Shale oil resources are abundant in China, such as in the Ordos Basin, Junggar Basin, Bohai Bay Basin and Songliao Basin, and the recoverable resources are 145×10⁸ t according to the evaluation by PetroChina in 2016^[1,2,3,4]. However, shale oil in China is mainly continental resource, and is characterized by strong reservoir heterogeneity, unstable thickness, unobvious abnormal pressure, heavy oil quality and low gas/oil ratio when compared with marine shale oil in North America^[5,6]. Shale oil exploration in China lags behind that in other places of the world and faces with difficult development problems. After years of research and exploration, great breakthroughs in shale oil exploration and development have been made in the Middle Permian Lucaogou Formation in the Junggar Basin, the Member 7 of the Triassic Yanchang Formation (Chang 7 Member) in the Ordos Basin, and the Permian Tiaohu Formation in the Santanghu Basin^[1], etc. Shale oil is expected to be an important strategic replacement resource in China^{[1, 7-8]}.

The Ordos Basin is the second largest sedimentary basin in China and the largest oil and gas production base in China, where oil and gas resources are low-permeability tight oil and gas. Annual hydrocarbon production in the Basin is more than 7000×10⁴ t (oil equivalent), among which 5000×10⁴ t is produced by the PetroChina Changqing Oilfield Company for seven years in a row, and has been increasing yearly. Oil produced by the company is 2416×10⁴ t in 2019 (102×10⁴ t oil from Class I shale oil reservoirs of Chang 7 Member), and gas 412.3×10⁸ m³, with an oil equivalent of 5701×10⁴ t.

The shale oil of the Chang 7 Member in the Ordos Basin are mainly self-sourced, and oil accumulated in the sandstones and argillaceous sandstones within source rock series in adsorption and free states, with no large-scale long-distance migration^[8,9]. Its main characteristics are as follows: coexistence of source rock and reservoir layers (reservoirs developed between source rock layers), continuous or quasi-continuous distribution of hydrocarbon, no obvious oil-bearing boundary, no obvious oil-water boundary, and no edge and bottom water^{[6, 10]}. Reservoirs are mainly lithologic traps, formed by differences in physical properties of reservoir rocks. Industrial oil output cannot be produced from this reservoir without reservoir reconstruction. Though it is able to be produced from vertical wells after fracturing, the production is not stable. Because hydrocarbon resources from this reservoir are developed by volume fracturing in horizontal wells^[1], they are termed as typical unconventional oil and gas resources. Preliminary evaluation of the geologic resources of Class I shale oil in Chang 7 Member is (40-60)×10⁸ t, and the prospective resources of Class II shale oil (30-40)×10⁸ t^[11].

The Changqing Oilfield Company reported 3.58×10⁸ t of newly increased proved initial oil in-place and 6.93×10⁸ t of possible oil in-place of Class I shale oil in source rock series of the Chang 7 Member in the Qingcheng region in the southwestern Yishaan Slope of the Ordos Basin in 2019, and discovered Qingcheng Oilfield with self-sourced unconventional shale oil of 10×10⁸ t. Large-scale horizontal well production testings have been carried out, promoting the economic development of the Qingcheng Oilfield.

This paper analyzes basic characteristics of the self-sourced reservoirs in the Chang 7 Member, and discusses the main controlling factors of hydrocarbon accumulation. And then the discovery of the Qingcheng Oilfield and the breakthrough of risk exploration in CY horizontal well group are showed as examples to discuss the exploration practice of different types of self-sourced reservoirs. This paper provides guidance for the exploration and development of unconventional continental oil and gas resources.

The Ordos Basin is located at the tectonic joint of the eastern and western domains in China. It was a part of the Great North China Basin during the Paleozoic period. The Indosinian Movement during the Late Triassic caused the northern margin of the Yangtze Plate to collide with the North China plate, and the large-scale inland depression lake basin in Ordos was developed under the coupling effects of basin and mountain. The Basin is divided into six secondary tectonic units (termed as Western Margin Thrust Belt, Tianhuan Depression, Yishaan Slope, Jinxi Flexural Fold, Yimeng Uplift, and Weibei Uplift) (Fig. 1a) according to the present structural morphology and evolution history of the basin.

The Triassic Yanchang Formation is a set of inland fluvial-delta-lacustrine clastic rock series (Fig. 1b), and is divided into the Chang 1 to Chang 10 members from top to bottom^[12,13]. During the depositional period of the Chang 7 Member, tectonic regions around the basin were active. Compressed by a strong force in southwest direction, a vertical uplift happened in northeast direction, and the basin experienced a rapid imbalanced depression process, resulting in a lake basin basement steep in the south and gentle in the north^[14,15]. This period was also the time when the lake basin most expanded. The water was deep and wide, with a semi-deep to deep lake region of 6.5×10⁴ km² (Fig. 1a). Source rock series with a thickness higher than 100 m developed in this area. The rocks are mainly dark mudstone and black shale, and laid a foundation for oil generation in the Mesozoic continental lake basin. The Chang 7 Member is mainly composed of argillaceous rocks, and with less than 20% of sand and formation ratio. From bottom to top, the Chang 7 Member can be further divided into three sub-members (Chang 7₃, Chang 7₂, Chang 7₁), and they are mainly semi-deep to deep lake sub-facies (Fig. 2). After the largest lake transgression period represented by Zhangjiatan shale in the Chang 7₃ sub-member, the lake basin shrank during the depositional periods of the Chang 7₂ and Chang 7₁ sub-members (Fig. 3). Due to the river influx and gravity flow sedimentation, a set of sedimentary sand bodies dominated by sandy clastic flow was deposited, where oil and gas were most accumulated^[16,17]. Widely distributed shale and silty-fine sandstone are in close contact or interbedded, forming a good combination of source rock and reservoir. Thus, oil and gas produced from the source rock can be charged into the reservoirs nearby at high pressure. The exploration potential is great^[18,19].

Commercial production layers of shale oil in Chang 7 Member in the Ordos Basin are mainly silty-fine and argillaceous sandstone layers interbedded between shale layers. Various types of self-sourced reservoirs with different sand development degrees have been formed under different sedimentation and hydrocarbon supply conditions. They are various in rock characteristics, reservoir physical properties, oil-bearing property, engineering mechanical properties and crude oil properties.

Water of the lake basin was deep and wide during the depositional period of the Chang 7 Member, and sediment filling rate was slow, resulting in argillaceous sedimentary rocks with a low sand ratio and single sand body thickness. According to the statistical data of more than 2000 wells in the basin, the average sand ratio of Chang 7 Member is 17.8%, and wells with a sand ratio lower than 30% account for 75.3%. Thickness of sand bodies have an average thickness of 3.5 m each, while 44.9% of the sand bodies are less than 2 m, 25.7% 2-5 m, and 29.4% higher than 5 m. Oil resources from source rock in the Chang 7 Member is typical shale oil according to the definition of shale oil in the national standard of shale oil

geological evaluation methods^[20]. Water level falls from the Chang 7₃ to Chang 7₁ periods, resulting in a decrease in accommodation space for sedimentary matters, and the gravity flow sedimentary sand bodies gradually advanced to the middle of the lake basin, where thickness of sand body and sand ratio increased. In the Chang 7₃ sub-member, wells with a sand ratio of 72.9% is lower than 10%, of 10%-30% is 18.0%, and that higher than 30% is 9.1%. In Chang 7₂, wells with a sand ratio of less than 10% accounts for 41.8%, 10%-30% is 28.1%, and that higher than 30% account for 30.1%. In Chang 7₁, wells with a sand ratio lower than 10% accounts for 32.7%, that between 10%-30% is 32.5%, and that higher than 30% accounts for 34.8%.

Shale oil in the Chang 7 Member are divided into three types according to lithologic association, sand ratio, and thickness of sand body^[11], and they are multiphase superimposed sandstone type (Class I), thick shale interbedded with thin silty-fine sandstone type (Class II) and pure shale type (Class III). Class I shale oil reservoirs are relatively high in sand amount with a ratio of 20%-30%, and the maximum thickness of single sand body is generally less than 5 m. They are mainly distributed in the zones of underwater distributary channels of delta front and gravity flow sedimentary areas of semi-deep to deep lake sandy debris flow in the Chang 7₁ and Chang 7₂ sub-members (Fig. 2a, 2b). This type of shale oil has been developed into commercial flow in large scale. By contrast, Class II shale oil reservoirs are relatively low in sand amount with, sand ratio of 10%-20%. Thickness of single sand body of this type is 2-4 m in general. They are mainly distributed in semi-deep to deep lake shale sedimentary zones of the Chang 7₃ sub-member (Fig. 2c). The first risk exploration well, CY horizontal well group, has made major breakthrough in this type. Class III shale oil reservoirs are mainly composed of organic-rich shale, with a sand ratio generally less than 10%, and the maximum thickness of single sand body is less than 2 m. They are mainly distributed in the deep lake shale sedimentary zone of the Chang 7₃ sub-member (Fig. 2c). At present, in-situ heating conversion technology is being used to recover oil from these reservoirs.

2.3.1. Source rocks

Mesozoic source rocks in the basin are mainly black shale and dark mudstone in the Chang 7 Member. They are different in sedimentary structure, organic geochemical hydrocarbon generation indexes and well logging response characteristics^[21]. The black shale is abundant in organic laminae, mainly type II₁ and type I organic matters; TOC ranges from 6% to 16% (13.81% on average); the amount of chloroform asphalt "A" ranges from 0.41% to 1.51% (0.78% on average), and S₁ ranges from 1.49 to 8.90 mg/g (4.02 mg/g on average). The black shale is characterized by abnormally high gamma ray value (>180 API), abnormally high resistivity (induction resistivity>50 Ω∙m), abnormally low density (<2.4 g/cm³) etc. Although dark mudstone contains less organic matter than black shale does, it is still treated as high-quality source rock in a continental basin with massive beddings and organic matters of type II₁ and type II₂. The amount of TOC ranges from 2% to 6% (3.74% on average), and chloroform asphalt "A" from 0.20%-1.17% (on average 0.65%), and its S₁ values from 0.51 to 4.34 mg/g (on average 2.11 mg/g). The dark mudstone is characterized by relatively high gamma ray (120-160 API), high resistivity (induction resistivity between 40-80 Ω∙m), and low density (2.4-2.5 g/cm³).

2.3.2. Petrological characteristics

The fine-grained sediments of Chang 7 Member are composed of five rock types: fine sandstone, siltstone, black shale, dark mudstone and tuff (Table 1), of which shale is the most abundant interbedded with thin layers of fine-grained sandstone. Reservoir rocks of the self-sourced reservoir are mainly sandstone and shale, while siltstone and shale are observed to be saturated with oil under core observation (Fig. 4).

Reservoirs of Class I shale oil are mainly composed of gray massive fine sandstone with sand body types of delta front underwater distributary channel, slope break zone sandy clastic flow and turbidite sediments. The gravity flow sedimentary sandstone usually contains 60%-70% of quartz and feldspar, with feldspar in dominance, and about 20% of clay mineral. Reservoir rocks of Class II shale oil are mainly thick shale with thin silty-fine sandstone interbedded with turbidite sediments, which was sedimented mainly by turbidite. The amount of quartz and feldspar in this reservoir is lower than 70% and relatively higher feldspar. The amount of clay mineral is 20%-30%. Class III shale oil reservoirs are mainly organic-rich massive dark mudstone and laminar black shale with semi-deep to deep lake facies, and are widely developed in the Chang 7₃ sub-member particularly. It is high in TOC (generally >2%), low in quartz and feldspar (about 30%), and clay higher than 60%.

2.3.3. Pore types

Pores in the self-sourced reservoirs of Chang 7 Member are intergranular pore, dissolution pore and clay mineral inter- crystal pore^[22]. Intergranular pores and dissolution pores, as well as micro and nano intercrystalline pores are developed in fine sandstone and siltstone reservoirs (Fig. 5a, 5b). This paper focuses on pore characteristics of the Class I shale oil reservoirs. According to more than 350 slice samples of the Qingcheng Oilfield, pore types are feldspar dissolution pores (0.65%), intergranular pores (0.27%), debris dissolution pores (0.10%), intergranular dissolution pores (0.07%), intercrystalline pores (0.01%) and microcracks (0.01%), with a total face porosity of 1.11%; feldspar dissolution pore and intergranular pore are the main types observed in those samples. There is less analysis about pore characteristics of the Class II shale oil reservoirs with more than 20 samples analyzed. Pore types in this reservoir are mainly feldspar dissolution pores (0.64%), intergranular pores (0.14%), and debris dissolution pores (0.06%) with face porosity of 0.89% and feldspar dissolution pore and intergranular pore are most observed. Rock type in Class III shale oil reservoirs is mainly shale, in which clay intercrystalline pore developed the most with few intergranular pores (Fig. 5c, 5d). Though small in pore-throat size, intercrystalline pores are high in number in shale, this enables the shale to have a certain storage capacity. Thermal evolution degree in the Chang 7 Member is low, so organic pores transformed from organic matters are rare. Pores in the tuff reservoirs are mainly dissolution pores (Fig. 5e), with strong heterogeneity and great differences in porosity and permeability among areas.

desorption gas test of organic-rich shale shows that gas content of shale reservoirs is about 1-3 m³/t, indicating a good gas-bearing property.

Intergranular pores are mainly developed in fine sandstone in Class I shale oil reservoirs. Small amounts of clay minerals are found at grain surfaces. Pore radius are 2-8 μm, porosity of 8%-1%, and permeability of lower than 0.3×10^-3 μm². Chlorite film is developed at the surface of intergranular pores of silty-fine sandstone in Class II shale oil reservoirs. Intercrystalline pores are developed in clay minerals, with radius of 1-5 μm, porosity of 6%-8%, and permeability of less than (0.01-0.10)×10^-3 μm². Clay intercrystal pores are developed in mudstone and shale in Class III shale oil reservoirs, of which pore radius in the dark mudstone is 40-110 nm, and in the black shale 30-100 nm (Fig. 6). Different types of reservoirs are various in their characteristics. Effective communication between micro-pores and nano-throats is the key to reservoir stimulation.

2.3.4. Fluids

PVT tests showed that temperature of oil-bearing layers in the Chang 7 Member is 61.0-66.2 °C, original formation pressure 14.3-16.0 MPa, saturation pressure 7.40-8.85 MPa, and formation-saturation pressure differences 5.45-8.60 MPa, indicating that these layers are unsaturated reservoirs. Densities of Class I and Class II crude shale oil in-situ are 0.73-0.78 g/cm³, viscosity 1.36-1.47 mPa·s, original gas-oil ratios 90-110 m³/t, and their volume coefficient 1.2. Density of crude oil on surface is 0.83 g/cm³, viscosities 3.72-3.89 mPa·s, initial boiling point 64 °C, and its freezing point is 16 °C. Clearly, the crude oil is of good quality: high gas-oil ratio, low density, low viscosity, low freezing point and no sulfur. Formation water type is CaCl₂, with its pH of 6.1, and total salinity of 44.8-53.2 g/L. Class III crude shale oil (pure shale type) produced from 13 wells have proved to show commercial oil flow with good oil quality. Density of the oil on surface is 0.83 g/cm³, viscosity 4.25-6.18 mPa·s, initial boiling point 70 °C, and its freezing point is 18-21 °C. Sealed core

2.3.5. Fractures

Natural fractures are well developed in the Chang 7 Member, including both macroscopic large-medium size fractures and microscopic small fractures. High-angle fractures which cut through both sandstone and shale strata are commonly seen in outcrop sections, and conjugate joints form on rock surface. Fractures are also commonly seen in drilling cores of sandstone and shale. Most of them are high conductive frac-tures, among which some are filled or half-unfilled (Fig. 7). Production practice proves that natural fractures in the Chang 7 reservoirs are an important factor for oil to accumulate to form "sweet spot". Besides, natural fractures help to form complex fracture network system for oil commercial development when volume fracturing is conducted. Statistical data of 379 tectonic fractures from southeast outcrop section of the basin shows that fractures with apertures of 0-1.5 mm account for 79.4%, and fracture apertures are generally small. 73.2% of fractures are not filled while 5.8% are half-unfilled, and 21.0% filled. Apparently, fracture effectiveness is good when most fractures are unfilled, providing important channels for primary migration of oil and gas.

Imaging logging of the horizontal section in the Chang 7₃ sub-member in Well CY 1 shows that fractures strike toward NEE-SWW, dip to NNW and SSE, and dip angles of the high conductive fractures range from 74°-90° (Fig. 7). Fractures occur in both sandstone and shale while most of the fractures occur in sandstone. High-angle fractures are mainly distributed in layers with less amount of clay and higher amount of quartz. As natural fractures occur in Chang 7 reservoirs and stress differences between two horizontal directions are moderate, complex fracture networks can be formed after artificial large-scale volume fracturing to improve seepage capacity of the self-sourced reservoirs. A large number of high-angle natural fractures and micro fractures have developed in the Chang 7 reservoirs of Qingcheng Oilfield, and these fractures connect superimposed sandstone layers improving percolation capacity of the reservoirs effectively. Most production wells in the self-sourced reservoirs are high in production.

Array acoustic logging and triaxial stress test show that brittleness index of Chang 7 sandstone ranges from 40% to 60% and brittleness varies between different blocks. Brittleness index of Chang 7 reservoirs in Qingcheng Oilfield averages 55%, and stress difference between two horizontal directions is 4-7 MPa, that between reservoirs and restraining barriers 5-8 MPa. These properties show that the reservoir is favorable for fracturing.

Hydrocarbon accumulation conditions of self-sourced reservoirs in Chang 7 Member in Ordos Basin are good, with favorable source rocks and good configuration of interbedded source rocks and reservoirs.

Large amounts of organic matter were gathered and preserved in the Chang 7 Member, forming a set of organic-rich black shale and dark mudstone, which are the most important high-quality source rocks in the Mesozoic. High productivity, anoxic preservation conditions, and low compensation rate of terrigenous debris are the key factors for accumulation of organic matter in the Chang 7 Member. Microscopic laminae and organic-rich phosphate nodules are commonly seen in shale in the Chang 7 Member, indicating a high primary productivity during its deposition time. Element geochemistry of source rocks reveals that some biological nutrient elements (P₂O₅, Fe, V, Cu, Mo, Mn, etc.) are obviously enriched in the organic-rich shale in the Chang 7 Member, and they are in good positive correlation with abundance of organic matter in the source rock (Fig. 8). This correlation indicates that the rich nutrients in water were the key factor for organism blooming and high oil productivity. Geological events, eutrophic lake basin and organic-rich shale are in spatiotemporal coupling, indicating that during the depositional period of Chang 7 Member in the lake basin, volcanic and earthquake events and hydrothermal activities happened frequently. Frequent geological event triggered high biological productivity and a large-scale of organic matter development. Anoxic environment is favorable for the preservation of organic matter^[23]. The organic-rich shale in the Chang 7 Member contains much globular pyrite and S^2-, indicating that the environment of bottom water and sediment surface is anoxic. The higher the anoxic degree, the higher the enrichment degree of organic matter is. The organic-rich shale in the Chang 7 Member is low in clay mineral (<40%), Al₂O₃ (averages 13.01%), SiO₂ (average 49.29%), and total rare earth (average 187×10^-6). These indexes are negatively correlated with the abundance of organic matter, reflecting that a low recharge rate of terrigenous debris facilitated the accumulation of organic matter.

According to data from more than 2000 wells in the Chang 7 Member in the basin, thickness of black shale in the middle of the lake basin is 10-35 m, averages about 20 m, and the maximum is about 60 m. The area of black shale is 4.3×10⁴ km². Thickness of dark mudstone is 10-50 m, averaging about 25 m, and the maximum is 120 m. The area of dark mudstone is 6.2×10⁴ km² (Fig. 9). Plane characteristics of both the black shale and dark mudstone are large-area and extensive-distribution, and the two are complementary in lithofacies. Black shale is well developed, the dark mudstone is thin or poorly developed, and vice versa. The widely distributed black shale and dark mudstone are excellent source rocks for large-scale hydrocarbon accumulation inside the source rock of the Chang 7 Member^{[21, 24-25]}.

Thermal simulation experiments of hydrocarbon gen-eration^{[21, 24]} shows that the hydrocarbon generation capacity of organic matters in the Chang 7 Member is very strong. Its hydrocarbon generation potential is about 400 kg/t. The hydrocarbon generation capacity of high-quality source rock samples has a hydrocarbon generation intensity of (400-600) ×10⁴ t/km² generally and 495×10⁴ t/km² on average. Maturity of the samples from Chang 7 source rock is 0.7%-1.1%^[26], and their maximum pyrolysis temperature is 447 °C. These indicate that source rocks have reached their mature stage and oil generation is at its peak point. The high-quality lacustrine source rock at its peak stage of hydrocarbon generation is strong in hydrocarbon expulsion, and its highest hydrocarbon expulsion efficiency is higher than 80%. The widely distributed high- quality source rocks, high-intense hydrocarbon generation, and high-efficient hydrocarbon expulsion altogether result in the accumulation of oil in self-sourced reservoirs.

Under the effect of frequent tectonic events, arenaceous clastic flow and turbidite deposits were developed in multiple stages in the semi-deep to deep lake environments, forming a unique fine-grained sedimentary association of interbedded organic- rich shale and silty-fine sandstone in the Chang 7 Member. The combination appears vertically as a superposition of arenaceous clastic flow, turbidite deposits and multi- stage arenaceous debris flow deposits, and on the plane as overlap of widely expanded sand bodies with a certain thickness^[10]. Drilling data showed that the fine-grained sandstone layers developed between thick organic-rich shale layers are usually oil-bearing, and are the main oil-bearing “sweet spots” in self-sourced reservoirs of the Chang 7 Member. Permeability is an important factor controlling oil enrichment in a sand body. Statistical data of physical properties show that in the Qingcheng Oilfield, only the oil-bearing sand bodies with permeability higher than 0.03×10^-3 μm² is able to produce commercial oil flow after fracturing. Permeability of most of the fine-grained sandstone reservoirs in the oilfield is higher than 0.03×10^-3 μm², making them important places for oil to be accumulated in large scale.

The Qingcheng Oilfield is located in the middle of the lake basin. Controlled by gravity flow sedimentation, multi-cycle interbedded sandstone and shale developed there, and this combination is favorable for oil accumulation. The Chang 7 Member, with overall low sandy contents, has multiple sets of thin sandstone, silty mudstone, argillaceous siltstone and dark mudstone stacked up, with argillaceous rocks taking dominance. The Chang 7₁ and Chang 7₂ sub-members have combination of fine sandstone, siltstone, black shale and dark mudstone, while the Chang 7₃ sub-member has the combination of black shale and dark mudstone interbedded with thin silty-fine sandstone (Fig. 10). Thickness of a single sand body is 2-5 m. Multi-stage sand bodies stack over each other in a wide distribution range with a total thickness of 10-15 m. Most of the sandstone reservoirs developed in the middle-upper Chang 7₁ and Chang 7₂ sub-members, of which the thickness of sand body in the Chang 7₂ sub-member is 5-15 m in total, and that in the Chang 7₁ sub-member is 10-20 m. These are the favorable oil-bearing “sweet spot” sections in the thick shale series (Fig. 10). Gravity flow sedimentary sand bodies are relatively developed in the Qingcheng Oilfield. They are stable in distribution, long in extension and good in continuity, providing favorable reservoir conditions for large-scale shale oil accumulation in the Qingcheng Oilfield. However, the oil layers are thin and unusually interbedded by other layers. Difficult to apply stimulation, this kind of reservoir is low in production.

With various rock types in reservoirs in the Chang 7 Member, fine sandstone and siltstone are the two dominant ones. Slice observations of more than 600 rock samples show that, the rock types of fine-grained sandstone reservoirs are mainly lithic feldspathic sandstone and feldspathic lithic sandstone. The amount of quartz ranges from 20% to 50% (on average 36.2%), and feldspar 10% to 40% (on average 25.3%). The debris types are mainly metamorphic rock debris, accounting for 5% to 25% (average 16.3%). The amount of interstitial materials is about 15%, and hydromica is the dominant type, followed by iron calcite and iron dolomite. Experiencing strong diagenetic processes such as compaction, cementation and clay mineral transformation, pore-throat sizes is small and pore structure is complex in sandstone reservoirs.

The fine-grained sandstone reservoirs were characterized by field emission scanning electron microscope, two-beam electron microscope, micro-nano CT scanning imaging and other testing techniques. Results show that the Chang 7 Member is abundant in micro-nano multi-scale pores with various types and shapes. Quantitative analysis shows that the pores in fine-grained sandstone reservoirs are continuously distributed. Proportions of the macro-pores and meso-pores are low, while those of small pores and micro-pores are relatively higher (Fig. 11a). Pore volume was applied to evaluate its contribution to the reservoir space in fine-grained sandstone reservoirs. It is found that small pores take the largest part of pore volume, followed by large pores, while micro-pores and nano pores take a small part. Normalized statistical results show that pores with radius of 2-8 μm in the fine-grained sandstone reservoirs take 65%-86% of the total pore volume (Fig. 11b). Combining CT imaging with digital core algorithm, the pore-throat network system of the fine-grained sandstone reservoirs was quantitatively characterized. Chang 7 reservoirs are low in pore coordination number. Reservoirs with a coordination number of 2-4 account for 83.1% with an average of 2.5. Sizes of pore-throat in fine-grained sandstone reservoirs in the Chang 7 Member are small. Pore radii are mostly 2-8 μm, and throats radii are mostly 20-150 nm. Though small in single pore volume, small-scale pores are large in number, these pores in the Chang 7 Member enable the self-sourced reservoirs a storage capacity equivalent to that in low-permeability reservoirs.

Comprehensive analysis reveals that micro-pores and nano- throats combine into pore-throat unit systems in independent clusters of the fine-grained sandstone reservoirs in the Chang 7 Member (Fig. 12). Though they are small in a single size, the pores are large in number and thus they obtain a certain level of storage capacity. During exploration practice of self-sourced reservoirs in the Chang 7 Member in the Ordos Basin, fine-grained sandstone reservoirs of Qingcheng shale oil field, whose reserves are 10×10⁸, are characterized by full-size distribution of micro-pores and nano-throats with large number and strong storage capacity.

Simulation results of hydrocarbon accumulation^[26] show that reservoir paleopressure during hydrocarbon accumulation

period was 18-26 MPa, and the pressure difference between source rock and sandstone was 8-16 MPa, providing a strong driven force for primary migration of hydrocarbon inside the reservoir and a short-distance migration from source rock^[22]. Short hydrocarbon migration distance and high differences in charging and accumulation pressure result in a high hydrocarbon filling in the self-sourced reservoirs in the Chang 7 Member. Under continuous high pressure, oil saturation in the reservoirs first increased rapidly and slowed down later. Hydrocarbon accumulation in reservoirs experienced two stages: rapid hydrocarbon accumulation and continuous charging, until finally reached the point where oil saturation was higher than 70%^[27].

Affected by charging force, oil in self-sourced, near-sourced and far-sourced reservoirs differs in microscopic occurrence state. NMR results show that both the macro-pores and micro-pores in self-sourced reservoirs Chang 7 Member are oil-bearing layers, while micro-pores in reservoirs far from source rock do not show oil. Oil saturation in the Chang 7 Member is 70% in general, with a 90% of maximum oil saturation. At the same time, other strata further from source rock are lower in oil saturation with about 50%. Original gas-oil ratio in the Chang 7 Member in Longdong area is 90-120 m³/t, and the measured gas-oil ratio in horizontal wells in the self-sourced reservoirs is 142-736 m³/t, averages 328 m³/t. Gas-oil ratio in the main Mesozoic oil layers in the basin is 40-120 m³/t, indicating a higher gas-oil ratio in the areas close to high-quality source rocks.

Self-sourced reservoirs in the Qingcheng Oilfield show characteristics of source rocks and reservoir rocks developing altogether, high-quality source rock development area matches well with the oil accumulation area. The abnormal residual pressure formed by clay mineral dehydration and hydrocarbon generation provided driving force for continuous oil and gas charging, making up the unfavorable conditions of small pores and throats in ultrafine-grained sandstone in the Chang 7 Member, thus forming extensive and continuous lithologic reservoirs. During the process of hydrocarbon expulsion from source rocks, the fine-grained sandstone reservoirs experienced primary and continuous charging. The distribution of high-quality source rocks controlled the distribution range of self-sourced reservoirs in Chang 7 Member; fine-grained sand bodies controlled the reservoir scale; storage space controlled the oil storage quantity; migration and accumulation power controlled the hydrocarbon charging degree^[26,27]. Effective combinations of these factors resulted in the large-scale hydrocarbon accumulation in Chang 7 Member, Ordos Basin (Fig. 3).

Exploration and early research about self-sourced reservoirs in the Chang 7 Member in the Ordos Basin can be dated back to the 1970s, while the large-scale exploration and development were conducted in the past decade. The giant Qingcheng Oilfield with 10×10⁸ t reserves was discovered in the Class I shale oil reservoirs, and great breakthrough has been made by risk exploration with the horizontal well groups in Class II shale oil reservoirs.

Early exploration and basic geological research on Class I shale oil reservoirs with multi-stage superimposed sand bodies in source rock of the Chang 7 Member by the Changqing Oilfield Company can be traced back to the 1970s. The large scale exploration and development process can be divided into three stages, with years of 2011 and 2017 as hallmarks: hydrocarbon generation evaluation and concurrent exploration stage before 2011, production and efficiency improvement by conduction of exploration and evaluation technologies from 2011-2017, and overall exploration, large-scale horizontal well development, and demonstration area construction from 2018 until now.

In the early 1970s, more than 40 wells encountered oil layers in the Chang 7 Member during the drilling in the Longdong area. Limited by geological understandings and technology at that time, the oil layers were evaluated as no value in oil production. After the 1990s, Chang 7 Member was explored during the time of Chang 8 exploration, when more than 100 wells obtained commercial oil flow during production test, with controlled reserves of 5132×10⁴ t and possible reserves of 6913×10⁴ t in the Chang 7 Member^[22].

From 2011 to 2017, several horizontal well pilot zones (X233, ZH183 and N89, etc.) were established successively in the Longdong area, and 25 horizontal wells were drilled, with a daily oil production average more than 1 million cubic meters in production test. Cumulative oil production in the test area is 45.38×10⁴ t, showing a stable production potential. With the understandings of the characteristics of Class I shale oil reservoirs and the factors controlling “sweet spot” accumulation, further exploration and development tests have been conducted. Guided by the idea of “controlling reservoirs by vertical wells and improving production by horizontal wells”, more vertical exploratory wells and appraisal wells were deployed, and "sweet spot" areas have been confirmed by vertical wells. A total of 248 vertical wells were drilled around the arenaceous “sweet spot” area in Chang 7 shale in Qingcheng area, among which 225 obtained commercial oil flow with a controlling favorable oil-bearing area of 3000 km². This was the hallmark of Class I shale oil exploration in Chang 7 Member. At the same time, long horizontal wells with horizontal section of 1500-2000 m were drilled at well spacing of 400 m, and oil was developed through large scale fracturing. Fracturing stages in horizontal wells increased from 12-14 to 22, and the quantity of fracturing fluid injected in single well increased from 1.2×10⁴ m³ to 2.9×10⁴ m³, volume of sand in a well increased from 1000-1300 m³ to 3500 m³. Initial single well production increased from 8-9 t/d to 17-18 t/d, thus formed the main development technologies.

Since 2018, large-scale horizontal well development has been carried out guided by the goal of "building a national-level development demonstration base, seeking industrialized operation in loess tableland landform, and forming a new framework of intelligent and information-based labor organization and management". The Qingcheng shale oil development demonstration zone has been built, with 154 horizontal wells drilled, and 97 put into production. The productivity of the demonstration zone is 114×10⁴ t at present, and daily oil production is 1003 t/d.

In 2019, the Qingcheng Oilfield, the largest shale oil field in China, was discovered in the self-sourced rock of the Chang 7 Member in the Qingcheng area, Ordos Basin (Fig. 1a), with newly proved and possible geological reserves of 3.58×10⁸ t and 6.93×10⁸ t respectively, and 10.51×10⁸ t in total.

Analogy of the geological conditions between the exploration area and the development area in the oilfield proved that when thickness of an oil layer in a vertical well reaches 4 m, daily oil production of a corresponding horizontal well is about 6.3 t in a favorable oil-bearing area. Comparing the geological conditions of oil layers in the developed area and the area planned to develop, it is believed that applying horizontal well development technology in the area planned to develop could improve the production effectively (Fig. 13). The 10×10⁸ t scale Qingcheng shale oil field is high in resource confirmation degree.

The discovery of the giant Qingcheng Oilfield has confirmed that the Class I shale oil reservoirs have an exploration potential, and are able to be developed economically in large scale by constructing large-scale development demonstration zone. Its preliminarily estimated resources have reached (40-60)×10⁸ t.

The Changqing Oilfield Company researched the self- sourced reservoirs within thick shale interbedded by thin silty-fine sandstone in the Chang 7₃ sub-member in 2019, and examined the geological conditions such as source rock thickness, lithology combination, thermal evolution degree, gas-oil ratio, burial depth, etc., to understand the exploration potential of class Ⅱ shale oil reservoirs. Two horizontal wells (Well CY 1 and Well CY 2) were drilled in Cheng 80 Block in the middle of the lake basin to apply risk exploration and production tests. High-yield oil flows with daily production of 121.28 t/d and 108.38 t/d were produced, respectively^[11], marking a substantial breakthrough in the risk exploration of Class II shale oil reservoirs, which has greatly promoted the exploration process of Class II shale oil reservoirs.

Class II shale oil reservoirs developed mainly in semi-deep to deep lake gravity flow sedimentary environments, with smaller sand bodies. Vertical thickness of a sand body is about of 1-5 m each. They are in isolated lenticular shape in lateral, with an extension length from 25 m to 50 m. The sandstone and mudstone appear alternately in both lateral and longitudinal directions. Fine sandstone and siltstone are the most favorable rock types for Class II shale oil reservoirs, and their porosity ranges from 6% to 12%, permeability less than 0.3×10^-3 μm². Shale is also capable of oil storage, but the storage capability of a single sand body is relatively low. Its porosity is generally less than 2% and permeability less than 0.01×10^-3 μm².

The breakthrough of risk exploration in two horizontal wells (Well CY 1 and Well CY 2) implies that Class II shale oil reservoirs in the Chang 7₃ sub-member, Ordos Basin is promising for large-scale exploration. According to the calculation unit taken from Cheng 80 Block, the preliminarily estimated prospective resources of Class II shale oil reservoirs in the Chang 7₃ sub-member in the basin (about 1.5×10⁴ km²) are up to 33×10⁸ t. With further exploration and improvement of key technologies, Class II of self-sourced reservoirs are expected to become a major replacement of unconven-tional oil and gas exploration in the basin.

Rock type of Class III shale oil reservoirs in the Chang 7 Member is pure shale. In-situ heating conversion might become a revolutionary technology for this type of shale oil reservoirs. The following conditions should be met when applying in-situ heating conversion on organic-rich shale layer: TOC of a shale concentration section is higher than 6%; R_o values from 0.5% to 1.0%; thickness of the shale section is higher than 15 m; burial depth of the shale is shallower than 3000 m; shale area is higher than 50 km²; tight sealing both at the top and bottom of the shale; the amount of formation water lower than 5%^[28,29]. The shale in the Chang 7 Member in the Ordos Basin is moderate in its maturity, high in organic matter abundance, large in its thickness, wide in area and shallow in burial depth. It is the most potential and representative area for in-situ conversion development in China. According to preliminary evaluation, at the oil price of 60-65 USD/bbl, by applying in-situ heating conversion, the recoverable oil and gas resources could be about (400-450)×10⁸ t, and (30-35)×10¹² m³ respectively when oil price stables at 60-65 USD/bbl, representing a large resource scale and bright exploration prospect^[30].

A large-scale Ordos inland depression lake basin was developed during the depositional period of Chang 7 Member at the background of tectonic activity during the middle stage of Indosinian Movement. Area of semi-deep to deep lake is about 6.5×10⁴ km² and water depth of the lake area is 60-120 m. A set of fine-grained sedimentary assemblage deposited in a wide distribution range. Rock type is mainly argillaceous rocks, and the thickness is higher than 100 m.

The fine-grained sediments in the Chang 7 Member are composed of fine sandstone, siltstone, black shale, dark mudstone and tuff, among which argillaceous rocks take the dominance. The average thickness of sandstone layers is about 3.5 m and average sand ratio is 17.8%. Types of organic matter of the dark shale are mainly types II₁ and I, and the average abundance is 13.81%. Types of organic matter of the dark mudstone are mainly types II₁ and II₂, and the average abundance is 3.74%. The black shale and dark mudstone rich in organic matter lay solid material foundation for large-scale hydrocarbon generation and accumulation in the Mesozoic.

The geological conditions of the Chang 7 Member were unique. Source rocks interbedded with reservoirs. Combination of factors leads to a large-scale hydrocarbon accumulation in the Chang 7 Member: The lacustrine black shale and dark mudstone are high in organic matter abundance thus are high in hydrocarbon generation and expulsion capacity, which are the material base for oil accumulation; the arenaceous rocks interbedded between organic-rich shale layers are “sweet spots” of oil enrichment; a large number of micro-pores and nano-throats in fine sandstone and siltstone reservoirs in multi-scales are strong in storage capacity; short-distance hydrocarbon charging under strong driven force in a self-sourced reservoir results in an oil saturation higher than 70% in a reservoir.

Three types of shale oil developed in the Chang 7 Member. The first type is multiphase superposed sandstone (Class I), which is the main develop target. Its thickness is usually less than 5 m and the sand ratio ranges from 20%-30%; the second type is thick shale interbedded with thin silty-fine sandstone (Class II). Thickness of a single sand body of this type is 2-4 m, and the sand ratio ranges from 5%-20%. This is the main target for risk exploration. The third type is pure shale (Class III). This kind of reservoir is mainly composed of thick organic-rich shale, with almost no sandstone basically. It is suitable to be developed by in-situ heating conversion.

The largest shale oil field in China, Qingcheng Oilfield, was discovered in the Chang 7 Member of Ordos Basin. The proved geological reserves increased by 3.58×10⁸ t and possible geological reserves by 6.93×10⁸ t. The discovery and large-scale economic development of the Qingcheng Oilfield has confirmed that the Class I shale oil reservoirs are a very promising hydrocarbon resource. The initial proved resources are about (40-60)×10⁸ t. It has a strategic significance and a demonstration role for the exploration and development of self-resourced oil in China.