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Shale oil saturated by high temperature (20 MPa) and high pressure (60 °C) conditions can not only realize the efficient saturation of shale, but also invert the shale oil return and drainage characteristics under the stratum temperature and pressure due to the heterogeneity of shale formations. In this study, the Chang 7 Member shale samples were collected, and the high-temperature and high-pressure containment device was utilized to saturate the shale oil efficiently under 20 MPa and 60 °C, and the differences of liquid hydrocarbon saturation and the degree of liquid hydrocarbon saturation for different types of pores and fractures in the shale were quantitatively characterized with a low-field nuclear magnetic resonance (NMR) technology. The results show that under the condition of formation temperature (60 °C) and pressure (20 MPa), shale oil saturation can be reached after 14 d of saturation in the shale samples. The shale oil saturation process can be roughly divided into three stages according to the various saturation rates: the rapid saturation stage, the slow saturation stage, and the second rapid saturation stage, and the degree of saturation of shale oil is characterized by a V-shape. The shale oil was distributed into four types of pore-fracture systems: adsorption pores, micropores, seepage fractures, and layer fractures. Additionally, the fluid dominantly distributes in the micropores and seepage fractures, the shale oil saturation degree of the micropores features a continuous increase, while that for the seepage fractures presents a V-shape, which finally determines the shale oil saturation characteristics of the shale. [ABSTRACT FROM AUTHOR]

The mineralogical development and diagenetic sequence of lacustrine shales in the Chang 7 Member of the Yanchang Formation in the Ordos Basin are detailed studied. A model of their depositional system and a diagenetic diagram are proposed in this study. Through detailed petrographic, mineralogical, and elemental analyses, four distinct shale types are identified: argillaceous shale, siliceous shale, calcareous shale, and carbonate, clay, and silt-bearing shale. The main diagenetic process in argillaceous shale is the transformation of illite to smectite, negatively impacting shale porosity. Siliceous shale undergoes carbonate cementation and quartz dissolution, contributing to increased porosity, particularly in mesopores. Calcareous shale experiences diagenesis characterised by carbonate formation and dissolution, with a prevalence of siderite. In carbonate, clay, and silt-bearing shale, the dissolution of K-feldspar contributes to illitization of kaolinite. Argillaceous shale, characterised by more clay minerals and lower mesopore volume, is identified as a potential hydrocarbon seal. Siliceous shale, with the highest pore volume and abundant inter-mineral pores, emerges as a promising shale oil reservoir. These findings contribute to a comprehensive understanding of shale properties, aiding in the prediction of shale oil exploration potential in the studied area. [ABSTRACT FROM AUTHOR]

With the global expansion of shale oil and gas exploration, analyzing the geochemical characteristics of different types of shale has seized extensive attention. Taking the Chang 7 Member and Shanxi Formation in the Ordos Basin as examples, this study investigated the geochemical and hydrocarbon expulsion characteristics of different types of shale. The results show that the Chang 7 shale is mainly type I. The total organic carbon (TOC) distribution of the Chang 7 shale in the study area was between 0.72% and 27.5%, with an average of 7.46%, and the vitrinite reflectance (Ro) ranged from 0.45% to 1.28%. It is a good source rock in immature to mature stages. The Shanxi shale is mainly type III. The TOC distribution of the Shanxi shale was between 0.6% and 58.5%, with an average of 8.92%, and the Ro distribution was between 0.42% and 2.39%, indicating good immaturity over mature source rock. The average hydrocarbon expulsion efficiencies of the Chang 7 shale calculated using the hydrocarbon generation potential method and the original hydrocarbon generation potential method were 82.49% and 94.81%, respectively. Simultaneously, a correlation analysis of hydrocarbon expulsion efficiency with organic matter abundance, maturity, and type found that when TOC was less than 5%, hydrocarbon expulsion efficiency increased with increased organic matter abundance and maturity. When the TOC exceeds 5%, the hydrocarbon expulsion efficiency is mainly related to maturity. The hydrocarbon expulsion efficiency of the sapropelic shale was much higher than that of the humic shale. This study is significant for evaluating shale gas and shale oil resources in similar areas. [ABSTRACT FROM AUTHOR]

A set of high-quality lacustrine shales at the bottom of the Chang 7 member of the Yanchang Formation in the Ordos Basin is one of the main source rocks of tight oil and gas and shale oil in the Yanchang Formation. Based on outcrop, core, drilling and seismic data, by the quantitative characterization of outcrops, fine characterization of logging facies and seismic facies, and geochemical tests, the lithofacies types, geophysical response characteristics and organic geochemical characteristics of this high-quality shale are clarified, and the formation paleoenvironment, including redox conditions, paleoclimate, paleosalinity and paleowater depth, is analyzed. The high-quality shale at the bottom of the Chang 7 member is divided into three lithofacies types: black shale, dark massive mudstone and silty mudstone. The organic matter in black shale is mainly interbedded or stratified, the organic matter in dark massive mudstone is dispersed and the organic matter content in silty mudstone is lower. The shale shows high gamma (more than 260 API), a high acoustic time difference (more than 280 μs/m), a high resistivity (more than 330 Ω m) well-logging phase and strong-amplitude parallel–subparallel seismic phase characteristics. Based on the logging and seismic facies characteristics, the plane distribution range of this set of shales is defined. The sedimentary thickness gradually increases from the edge (5–10 m) to the center of the basin, among which the Jiyuan–Huachi–Yijun black shale has the largest thickness (more than 30 m). This set of high-quality shales was mainly formed under a warm and humid paleoclimate, in water depths of 60–120 m, and in an anaerobic reducing and continental freshwater paleoenvironment. The fine identification, distribution range and formation conditions of black shale lithofacies are of practical significance for predicting the distribution of favorable lithofacies of shale oil and gas and the deployment of horizontal wells. [ABSTRACT FROM AUTHOR]

To quantify the pore characteristics of various macerals in Chang 7 lacustrine shales, macerals were effectively identified according to their optical and morphological characteristics, and the nanoscale pore structure of macerals was observed by scanning electron microscope. Meanwhile, the reflectances of different positions in the same pieces of vitrinite or solid bitumen with heterogeneous pores development were measured. The results showed that the average contents of sapropelinite, liptinite, vitrinite, inertinite and solid bitumen are 42.7%, 8.7%, 13.6%, 13.8% and 21.2%, respectively, which suggests that the source of the organic matter of the Chang 7 shales is a mixed source input. The organic pores of Chang 7 shales are enriched, and the pore shapes are mostly round or elliptical. The pore size of organic pores has a wide distribution, mainly concentrate in the range of 100e400 nm, and the average plane porosity of organic pores is 10.13%. The size order of the organic pores in various macerals is: solid bitumen < bituminite < alginite < vitrinite < fusinite < liptinite. The abundance order of organic matter pores of each maceral is as follows: alginite > fusinite > bituminite > solid bitumen > vitrinite > liptinite. OM pores are mainly contributed by bituminite, solid bitumen and fusinite. The plane porosity of bituminite increases with maturity. In the process of thermal evolution, the plane porosity of fusinite is distributed in the two ranges of 20%e28% and 1%e7%. The former is mainly the primary pores of the fusinite itself, and the latter is the secondary pores formed in the thermal evolution. As for the organic pores of other macerals, no obvious thermal evolution law was found. Meanwhile, the surface imperfections of vitrinite or solid bitumen is enhanced by the enrichment of organic pores (an increase in pore size or pore number), which may result in the underestimation of their reflectances. [ABSTRACT FROM AUTHOR]

Recent studies dispute the long-standing belief that highly organic-rich shales are predominantly linked to condensed section (CS) at the maximum flooding surface (MFS), proposing instead that they more commonly develop in shallow waters along basin margins. This hypothesis, however, has not been thoroughly investigated. Our study, focusing on the pivotal regions of the Longmaxi Formation and the Chang 7 Member for marine and continental shale exploration in China, integrates sequence stratigraphy with analyses of total organic carbon (TOC) and elemental geochemistry to pinpoint the environments conducive to the formation of organic-rich shales. The most organic-rich intervals, specifically the bottom of the Longmaxi Formation and the mid-to-lower Chang 7 3 sub-member, are located within the transgressive systems tract (TST), showing higher TOC levels in the middle to lower TST, compared to the CS of the MFS. The formation of high organic matter shale is related to warm, humid, and dysoxic shallow waters with minimal terrigenous influx and high primary productivity. We found a pronounced inverse relationship between organic matter concentration and water depth, with deeper waters promoting the decomposition and recycling of organic matter during sedimentation, thereby diminishing the TOC content. Moreover, volcanic activity also affected the organic enrichment in the Chang 7 Member. With high primary productivity, organic matter tends to accumulate in dysoxic slope zones. Consequently, high-organic content shale intervals are generally situated in shallow, dysoxic water environments, suggesting that the CS of the MFS may actually represent organic-poor intervals with limited source rock potential. The results of this study are of great significance for shale oil and gas exploration in China, and provide a new direction for the discovery of shale oil and gas sweet spots. [Display omitted] • Shales in the middle to lower TST exhibit high TOC content and relatively low MFS. • Shale samples with high organic matter were developed in shallow water suboxic environment under warm and humid climatic conditions. • Increasing water depth correlates with a decrease in clay mineral content, resulting in reduced TOC levels. • The CS of MFS typically exhibit low organic matter content and limited potential as effective source rocks. [ABSTRACT FROM AUTHOR]

Organic matter (OM) type critically controls the hydrocarbon generation potential and organic pore development in black shales. However, maceral variation in lacustrine shales and its control on hydrocarbon generation potential and organic pore development are not yet well understood. In this study, 15 Chang 7 Member shale samples of the Yanchang Formation, Ordos Basin, were investigated with organic petrography, Rock-Eval pyrolysis, and a scanning electron microscope to study the maceral composition, hydrocarbon generation potential, and organic pores in this black shale succession. The results show that the studied shales are in the oil window (Ro~0.70%). OM belongs to Type I and Type III kerogen, as demonstrated by Rock-Eval pyrolysis. Macerals in the Chang 7 Member shales are composed of amorphous OM, alginite, sporinite, liptodetrinite, vitrinite, inertinite, and solid bitumen. Amorphous OM and alginite are major hydrocarbon-generating macerals, and their content determines the hydrocarbon potential of shales. Secondary organic pores were not observed in the studied Chang 7 Member shales due to either a low thermal maturity or a dominance of terrigenous OM. Maceral variation can affect the reliability of using Rock-Eval Tmax as a thermal maturity indicator. This study provides important insights into maceral control on hydrocarbon generation and organic pore development in black shales, calling for a critical evaluation of OM in black shale successions with organic petrography. [ABSTRACT FROM AUTHOR]

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Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The lacustrine Chang 7 organic‐rich shale of the Upper Triassic Yanchang Formation is a very important hydrocarbon source rock and contains high total organic carbon content (average = 5.54 wt.%). Previous studies suggest that deposits of the Chang 7 accumulated slowly under dominantly oxic–suboxic bottom‐water conditions occasionally interrupted by the establishment of anoxic conditions. However, questions remain regarding the role of palaeoproductivity in accumulation of the Chang 7 organic matter. Palaeoproductivity indicators, including P/Al, Ni/Al, Cu/Al, and Zn/Al, suggest that the deposition of the Chang 7 period was accompanied by high primary productivity level. Furthermore, productivity appears to have been highest in the middle and south‐east region of the lake, diminishing to the north‐east, the north‐west, and the south‐west. Elevated productivity resulted in the flux of large amounts of organic matter to the lake floor. Intermittent anoxic bottom‐water conditions caused by oxygen consumption during organic matter degradation favoured enhanced preservation of organic matter even under oxic–suboxic water column. Low sedimentation rate made the organic matter in sediments avoid dilution of terrigenous clastics, which made it play a positive role in the accumulation of organic matter. [ABSTRACT FROM AUTHOR]

The organic-rich shale of the Chang 7 member is the most important source rock in the Ordos basin. The sedimentary environment and the controlling factors of organic matter enrichment, however, are still in contention. In this investigation, the Yishicun outcrop, located on the south margin of the Ordos basin, has been considered for the study. X-ray diffraction, polarizing microscopy, field emission scanning electron microscopy and cathodoluminescence (CL) were used to investigate the petrological features of the organic-rich shale. The content of volcanic ash and the diameter of pyrite framboid pseudocrystals were measured to illustrate the relationship between oxygen level, ash content and the enrichment of organic matter. It has been found that the diameter of pyrite framboid pseudocrystals has a strong correlation with the total organic carbon, demonstrating that the redox status degree of the water column has a positive impact on the enrichment of organic matter. Additionally, with an increase in the ash content, the content of organic matter increased at first and then decreased, and reached a maximum when the ash content was about 6%, illustrating that the ash input has a double effect on the enrichment of organic matter. [ABSTRACT FROM AUTHOR]