Shale oil accumulations have been found in many regions around the world, with a total amount of geological resources of about 4090×10⁸ t, showing good exploration prospects ^[1]. In particular, shale oil exploration and development in the United States have grown rapidly. In 2017, the oil production from shale strata in the United States was about 2.34×10⁸ t, accounting for 47.6% of its crude oil production, which highlights its strategic position ^[2].

In recent years, China has actively engaged in shale oil exploration and tackled key technical problems for exploration and development experiments in shale, dolomitic mudstone, argillaceous siltstone, hybrid sedimentary rocks and other source rock reservoirs. Volume fracturing of the vertical well and horizontal well has been exploited in Xinjiang, Dagang, Turpan-Hami, Changqing, and Daqing oil fields. Some exploration wells have obtained industrial oil flows. Industrial development of shale oil has proceeded in the Permian Lucaogou Formation in the Jimusar Sag, the Member 7 of the Triassic Yanchang Formation in the Ordos Basin (hereinafter referred to as Chang-7 Member) and the Member 2 of the Paleogene Kongdian Formation in the Cangdong sag (hereinafter referred to as Kong-2 Member). It is preliminarily confirmed that the oil resources in continental shale source rocks in China have great potential and are potential strategic replacements^{[3⇓⇓⇓⇓⇓-9]}.

Compared with the geological conditions of shale oil/tight oil in North America, the continental shale oil in China is more complex in geological context ^[10-11]. Source rocks in China are different in hydrocarbon generation capacity, reservoir preservation, and distributions of source rocks and reservoirs. It is urgent to study the geological characteristics and reveal the distribution patterns of continental shale oil in China to provide a basis for locating "sweet spots” for exploration and development and for promoting the key technologies for large-scale exploration and efficient development. In this study, we compare the geological conditions, quality, and sedimentary environment of source rocks and reservoirs of various lacustrine shale oil plays in China to reveal the heterogeneity in the formation of medium-high maturity shale oil. We summarize the characteristics in reservoir-forming processes and enrichment based on the study of source-reservoir combination and hydrocarbon generation, expulsion, and migration of source rocks to provide a reference for the evaluation of "sweet spot" areas and selection of exploration domains of continental shale oil in China.

In recent years, shale oil has become one of the hotspots of petroleum geology research worldwide^{[12⇓⇓⇓-16]}. Many scholars and relevant scientific research institutions believe that shale oil refers to the oil resources existing in organic-rich source rocks, including shale, carbonate rock, siltstone, and other types of tight reservoirs, which are difficult to be extracted by conventional technologies ^{[12⇓⇓⇓-16]}. Tight oil refers to the oil stored in thick layered tight reservoirs after secondary migration, which has independent development units and strata. Commercial oil flow can be obtained through certain technical implements. Shale oil is the oil resource existing in pure shale or organic-rich mudstone to form in-situ and continuous distributed oil storages ^[10-11]. Hence, shale oil highlights the resource potential, types, engineering technologies, strategic positioning, and exploration prospect. Therefore, whether the oil is inside the source rock and whether it has experienced long-distance secondary migration are the keys to differentiating the shale oil reservoir.

The continental medium-high maturity shale oil in China discussed here refers to the oil resources in shale source rocks that have entered the oil generation window and generated massive liquid hydrocarbons with vitrinite reflectance larger than 0.7% (mostly larger than 0.9%) ^[17]. It exists in three types of lithologies, including shale, transitional lithology, and tight reservoir-related rocks. From the perspective of resource type, shale oil refers to the liquid hydrocarbons that remain in the shale layers and have not been expelled, excluding the organic matter that has not been converted into oil. From the perspective of resource distribution, shale oil reservoir may preserve in the organic-rich source rock (source reservoirs), transition lithologies (sandy dolomite, dolomitic siltstone, dolomitic mudstone, argillaceous dolomite, etc.), and tight reservoir (siltstone, etc.), which are characterized by in-situ accumulation, source-reservoir-in one and source-reservoir closely contacted. Shale oil requires multi-stage fracturing in horizontal wells technology for recovery which is different from conventional oil and gas development. Volume fracturing in vertical well and horizontal well has been applied to mature shale and dolomitic mudstone, argillaceous siltstone, tuff, and other transition lithologies in Xinjiang, Dagang, Turpan-Hami, and other oil fields. The experiment has made new progress in shale oil production.

Meso-Cenozoic continental basins in China experienced multicycle tectonic evolution ^[18], forming multi-types of continental lake basins in multi-stages. The lake basins went through multi-stages of expansion and shrinking, providing an opportunity for large-scale development of continental shale oil. Freshwater, saline, and brackish lake basins have been formed since the Permian during the expansion of continental rift and depression lake basins (Table 1), which allows for the formation of continental organic-rich shale and multi-source sedimentary reservoirs. For example, Cretaceous Qingshankou Formation of the Songliao Basin shows cyclic expansion and shrinking of lake basins. During the deposition of Member 1 of the Qingshankou Formation (hereinafter referred to as Qing-1 Member), the lake basin expanded to 8.7×10⁴ km². The dark mudstone deposited in this period is more than 40 m thick and 5.1×10⁴ km² in area (Fig. 1), forming a high-quality shale section with an average TOC of 3.8%. During the deposition of Member 3 of the Qingshankou Formation (hereinafter referred to as Qing-3 Member), the lake basin shrank to only 3.5×10⁴ km².

Research shows that North American shale oil mainly accumulated in the margins of marine cratons or slope areas of foreland-depressions with a stable distribution. In contrast, continental shale oil accumulations in China mostly developed in depression or rift basins with complex and diverse geological conditions. Strong heterogeneity is reflected by the diversity of lithology and organic matter abundance of source rocks, reservoir types, and complex source-reservoir combinations, which in turn makes the differentiation of enrichment patterns of continental shale oil.

The marine source rocks of shale oil in North America generally have high quality, with TOC values of 5%-17% (on average 6%), R_o value of 0.6%-1.5% (on average 1.2%), and high hydrocarbon generation potential. They are primarily black shale with a high abundance of organic matter and a high degree of thermal evolution. For example, two sets of laminated black shale (the upper member and lower member of the Bakken Formation of the Upper Devonian-Lower Carboniferous in the Williston Basin, North America) formed in a marine anoxic environment have TOC values up to 10%-14%, and hydrogen index (HI) up to 900 mg/g, indicating huge hydrocarbon generation potential. The Permian Basin stretching from western Texas to southeastern New Mexico has source rocks and hydrocarbon discoveries from the Cambrian to the Cretaceous, and several sets of shale oil layers developed. Among them, the source rock of the Permian Wolfcamp Formation has a high organic matter abundance, with an average TOC value of 3.25%. More than 80% of source rock with a TOC value greater than 2%, indicating a superior hydrocarbon generation potential^[19⇓-21].

In contrast to the shale oil in North America, source rocks of China's continental shale oil vary widely in lithology, quality, and microstructure. Source rocks consist of black shale, mudstone, and dolomitic mudstone (Fig. 2). They have TOC values of 0.6%-32.0%, R_o values of 0.50%-1.67%, S₁ values of 0.01-10.00 mg/g, and S₂ values of 0.06-110.00 mg/g. Organic matter, clay, and other minerals are highly heterogeneous and deposited in laminae, bed, or dispersion.

The source rocks of continental shale oil in China are diverse in lithology. Freshwater lake basins (such as Chang-7 Member in Ordos Basin ^{[22⇓⇓-25]}) mainly deposited shale and mudstone, made up of feldspar, quartz, clay, and a small amount of pyrite and volcanic ash. The clay contents are between 30% and 50%, with some over 50%, reflecting the continental sedimentary supply system. The source rocks of shale oil in the salinized lake basins are mainly autochthonous rocks, including argillaceous dolomite, dolomitic mudstone, and tuffaceous mudstone. They are characterized by low clay mineral content and high carbonate mineral content. For example, the Santanghu Basin at the northeast margin of the Junggar Basin was an intermontane basin formed on the basement of the Paleozoic fold during the late Paleozoic - middle Cenozoic period. Permian Lucaogou Formation was deposited in a deep lake environment with constant and prolonged sediment supply ^{[27⇓⇓-30]}. Source rock of the Lucaogou Formation was deposited in a saline lake environment ^{[26⇓⇓-29]}, which is supported by the high contents of gammacerane and carotane in the organic extracts (Fig. 3). The source rocks commonly contain volcanic materials and are marked by complex lithological composition, including argillaceous dolomite, dolomitic mudstone, tuffaceous mudstone, tuffaceous siltstone, and interbedded structure. Dolomite contents take up 20%-40% and clay minerals are generally less than 10%. Source rocks in freshwater to saline lake basin (such as Qingshankou Formation in Songliao Basin) mainly consist of felsic shale, mixed with a small amount of silty mudstone and shell limestone. It is mainly made up of quartz and clay with minor feldspar and carbonate minerals. Quartz and clay minerals take up 60%-75% and carbonate minerals hold about 10% in the Qingshankou Formation shale samples in the Gulong Sag.

Therefore, shale oil source rocks in freshwater lake basins are mainly clay in general, with low content of felsic and carbonate minerals. In contrast, salinized lake basins mainly deposit dolomitic and felsic sedimentary rocks with low clay mineral content and good brittleness. Transitional source rocks between the above two types contain mineral composition in between them too (for example, the contents of clay minerals and felsic account for about one-third respectively).

The shale of Chang-7 Member in the Ordos Basin was deposited in a freshwater environment, showing lower contents of gammacerane and carotane, with a thickness of 20-100 m ^[31]. Kerogens are dominated by type I-II₁ represented by sapropel (Fig. 4) (accounting for 65% in volume), with organic carbon contents of 3%-32% (mostly between 10% and 15%), HI of 300-600 mg/g, S₁ of 0.2-7.1 mg/g, S₂ of 0.3-46.1 mg/g, and strong hydrocarbon generation capacity. Among them, high-quality source rock with TOC greater than 8% occupies 1.41×10⁴ km², and source rock with TOC greater than 2.5% is up to 5.6×10⁴ km². The source rock layers have thermal evolution degrees (R_o) of 0.7%-1.3% and maximum pyrolysis temperatures (T_max) of 440-460 °C, indicating a stage of the oil window. The black shale has an average hydrocarbon generation intensity of 235.4×10⁴ t/km² and a hydrocarbon generation amount of 1012.2×10⁸ t. The dark mudstone has an average hydrocarbon generation intensity of 34.8×10⁴ t/km² and a hydrocarbon generation amount of 216.4×10⁸ t. The total amount of generated hydrocarbon by black shale and dark mudstone is 1228.6× 10⁸ t.

Compared with the Chang 7 Formation in the Ordos Basin, the Permian Lucaogou Formation source rock in the Santanghu Basin deposited in saline lake basin with organic carbon abundance of 1.1%-13.4% (on average 4.9%), R_o of 0.5%-1.3%, HI of 600-800 mg/g, S₁ of 0.01-3.00 mg/g, and S₂ of 0.06-110.00 mg/g. Organic matter is dominated by algae and spores (Fig. 4) that form type I-II₁ kerogens. The deposition has a thicknesses of 100-240 m which makes it a potential high-quality source rock. During the sedimentation of the Qing-1 and Qing-2 members in the Songliao Basin, planktonic algae bloomed and act as the main source of organic matter. Source rocks were deposited in anoxic sedimentary environment and contained type I organic matter, with an average TOC of 1.8%-4.5% and HI values of 600-800 mg/g, indicating high oil generation potential. The shale in the lower Qing-1 Member has the highest S₁ value (generally >8 mg/g), followed by the shale in the upper Qing-1 Member and Qing-2 Member (generally >6 mg/g). They are high-quality source rock of Daqing Gulong shale oil ^[32].

Source rocks deposited in a freshwater environment (such as Chang-7 Member of the Ordos Basin) are large in thickness, widely distributed, and highly abundant in organic matter, suggesting huge oil generation potential and large shale oil resource potential. Source rocks deposited in a saline environment (such as Jimusar Sag) have a lower abundance of organic matter than those deposited in the freshwater environment but high-quality organic matter and high hydrogen index, so they also have high oil generation potential and shale oil resource potential.

Compared with the monotonous microstructure of marine organic-rich shale, continental organic-rich source rocks in lake basins have a more complex microstructure and massive laminated textures due to complex sedimentary types and rapid changes in a dynamic environment (Table 2). The argillaceous source rocks have a massive texture with lamination poorly developed. They are rich in clastic minerals, with low content of organic carbon (mostly less than 2%). The source rocks with horizontal laminar texture have TOC of more than 5%, with high contents of organic matter and clay and commonly preserved algae and pyrite. Source rocks with occasionally developed lamination contain inorganic minerals, organic matter, and clay laminae, with sparse organic laminae and organic carbon contents of 2%-5%. The abundance and types of laminations in source rocks vary with depositional environments. Given that freshwater lake basins are mainly sourced by continental sediments, dark thin laminae of clay and organic matter are well developed, with a space of 0.01-0.05 mm between two laminae. Salinized lake basins are dominated by autochthonous chemical deposition. For example, the shale successions in the Lucaogou Formation of the Jimusar Sag and the Cretaceous of the Jiuquan Basin developed micritic or dolomitic laminae. Laminae are less abundant than those in the freshwater lake basin, with a space of 0.5-2.0 mm between two laminae. Shale in brackish lake basins, such as the Qingshankou Formation in the Songliao Basin, also has well-developed clay laminae and minor laminae of shell fragments. The laminae are medium in abundance, with a space of 0.2-2.0 mm between two laminae, which falls between those in saline and freshwater lakes. In short, the content of organic carbon is high in the source rock with a high abundance of organic laminae, indicating a relatively quiet reductive sedimentary environment.

Shale oil in North America is mainly preserved in marine fine sandstone, dolomitic siltstone and dolomite with relatively monotonous mineral composition and a porosity greater than 6% ^[15]. However, shale oil in China is mainly formed in mudstone, argillaceous siltstone, silt-fine sandstone, dolomitic mudstone, clastic dolomite, and dolomite. These rocks are characterized by complex mineral composition, fine laminae, tight structure, low porosity, variable pore-throats, and strong heterogeneity in physical properties (Table 1).

Reservoir rocks in lake basins in China are mainly derived from two sources: continental source and authigenic source (including volcanic source) (Table 3), which are characterized by mudstone, transitional rocks, and tight oil-related rocks, providing various shale oil accumulation systems.

Continental shale oil reservoirs are mainly developed in large-scaled depressional lake basins. The rapid subsidence and multi-stage tectonic activities on the edge of the basin form deltaic and semi-deep lake depositional systems, which benefit the formation of large shale oil reservoirs. The reservoir rocks are composed of shallow-semi deep lacustrine shale and mudstone, pre-deltaic argillaceous siltstone or silty mudstone, siltstone of delta front, represented by the Chang-7 Member in the Ordos Basin and the Qing-1 Member in the Songliao Basin (Table 3). These reservoirs are mainly siltstone, argillaceous siltstone, and shale, which can be divided into two types, interbedded sandy mudstone, and sandstone with fine mud laminae. In contrast, autochthonous shale oil reservoirs (including volcanic sediments) are mostly in the brackish-saline lake of rift basins. The semi-deep to deep water setting allows for the deposition of limestone and dolomite in lake platform and peperite, tuff, and turbidite in deep water (Fig. 5). Felsic and dolomitic peperite is mainly deposited in the Lucaogou Formation of the Junggar Basin and the Santanghu Basin and the Kongdian Formation of the Paleogene Bohai Bay Basin. Carbonate reservoirs are mainly developed in the Paleogene Shahejie Formation and Kongdian Formation in the depressional area of the Bohai Bay Basin.

Continental shale oil reservoirs are mostly developed in delta front to the semi-deep lake, with a sufficient supply of terrigenous clasts. Source rocks are composed of detrital flow, gravity flow, and underwater distributary sand that have good reservoir physical properties. For example, detrital sandstone in the Chang-7 Member of the Longdong area in the Ordos Basin has an average porosity of 8.50%, an average matrix permeability of 0.10×10^-3 μm², and pore-throat of 0.07-0.32 μm in diameter. The Chang-7 Member siltstone reservoirs of delta front facies in the Shanbei area have relative good reservoir quality, with permeability generally of 0.08×10^-3 μm², matrix porosity of 5%-8%, pore-throat diameters of 0.08-0.50 μm, and high movable fluid saturation ratio (25%-50%). Argillaceous the siltstone and silty mudstone developed in pre-delta and shallow lakes have poor reservoir quality, with permeability less than 0.03×10^-3μm², matrix porosity less than 5%, median pore radius less than 0.05 μm, and a movable fluid saturation ratio less than 25%. The mudstone deposited in semi-deep to a deep lake in the Longdong area has a permeability of mainly 0.003×10^-3 μm² and matrix porosity of 0.50%- 4.00%. Among the above source rocks, black shale and dark mudstone have mode pore radius of 88 nm and 150 nm, respectively^[8]. Deep lacustrine deposits of the Cretaceous Qingshankou Formation in the Yingtai area of the Songliao Basin contain various types of sedimentary lithofacies. Black shale and mudstone are interbedded with gravity flow deposits, turbidite, and bottom flow. A single series of gravity flow deposits occupy 0.2-15.0 km², with an accumulative area of 161 km². Qing-1 Member in Well Ying 47, oil is composed of gravity flow deposits, of which sandstone is 6.6 m thick, with oil production of 5.34 t/d in testing, showing good exploration potential.

Autochthonous shale oil reservoirs (including volcanic sediments) are developed in carbon rocks in gentle slope and underwater uplift areas. Diamictite of the Lucaogou Formation in the Junggar Basin and the Santanghu Basin are deposited in the slope area of the semi-deep lake, with a porosity range of 4%-14%, a permeability range of (0.01-1.00)×10^-3 μm², and a throat radius range of 0.01-1.00 μm, indicating relatively high reservoir quality. Diamictite of the Kong-2 Member in Cangdong Sag is deposited in the semi-deep-deep lake area, with carbonate mineral contents of 10.0%-58.0% (32.6% on average) and a porosity range from 2% to 5%. Nano-scaled intercrystalline pores, organic pores, and microfractures are well developed, with pore-throat diameters ranging from 450 to 1500 nm ^[33], showing excellent reservoir quality.

The continental basins in China have experienced multi-stage and multi-cycle complex tectonic evolution ^[18]. The mixing of terrigenous, autochthonous, and volcanic sources results in complex source-reservoir combinations that are rich in liquid oil and various unconverted organic matter.

Besides the in-situ retained oil in shale, there are two types of source-reservoir combinations, based on source- reservoir contact relations. The first type is the source-reservoir-in-one accumulation with unclear source-reservoir boundaries. Source rocks and reservoir rocks are frequently interbedded, showing similar hydrocarbon generation and reservoir capacity. This type of shale oil accumulation is mainly composed of shale and interbedded argillaceous dolomite and tight carbonate rock, marked by complex lithology, thin bed, well-developed laminae, and frequent interbedding. Most rocks have considerable hydrocarbon generation and accumulation capacities, which allow the formation of multiple sweet spots. In general, shale has high organic matter abundance and strong hydrocarbon generation capacity, which determines the extension of the shale oil reservoir. Argillaceous dolomite, tight carbonate rock, and siltstone have better reservoir performance and determine the abundance of shale oil sweet spots. This type of shale oil is mainly represented by the Lucaogou Formation in the Jimusar Sag and the Kongdian Formation in the Bohai Bay Basin. 21 sub-layers have been identified from a 400m thick high resistivity fine-grained deposition in the Kong-2 Member of the Cangdong Sag in the Bohai Bay Basin. Dolomite, fine- grained felsic sedimentary rock, and fine-grained hybrid sedimentary rock are frequently interbedded. This section has 7 sweet spot sections, along with vertical depiction, featured by reservoir-source-in-one and entirely oil- bearing ^[5]. The second type is shale oil accumulation is where source rocks are adjacent to the reservoir rocks with clear division. Fine sandstone and carbonates without hydrocarbon generation potential are interbedded with massive shale. The vertical superposition of source rocks and reservoir rocks is the key shale oil accumulation. For example, shale and siltstone are vertically interbedded in Chang-7₁ Sub-member and Chang-7₂ Sub-member of the Ordos Basin, the Qing-1 Member in the Songliao Basin, and the Lower Neogene in the Qaidam Basin.

Conventional petroleum geology mainly focuses on hydrocarbon migration from source rocks to reservoirs after the hydrocarbon reaches the oil or gas window. However, hydrocarbon migration within the source rocks has rarely been addressed. The source rock of continental medium-high maturity shale oil has undergone significant hydrocarbon expulsion. Hydrocarbon is mainly expulsed from organic-rich shale. Organic-poor shale, siltstone, and diamictite mainly receive hydrocarbons migrated from source rocks and show large vertical heterogeneity. In addition, the hydrocarbon expulsion efficiency of source rocks is affected by the sedimentary environments, textures, and development of laminae. The amount of hydrocarbon generated, expelled, and retained in the process of source rock maturation can be evaluated by the mass balance principle ^{[34⇓⇓⇓-38]}. Based on the equation in the reference [39], the average hydrocarbon expulsion efficiency of the Chang-7 Member shale from typical wells in the Ordos Basin is 34% (Fig. 6). The average hydrocarbon expulsion efficiency of salinized lacustrine source rock (Lucaogou Formation in the Santanghu Basin) is even higher at about 40%-50%. The classical calculation method of hydrocarbon expulsion efficiency generally regards the source rock as a whole and measures the degree of hydrocarbon expulsion of the whole source rock ^[40]. This work calculated the hydrocarbon expulsion of the samples to surrounding rocks in each selected depth to understand expulsion during hydrocarbon migration. This research method is more effective for highly heterogeneous source rocks. The mudstone samples with a higher silt/sand ratio and lower TOC do not expel hydrocarbons but act as reservoirs for hydrocarbon storage, so they have negative hydrocarbon expulsion efficiency. The reservoir successions are generally sections with high physical properties and sand contents, which are similar to conventional reservoirs. The sections with stronger hydrocarbon expulsion often have higher TOC and more laminae. The higher the TOC value, low the clay mineral content and the higher the hydrocarbon expulsion efficiency of the source rock will be.

Continental medium-high maturity source rock not only discharges hydrocarbons to the tight reservoirs but also can be a reservoir itself. Shale with high organic matter abundance is the key for migration and accumulation of hydrocarbons inside the source rock. Argillaceous silt layers, siltstone, argillaceous dolomite, fine sandstone, and dolomitic siltstone with low organic abundance can be sweet spots. Chang-7₃ Sub-member in Well G135 of the Ordos Basin is a source-reservoir-in-one shale oil system, mainly composed of shale with high organic matter abundance and TOC of up to 17%. Chang-7₁ Sub-member and Chang-7₂ Sub-member consist of shale intercalated with siltstone and argillaceous siltstone, representing interbedded source-reservoir shale oil accumulations. According to the stratum chromatogram effect ^[41], saturated and aromatic hydrocarbons have higher mobility in the source rock than polar components. Hence, polar components in retained hydrocarbons are highest in the Chang-7₃ Sub-member have the highest polar components and the lowest in the Chang-7₁ Sub- member and Chang-7₂ Sub-member. Saturated hydrocarbon and aromatic hydrocarbon contents are highest in Chang- 7₁ Sub-member and Chang-7₂ Sub-member and lowest in Chang-7₃ Sub-member. This opposite distribution pattern of retained hydrocarbon indicates that the hydrocarbons generated from organic-rich Chang-7₃ Sub-member have been expelled and migrated to the Chang-7₁ Sub-member and Chang-7₂ Sub-member with lower organic contents. Moreover, the Chang-7₃ Sub- member has an average organic carbon content of 8.32% and an average free hydrocarbon content of 3.81 mg/g. The Chang-7₁ Sub-member and Chang-7₂ Sub-member have average organic matter contents of 2.62% and 4.61%, and average free hydrocarbon contents of 2.21 mg/g and 3.19 mg/g, respectively. The organic-rich Chang-7₃ Sub- member has free hydrocarbon content equal to or even lower than Chang-7₁ Sub-member and Chang-7₂ Sub-member with lower organic contents, suggesting that enriched organic matter in the Chang-7₃ Sub-member is the main source for hydrocarbon expulsion. Argillaceous silt layers, siltstone, argillaceous dolomite, fine sandstone, and dolomitic siltstone with lower organic content are sweet spots.

Jarvie suggested that the oil saturation index (OSI) can be used as the evaluation index to preliminarily screen out potential sweet spots of shale oil. When the OSI value exceeds 100 mg/g, the retained hydrocarbons in shale may get rid of the absorption to hydrocarbons from the complex network structure of organic matter and thus can be effectively exploited ^[42]. The OSI value of mudstone in 1740-1755 m and 1780-1795 m intervals exceeds 100 mg/g, indicating that argillaceous siltstone laminae, siltstone, argillaceous dolomite, fine sandstone, and dolomitic siltstone with low organic matter abundance have higher hydrocarbon mobility and better development potential. Thus, it can be taken as the main target layers of exploration.

As mentioned above, from the perspective of source-reservoir configuration, the continental shale oil in China includes two types: source-reservoir-in-one (pure mudstone and diamictite) and closely contacted source-reservoir systems. For the former type, hydrocarbon is retained in the source rocks. For the latter, its resource includes two parts: the liquid hydrocarbons retained in the source rock and the liquid hydrocarbons accumulating in the silt/sand layers adjacent to the source rock after short-distanced migration. Due to the stratum chromatogram effect, the liquid hydrocarbons after short distance migration is often better in quality, and have more efficient development potential for middle-high maturity shale oil.

The higher the efficiency of liquid hydrocarbons expulsion from source rock to the nearby reservoir, the more conducive it is to the enrichment of shale oil. The above research shows that the better the laminae develop in source rock, the higher the efficiency of hydrocarbon generation and migration is. The shale with well-developed horizontal laminae and enriched organic matter often has higher hydrocarbon generation potential than massive source rock (Table 2). Moreover, organic laminae can act as an effective pathway for the migration and accumulation of liquid hydrocarbons. Due to special mineral composition and sedimentary texture, laminar fine-grained rocks often have several types of reservoir spaces. In addition, the lamina is usually the interface of abrupt mineral composition variation and sedimentary transformation, and the weak surface in rock mechanics. Therefore, the development of laminae is an important factor for the compressibility of shale and controls the fracture propagation mode in the process of shale fracturing ^[43]. Therefore, the organic-rich shale with rich laminae is the ideal target for shale oil exploration^[44].

The maturity of the source rock is another important factor controlling the enrichment of shale oil. When the R_o value is 0.8%-1.1%, the amount of retained hydrocarbons in shale is larger. Most of the shale oil plays that have been put into large-scale commercial development in the United States are medium-high maturity stage in the oil window. The main part of Bakken Formation shale in the Williston Basin has R_o values between 0.6% and 1.1%, and multiple sets of source rocks in the Permian Basin are in the oil window at present. For the continental shale oil in China, the source rocks in the salinized lake basins have the largest hydrocarbon index (up to 557 mg/g) at the R_o value of 0.9%, while source rocks in freshwater lake basin have the largest hydrocarbon index (201 mg/g) at the R_o value of 0.8%.

For medium-high maturity shale oil, the source rock with high organic matter abundance and a large amount of retained hydrocarbons is conducive to the development of sweet spot sections inside the source rock (Fig. 7). At the TOC of more than 2.5%, freshwater 0.8%-1.1% source rock has a high content of retained hydrocarbons, which is favorable for the development of a sweet spot inside the source rock. The TOC value between 0.5% and 2.5% takes second place. Among the saline lake basin source rocks, tuffaceous mudstone/micritic dolomite with TOC values of 2%-10% has a high content of retained hydrocarbons and are favorable zones of sweet spots.

Continental shale oil accumulations mostly occur in the center of lake basins and their surrounding areas, and mainly in fine-grained sediments (such as shale, siltstone, dolomite, carbonate rock, and tuff) or transition lithologies. Among them, shale, argillaceous dolomite, and other source rocks with stronger hydrocarbon generation capacity, and siltstone, carbonate rock, and tuff with better reservoir capacity combine into favorable source-reservoir systems. Hydrocarbon migration and accumulation are not only controlled by source-reservoir characteristics but also affected by source-reservoir configuration.

From the microstructure of source rock, the organic-rich source rock has more laminae and interbeds of organic matter and inorganic mineral particles, resulting in significant permeability anisotropy. The laminar silty mudstone of Chang-7₃ Sub-member in the Ordos Basin has a horizontal permeability range of (0.000 090-0.000 250)×10^-3 μm², and a vertical permeability range of (0.000 05-0.000 10)×10^-3 μm² under different effective stresses (Fig. 8a), starting pressure gradient along with bedding of about 2.16 MPa/cm, and starting pressure gradient perpendicular to the bedding of as high as 8.74 MPa/cm (Fig. 8b). Therefore, liquid hydrocarbons generated by organic-rich source rocks are more likely to migrate along the horizontal direction of diagenetic laminae and beddings. Local high-angle faults or coarser-grained rock layers can act as pathways of vertical hydrocarbon migration. From the perspective of macroscopic lithofacies of source and reservoir rocks, hydrocarbons migrate along with horizontal laminae of shale and accumulate in silt-fine sandstone, carbonate rock, tuff, and other lithologies with better reservoir physical properties. The complexity of different source-reservoir combinations makes the reservoir-forming mechanism and enrichment pattern different, which reflects the characteristics of migration and accumulation mechanism of shale oil within and near-source rock.

From the distribution of the continental shale oil resources in China, large-scale inland depression and rift lake basins are the main body of shale oil resources. Hereinto, the Ordos Basin, the Songliao Basin, the Junggar Basin, the Bohai Bay Basin, etc. are rich in shale oil resources and contain a large amount of retained liquid hydrocarbons, thus they are important exploration objects of medium-high maturity shale oil in China in the future ^[45⇓-47].

Physical simulation experiments of hydrocarbon generation, expulsion, and retention of continental shale show that at the thermal evolution maturity of greater than 1.0%, a large amount of convertible organic matter in shale is transformed into oil and gas, the proportion of retained oil in shale is higher at about 20%-40%; unconverted organic matter accounts for 20%-40%; and retained hydrocarbons exist in organic micropores, fractures or dissolution micropores. With a high gas-oil ratio and good mobility, the retained hydrocarbons in this stage are practical resources for shale oil exploration and development.

The exploration and development of medium-high maturity shale oil in the Ordos, Junggar, Bohai Bay, and other basins have achieved encouraging results, with proved reserves of 12 655×10⁴ t submitted. By adopting volume fracturing of horizontal well technology in development, newly production capacity of 216×10⁴ t has been constructed. In these basins, shale layers of transitional lithologies interbed with carbonate rock, mudstone, siltstone, tuff, and diamictite, multiple sets of sweet spot sections turn up, reservoirs have good physical properties, high oil saturation, and high enrichment degree of retained hydrocarbons. Dolomitic mudstone in Kong-2 Member of the Cangdong Sag, Bohai Bay Basin has oil resources of 8.24×10⁸ t and 15 wells have obtained industrial oil flows. The Lucaogou Formation in the Jimusar Sag, Junggar Basin has well-controlled shale oil resources of 11.12×10⁸ t and proved oil reserves of 2546×10⁴ t, showing bright exploration prospects, and will become a key area for large-scale shale oil production.

The continental medium-high maturity shale oil in China refers to the oil resources in source rocks and tight reservoirs in the continental shale successions, in which the source rocks have entered the oil generation window and generated large amounts of liquid hydrocarbons, with corresponding vitrinite reflectance of greater than 0.7% (mostly >0.9%).

The heterogeneity of geological conditions of China's continental medium-high maturity shale oil appears as differential development and distribution of source rocks and reservoirs in sedimentary successions. The sedimentary water environment of source rocks can be divided into freshwater, saline water, and brackish water. The source rocks developed in the freshwater environment are mostly fine-grained, with high contents of clay minerals and organic matter. The source rocks developed in the saline environment are mostly diamictite, with high contents of dolomite and carbonate minerals. In addition, source rocks have various structures, lithofacies, and hydrocarbon generation potential. Reservoirs are divided into two types: terrigenous source and authigenic source ones. There are three types of effective reservoirs: argillaceous rock, transition lithology, and tight reservoir. The physical properties and source-reservoir assemblages are also diverse.

The heterogeneity of source rock controls the differentiation in hydrocarbon generation and expulsion. The diverse reservoir types make reservoir quality different and source-reservoir configurations complex, finally resulting in differences in hydrocarbon enrichment patterns. Hydrocarbon generation and expulsion capacity of high- quality source rocks affect the accumulation of shale oil. Freshwater source rocks with TOC>2.5% and high retained hydrocarbon contents, indicating sweet spots inside the source rock. Salinized source rocks have TOC between 2% and 10%, composed of tuffaceous mudstone and micritic dolomite, with high retained hydrocarbon contents, indicating sweet spot sections inside the source rock.

China has reported the proven medium-high maturity shale oil reserves of 12 655×10⁴ t, indicating broad exploration prospects. Transitional lithologies are represented by interbedded shale, carbonate rock, mudstone, siltstone, tuff, and diamictite. Shale oil reservoirs developed multiple sets of sweet spots, good physical properties, high oil saturation ratio, and high enrichment of retained hydrocarbon. Shale oil exploration has made progress in several basins, such as the Kong-2 Member in the Cangdong Sag and the Lucaogou Formation in the Jimusar Sag, which will be the hot spot in shale oil exploration in the future.

HI—hydrogen index, mg/g;

OSI—oil saturation index, mg/g;

S₁—free hydrocarbon content, mg/g;

S₂—pyrolytic hydrocarbon content, mg/g;

T_max—highest pyrolysis temperature, °C.