By comprehensively analyzing the current classification scheme of shale oil at home and abroad, and comparing the exploration and development history, shale oil reserves and output status, and production performance of horizontal wells in China and the United States, the differences in geological conditions, resource quality, recoverability, economy and scale of shale oil between the two countries are analyzed. It is proposed to define the continental shale oil zones in China by using the nomenclature of “geographical location + shale oil” or “geographical location + stratigraphic unit + shale oil”. Furthermore, the shale oil types are classified according to two levels: Firstly, the shale oil is divided into three types based on the sandstone-to-formation ratio (the ratio of source rock to reservoir) or sedimentary facies zones, namely tight oil type shale oil, transitional type shale oil, and pure shale oil; Then the pure shale oil is further subdivided into medium-high mature shale oil and medium-low mature shale oil, or in another way, it is subdivided into light oil, thin oil, heavy oil, and viscous oil zones based on the maturity or oil properties. For the medium-high mature continental shale oil targets, efforts should be focused on the evaluation of enrichment zones/intervals in high-pressure zones, as well as trial production, and research on optimal production technology before 2025, so as to reduce costs to the maximum extent and improve initial single-well production and cumulative output. It is estimated that the annual shale oil output in China will be 600×104t to 1000×10⁴t. During the 2025-2035, technology will be further upgraded and optimized to reduce costs, and the annual shale oil output is expected to be 1200×10⁴t to 1500×10⁴t, growing to be a major supplement to crude oil production with an annual capacity of 2×10⁸t/a. As for medium-low mature shale oil, pilot test on the in-situ conversion of shale oil in Chang 7₃ sub-member in Ordos Basin is focused at present; At around 2030, the critical equipment and core technologies will be localized hopefully, and the large-scale and commercial development of shale oil will be achieved, with an annual shale oil output of ten millions of tons.

Based on the study of source rock, sediment source system, tectonic evolution and dynamic matching relationship between hydrocarbon source and reservoir, the Permian oil and gas bearing system in Jimsar Sag is analyzed in detail. The study results indicate that the Permian in Jimsar Sag has favorable geological conditions for forming full oil and gas system. On the plane, shale oil, tight oil, conventional glutenite oil reservoirs are distributed in sequence from the central to the marginal sag, and tight oil, shale oil, conventional glutenite oil reservoirs are distributed upward around the high-quality source rock in Lucaogou Formation, showing the overall hydrocarbon accumulation characteristics of “full oil and gas system”. Guided by this understanding, breakthroughs have been made in the exploration of three types of oil reservoirs, basically forming a pattern of oil bearing in the whole sag and orderly co-occurrence and superposition of multi-layer and multi type oil reservoirs. By applying the large platform and multi-layer stereoscopic development mode, the exploration idea and technology for maximizing the benefit resource utilization are gradually mature in practice. The detailed analysis of geological characteristics, hydrocarbon accumulation pattern, and exploration and development practice of the Permian full oil and gas system in Jimsar Sag is of great significance for enriching the exploration theory of the full oil and gas system and guiding the exploration and development in similar oil rich sags.

The large-scale and benefit development of shale oil in the second member of Kongdian Formation (Kong 2 member) in Cangdong Sag in Huanghua Depression face multiple challenges, such as evaluation and optimal selection of main production layers of medium-low mature shale oil, stereoscopic development and production of multiple vertical sweet spots, optimal and fast drilling and completion of horizontal section with a length of 2000 m in complex fault blocks, high-efficiency volumetric fracturing of laminated shale reservoir and high-efficiency lifting of shale oil with high viscosity and high wax content. Go through four stages of shale oil exploration, i.e., research, breakthrough in horizontal wells and pilot test of production capacity evaluation, 60% of the horizontal wells in production have a single well EUR of less than 2.0×10⁴t, and the low single well oil rate has become a bottleneck problem restricting the benefit development of lacustrine shale oil. By comprehensively analyzing wireline logging, mud logging, fracturing parameters and production data of Kong 2 member shale oil in 47 production wells, the systematic research has been conducted on sweet spot evaluation and optimal selection of sealing box, well pattern deployment, geosteering while drilling, fracturing reconstruction, and production regime for the lacustrine shale oil in complex fault blocks, and three sweet spot layers (C1, C3 and C5) have been identified for benefit shale oil development in Guandong area, with an favorable area of 42-53.3 km² and the estimated Class Ⅰ resources of 1.54×10⁸t. The pilot test of benefit shale oil development has been conducted on two 10 m-level sweet spot layers (C1③ and C3⑧) in Kong 2 member on No.5 platform in Cangdong Sag, in which the drilling difficulty of long horizontal section by water-based drilling fluids in complex fault blocks has been solved, the high-efficiency fracturing technology for laminated shale oil has been innovated, and permeability improvement, swelling energization and viscosity reduction of miscible fluids have been achieved of the laminated shale reservoir, with the tested single well peak oil output of 39.6-122.3t. As of June 22, 2023, the five pilot wells had been put into production for 174-201 days, and the cumulative oil output was 3.34×10⁴t. The predicted cumulative oil output in the first year is 6.5×10⁴t, and the average EUR of a single well is 4.11×10⁴t, which indicate that the large-scale and benefit development mode of lacustrine shale oil has been established in complex fault blocks.

A major breakthrough has been made in the Carboniferous-Permian carbonate rocks in Well Qiatan 1 in Wuqia structural zone in the West Tianshan Thrust Belt, marking the discovery of a major replacement formation in the piedmont southwestern Tarim Basin. Combined with the regional geological data, studies on field outcrops, drilling, and seismic data, as well as the comprehensive analysis of laboratory experimental and test data in wells Qiatan 1 and Kuntan 1, hydrocarbon accumulation conditions in the Carboniferous-Permian in the piedmont southwestern Tarim Basin are re-recognized. The study results show that a “horseshoe-shaped” platform margin zone was developed in the Late Carboniferous-Early Permian in the piedmont southwestern Tarim Basin, and a favorable high-energy beach sedimentary facies zone was formed in belts, in which source rocks of bay lagoon facies were deposited in the Permian Qipan Formation, with a thickness of up to 500 m, type Ⅰ-Ⅱ organic matter, and high organic matter abundance. Jointly controlled by sedimentary facies and karstification, the fractured-vuggy type carbonate reservoirs were developed in the Late Carboniferous-Early Permian, forming a favorable reservoir and cap rock assemblage with the overlying thick mudstone in the Permian Qipan Formation. As a result, a new hydrocarbon accumulation pattern of “lower source rock and upper reservoir, hydrocarbon transport by fault and near-source accumulation” is established. The comprehensive analysis indicates that the fault anticline traps in the Carboniferous-Permian are distributed in rows and belts, which have the advantage of near-source hydrocarbon accumulation, and a number of favorable exploration targets such as wells Yetan 1 and Abei 1JS are optimally selected after evaluation, showing good exploration prospects, which are expected to be the major replacement field for large-scale gas exploration and discovery.

Influenced by multi type and high density laminae of continental shale, the differential intrasource micro-migration of hydrocarbons is an important factor controlling the oil-bearing property and mobility of the laminated shale with various lithofacies. By taking the systematic shale coring sections in 10 wells in Ordos Basin as the research object and using laboratory experimental results such as XRD, thin section observation, SEM, nitrogen absorption, laser scanning confocal, geochemical test, as well as wireline logging and well testing results,the hydrocarbon micro-migration inside shale section in the third sub-member of the seventh member of the Triassic Yanchang Formation (referred to as Chang 7₃ sub-member) and the mechanism are analyzed. The study results show that the micro-millimeter scale migration generally occurred of crude oil generated by organic-rich laminated shale. Among them, hydrocarbon generation pressurization provided forces for micro-migration; The frequently developed brittle laminae provided a large amount of reservoir space for the micro migrated hydrocarbon; The hydrocarbon generation pressurization fractures, foliation fractures, and micro fractures provided pathways for micro-migration; The different mineral compositions of the laminae resulted in hydrocarbon differentiation. The combination type of laminae and differential micro-migration mechanism determine that the felsic laminated shale and tuffaceous laminated shale have relatively good oil-bearing property and mobility, which are the main lithofacies types for risk exploration of medium-high maturity laminated type shale oil. The commercial oil flow of more than 10 t/d was obtained in pilot test of Chang 7₃ sub-member shale in seven vertical wells, confirming that shale section in Chang7₃ sub member has good oil-bearing property, and there is a high potential of production increase by horizontal well development, which is expected to be a replacement field for reserve and production increase of shale oil in Ordos Basin.

The global technical recoverable shale oil resources are 2512×10⁸ t, and the cumulative crude oil output from shale series has reached up to 3.9×10⁸ t in 2021, which has grown to be one of the major fields for global petroleum exploration and development. Based on the geological conditions and sedimentary characteristics, the “sweet spot” reservoir of continental shale oil in China is classified into three categories, i.e., interlayer type, mixed type and shale type, among which the interlayer type and mixed type shale oil are the main targets for global shale oil exploration and development. Recently, breakthrough has been achieved in the exploration of shale type shale oil. However, the sweet spot evaluation urgently needs to be deepened to provide a basis for achieving benefit shale oil development. Therefore, the geological characteristics between the Carboniferous Wolfcamp D marine shale oil in Permian Basin in north America and the Cretaceous Qingshankou Formation Gulong continental shale oil in China are compared, and the key indicators for sweet spot evaluation of shale type shale oil are discussed, so as to provide reference for the exploration and development of continental shale type shale oil in China. The comparative study indicates that the two sets of shales are dominated by felsic shale and clayey shale with high clay minerals and quartz contents, and show similar geological characteristics of formation overpressure, relatively light oil quality and high brittle mineral content; The reservoir space of Gulong shale oil is dominated by composite organic matter pores and bedding joints related to a large number of clay minerals, showing better connectivity than Wolfcamp D shale oil but slightly poorer organic carbon abundance. The geological sweet spot of Wolfcamp D shale includes organic-rich clayey shale and siliceous shale, while the engineering sweet spot is the organic-rich siliceous shale based on the fracturing results. Similarly, the geological sweet spot of Gulong continental shale oil includes organic-rich clayey shale and felsic shale by using evaluation indicator of retained hydrocarbon, and the engineering sweet spot is the silicic felsic shale.

In recent years, major breakthroughs have been made in the exploration of the Jurassic lacustrine shale oil in Sichuan Basin, showing great exploration potential. In order to support the further deepening exploration, the field outcrop, logging, well drilling, coring and systematic core sample experimental data are combined to analyze the geochemical characteristics, rock brittleness, pore structure and oil and gas bearing properties of shale with various lithologic combinations in the Jurassic Lianggaoshan Formation, and identify the lithologic combination types of the dominant shale. The results indicate that the lithologic combination of shale interval in Lianggaoshan Formation is classified into three categories and five types, i.e., pure shale combination, shell type shale combination, and siltstone type shale combination; Among the five types of lithologic combinations, shale is in the mature stage, but there are differences in organic matter content, type, and brittle mineral content. The shale of pure shale combination has an average TOC of greater than 1%, Type Ⅱ₁ organic matter, and brittle mineral content of higher than 52%; The organic matter characteristics of the shale of the shell type shale combination are similar to those of the pure shale combination, but the content of brittle minerals is greater than 65%; The shale of the siltstone type shale combination has an average TOC of less than 1%, Type Ⅱ₂ organic matter type, and the brittle mineral content of greater than 65%. The main reservoir space of various combinations is composed of microfractures and inorganic pores (pore diameter of 10-500 nm). The siltstone type shale combination has a larger pore throat and better pore connectivity than the pure shale combination and shell type shale combination. The free hydrocarbon content of the pure shale combination is greater than 1 mg/g, showing good oil-bearing property. OSI of the pure shale combination and the siltstone type shale combination is greater than 100 mg/g of some samples, indicating good oil movability. The comprehensive analysis shows that pure shale combination and siltstone type shale combination are favorable lithologic combination for shale oil exploration. Benefiting from the good fracability, the siltstone type shale combination is the main sweet spot for shale oil exploration in Lianggaoshan Formation.

During the deposition period of the third sub-member of the seventh member of the Triassic Yanchang Formation (Chang 7₃ sub-member), Fuxian area was located in the depo-center of the eastern lake basin, showing abundant shale oil resources and good exploration prospects. However, few studies have been conducted on shale oil reservoir characteristics, and there is a lack of the study on reservoir space characteristics and oil-bearing property, which restrict shale oil exploration and deployment practice. By using core observation, whole rock mineral XRD analysis, chloroform extraction, NMR, organic carbon abundance and porosity test data, the petrological and mineralogical characteristics, reservoir space, pore structure and oil and gas bearing property are analyzed in Fuxian area, which indicates that three rock categories are observed in Chang 7₃ sub-member, such as shale, silty-fine sandstone and tuff. Based on the development degree of lamina, the shale category is subdivided into four rock types. The main mineral composition of shale section includes quartz, feldspar, carbonate and clay minerals, as well as a small amount of pyrite. The reservoir is dominated by four types of pores, i.e., intergranular pore, intragranular pore, organic pore and microfracture. The meso-macropores are relatively developed in silty-fine sandstone, and the NMR T₂ spectrum is characterized by multi-peak and left-biased single-peak types. The micropores are relatively developed in black shale, with relatively few macropores, and the NMR T₂ spectrum is mainly multi-peak and right-biased single-peak types. The silty-fine sandstone with good oil-bearing property and high oil saturation index is the optimal shale oil reservoir in Chang 7₃ sub-member, followed by the laminated shale. The comprehensive study indicates that both silty-fine sandstone and shale targets have certain exploration potential, which are potential fields for increasing reserves and production in the future.

A total of 31 horizontal wells have been drilled on Hua H100 platform in Qingcheng Oilfield in Ordos Basin. For the first time, the cross-layer fracturing is tested in four horizontal wells on this platform with low drilling rate of reservoir. Meanwhile, by applying tracer monitoring technology, the engineering feasibility of penetrating mudstone and communicating the adjacent sandstone reservoirs and the effectiveness of flow pathway after fracturing are analyzed, and the production contribution of the cross-layer fracturing stage is evaluated. The pilot test results show that when the distance between the horizontal well trajectory in the non-reservoir interval and the sandstone reservoir is less than 3 m and the clay content is lower than 40%, the cross-layer fracturing engineering is feasible. A total of 14 stages have been tested and good results have been achieved in 12 stages, with a success rate of 85.7%, confirming the feasibility of cross-layer fracturing operation. The tracer detection and interpretation results show that continuous water flowback has been monitored in cross-layer fracturing stages, which indicate that an effective flow pathway has been formed after soaking the well. The varying amounts of oil production in various cross-layer fracturing stages also demonstrate that the sandstone reservoirs have been communicated by cross-layer fractures, with a definite production contribution. Compared with another four offset wells with drilling rate of higher than 80% on the same platform, the cumulative oil production of cross-layer fracturing wells is above the average level. The successful pilot test of Hua H100 platform proves that the cross-layer fracturing is an effective engineering means to increase single well production of horizontal wells with low drilling rate of reservoir.

The limy dolomite and shale are well developed in Lower Ganchaigou Formation shale series in Yingxiongling area in Qaidam Basin, and the favorable reservoirs of limy-dolomitic flat are superimposed and interbedded with high-quality source rocks, showing huge exploration and development potential of shale oil resources. However, there are some difficulties in reservoir reconstruction, such as high field stress, high stress difference, high-frequency sedimentary cycle, well-developed lamination in vertical direction, and strong reservoir heterogeneity. Therefore, it is urgent to develop a highly applicable and high-efficiency fracturing technology system to support the benefit exploration and high-efficiency development. By conducting geomechanical experiments, the law of shale rupture and fracture propagation is clarified, and the reservoir reconstruction idea of “controlling nearby fractures and propagating long fractures” is formulated; A new algorithm of 1D geomechanics and fracability is researched, and a 3D fine geomechanical model is established; The combination of model simulation optimization and data-driven optimization is applied to form a network fracturing technology template for vertical wells; By benchmarking the mainstream practices and key parameters of volume fracturing of continental shale oil in China, the concept of geology and engineering integration is implemented in cluster setting and fracturing parameters optimization, achieving the upgrade of volume fracturing technology with the core of “high-density cutting, extremely limiting flow perforation, large displacement, large scale, high-intensity and continuous slickwater sand addition, inverse composite ‘controlling nearby fractures and propagating long fractures’, and high quartz sand percentage”. A total of 37 fracturing operations were conducted in vertical wells, with an oil rate of 2.1-44.9 m³/d, and the wells with commercial oil flows accounted for 97.2%; Another six fracturing operations were conducted in horizontal wells, with the maximum oil rate of 113.5 m³/d, and the proven shale oil geological reserves of 5×10⁸ t in the upper member of Lower Ganchaigou Formation.

In 2021, breakthrough of high-yield shale oil production was achieved in Well CP1 in Yingxiongling area in Qaidam Basin. However, it is still necessary to determine the dominant sweet spot lithofacies between the two preliminarily identified sweet spots, i.e., laminated limy-dolomitic shale and layered limy-dolomitic shale. The geological and engineering sweet spot lithofacies of shale oil in Yingxiongling area are systematically analyzed from the perspectives of organic matter abundance, physical properties, oil-bearing property, fracture complexity after fracturing, tracer, and production performance. The results show that: (1) The indicators characterizing the geological sweet spots of Yingxiongling shale oil include carbonate content, oil saturation, and TOC, while the fracability is a key parameter to characterize the engineering sweet spots. (2) Compared with the layered limy-dolomitic shale, the laminated limy-dolomitic shale has advantages of oil saturation, horizontal permeability, fracture complexity, imbibition ability, oil phase flow ability, and oil breakthrough rate. (3) The laminated limy-dolomitic shale is geological and engineering sweet spots in Yingxiongling area. (4) In practice, it is suggested that the No.3 thin layer of the No.11 layer in the IV oil group of the upper part in Lower Ganchaigou Formation (E32-Ⅳ-11-3) is targeted for shale oil exploration and development.

XAB Oilfield in Ordos Basin is located in Dingbian, Shaanxi Province. The main target layer is the shale interval in the seventh member of the Upper Triassic Yanchang Formation (Chang 7 member), with tight shale reservoir and high-angle fractures widely developed. Since 2011, rolling capacity construction and development by horizontal wells have been applied to produce the shale oil reservoir, which have the problem of low oil recovery degree and low production rate. In recent years, in order to research the high-efficiency shale oil development mode, tests of inter-well infill adjustment in horizontal well development zone have been conducted, and good results have been achieved in increasing oil production. Based on understanding of infill adjustment tests in the early stage, a “matrix-fracture” dual medium numerical model is established by fully considering the natural fracture features in the study area, and the infill adjustment parameters are optimized for horizontal well development zones with various production years, with the main parameters including infill timing, hydraulic fracture layout, horizontal well spacing and half-length of hydraulic fractures. A total of 76 infill adjustment schemes are designed to simulate and calculate the cumulative oil production of various schemed during the evaluation period. Furthermore, the financial net present value ratio is calculated and the economically beneficial schemes are ranked and optimally selected by using the financial cash flow method. The study results show that: (1) The longer the production period of the horizontal well development zone, the poorer results of infill wells. (2) The staggered layout pattern is better than the symmetrical layout pattern of hydraulic fractures. (3) The optimal infill adjustment parameters vary given the different infill timings. When the infill timing is 2-10 years, the optimal infill well spacing is 260 m, and the optimal half-length of the hydraulic fracture ranges in 140-180 m. On this basis, the horizontal infill adjustment design for Chang 7 member shale oil in horizontal well development zone in XBA Oilfield is conducted. After infill adjustment, the oil recovery factor and net present value ratio are expected to increase by 1.05% and 0.013 respectively, which supports to improve the shale oil development results and achieve good economic benefits.