved good deep (greater than 3.5 km) hydrocarbon exploration results. However, the hydrocarbon formation mechanisms and the diagenetic evolution of these high‐quality sandstone reservoirs remain unclear. Mineralogical, petrographic, and fluid inclusion data were selected to investigate the deeply buried sandstone reservoir physical properties, pore systems, and diagenetic features and the main formation mechanisms of the deep high‐quality reservoirs of the Es1 (the top member of the Paleogene Shahejie Formation) sandstones in the Nanpu Sag, eastern China. The results show that the Es1 sandstone reservoirs are dominated by lithic arkoses and have low porosities (average porosity 13.4%) but medium to high permeability (average permeability 188 mD). The pore systems consist of primary intergranular pores, secondary dissolution pores, and microfractures. Loss of porosity is greater due to compaction (average 20.89%) than due to cementation (average 4.82%), and dissolution contributes an improvement in porosity of approximately 5.85%. Two peaks in the inclusion homogenization temperatures were observed: The first fluid inclusion homogenization temperature term ranges from 130°C to 145°C and exhibits yellow fluorescence; the other peak occurs from 155°C to 165°C, showing green fluorescence. Eodiagenesis is due to mechanical compaction, precipitation of early calcite cements and quartz overgrowths, and dissolution of feldspar and carbonate cements. Mesodiagenesis occurs due to mechanical compaction, framework grain (mainly feldspar) dissolution, and precipitation of quartz and late carbonate cements. The main factors controlling the formation of the deep high‐quality reservoirs are the presence of a high hydrodynamic depositional environment (subaqueous distributary channel), the type of provenance (Archean granites), and the creation of secondary porosity by feldspar dissolution. [ABSTRACT FROM AUTHOR]

Reservoir quality is critical in tight gas exploration. A systematic study of the petrological, petrophysical, and diagenetic characteristics of the Shanxi Formation sandstones in the north‐eastern Ordos Basin (China) was undertaken to characterize the pore system of these tight sandstones and recognize the distribution of reservoirs favourable for hydrocarbon accumulation. The results suggest that the Shanxi Formation sandstone has low compositional and moderate textural maturity, with its porosity ranging from 0.7% to 11.8% (average 5.97%) and permeability ranging from 0.001 to 2.77 mD (average 0.36 mD). In addition, thin section and SEM observations suggest that the sandstone is dominated by secondary dissolution pores, primary intergranular pores as well as microfractures, and has undergone compaction, cementation, and dissolution. Diagenetic minerals, such as carbonate cements, authigenic quartz, clay minerals, and dissolved feldspar, are identified. Fluid inclusions are observed in healed microfractures of quartz grains and in quartz overgrowths, while the homogenization temperatures of fluid inclusions in healed microfractures and in quartz overgrowth are in the ranges of 92.8–179.1°C (average 134.8°C) and 104.7–169.1°C (average 145.7°C), respectively. Authigenic kaolinite is sourced from the process of K‐feldspar dissolution, authigenic illite is sourced from transformation of smectite and kaolinite, while sources for quartz cements include mineral alteration and pressure dissolution. Selective dissolution of K‐feldspar in the presence of carbonate minerals is observed in the Shanxi Formation tight sandstones due to the different equilibrium constants of carbonate minerals and K‐feldspar leaching, resulting in dissolution pores associated with K‐feldspar. Reservoir quality in the Shanxi Formation tight sandstones is greatly influenced by diagenesis, as compaction and cementation are responsible for the loss of porosity while dissolution accounts for the enhancement of secondary porosity. [ABSTRACT FROM AUTHOR]