Oil shale in the Sinopec exploration areas is abundant and serves as an important strategic reserve and supplementary energy source for the country. Accelerating the exploration and development of oil shale is crucial for improving China’s energy structure and ensuring national energy security. To achieve large-scale exploration and cost-effective development of oil shale, the technologies of in-situ exploitation field tests successfully conducted both domestically and internationally were reviewed and summarized. Based on this review, the characteristics of test areas, geological and engineering adaptability, and selection layer requirements were analyzed. It was concluded that field pilot tests of Shell’s electric heating method, Jilin Zhongcheng Company’s in-situ fracturing chemical retorting technology, and Jilin University’s local chemical reaction-based in-situ pyrolysis technology have been successfully carried out. However, the maturity and feasibility of two technologies in China need to be further studied and validated, and the adaptability of existing in-situ exploitation technologies to deep oil shale remains unverified. The technical characteristics, geological resource conditions, and exploitation engineering conditions of in-situ oil shale exploitation were reviewed and analyzed. Based on the key factors restricting in-situ exploitation of oil shale in China and the heating method, four geological parameters, six engineering parameters, and classification evaluation limits were determined. Additionally, the weights of each parameter were assigned according to the degree of constraints on in-situ exploitation and utilization of oil shale. A two-factor evaluation model of geological and engineering for identifying favorable areas for in-situ oil shale exploitation was then established, leading to the selection of 15 Class Ⅰ favorable areas in Sinopec exploration areas and adjacent areas. The effects of key factors, including roof and floor, fractures, and movable water, on the selected favorable areas were further analyzed. Through comprehensive evaluation, four target areas were selected: the Xunyi mining area on the southern margin of the Ordos Basin, the Shanghuangshan Street mining area on the southern edge of the northern piedmont of the Bogda Mountains, the Dianbai mining area in the Maoming Basin, and the Fushun mining area in the Fushun Basin.

Shale oil resources in the Subei Basin show significant potential. The second and fourth members of the Funing Formation (hereafter referred to as Funing Member 2 and Funing Member 4) are the main target layers for exploration. These layers are characterized by substantial thickness, wide distribution, high content of brittle minerals, well-developed laminated structure, and favorable organic matter types, with typical geological features, including low to medium total organic carbon (TOC), complex tectonics and lithology, and developed faults/fractures. Since 2011, Jiangsu Oilfield has strengthened basic research and exploration practices, leading to the establishment of the theory of differential enrichment of shale oil in complex fault blocks with low to medium TOC. Key technologies for exploration and development have been integrated and innovated, green and low-carbon development models have been explored, and significant breakthroughs have been achieved in shale oil exploration of Funing Member 2 and Funing Member 4 in the Gaoyou Sag. However, there are still many challenges, such as an unclear understanding of the patterns of shale oil enrichment and high yield, insufficient adaptability of engineering technologies, undefined technical policies for cost-effective development, and high development costs. Main approaches to achieving large-scale production and cost-effective development of shale oil include: deepening the fundamental research on the main controlling factors of shale oil enrichment and high yield, tackling main challenges and advancing key technologies, optimizing integrated organizational management and operation mechanisms, and maximizing the drilling success rate in high-quality reservoirs, the utilization rate of shale oil reserves, and the recovery efficiency to further reduce costs and improve efficiency.

The Subei Basin is characterized by a complex structure and well-developed faults. The shale in the second member of the Funing Formation has relatively low organic matter abundance, with a total organic carbon (TOC) generally below 1.5%. This shale exhibits diverse lithofacies types, complex pore structures, strong reservoir heterogeneity, and significant lateral variations in pressure coefficients. Using the shale oil reservoir in Block H of the second member of the Funing Formation in the Subei Basin as an example, this paper analyzed the characteristics of regional logging responses based on the results of rock physics experiments. By integrating conventional and specialized logging methods, a logging interpretation model was developed to evaluate the lithology, reservoir properties, oil content, mobility, and fracability of the shale oil reservoir. The model’s calculations aligned well with core analysis results. Building on this, sensitive parameters were optimized to establish evaluation and classification standards for the shale oil reservoirs in the block, and a comprehensive “sweet spot” evaluation of the reservoir was conducted. Exploration practices involving multiple wells have verified that this logging evaluation technology is regionally adaptable. It effectively classifies shale oil reservoir types, supports the optimal selection of “sweet spots”, and provides reliable technical support for the exploration and development of shale oil in the Subei Basin.

The shale lithofacies in the second member of the Funing Formation (hereafter referred to as Funing Formation Member 2) in the Gaoyou Sag of Subei Basin exhibits significant heterogeneity, with complex lamination types that are challenging to quantify using well logging, thus limiting the identification of favorable “sweet spots” for shale oil. Therefore, this study investigates the methods for quantitative characterization of shale lamination types and their development in Funing Formation Member 2 of Gaoyou Sag, by integrating data from core thin sections, whole rock diffraction, elemental logging, and well logging, based on the climatic and environmental evolution during different sedimentary stages. The results show that shale lamination types mainly include quartz-enriched, clay-enriched, calcite, and dolomite bands. Influenced by ancient climatic evolution, the proportions of different lamination types vary across intervals, and the vertical superposition and coupling of these lamination types lead to differential shale oil enrichment, with more developed laminations corresponding to higher oil enrichment. During the deposition of intervals Ⅴ-6 to Ⅴ-10, the sediments exhibit a high aridity index, low Sr/Cu ratio, significant variation in the Sr/Ba ratio, and high V/(V+Ni) ratio. These characteristics suggest a strongly reducing, semi-arid to arid saline water environment with fluctuating water depths and periodic variation in lake nutrients. Saline stratification and diagenesis facilitate the development of abundant bright calcite layers, fibrous calcite layers, and dolomite layers, providing favorable reservoir properties for shale oil. During the deposition of intervals Ⅴ-1 to Ⅴ-5, the Sr/Cu ratio increases significantly while the aridity index decreases. The overall environmental characteristics indicate a strongly reducing, arid saline water environment. The shale is predominantly composed of clay-rich to sandy-mixed lithology, with clay-enriched layers and clay-rich laminations as the dominant lamination types. Due to the influence of recrystallization degree, the proportion of bright calcite layers decreases while the proportion of mudstone-like calcite layers increases. During the deposition of intervals Ⅳ-5 to Ⅳ-8, the Sr/Cu ratio exhibits a periodic variation of “decrease followed by increase”, indicating a decrease in lake water salinity. The lithology primarily consists of clay-rich to sandy-mixed shale, with the development of clay-enriched layers, clay-rich laminations, bright calcite layers, fibrous calcite layers, and dolomite layers. These intervals demonstrate excellent reservoir properties and are regarded as high-quality sweet spot layers for shale oil. During the deposition of intervals Ⅳ1-Ⅳ4, the Sr/Cu ratio increases, indicating intensified arid conditions. The climate characteristics suggest a strongly reducing, arid saline environment. The recrystallization degree of calcite is higher, leading to the development of bright calcite, fibrous calcite, and dolomite layers. Additionally, the proportion of mudstone-like calcite layers increases, indicating a higher overall carbonate mineral content influenced by the depositional environment. During the deposition of the subinterval Ⅲ, the climate alternates between humid and arid conditions, with a higher degree of calcite crystallization and the development of bright calcite layers. Subintervals Ⅱ and Ⅰ exhibit a significant decrease in Fe/Mn and Sr/Ba ratios, indicating intensified humid conditions. Water depth increases, and the shale gradually transitions to blocky structure. The content of gray and muddy minerals decreases, limiting the development of gray and muddy laminations. The study further confirms a positive correlation between the degree of shale lamination development and shale oil enrichment. Based on the geological characteristics of the shale lamination distribution, further analysis was conducted using methods such as edge detection from electrical imaging well logging and shale deposition rate calculation. The study identified intervals Ⅳ-3 to Ⅳ-7 and Ⅴ-6 to Ⅴ-8 in Funing Formation Member 2 as having well-developed laminations and higher total organic carbon (TOC) compared to other intervals, marking them as vertical shale oil sweet spot layers. The image edge detection method using electrical imaging well logging offers high accuracy for shale bedding identification and is suitable for detailed geological evaluation of vertical shale oil sweet spot layers in different blocks. Furthermore, as the climate change during shale deposition becomes more frequent and the sedimentation rate varies more drastically, the vertical heterogeneity and lamination development of shale increase. Thus, sedimentation rate variations can serve as an indicator of shale lamination development. An analysis of stratigraphic cycles in the Huazhuang area's Funing Formation Member 2 revealed that natural gamma MTM spectrum analysis of well Huaye 7 identified eight dominant frequencies, corresponding to cycle thicknesses of 39.84, 11.76, 9.43, 4.20, 3.19, 2.32, 2.13, 1.82 m. The ratio of cycle thicknesses is 21.91:6.47:5.19:2.13:1.76:1.28:1.17:1.00, which is close to the theoretical cycle ratio of 21.32:6.58:5.26:2.74:2.00:1.21:1.16:1.00 for this period. Therefore, the shale deposition process of the Funing Formation Member 2 is controlled by the Milankovitch astronomical cycle. The optimal sedimentation rate for this interval was determined to be 10.8 cm/kyr. Using this optimal rate, the eCOCO statistical method was applied to track and analyze sedimentation rate variations in the Funing Formation Member 2. The results indicate significant differences in sedimentation rates among different sub-layers of the Funing Formation Member 2 due to the influence of periodic climatic fluctuations. Moreover, the degree of lamination development indicated by the sedimentation rate variation correlates well with the overall proportion of lamination development obtained from thin section analysis, and is consistent with the lamination development detected by imaging logging in different intervals. Consequently, this method can predict the spatial distribution of lamination development, providing guidance for three-dimensional shale oil exploration. In summary, this study provides insight on the lithological heterogeneity and quantitative logging characterization of the Funing Formation Member 2 in the Gaoyou Sag, Subei Basin. These findings contribute to the identification and evaluation of shale oil sweet spot layers, promoting shale oil exploration and development.

In the deep shale reservoirs of the southern Sichuan Basin, the development of fractures directly impacts the engineering construction and effective production of horizontal shale gas wells. Taking the shale cores in the Wufeng-Longmaxi Formation in the southern Sichuan Basin as a case study, rock physics experiments and numerical simulations were conducted to obtain the acoustic response characteristics of fractures at different scales, orientations, and fillings. The study analyzed the factors affecting the attenuation capability of acoustic waves on fractures and established a set of fracture identification and effectiveness evaluation methods for horizontal shale gas wells. The results showed that the amplitude attenuation of P-waves, S-waves, and Stoneley waves was influenced by both the fracture dip angle and fracture width, with attenuation capacity exponentially increasing with fracture width and decreasing with the dip angle. Stoneley waves were sensitive to fluid-filled fractures and could be used to identify and evaluate gas-bearing and water-bearing effective fractures. P-waves and dipole S-waves were sensitive to calcite-filled fractures, able to identify and evaluate ineffective calcite-filled fractures. The fracture identification results based on reflected wave imaging were consistent with the results obtained from imaging logging and core identification, verifying the reliability of the effectiveness evaluation method. The research findings were applied to the actual data from horizontal shale gas wells, thoroughly evaluating the fracture risk positions in horizontal shale gas wells and effectively ensuring the optimized and tailored design for fracturing segments.

Significant differences exist in the geological and production characteristics between normal-pressure shale gas reservoirs and high-pressure shale gas reservoirs. The current shale gas reserve calculation standards are primarily based on the initial investment in shale gas exploration and the production patterns of high-pressure shale gas. With breakthroughs in normal-pressure shale gas exploration in blocks such as Wulong and Daozhen in southeastern Chongqing, it is imperative to conduct targeted research on reserve calculation standards by considering the geological characteristics, technical and economic indicators, and production patterns of normal-pressure shale gas. Based on the break-even principle, this study analyzed the sensitivity of economic parameters to reserve calculation standards and developed a calculation model tailored to the characteristics of normal-pressure shale gas according to the production performance of atmospheric shale gas. Taking Wulong block as an example, the reserve calculation standards were estimated at different burial depths by incorporating technical parameters and economic factors such as investment, costs, and gas prices. The variation patterns in the calculation results were analyzed. It was suggested that the reserve calculation standards for single well in normal-pressure shale gas reservoirs ranged from 2.0×10⁴ m³/d to 5.5×10⁴ m³/d when the burial depth was 2 000 m to 7 000 m. The study of reserve calculation standards for normal-pressure shale gas provides valuable insights for revising relevant standards, facilitating the addition of proven reserves and large-scale production of normal-pressure shale gas outside the Sichuan Basin.

The Shunbei No. 4 fault zone is located in the central part of the Shuntuoguole Low Uplift. The reservoir type in this zone is a condensate gas reservoir controlled by fracture zones. This type of reservoir is rare both domestically and internationally. The phase characteristics of the condensate gas reservoir exhibit a clear north-south differentiation. In-depth research on the phase characteristics and the main controlling factors of this differentiation can provide valuable insights for the exploration and development of similar condensate gas reservoirs. This study employed various technical methods, including PVT (pressure-volume-temperature) high-pressure physical property experiments, organic geochemical analysis, and fluid inclusion analysis and testing. The results showed that the crude oil in the Shunbei No. 4 fault zone was characterized by a low freezing point, low sulfur content, and medium-to-high wax content. Within the zone, natural gas exhibited differential distribution in terms of methane molar fraction, gas-oil ratio, gas dryness coefficient, and CO₂ molar fraction. PVT experiments indicated that the reservoir was a condensate gas reservoir with a large difference between formation pressure and dew-point pressure, classifying it as an unsaturated reservoir. The critical temperature and pressure in the northern section were significantly higher than in the middle and southern sections, showing a decreasing trend from north to south. From the perspective of hydrocarbon source rocks and reservoir formation, the differential enrichment of condensate gas reservoirs in the Shunbei No. 4 fault zone is primarily controlled by multiple sources of hydrocarbon supply and multiple phases of reservoir formation.

The C-2 oilfield, located in the Bohai Bay Basin, is a fluvial-facies offshore oilfield primarily developed with horizontal wells. It is characterized by thin reservoir layers, vertically stacked multi-phase sandbodies, and rapid lateral facies transitions, leading to complex internal reservoir structures and connectivity. The combined effects of complex reservoir structures and well-seismic data make reservoir characterization challenging under sparse well patterns, hindering refined development. Conventional seismic inversion fails to meet the requirements for high-resolution prediction of thin reservoirs and detailed characterization of internal reservoir structures. To study the structural characteristics of braided river reservoirs in the oilfield, a stacking ensemble learning method based on seismic attributes was applied to predict the complex fluvial-facies reservoir structures. This approach significantly improved prediction accuracy compared to a single machine learning model. By integrating multi-dimensional information such as geology, geophysics, and reservoir dynamics, iterative optimization was conducted to further reduce the uncertainty in subsurface reservoir prediction and structural understanding. This enabled the precise characterization of the complex braided river reservoir structures in the study area, providing a basis for refined exploitation of remaining oil and potential sandbodies in the oilfield. The study demonstrates that the reservoir prediction method based on stacking ensemble learning not only enhances seismic vertical resolution, but also exhibits strong horizontal phase-control capabilities. The prediction results include sandbody stacking relationships and internal reservoir structures, making it more suitable for the prediction and fine characterization of continental fluvial sedimentary systems with rapid facies transitions and complex spatial architectural structures. This method can serve as a reference for the detailed characterization of fluvial-facies reservoir configurations during the middle and late development stages of offshore oilfields with sparse well patterns.