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Copyright of Petroleum Geology & Experiment is the property of Petroleum Geology & Experiment Editorial Office 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.)

As the second-largest sedimentary basin in China, the Ordos Basin has enormous potential for oil and gas exploration. The Chang 7 member of the Triassic Yanchang Formation in the basin is extensively distributed with organic-rich source rocks, covering an area of 40 000 to 50 000 km2. These source rocks are characterized by high organic content, with organic matter types mainly being type Ⅰ to Ⅱ1. The Ro values mostly range from 0.9% to 1.2%, indicating that they are in thermal maturity stage. The high content of retained hydrocarbons provides a strong material basis for the development of large-scale shale oil and gas reservoirs. Although the organic-rich mud shale layers in the Chang 73 sub-member have good gas-bearing properties, a systematic analysis of their gas-bearing characteristics is lacking. Using the southern part of the basin's western margin as a case study, multiple analytical methods such as rock geochemistry, organic geochemistry, and isotope analysis were used to identify the geological and geochemical characteristics of the source rock reservoirs in the Chang 7 member. The gas-bearing characteristics and shale gas occurrence states of the Chang 7 member were analyzed, and the shale gas resource potential was preliminarily calculated. The results indicated that the black shale of the Chang 73 sub-member in the study area exhibited good gas-bearing properties, with dissolved shale oil gas being the main component, along with minor amounts of adsorbed gas of kerogen clay minerals and free gas. The average volume of desorption gas was calculated to be 1.91 m3/t. The Chang 73 sub-member contained both shale oil and gas resources, with approximately equivalent quantities. It is recommended to consider both oil and gas development in future exploration and development research. In the central part of the lake basin, where thick black shale of the Chang 7 member occurred, gas content reached 2 m3/t. These reservoirs contained rigid minerals, micropores, and fractures, with high gas abundance and substantial resource potential, making this a favorable area for shale gas exploration. The total shale gas resource of the Chang 7 member was preliminarily estimated to be about 4.25×1012 m3, indicating promising exploration prospects. The favorable exploration areas were identified in the Jiyuan, Gucheng and Zhengning area.

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]

In recent years, the Chang 7 member of the Mesozoic Triassic Yanchang Formation in the Ordos Basin has emerged as a significant repository of abundant and distinctive unconventional oil resources. The Heshui area boasts substantial shale oil reserves, with reported third-level reserves surpassing 600 million tons. However, the region in the southern part of Heshui is marked by pronounced variability in reservoir quality, intricate oil–water dynamics, low formation energy, and suboptimal fluid properties, leading to divergent development outcomes for horizontal wells. There is an imperative need to devise and refine new geological models to underpin the efficient exploitation of shale oil in the southern Heshui area. This study focuses on the shale oil reservoir of the Chang 7 member in the southern Heshui area of the Ordos Basin, conducting detailed stratigraphic correlation and establishing a refined isochronous stratigraphic framework. Utilizing PetrelTM modeling software (version 2018), we integrate deterministic and stochastic modeling approaches, adhering to the principles of isochronous and phased modeling. By assessing the thickness of sand and mudstone layers and the overall stratigraphic sequence, we derive a geological probability surface. Subsequently, this surface is harnessed to constrain the lithofacies, yielding a constrained lithofacies model. Employing sequential indicator simulation and sequential Gaussian stochastic simulation, we develop a reservoir attribute model that is anchored in the lithofacies model and its controls, culminating in a robust and dependable static model. Employing the geological probability surface constraint method, we meticulously construct the reservoir matrix model, amalgamating individual well data with the inherent certainty and randomness of reservoir plane thickness. This approach further enhances the model's accuracy and mitigates the uncertainty and randomness associated with inter-well interpolation to a significant degree. [ABSTRACT FROM AUTHOR]

Based on a large number of newly added deep well data in recent years, the subsidence of the Ordos Basin in the Mid-Late Triassic is systematically studied, and it is proposed that the Ordos Basin experienced two important subsidence events during this depositional period. Through contrastive analysis of the two stages of tectonic subsidence, including stratigraphic characteristics, lithology combination, location of catchment area and sedimentary evolution, it is proposed that both of them are responses to the Indosinian Qinling tectonic activity on the edge of the craton basin. The early subsidence occurred in the Chang 10 Member was featured by high amplitude, large debris supply and fast deposition rate, with coarse debris filling and rapid subsidence accompanied by rapid accumulation, resulting in strata thickness increasing from northeast to southwest in wedge-shape. The subsidence center was located in Huanxian–Zhenyuan–Qingyang–Zhengning areas of southwestern basin with the strata thickness of 800–1 300 m. The subsidence center deviating from the depocenter developed multiple catchment areas, until then, unified lake basin has not been formed yet. Under the combined action of subsidence and Carnian heavy rainfall event during the deposition period of Chang 7 Member, a large deep-water depression was formed with slow deposition rate, and the subsidence center coincided with the depocenter basically in the Mahuangshan–Huachi–Huangling areas. The deep-water sediments were 120–320 m thick in the subsidence center, characterized by fine grain. There are differences in the mechanism between the two stages of subsidence. The early one was the response to the northward subduction of the MianLüe Ocean and intense depression under compression in Qinling during Mid-Triassic. The later subsidence is controlled by the weak extensional tectonic environment of the post-collision stage during Late Triassic.

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]

The Carnian Pluvial Episode (CPE) fingerprints global environmental perturbations and biological extinction on land and oceans and is potentially linked to the Wrangellia Large Igneous Province (LIP). However, the correlation between terrestrial environmental changes and Wrangellia volcanism in the Ordos Basin during the CPE remains poorly understood. Records of negative carbon isotopic excursions (NCIEs), mercury (Hg), Hg/TOC, and Hg enrichment factor (HgEF) from oil shales in a large‐scale terrestrial Ordos Basin in the Eastern Tethys were correlated with marine and other terrestrial successions. The three significant NCIEs in the study section were consistently correlated with those in the CPE successions of Europe, the UK, and South and North China. The U‐Pb geochronology indicates a Ladinian–Carnian age for the Chang 7 Member. A comprehensive overview of the geochronology, NCIE correlation, and previous bio‐ and chronostratigraphic frameworks shows that the Ladinian–Carnian boundary is located in the lower part of Chang 7 in the Yishicun section. HgEF may be a more reliable proxy for tracing volcanic eruptions than the Hg/TOC ratio because the accumulation rates of TOC content largely vary in terrestrial and marine successions. The records of Hg, Hg/TOC, HgEF, and NCIEs in the Ordos Basin aligned with Carnian successions worldwide and were marked by similar anomalies, indicating a global response to the Wrangellia LIP during the CPE. Anoxia, a warm‐humid climate, enhancement of detrital input, and NCIEs are synchronous with the CPE interval in the Ordos Basin, which suggests that the CPE combined with the regional Qinling Orogeny should dominate the enhanced rate of terrigenous input and paleoenvironmental evolution in the Ordos Basin. [ABSTRACT FROM AUTHOR]

Based on a large number of newly added deep well data in recent years, the subsidence of the Ordos Basin in the Mid-Late Triassic is systematically studied, and it is proposed that the Ordos Basin experienced two important subsidence events during this depositional period. Through contrastive analysis of the two stages of tectonic subsidence, including stratigraphic characteristics, lithology combination, location of catchment area and sedimentary evolution, it is proposed that both of them are responses to the Indosinian Qinling tectonic activity on the edge of the craton basin. The early subsidence occurred in the Chang 10 Member was featured by high amplitude, large debris supply and fast deposition rate, with coarse debris filling and rapid subsidence accompanied by rapid accumulation, resulting in strata thickness increasing from northeast to southwest in wedge-shape. The subsidence center was located in Huanxian-Zhenyuan-Qingyang-Zhengning areas of southwestern basin with the strata thickness of 800-1 300 m. The subsidence center deviating from the depocenter developed multiple catchment areas, until then, unified lake basin has not been formed yet. Under the combined action of subsidence and Carnian heavy rainfall event during the deposition period of Chang 7 Member, a large deep-water depression was formed with slow deposition rate, and the subsidence center coincided with the depocenter basically in the Mahuangshan-Huachi-Huangling areas. The deep-water sediments were 120-320 m thick in the subsidence center, characterized by fine grain. There are differences in the mechanism between the two stages of subsidence. The early one was the response to the northward subduction of the MianLüe Ocean and intense depression under compression in Qinling during Mid-Triassic. The later subsidence is controlled by the weak extensional tectonic environment of the post-collision stage during Late Triassic. [ABSTRACT FROM AUTHOR]

The study aims to investigate the rock mechanical properties of the Chang 7 member tight oil reservoir in the Longdong region of the Ordos Basin, China, with the goal of enhancing the efficiency of oilfield development. Despite numerous contributions in the field of rock mechanics, challenges persist in reconciling experimental results with actual geological conditions and achieving comprehensive understanding of rock mechanical properties in tight oil reservoirs. To address this, a variety of experimental methods were employed to systematically assess the mechanical properties of the target reservoir. Rock density was measured using volumetric methods, tensile strength was evaluated through the Brazilian test, uniaxial and triaxial compression tests were conducted to assess rock mechanics properties, and dynamic elastic modulus and Poisson's ratio were obtained via sonic velocity measurements. Furthermore, differential strain analysis and imaging log analysis were employed to determine the magnitude and direction of geostress. The results revealed that fine-grained sandstone exhibited higher rock density and relatively higher tensile strength, while muddy siltstone exhibited comparatively lower values in these aspects. Analysis of the influence of confining pressure on rock mechanics properties demonstrated a strong positive correlation between compressive strength and Young's modulus with confining pressure, while Poisson's ratio exhibited more irregular variations. Additionally, a mathematical relationship between dynamic and static rock mechanical parameters was established. Lastly, based on the characteristics of geostress, reliable foundations for optimizing hydraulic fracturing and wellbore layout were provided. This study has enriched and complemented the investigation of rock mechanical properties in tight reservoirs, offering vital parameters and theoretical support for the development of tight oil reservoirs. This bears significant importance in enhancing petroleum extraction efficiency and optimizing engineering design. [ABSTRACT FROM AUTHOR]

The identification of the origins of source rock palaeo-overpressure based on logging response parameters is frequently influenced by the presence of organic matter. To accurately identify the paleo-overpressure origin of source rocks and employ an appropriate method for paleo-pressure calculation, this study proposes a quantitative approach to assess the influence of organic matter content on overpressure analysis. The findings demonstrate that the presence of organic matter in Chang 7 shale leads to an increase in acoustic time difference and a decrease in density. After incorporating the multicomponent rock equivalent volume model correction, it is evident that the paleo-overpressure observed in the Chang 7 member is not attributed to undercompaction but rather to fluid expansion mechanisms. In conjunction with the acoustic velocity-density palte method, the paleo-overpressure of the Chang 7 member is attributed to undercompaction and expansion resulting from hydrocarbon generation. The pressure evolution of the Chang 7 member has been reconstructed based on the identification of its paleo-overpressure origin. The distribution characteristics of paleo-pressure recovered based on hydrocarbon generation expansion exhibit a strong correlation with the reservoir. The reliability of the correction method for organic matter influence is demonstrated. The proposed method is not only applicable for the investigation of paleo-pressure, but also holds significant implications for the assessment of pressure within organic-rich intervals. [ABSTRACT FROM AUTHOR]

The Chang 7 member of the Triassic Yanchang Formation in the Ordos Basin is a significant continent shale oil reservoir in China. Therefore, conducting an in-depth investigation into the pore structure and fluid mobility characteristics of the Chang 7 shale oil reservoir holds immense importance for advancing shale oil exploration. This study conducts a detailed analysis of the pore structures and their impact on fluid mobility of the Chang 7 shale oil reservoir using multiple methodologies, i.e., a cast thin section, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTNA), and nuclear magnetic resonance (NMR). The results show that the sandstone in the Yanwumao area of the Chang 7 shale oil reservoir consists mainly of lithic arkose and feldspathic litharenite, which can be classified into three lithofacies (massive fine-grained sandstone (Sfm), silt-fine sandstone with graded bedding (Sfgb), and silt-fine sandstone with parallel bedding (Sfp)). Moreover, three pore structures (Type I, II, and III), and four pore spaces (nanopores, micropores, mesopores, and macropores) can be characterized. Pore structure Type I, characterized by large pores, exhibits bimodal pore diameter curves, resulting in the highest levels of movable fluid saturation (MFS) and movable fluid porosity (MFP). Pore structure Type II demonstrates unimodal pore structures, indicating robust connectivity, and higher MFS and MFP. Pore structure Type III primarily consists of dissolved and intercrystalline pores with smaller pore radii, a weaker pore configuration relationship, and the least fluid mobility. Furthermore, a correlation analysis suggests that the pore structure significantly impacts the fluid flowability in the reservoir. Favorable petrophysical properties and large pores enhance fluid flowability. Micropores and mesopores with high fractal dimensions have a greater impact on reservoir fluid mobility compared to macropores and nanopores. Mesopores mainly control MFS and MFP, while micropores govern the shift from bound fluid to movable fluid states. Among the lithofacies types, the Sfm lithofacies exhibit the highest fluid mobility due to their significant proportion of macropores and mesopores, whereas the Sfgb lithofacies have lower values because they contain an abundance of micropores. The Sfp lithofacies also dominate macropores and mesopores, resulting in medium fluid mobility levels. This study combines lithofacies types, micro-reservoir pore structure characteristics, and mobile fluid occurrence characteristics to better understand the dominant reservoir distribution characteristics of the Chang 7 shale oil reservoirs in the Ordos Basin and provide theoretical information for further optimization of production strategies. [ABSTRACT FROM AUTHOR]

The study aims to investigate the rock mechanical properties of the Chang 7 member tight oil reservoir in the Longdong region of the Ordos Basin, China, with the goal of enhancing the efficiency of oilfield development. Despite numerous contributions in the field of rock mechanics, challenges persist in reconciling experimental results with actual geological conditions and achieving comprehensive understanding of rock mechanical properties in tight oil reservoirs. To address this, a variety of experimental methods were employed to systematically assess the mechanical properties of the target reservoir. Rock density was measured using volumetric methods, tensile strength was evaluated through the Brazilian test, uniaxial and triaxial compression tests were conducted to assess rock mechanics properties, and dynamic elastic modulus and Poisson’s ratio were obtained via sonic velocity measurements. Furthermore, differential strain analysis and imaging log analysis were employed to determine the magnitude and direction of geostress. The results revealed that fine-grained sandstone exhibited higher rock density and relatively higher tensile strength, while muddy siltstone exhibited comparatively lower values in these aspects. Analysis of the influence of confining pressure on rock mechanics properties demonstrated a strong positive correlation between compressive strength and Young’s modulus with confining pressure, while Poisson’s ratio exhibited more irregular variations. Additionally, a mathematical relationship between dynamic and static rock mechanical parameters was established. Lastly, based on the characteristics of geostress, reliable foundations for optimizing hydraulic fracturing and wellbore layout were provided. This study has enriched and complemented the investigation of rock mechanical properties in tight reservoirs, offering vital parameters and theoretical support for the development of tight oil reservoirs. This bears significant importance in enhancing petroleum extraction efficiency and optimizing engineering design.

Copyright of Acta Sedimentologica Sinica is the property of Acta Sedimentologica Sinica Editorial Office 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.)

Organic geochemistry experiments, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), N2 adsorption, CO2 adsorption, and two-dimensional nuclear magnetic resonance (2D NMR) were performed on ten Chang 7 shale samples (Longdong area, Ordos Basin) to elucidate their pore structure and the characteristics of oil occurrence. Moreover, the factors influencing free oil were discussed, and an occurrence model was established. FE-SEM analysis reveals that the pore types include interparticle pores, intraplatelet pores within clay aggregates, rare fracture pores, and organic matter (OM) pores. The pores are predominantly slit-shaped. The development of micropores was mainly contributed to by OM. Quartz and clay minerals influence the development of macropores and mesopores, feldspar mainly controls macropore development, and pyrite most strongly affects micropore development. Micropores and mesopores constitute the main total pore specific surface area, while mesopores and macropores are the main contributors to the total pore volume. Pores > 2 nm are the main storage spaces for shale oil, and free oil mainly occurs in pores > 20 nm. Adsorbed oil and free oil were assessed by NMR T1–T2 mapping. The adsorbed oil signal intensities range from 7.5–23.4 a.u. per g of rock, and the free oil signal intensities range from 4.4–23.2 a.u. per g of rock. The free oil proportions are 15.9–70.6% (average of 44.2%). The free oil proportion is negatively correlated with the clay mineral content and total organic carbon (TOC) content but positively correlated with the saturated hydrocarbon content and volume of pores > 20 nm. The results of this study could help optimize favorable shale oil target areas. [ABSTRACT FROM AUTHOR]

Pore structure, as the focus of shale reservoir research, has important influence on the occurrence of shale oil. In this paper, 10 shale core samples from the 7th member of Triassic Yanchang Formation (Chang 7 member) in Huachi area, Ordos Basin were selected to carry out scanning electron microscope (SEM) observation and low-temperature nitrogen adsorption experiment, and the fractal dimension was calculated to quantitatively characterize the pore structure of shale in the research area combined with fractal FHH model. On this basis, the relationship between fractal dimension and pore structure parameters and oil-bearing parameters was discussed, and the main influencing factors of pore development of shale in the Chang 7 member were determined. The results show that the shale of the Chang 7 member in Huachi area has high organic matter abundance, which are good-to-excellent source rocks mainly composed of quartz and clay minerals, with good oil-bearing property (up to the middle oil-bearing level) and mobility. The reservoir space is dominated by intergranular pores, intragranular pores and a small amount of organic pores. There are two types of pore morphology, namely, parallel plate-shaped slit+unilateral slit and ink bottle-shaped + parallel plate-shaped slit. The pores are mainly micropores and mesopores, with macropores less developed. Most samples have fractal characteristics. The fractal dimension D1 of small pores is between 2.264 7 and 2.714 9, and the fractal dimension D2 of large pores is between 2.373 3 and 2.77 7. Among them, D2 has a good correlation with surface area, pore volume, average pore diameter and S1, which can characterize the development characteristics of pore structure and oil-bearing property of shale, while D1 can only characterize the mobility of shale oil. The pore development of shale is mainly controlled by TOC and quartz content, and has a certain relationship with feldspar content, while clay mineral content is not the main influencing factor.

In recent years, the Chang 7 member of the Mesozoic Triassic Yanchang Formation in the Ordos Basin has emerged as a significant repository of abundant and distinctive unconventional oil resources. The Heshui area boasts substantial shale oil reserves, with reported third-level reserves surpassing 600 million tons. However, the region in the southern part of Heshui is marked by pronounced variability in reservoir quality, intricate oil–water dynamics, low formation energy, and suboptimal fluid properties, leading to divergent development outcomes for horizontal wells. There is an imperative need to devise and refine new geological models to underpin the efficient exploitation of shale oil in the southern Heshui area. This study focuses on the shale oil reservoir of the Chang 7 member in the southern Heshui area of the Ordos Basin, conducting detailed stratigraphic correlation and establishing a refined isochronous stratigraphic framework. Utilizing PetrelTM modeling software (version 2018), we integrate deterministic and stochastic modeling approaches, adhering to the principles of isochronous and phased modeling. By assessing the thickness of sand and mudstone layers and the overall stratigraphic sequence, we derive a geological probability surface. Subsequently, this surface is harnessed to constrain the lithofacies, yielding a constrained lithofacies model. Employing sequential indicator simulation and sequential Gaussian stochastic simulation, we develop a reservoir attribute model that is anchored in the lithofacies model and its controls, culminating in a robust and dependable static model. Employing the geological probability surface constraint method, we meticulously construct the reservoir matrix model, amalgamating individual well data with the inherent certainty and randomness of reservoir plane thickness. This approach further enhances the model’s accuracy and mitigates the uncertainty and randomness associated with inter-well interpolation to a significant degree.

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]

To explore the genesis of the laminated calcite veins developed in the black shale of Chang 73 submember of the Ordos Basin, the petrology, microstructure and geochemistry of calcite veins are studied using thin sections, fluid inclusions, trace elements and isotopic geochemistry. The source, the time of formation, the mechanism of formation, and the dynamic background of the veins are discussed. The veins are mostly made up of calcite, mixed with lenticular or spindle‐shape solid wall rock inclusions. Three structures are identified in the calcite minerals: fibrous, rhombic cleavage, and wedge‐like structure. Trace elements and isotopes of carbon and oxygen confirm that the calcite veins were formed from a high density hydrothermal fluid. It is assumed that calcite veins formed prior to wall rock consolidation during the formation of the Qinling Orogenic Belt in the Middle and Late Triassic. The results show that the sedimentary sources of Chang 73 submember were influenced by hydrothermal materials besides terrigenous detritus. The fibrous calcite is different from the fibrous calcite reported in the morphology and mechanism of formation. In this way, the research complements and improves the morphology and the mechanism of formation of fibrous calcite. [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]

Shale oil in the Chang 7 member is the main target for further exploration in the Ordos Basin. However, the lack of research on the characteristics of the Chang 7 member's mudstone and shale reservoir has seriously affected the next stage of exploration for shale oil in the Chang 7 member. This study analyzed in detail the changes in the pore structure of different types of reservoirs and their influencing factors in detail through experiments such as gas adsorption, nuclear magnetic resonance, and high-pressure mercury intrusion. The results showed that black shale is mainly composed of micropores, with a micropore ratio of up to 79.88%. Dark mudstone and silty mudstone are mainly composed of mesopores, with a mesopore ratio greater than 30%. Argillaceous sandstone is mainly composed of large pores, accounting for 78.57%. From black shale to muddy sandstone, the proportion of micropores decreases, while the proportion of macropores increases. The micropore volume is mainly controlled by the organic matter content and the inorganic mineral content related to the organic matter content. Inorganic minerals such as quartz and feldspar mainly control the degree of development of mesopores and macropores. For rocks with a strong hydrocarbon-generating ability (black shale and dark mudstone), the ratio between the total pore volume and micropore volume is the main factor restricting their oil content. For rocks with a weak hydrocarbon-generating capacity (silty mudstone and argillaceous sandstone), the macropore volume and total pore volume are the main controlling factors for oil content. The results of this study have important guiding significance for the next step of exploration and development for shale oil in the Chang 7 member and, to a certain degree, for the exploration of continental shale oil in other basins. [ABSTRACT FROM AUTHOR]

Ordos Basin is rich in shale oil resources. Fuxian area in the southern Ordos Basin is an important area where Sinopec is expected to make a breakthrough in the exploration and development of lacustrine shale oil. To further identify the exploration target of shale oil in Chang 7 Member of Fuxian area, Ordos Basin, this study systematically analyzed the sedimentary characteristics, source rock characteristics, reservoir characteristics, oil-bearing characteristics, fracability and preservation conditions of Chang 7 Member in this area. The results show that: (1) the organic matter abundance of shale in the Chang 7 Member is high, and its type is dominated by type I-II1; the maturity is also relatively high, with Ro generally greater than 0.8% in the central and western parts, which is favorable for the generation and accumulation of shale oil. The development of fractures is pretty low, and thus the structure preservation conditions are relatively favorable. Delta front deposits are mainly developed in the northeastern part, while deep lake - semi-deep lake mudstone and gravity-flow deposits are developed in the western part, and the shale oil types are mainly type II and III. (2) Comparing with the western Ordos Basin, the reservoir in Fuxian area has rapid lateral variation and relatively thin cumulative thickness; the thickness of high-quality shale and maturity are low, resulting in low intensity of hydrocarbon generation and low mobility of oil-bearing saturation. Therefore, there is still a need for further improvement and refinement in the exploration evaluation, effective development and engineering techniques. (3) Integrating sedimentary characteristics with shale oil types, source conditions, oil-bearing properties, mobility, fracability and preservation conditions, it is clarified that the Type II shale oil of Chang 72 sub-,Member in Fuxian area is the current realistic exploration type of shale oil, and its favorable target is located in the development area of delta front in the central part and gravity flow in the western part.

A large number of sandstone interlayers were developed in the shale strata of the 7th member of the Triassic Yanchang Formation (Chang 7 member) in the Ordos Basin. These sandstone interlayers are frequently accompanied by abnormally high natural gamma values as high as 330.5 API. As a result, they are undistingui shable from shale on well profile. To characterize the high-gamma sandstones, the clastic rocks of the Chang 7 member were classified into fine sandstone, siltstone, argillaceous siltstone, silty mudstone, mudstone and shale according to grain size and structural characteristics. Conventional logging data were recalculated and combined into two new parameters—A and B. The new intersection chart composed of parameters A and B and natural gamma logging data can effectively distinguish and identify normal sandstone and high-gamma sandstone in the Chang 7 member. The plane distribution of cumulative thickness of high-gamma sandstone in the Chang 7 member shows that the high-gamma sandstone is mainly located in the southwest of the study area. The finger-like distribution of high-gamma sandstone from the edge to the center of the lake basin indicates that its formation may be related to the volcanic eruption during sedimentary period.

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The tight oil reservoir of the Chang 7 member, Yanchang Formation in the Yan'an area of the Ordos Basin has the characteristics of oil and gas "co-existence" and strong heterogeneity. Unclear understanding of special geological characteristics and reservoir characteristics hinders the exploration and development of continental shale oil and gas. In this paper, a large number of core observations, thin sections, scanning electron microscopy, conventional mercury injection, and nuclear magnetic resonance experiments were used to characterize the reservoir properties of the Chang 7 oil shale. The experimental results show that the main mineral types of shale in the study area are clay minerals, quartz and feldspar, in addition to a small amount of carbonate and pyrite. The average size of rigid particles in argillaceous shale is 5.4 μm, and there are no sand-grade particles. Mud particles (≥8) can account for more than 50% of the medium particles. The TOC difference of shale, laminar shale and laminar shale is obvious, indicating that the TOC of shale has strong heterogeneity. The gas-bearing porosity occupied by shale gas in silty laminae is larger than that of shale. Moreover, the pore development characteristics are mainly intergranular pores and intra granular pores. Before oil washing, the porosity ranges from 0.27% to 8.06%, with an average of 2.35%. The Chang 7 tight oil reservoir contains oil in the pores from macropores to nano-pores, and the pores from micropores to nano-pores are full of oil. The micro-cosmic understanding of the reservoir characteristics of the Chang 7 tight oil reservoir is of great significance to the exploration and development of unconventional oil and gas resources in the Ordos Basin. [ABSTRACT FROM AUTHOR]

The Chang 7 member of the Triassic Yanchang Formation in the Ordos Basin is a significant continent shale oil reservoir in China. Therefore, conducting an in-depth investigation into the pore structure and fluid mobility characteristics of the Chang 7 shale oil reservoir holds immense importance for advancing shale oil exploration. This study conducts a detailed analysis of the pore structures and their impact on fluid mobility of the Chang 7 shale oil reservoir using multiple methodologies, i.e., a cast thin section, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTNA), and nuclear magnetic resonance (NMR). The results show that the sandstone in the Yanwumao area of the Chang 7 shale oil reservoir consists mainly of lithic arkose and feldspathic litharenite, which can be classified into three lithofacies (massive fine-grained sandstone (Sfm), silt-fine sandstone with graded bedding (Sfgb), and silt-fine sandstone with parallel bedding (Sfp)). Moreover, three pore structures (Type I, II, and III), and four pore spaces (nanopores, micropores, mesopores, and macropores) can be characterized. Pore structure Type I, characterized by large pores, exhibits bimodal pore diameter curves, resulting in the highest levels of movable fluid saturation (MFS) and movable fluid porosity (MFP). Pore structure Type II demonstrates unimodal pore structures, indicating robust connectivity, and higher MFS and MFP. Pore structure Type III primarily consists of dissolved and intercrystalline pores with smaller pore radii, a weaker pore configuration relationship, and the least fluid mobility. Furthermore, a correlation analysis suggests that the pore structure significantly impacts the fluid flowability in the reservoir. Favorable petrophysical properties and large pores enhance fluid flowability. Micropores and mesopores with high fractal dimensions have a greater impact on reservoir fluid mobility compared to macropores and nanopores. Mesopores mainly control MFS and MFP, while micropores govern the shift from bound fluid to movable fluid states. Among the lithofacies types, the Sfm lithofacies exhibit the highest fluid mobility due to their significant proportion of macropores and mesopores, whereas the Sfgb lithofacies have lower values because they contain an abundance of micropores. The Sfp lithofacies also dominate macropores and mesopores, resulting in medium fluid mobility levels. This study combines lithofacies types, micro-reservoir pore structure characteristics, and mobile fluid occurrence characteristics to better understand the dominant reservoir distribution characteristics of the Chang 7 shale oil reservoirs in the Ordos Basin and provide theoretical information for further optimization of production strategies.

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]

In-situ conversion technology has been more popular lately as an effective way to realize the industrial development of extensive medium- and low-mature shale oil/oil shale resources in China. The Ordos Basin has been recognized as the main basin containing the largest in-situ conversion recoverable resources of China. Researchers have revealed the hydrocarbon generation kinetics of low mature oil shales in the 7th Member of the Yanchang Formation (Chang 7 Member). However, the kinetics of medium mature shales may be different from those of low mature shales, and they can also be used as potential targets for in-situ conversion. At present, there is a lack of research on the in-situ hydrocarbon generation kinetics of medium- and low-mature shale oil/oil shale. In this paper, open system pyrolysis experiments were carried out on natural shales and shale samples derived from semi-open system pyrolysis with different maturities in the Chang 7 Member of the Ordos Basin respectively. Using the parallel first-order reaction theory, the frequency factor of low-mature shale was calculated to be 5.47 × 1010 s−1, and the distribution of activation energy ranged from 38 kcal/mol to 61 kcal/mol. The main peak of activation energy was 49 kcal/mol, and accounted for 66.91% of all shales. With the increase in maturity, the average activation energy becomes higher, and therefore more in-situ conversion energy is needed. At the same time, the hydrocarbon generation potential characterized by S2 peak of pyrolysis decreases during the maturation process. The activation energy is divided into three groups according to its distribution characteristics: low, main peak and high activation energy groups, representing 52 kcal/mol respectively. The proportion of low and high activation energy groups increased with the increase in maturity, while the proportion of main peak activation energy groups decreased. When the kinetic parameters are extrapolated to the condition of in-situ conversion, it is better to choose shales with low maturity (RO

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]

The Chang 7 member of the Upper Triassic Yanchang Formation is an important source rock in the Ordos Basin, NW China. Previous studies of the unit have in general focused on the relatively deep‐water (∼25 m) anoxic, OM‐rich lacustrine mudstones which form the main Mesozoic source rock at fields in the south of the basin. However, this paper presents an integrated geochemical investigation of the relatively shallow‐water Chang 7 facies in a study area around Jiyuan field in the western part of the basin in order to evaluate its source rock potential. The results show that the Chang 7 source rock in the study area has a high content of Type II OM, and is interpreted to have been deposited in a suboxic‐anoxic lacustrine setting with a mixed input of aquatic and terrigenous organic matter. The results of 1D modelling of a well in the study area showed that the Chang 7 member entered the oil generation window from the Middle Jurassic. Oil‐oil correlations based on hierarchical cluster analysis and correspondence analysis showed that crude oils generated by deep‐water Chang 7 source rocks from wells in the south of the basin can be distinguished from Soxhlet‐extracted petroleum from reservoir sandstones in the study area. The compositional differences are inferred to be due to variations in source rock facies compositions. An oil – source rock correlation study showed that the shallower‐water Chang 7 source rock in the western part of the basin generated the hydrocarbons in core extracts of reservoir sandstones from wells in this area. The shallow‐water Chang 7 facies may therefore constitute an effective oil‐prone source rock in the western Ordos Basin. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs are widely developed in the Mesozoic Yanchang Formation of the Ordos Basin, China. There is a lack of understanding on the sedimentary setting, source-reservoir relationship and oil accumulation conditions in this area. In this study, through the comprehensive analysis of the distribution of tight oil, we evaluated the properties and petrological features of reservoir, geochemical characteristics of source rocks, the source-reservoir relationship, as well as the trapping, preservation and accumulation conditions of tight oil in the Chang 7 Member, and predicted the sweet spots of tight oil in the study area. The results show that the Chang 7 Member is a typical low-porosity and ultra-low permeability reservoir with great tightness, small pore throat and high capillary pressure, and must have been of near-source accumulation. The source rocks are mainly developed in the Chang 73 submember, and the reservoirs mainly occur in the Chang 71 and Chang 72 submembers, forming a combination mode of "lower source rock and upper reservoir". Sandbodies with good connectivity and fractures being well developed in local areas are the main hydrocarbon transport systems. The abnormal high pressure caused by hydrocarbon generation and pressurization is the main driving force of tight oil accumulation. The mode of hydrocarbon transportation is dominated by the vertical or lateral migration from underlying source rocks or adjacent source rocks to reservoirs within a short distance. Following the integrated evaluation of lithology, physical properties and oil saturation of reservoirs and geochemical characteristics of source rocks, we grouped the sweet spots of Chang 7 Member into three types: Type I, Type II and Type III. Among others, the Type I sweet spots are the best in terms of porosity, permeability and source rock thickness and hydrocarbon enrichment which should be the focus of oilfield development. This study lays an important foundation for the economic and efficient development of tight oil in the Chang 7 Member of Heshui area, and has important implications on tight sandstone reservoirs in other regions of Ordos Basin in China. [ABSTRACT FROM AUTHOR]

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During the Late Triassic, the southern margin of the Ordos Basin was influenced by the Indosinian Movement, in response to closure of the Paleo-Tethys Ocean. Thinly bedded tuff layers are exposed near the base of the Middle–Upper Triassic formations (Chang 8/7/6 members) in the Ordos Basin. Tuff samples from drilling core (well An-35) and outcrop (Motiangou section) samples in the Yanchang Formation were studied by the detailed field work. Igneous zircon crystals were picked and analyzed by the instruments of cathodoluminescence and laser ablation inductively coupled plasma mass spectrometery (LA-ICP-MS). The timing of volcanic activity was determined by means of high-precision zircon age. Combining tectonic setting and paleogeographic reconstruct, the ages of tuff layers indicate that volcanic activities occurred at three peaks (241.6 ± 1.1 Ma, 240.2 ± 1.1 Ma, and 233.5 ± 1.2 Ma) from the top of Chang 8 Member to the Chang 7 Member. The results implied that the volcanic activities lasted from Ladinian to Carnian, as well as define the sedimentary duration of the Chang 7 Member with a deposition rate of 1.34 cm/ka.

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High organic matter abundance is necessary for large-scale hydrocarbon accumulation and enrichment in shale oil. Comparing conventional marine basins with lacustrine ones, the formation of organic matter in the latter usually changes rapidly with strong heterogeneity, which brings great challenges to identify sweet spot prediction and resource evaluation. In addition, water columns with different redox conditions also affect the burial and preservation of organic matter during the deposition of lacustrine shales, especially under the impact of ephemeral critical geological events such as volcanic activities. Therefore, determining the properties and the influencing factors on the water columns is one of the key scientific issues in revealing the differential enrichment of organic matter in such basins. By comparing the petrological and geochemical characteristics of organic-rich shales between a typical freshwater and a saline lacustrine basin, this study analyzes the depositional environment and water column properties during the burial and preservation of organic matter in the Ordos and Junggar basins. The results demonstrate that volcanic activity intensifies the degree of hypoxia during the formation of organic-rich matter, which in turn affects organic matter preservation. The sulfate reduction index (SRI) indicates that the organic-rich shale of the Chang seven Member (Ch7) of Yanchang Formation in the Ordos Basin and the Lucaogou Formation (P2l) in the Jimsar Sag of Junggar Basin in freshwater-saline environments have a certain intensity of sulfate reduction (BSR) (SRI 1.375) of the Fengcheng Formation (P2f) in saline lake of the Mahu Sag in Junggar Basin. This indicates that a large amount of organic matter was degraded and consumed, causing more iron oxides (and other iron-containing minerals) dissolve gradually to form pyrite, reducing the TOC content. This study enabled us to understand the shale oil enrichment, sweet spot prediction and evaluation, and propose a better theoretical and practical model for hydrocarbon accumulation in shale formations, to guide future exploration efforts.

The microscopic pore-throat structure characteristics of tight oil reservoirs are the focus and difficulty of tight oil reservoir research, as well as the key to restrict the development of tight oil. After years of development, there are currently many problems such as large differences in horizontal well production and severe production stabilization. Therefore, it is necessary to do in-depth research of reservoir characteristics and comprehensive evaluation of the reservoir. The result of comprehensive reservoir evaluation will directly affect the effect of reservoir development. Taking the Chang 7 Member tight oil reservoir in the northern Shaanxi area of the Ordos Basin as an example, this research was guided by the theory of reservoir geology, sedimentology and logging geology, and basic geological data, logging data, production data and fluids data were used. According to the data, the microscopic pore throat structure of the reservoir is analyzed through observation under microscope and mercury intrusion experiment. According to the observation of casting thin section and scanning electron microscope, the tight sandstone reservoir of Chang 7 Member in the research area mainly developed primary remaining intergranular pores, intragranular dissolution pores, intergranular dissolution pores, inter crystalline micropores and microcracks, among them, the intergranular dissolution pores and intragranular dissolution pores are the main ones. The high-pressure mercury intrusion analysis identified four types of Chang 7 Member tight sandstone reservoir pore structures, of which type I and type II pore throat radii were larger, and the maximum connected pore throat radius was 0.593µm-0.756µm, which showed good connectivity. [ABSTRACT FROM AUTHOR]

A set of shale-dominated source rocks series were deposited during the heyday of lake basin development in the Member 7 of Triassic Yanchang Formation, Ordos Basin, and the thickness is about 110 m. Aimed at whether this layer can form large-scale oil enrichment of industrial value, comprehensive geological research and exploration practice have been carried out for years and obtained the following important geologic findings. Firstly, widely distributed black shale and dark mudstone with an average organic matter abundance of 13.81% and 3.74%, respectively, lay solid material foundation for the formation of shale oil. Secondly, sandy rocks sandwiched in thick organic-rich shale formations constitute an oil-rich “sweet spot”, the average thickness of thin sandstone is 3.5 m. Thirdly, fine-grained sandstone and siltstone reservoirs have mainly small pores of 2–8 μm and throats of 20–150 nm in radius, but with a large number of micro-pores and nano-throats, through fracturing, the reservoirs can provide good conductivity for the fluid in it. Fourthly, continued high-intensity hydrocarbon generation led to a pressure difference between the source rock and thin-layer reservoir of up to 8–16 MPa during geological history, driven by the high pressure, the oil charged into the reservoirs in large area, with oil saturation reaching more than 70%. Under the guidance of the above theory, in 2019, the Qingcheng Oilfield with geologic oil reserves of billion ton order was proved in the class I multi-stage superimposed sandstone shale reservoir of Chang 7 Member by the Changqing Oilfield Branch through implementation of overall exploration and horizontal well volume fracturing. Two risk exploration horizontal wells were deployed for the class II thick layer mud shale interbedded with thin layers of silt- and fine-sandstones reservoir in the Chang 73 submember, and they were tested high yield oil flows of more than 100 tons per day, marking major breakthroughs in petroleum exploration in class I shale reservoirs. The new discoveries have expanded the domain of unconventional petroleum exploration.

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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.

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Abstract 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.

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A set of shale-dominated source rocks series were deposited during the heyday of lake basin development in the Member 7 of Triassic Yanchang Formation, Ordos Basin, and the thickness is about 110 m. Aimed at whether this layer can form large-scale oil enrichment of industrial value, comprehensive geological research and exploration practice have been carried out for years and obtained the following important geologic findings. Firstly, widely distributed black shale and dark mudstone with an average organic matter abundance of 13.81% and 3.74%, respectively, lay solid material foundation for the formation of shale oil. Secondly, sandy rocks sandwiched in thick organic-rich shale formations constitute an oil-rich "sweet spot", the average thickness of thin sandstone is 3.5 m. Thirdly, fine-grained sandstone and siltstone reservoirs have mainly small pores of 2–8 μm and throats of 20–150 nm in radius, but with a large number of micro-pores and nano-throats, through fracturing, the reservoirs can provide good conductivity for the fluid in it. Fourthly, continued high-intensity hydrocarbon generation led to a pressure difference between the source rock and thin-layer reservoir of up to 8–16 MPa during geological history, driven by the high pressure, the oil charged into the reservoirs in large area, with oil saturation reaching more than 70%. Under the guidance of the above theory, in 2019, the Qingcheng Oilfield with geologic oil reserves of billion ton order was proved in the class I multi-stage superimposed sandstone shale reservoir of Chang 7 Member by the Changqing Oilfield Branch through implementation of overall exploration and horizontal well volume fracturing. Two risk exploration horizontal wells were deployed for the class II thick layer mud shale interbedded with thin layers of silt- and fine-sandstones reservoir in the Chang 7 3 submember, and they were tested high yield oil flows of more than 100 tons per day, marking major breakthroughs in petroleum exploration in class I shale reservoirs. The new discoveries have expanded the domain of unconventional petroleum exploration. [ABSTRACT FROM AUTHOR]

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