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Tight-gas reservoirs, characterized by low porosity and low permeability, are widely considered to be the product of post-depositional, diagenetic processes associated with progressive burial. This study utilizes a combination of thin section petrography, scanning electron microscopy, microprobe and back scatter electron analysis, stable isotope geochemistry and fluid inclusion analysis to compare the diagenetic history, including porosity formation, within sandstones of the second member of Carboniferous Taiyuan Formation (C 3 t 2 ) and the first member of Permian Xiashihezi Formation (P 1 x 1 ) in the Ordos Basin in central China. In the P 1 x 1 member, relatively high abundances of metamorphic rock fragments coupled with a braided river and lacustrine delta environment of deposition, produced more smectite for transformation to illite (50–120 °C). This reaction was driven by dissolution of unstable minerals (K-feldspar and rock fragments) during the early to middle stages of mesodiagenesis and consumed all K-feldspar. Abundant intragranular porosity (average values of 2.8%) and microporosity in kaolinite (average values of 1.5%) formed at these burial depths with chlorite and calcite developed as by-products. In the C 3 t 2 member, relatively low abundances of metamorphic rock fragments coupled with an incised valley-coastal plain environment of deposition resulted in less smectite for transformation to illite. High K + /H + ratios in the early pore waters related to a marine sedimentary environment of deposition promoted this reaction. Under these conditions, K-feldspar was partially preserved. During the middle to late stages of mesodiagenesis, K-feldspar breakdown produced secondary intragranular (average values of 1.4%) and intergranular pores (average values of 1.2%). Release of K + ions promoted illitization of kaolinite with quartz overgrowths and ferrous carbonates developed as by-products. This study has demonstrated that whereas both members are typical tight-gas sandstones, they are characterized by quite different diagenetic histories controlled by the primary detrital composition, especially during mesodiagenesis. Types of secondary porosity vary between the two members and developed at different stages of progressive burial. The content of unstable detrital components, notably feldspar, was the key factor that determined the abundance of secondary porosity. [ABSTRACT FROM AUTHOR]

With the recent discovery of new oil fields around the Dongying Depression, Bohai Bay Basin, a better understanding of the origin of its high-quality lacustrine carbonate reservoirs has become important. The lacustrine carbonates of the upper fourth member of the Eocene Shahejie Formation (ES4s) have been examined using a variety of methods, including core and thin section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The δ13C and δ18O values of the Dongying carbonates differ significantly from Eocene marine values, and the 87Sr/86Sr ratios of carbonate minerals are higher than Eocene marine water and the same as the surrounding Palaeozoic carbonate bedrock. Additionally, the distribution of the carbonates is controlled by the surrounding Palaeozoic carbonate bedrock, all of which indicate their lacustrine origin. Six major carbonate lithofacies are distinguished in the southern gentle slope zone. Bioclastic, oolitic and intraclastic grainstones dominate the littoral to sublittoral zones, while mudstones are mainly distributed in the profundal zone. The ES4s interval is composed of a third-order depositional sequence that contains several small-scale, higher frequency depositional cycles. Overall, stacking patterns of high-frequency depositional cycles are consistent with the general facies variation from basin to shore. Shallow-water grainstones contain both primary intergranular and secondary dissolution pores, whereas the dolomitized mudstones in the profundal zone contain intercrystalline pores and fractures. Diagenetic processes that improve reservoir quality include dissolution and dolomitization. Reservoir quality was diminished by burial calcite cementation and compaction. High-quality reservoirs are mostly distributed in dolomitized grainstones and wackestones, which are deposited in the shoal facies of shallow lakes. Variations in reservoir quality in the Dongying Depression are due to a combination of lithofacies type, dissolution intensity, early dolomitization and tectonic-related deformation. Due to the influx of meteoric waters and shallow burial depth, the grainstones in the littoral zone have the best reservoir quality. Integrated studies have helped to understand the reservoir heterogeneity and hydrocarbon potential of the Dongying lacustrine carbonates. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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

The Es 4 s shale is taken as the main research object to understand and describe the reservoir characteristics in Boxing Sag, Dongying Depression. Through core observation, X-ray diffraction, thin section observation, field-emission scanning electron microscopy analysis, and low-pressure nitrogen gas adsorption experiment, the reservoir space of the Es 4 s shale including pore types, pore size, and pore structure characteristics was elucidated. The study shows that the shale in the Boxing Sag has the following characteristics: (1) the reservoir space in the study area is diverse, with the development of inorganic pores, organic pores, and microfractures. The higher the content of calcite and organic matter, the more favorable the development of intergranular pores, dissolution pores, and organic matter pores and (2) complex pore structure. The average Barrett-Joyner-Halenda pore size of calcareous shale is 6.5-22.8 nm, and the Brunauer-Emmett-Teller cumulative specific surface area is 0.7588-4.744 m 2 /g. According to the morphological analysis of the adsorption and desorption curve, it is found that the shale samples in the target interval are mainly ink-bottle-shaped, cylindrical, and slit-shaped pores. The shale samples with relatively well-laminated intervals are mainly composed of ink-bottle-shaped and cylindrical pores, while the samples with relatively unlaminated intervals and high clay mineral content are mainly composed of slit-shaped pores. The contents of clay minerals and calcite are correlated with pore volume, specific surface area, and pore size, which further indicates the controlling effect of clay minerals and carbonate mineral components on pore structure parameters. This study not only helps us to understand the distribution of various micropores/nanopores in the lacustrine shale but also acts as a guiding note for the characterization of the shale oil reservoir, which ultimately offers a theoretical foundation for lacustrine shale oil exploration and development., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)