Exploration and development practices in China and abroad show that the amount of shale oil resources inside oil source is much greater than the amount of conventional oil resources outside oil source. Exploration target transferring from outside oil source to inside oil source is inevitable for the sustainable development of the petroleum industry. Shale oil is an important field for the scaled growth of future oil and gas reserves and production ^{[1,2,3,4,5,6,7,8]}. Different scholars have different understandings on the definition and connotation of shale oil, especially in the definition and distinction of tight oil ^{[9,10,11,12,13]}. In fact, the concepts of shale oil and tight oil are not clearly distinguished and defined abroad. The two concepts are used in a mixed-up way in published literatures, academic conferences and authoritative energy reports, and they are handled as unconventional oil resources, with less attention being paid to the difference between the two terms. Currently in China, there are basically two understandings about the definition of shale oil: (1) shale oil in a narrow sense, which refers to the oil resources existing in organic-rich shale and sandwiched thin interlayers of carbonate rocks and sandstones (generally less than 3 m thick); (2) shale oil in a broad sense, which includes shale oil in a narrow sense and tight oil ^{[9, 11]}.

Shale oil refers to the oil existing in organic-rich shale series ^[14]. The thickness of a single layer of siltstone, fine sandstone and carbonate rock in the source rock of organic-rich shale series is no more than 5 m, and the cumulative thickness accounts for less than 30% of the total thickness of shale series. It usually refers to those resources with no natural production capacity or with a natural production capacity lower than the lower limit of industrial oil production, or industrial oil production required special process and technical measures. In the exploration and development practices in China, based on the maturity, shale oil is usually classified into two categories: medium-low maturity and medium-high maturity. According to source reservoir characteristics, medium-high maturity shale oil can be further divided into three sub-categories: source-reservoir integration, source reservoir separation and pure shale oil ^{[4, 5, 11, 15-17]}, or another classification scheme into interlayer, mixed sedimentation and shale types ^[8].

Since the early 1980s, oil and gas exploration in the Yingxiongling area has been based on the idea of structural oil and gas reservoir, mainly conducted in the two structural traps of upper member of the Paleogene Lower Ganchaigou Formation of deep Shizigou (E_2-3xg₂). Wells Shi20, Shi24, Shixin 28, etc. achieved high production with a daily oil production of 200-500 t. Then, 16 deep wells were successively drilled around the main area of Shizigou, showing visible oil and gas. However, limited by the quality of seismic data, trap evaluation was not accurate, and no great progress was made. From the 1990s to the beginning of 21th century, wells Shi35, Shi36, Shaxin 1, etc. were drilled. The wells showed good oil and gas display in the upper member of the Lower Ganchaigou Formation, but the conventional oil test failed to reach the industrial oil and gas flow. Since 2014, based on the three-dimensional seismic research results, the effective source rock distribution in the sedimentary period of the upper member of the Lower Ganchaigou Formation was finely characterized, and the oil and gas areas in the Yingxi and Yingzhong sources had been found one after another. Nine high-yield wells with 1000 t oil and gas production and eight wells with 100 t oil and gas production were obtained. The submitted proven oil geological reserves in 2019 reached 2899×10⁴ t ^[18,19]. Since 2021, based on the research on the formation mechanism of oil and gas intra-source and systematic coring evaluation, it has been determined for the first time that typical shale oil reservoir has developed in the upper member of the Lower Ganchaigou Formation in the Yingxiongling area. Based on the three-dimensional seismic continuous interpretation results of the Yingxiongling, the Ganchaigou area with moderate burial depth and more stable structure than Yingxi-Yingzhong area was selected and explored. Thus, a strategic breakthrough in shale oil exploration in the basin was achieved. Exploration of horizontal wells was conducted, obtaining 100 cubic meters of high-yield oil flow per day.

Theoretically, since the source rocks have quite low overall organic matter abundance, according to the parameter standards of shale oil reservoir selection evaluation and "sweet spot" evaluation by most scholars ^[1,11,16-17], it can be inferred that Chaixi Depression should have no potential of shale oil exploration, which is obviously not consistent with the exploration practice. This contradiction indicates that Yingxiongling shale oil reservoir has unique value for both theoretical research and exploration and development. Combining the data analysis of lithologic assemblage, source rock characteristics, hydrocarbon generation potential, reservoir property, source-reservoir assemblage, oil-gas potential, reservoir distribution and pressure distribution of the upper member of Lower Ganchaigou Formation, according to national standards, this paper defines that Yingxiongling shale oil reservoir is distributed in the Yingxiongling area of Qaidam Basin and occurs in the source rock series of the upper member of Lower Ganchaigou Formation, with shale and mixed types of shale oil reservoirs, characterized by high-frequency interaction between organic-rich laminar shale and calcareous dolomite. Industrial oil flow can be obtained by vertical well fracturing, and high yield can be obtained by horizontal well volumetric fracturing.

The research team focuses on the shale and mixed types of shale oil reservoirs in the Yingyingling area of Qaidam Basin, which are characterized by the high frequency of organic-rich laminated shale and limy dolomite. We established evaluation standards for shale oil in the Yingxiongling area and evaluated its resource potential using seismic, laboratory data analysis and comprehensive research, in order to provide a basis for evaluation and exploration of shale oil in the upper part of the Paleogene Xiaganchaigou Formation in the Yingxiongling area.

Qaidam Basin, located in the north of Qinghai Tibet Plateau, is a large continental Mesozoic-Cenozoic intermountain petroliferous basin in Western China ^[20,21]. Affected by plateau uplift and peripheral strike slip orogeny, its shape is an irregular diamond wide in the west and narrow in the east. According to the difference of tectonic deformation and geophysical characteristics in different areas of the basin, it is divided into three first-order structural units including Chaixi depression, Chaibei fault depression and Sanhu depression (Fig. 1). Since the Mesozoic-Cenozoic, Qaidam Basin has experienced multi-stage tectonic movements. The two tectonic movements of late Yanshanian and late Himalayan had the biggest impact on the basin. The regional tectonic evolution has experienced three major evolution stages: Paleocene-Eocene rifting, Oligocene-Miocene weak compression and Pliocene-Quaternary strong compression.

At present, folds and thrust faults developed in the structural belt of the Qaidam Basin, and the plane structural deformation is strong in the west and south, but weak in the east and north. The Qaidam depression generally presents a structural pattern of alternating uplift and depression. The main body of the Yingxiongling area is located in the west part of the Mangya Depression, which is a typical late-stage uplift (Fig. 1). This area was the sedimentary center of the Chaixi Depression in the Paleogene. At the end of the Neogene, it was reversed and uplifted into a mountain under the influence of the late Himalayan tectonic movement. Under the influence of differential tectonic uplift, Yingxiongling sank from the front of Arkin mountain to the abdomen of the basin.

The sedimentary strata cover mainly the Cenozoic sedimentary sequences in the Yingxiongling area. Current drilling reveals four sets of strata vertically from the bottom to the top (Fig. 2), including: the Paleocene-Eocene Lulehe Formation (E₁₊₂) variegated glutenites interbedded with mudstones; Lower Ganchaigou Formation (E_2-3xg) of the Eocene-Oligocene. The lower part of its lower member (E_2-3xg₁) is mainly composed of brownish mudstone, dolomite mudstone and argillaceous siltstone, whereas the upper part is composed of argillaceous dolomite and dolomite mudstone; and the upper member (E_2-3xg₂) dark organic-rich shale is mainly developed in the lower part, and calcareous dolomite and limy mudstone are superimposed in the upper part with multiple sets of salt rock layers; the Oligocene-Miocene Upper Ganchaigou Formation (E₃-N₁sg) consisted mainly of brownish yellow sandstone and mudstone, mixed with gray and dark gray carbonate rocks; Miocene Lower Youshashan Formation (N₁xy) developed mainly brownish yellow and gray siltstone and mudstone.

In general, the Cenozoic Qaidam Basin experienced a paleoclimate evolution process from drought to humidity and then to drought. Vertically, it can be divided into five sequences (Fig. 2). Controlled by paleoclimate change, E₁₊₂ and E_2-3xg₁ in the Lower Paleogene are mainly sandstone and conglomerate deposits formed in dry and hot environment; the climate in the upper member of the Lower Ganchaigou Formation turned wet in the early stage of the deposition ^[22], developing the largest lake transgression in the Qaidam Basin since the Cenozoic. The relatively warm and humid climate in this period facilitated the massive reproduction of organisms and laid a material foundation for the enrichment and preservation of organic matter. At the same time, the Yingxiongling area was the sedimentary center during this period, providing good conditions for large-scale stable deposition of shale (Fig. 3). In the late deposition of the upper member of the Lower Ganchaigou Formation, the lake basin shrank rapidly in the strong evaporation environment due to the dry and cold climate, forming vertically superimposed and horizontally continuous evaporite (salt rock) layers. It is considered as favorable regional caprocks, which provides an important condition for the enrichment and preservation of high-pressure shale oil in the area.

In recent years, according to the shale oil exploration idea and in-depth research and evaluation of source rocks, the exploration was carried out in the Ganchaigou and Chaishen, two areas with the most developed high-quality salt rock caprocks in the Yingxiongling area, and remarkable results were achieved in shale oil exploration. In the Ganchaigou area, a total of 12 vertical wells and 6 oil testing wells were deployed for the shale oil of IV-VI oil-bearing formation in the lower part of the upper member of Lower Ganchaigou Formation, and industrial oil flow has been obtained in 9 layers (Table 1). Among them, Well Chai 902 has a daily oil production of 32.53 t, gas production of 2582 m³, a daily oil production from a testing well of 10-12 t and a cumulative oil production of 2113.7 t, cumulative gas production 78.2×10⁴ m³ in 170 d at 2800-2803 m (IV oil layer). This proves that vertical wells for shale oil exploration in the Ganchaigou area also have good stable production capacity. At the same time, in order to explore the efficient production mode of shale oil, the “horizontal well + volume fracturing” test was carried out in the Ganchaigou area. Well Chaiping 1 was drilled for oil layer IV, with a completed drilling depth of 3924.33 m and a horizontal section length of 997.33 m. It was divided into 21 sections and 124 clusters for fracturing. After fracturing, the well was soaked for 16 d, and the oil could be seen immediately after opening. Different working systems were adopted for blowout output. Consequently, the production began to increase gradually. At present, the flowback rate is only 4.9%. The daily oil production using 4 mm nozzle is 103.97 m³, with the gas production of 15 025 m³, the gas oil ratio of 139.65 m³/m³, and the cumulative oil production in 39 d of 2240.44 m³, gas production of 262 915 m³. In order to further explore the shale oil potential in the saline lacustrine sedimentary center, Well Shi303 was drilled 20 km away from Chaishen area in the hinterland of Yingxiongling. The pressure coefficient of the 5336-5350 m well section is as high as 2.48, and the daily oil production of conventional perforation with 4 mm nozzle exceeds 100 m³ (daily oil production of 227.4 t and gas production of 6.6×10⁴ m³).

According to a large number of drilling and analysis data in the Yingxiongling area, it is considered that Yingxiongling shale oil has the following six characteristics.

The upper member of Lower Ganchaigou Formation (E_2-3xg₂) is the best source rock in the Chaixi Depression, with an effective source rock distribution area of nearly 3650 km². The average organic carbon content is 0.91%; chloroform asphalt "A" is concentrated between 0.05% and 1.00%; R_o is 0.6%-1.3%; and the organic matter type is mainly type I-II₁. Through TOC and hydrocarbon generation potential of the low mature source rock in different basins, it is found that the TOC value of source rocks in the upper member of the Lower Ganchaigou Formation in Chaixi Depression is low, but the hydrocarbon generation potential and the hydrogen index (HI) are higher compared with other basins (Figs. 4 and 5). Elemental analysis shows that the H/C atomic ratio of the source rocks is much higher than that of other types, and they are mainly consisted of Type I organic matter. Although the TOC value of the upper member of Lower Ganchaigou Formation in the Yingxiongling area is generally low (mostly less than 1%), the organic matter is rich in hydrogen. Therefore, the hydrocarbon generation potential of per unit organic carbon is large, and the organic carbon conversion rate is as high as 80% ^[23]. For example, the hydrocarbon generation potential of per unit organic carbon in the source rock with TOC value of 1.0% in the upper member of the Lower Ganchaigou Formation is equivalent to that of the source rock with TOC value in the range of 3%-5% in the fresh water-brackish water depositional environment (Fig. 5). The above characteristics of the source rock in the upper member of Lower Ganchaigou Formation in the Chaixi Depression have an obvious particularity of a salty lake, which is the material basis for the Yingxiongling shale oil to be different from other shale oils.

Generally, the soluble organic matter in the shale of saline lacustrine basin is easier to be preserved and can generate hydrocarbon on a large scale in the low maturity stage. It is estimated that the contribution of the soluble organic matter to the yield of liquid hydrocarbon is as high as 60%. The source rocks in the Yingxiongling area, which was deposited in the saline lacustrine basin, are characterized with a "two-stage" oil generation model, that is, the early low mature hydrocarbon generation of soluble organic matter and the late mature hydrocarbon generation of insoluble organic matter in the upper member of the Lower Ganchaigou Formation. The source rock was "early generation and early drainage", forming shallow immature-low mature reservoir; kerogen generates a large amount of oil in the later stage, which is the main stage of oil generation and accumulates in the deep. Therefore, under the joint action of early hydrocarbon generation of the soluble organic matter and kerogen degradation, saline lacustrine source rocks have very high hydrocarbon conversion rate and hydrocarbon generation intensity, which changed the original understanding and greatly expanded the exploration field. Thermal simulation shows that the R_O value of saline lacustrine source rocks in the upper member of the Lower Ganchaigou Formation in the Yingxiongling area is 0.6%-1.3%, which is dominated by oil generation, and the maximum oil generation quantity is 350 mg/g, which lays a material foundation for shale oil enrichment in the Chaixi area ^[24].

There are three types of oil and gas reservoirs developed in the upper member of the Lower Ganchaigou in the Yingxiongling area: limy shale, argillaceous shale and clastic rock ^[25]. They are characterized by various types of reservoir space and good reservoir performance. The reservoir space is dominated by inorganic pores, mainly including intergranular pores, intergranular/intercrystalline dissolution pores, breccia pores (holes) and reticular fractures. It includes intergranular pores, mold pore, and bedding fractures as well (Fig. 6). Dissolution vugs and fractures above millimeter level are developed in the high porosity and permeability reservoirs; fractures generally do not develop or be filled with sediments in the low porosity and permeability reservoirs, and dolomite intergranular pores and micron level dissolution pores are developed.

Cluster development of nano intercrystalline pores, micron sized dissolved pores, bedding fractures and breccia fractures (holes) can ensure that the shale has high and stable production capacity. The intercrystalline pores of dolomites are the major type of reservoir space of the Yingxiongling shale oil. It formed mainly because the volume of carbonate rocks shrinks as a result of ion exchange in the lattice during the quasi syngenetic dolomitization process, forming a large number of nano and micron sized intergranular pores of dolomite. The dissolution pores are affected by fluid exchange during diagenesis, acid expulsion during hydrocarbon generation and sulfate thermal reduction reaction during deep burial period, and they are only locally developed. Inter breccia pores (holes) are of structural origin and mainly develop along a fault fracture zone and interlayer crumple area. They are often associated with dissolution pores and structural fracture network. The size of pores and vugs is large and the radius is mostly 1-4 mm ^[26]. Cracks are generally developed in stress concentration areas, especially soft (salt rock) and hard (carbonate rock) rock interbedding. Fracture network caused by brittleness difference is often compounded with matrix pores.

Compared with shale oil reservoirs in other continental basins, the reservoir space types of the Yingxiongling shale oil are more diversified, with pore sizes in the millimeter, micron and nanometer scale, and better reservoir capacity. The core analysis porosity is 3.1%-11.5%, and the average value is 5.1%. Core analysis permeability is (0.05-0.62)×10^-3 μm², with the average permeability of 0.4×10^-3 μm². The samples with permeability higher than 1 ×10^-3 μm² account for 4.6%. In addition, widely developed bedding fractures provide good oil and gas migration channels and reservoir space. Scanning electron microscope analysis shows that the diameter of dolomite intergranular pore is about 150 nm. Mercury injection and digital core test show that the throat radius is fine (mostly less than 100 nm), which belongs to typical small hole-fine throat type.

The semi-deep-deep lacustrine shale lamina in the upper member of the Lower Ganchaigou Formation of the Paleogene in the Yingxiongling area is a typical light- dark interactive seasonal lamina. The lamina is stable and continuous, and the rhythm is mainly the high-frequency interaction between pure carbonate rocks and dark organic-rich laminae, in which the pores in the carbonate rocks are relatively well developed ^[27,28]. The thickness of thin carbonate rock is generally less than 1 m. It can be divided into two types of combinations: mixed sedimentation and shale, forming two types of shale oil. In the upper member of the Lower Ganchaigou Formation in the Yingxi area, the thickness of shale series is 1000-2000 m, with many longitudinal oil-bearing sections. According to the statistical data of Well Chai 2-4, it is found that the thickness of single lamina is mainly 100-200 μm, and the density is 4000-4500 laminae/m. The total thickness of oil-bearing layers is more than 1000 m. For example, the coring section of the upper member of the Ganchaigou Formation in Well Chai 2-4 is 50.3 m, and the lithology is the shale composed of limy dolomite lamina and organic-rich lamina, with an oil- bearing core thickness of 27.7 m. The highest level of oil- bearing is saturated with oil. The limy dolomitic laminae (with a thickness of 100-200 μm) and thin-layer calcareous dolomite (with a thickness mostly less than 1 m) are in high oil-bearing grade with the oil saturation of core analysis of 46%-88%, among which, the oil saturation of limy dolomitic laminae is the highest. The 3D laser confocal analysis shows that there are a large number of liquid hydrocarbons in shale, mainly came from in-situ retention and micro migration, and are generally distributed in layers (Fig. 7). To sum up, the Yingxiongling shale oil is characterized by well-developed laminae, thin single cycle, large cumulative thickness and high oil-bearing grade.

The Late Paleogene of the Yingxiongling area was deposited in a salt lake sedimentary system. The top of the upper member of the Lower Ganchaigou Formation is a salt rock. This set of salt rock is widely developed, with a single layer thickness of 1-10 m and a cumulative thickness of 200-300 m, which makes it a good caprock in this area. The sealing of the salt rock makes the upper member of the Lower Ganchaigou Formation form a self-sealing system. The superior caprock conditions resulted in the generally developed abnormal high pressure in the region. The pressure coefficient reaches 1.7-2.4 (Fig. 8). The abnormal high pressure in the formation indicates that the formation energy is sufficient to ensure the long-term flowing production of shale oil, which is the power basis for the stable and high yield of the Yingxiongling shale oil. For example, the conventional oil test in Well Shixin 58 in the 5502-5514 m section shows a daily oil output of 205 t, a daily gas production of 70 229 m³ and an oil pressure of 27 MPa with a 4 mm nozzle; after 2.5 years of production, the cumulative oil and gas production has been 10.6×10⁴ t and 4478×10⁴ m³, respectively. At present, the 5.5 mm nozzle is used for a stable production, with a daily oil output of 130 t and a daily gas output of 4.7×10⁴ m³, oil pressure of 25 MPa.

Under the uplift of Qinghai Tibet Plateau, the Cenozoic sedimentation rate in the Chaixi Depression is large. The Yingxiongling area experienced deep burial in the Early Paleogene and formed large-scale high mature oil and gas. The crude oil has high gas oil ratio (40-300 m³/m³), light oil quality (density of 0.78-0.85 g/cm³) and good fluidity (viscosity of 4.86 mPa•s), and the content of naphthenic hydrocarbon is relatively high (Fig. 9).

According to the geochemical characteristics of Yingxi- Yingzhong crude oil, the maturity of crude oil is low in the west and high in the east, showing a consistent trend with the maturity of source rock, and the retention and accumulation characteristics in shale oil source are obvious. There is a positive correlation between crude oil maturity and gas oil ratio. As the burial depth increases, the the maturity of the crude oil is higher, and the density and viscosity is lower ^[29]. The shale oil in the hinterland of the Yingxiongling area obviously has the characteristics of high maturity and light crude oil color.

The Yingxiongling area is located in the center of the Chaixi Depression. Multiple stages of shales in semi-deep to lake-deep lake facies have developed in the upper member of the Paleogene Lower Ganchaigou Formation, forming two types of shale: laminar type and mixed type. The analysis of mineral composition of the whole rock reveals that the upper member of the Lower Ganchaigou Formation has obvious lithologic mixing characteristics. The rock types include limy/dolomitic shale, argillaceous shale and clastic rock, mainly limy/dolomitic shale. The content of brittle minerals is high. The main mineral components are silt-sized quartz, feldspar, calcite and dolomite (60%-90%), and the contents of clay and plastic minerals are generally low (10%-40%). Core scanning logging combined with X-ray diffraction analysis shows that the mineral components of the upper member of the Lower Ganchaigou Formation include clay, quartz, feldspar, calcite, dolomite, gypsum and other minerals, with a carbonate content of 40%-60%. The whole rock mineral composition analysis is carried out for the upper member of the Lower Ganchaigou Formation in Well Chai 2-4 in the Ganchaigou area. The rocks are mainly consisted of carbonate minerals, showing obvious characteristics of mixed sedimentation (Fig. 10).

The unique plasticity and brittleness of rock mechanical properties are determined by mineral compositions and structures ^[30]. Generally, shale with a low clay mineral content and a high brittle mineral content tends to produce fractures more easily and has strong fracture making ability, which is good for large-scale volume fracturing. Jarvie et al. concluded that the higher the content of quartz and calcium is, the greater the brittleness is ^[31]. The content of carbonate and other brittle minerals in the Yingxiongling shale is generally higher than that of the Cretaceous Qingshankou Formation shale in Songliao Basin, Triassic Chang 7 shale in Ordos Basin and Paleogene Shahejie Formation shale in Bohai Bay Basin ^[17]. Therefore, high brittle mineral content leads to the good fracturability of the Yingxiongling shale oil, which is good for large-scale fracturing.

The shale oil in the upper member of the Lower Ganchaigou Formation in the Yingxiongling area has the "unique" characteristics of low TOC value, thin single layer thickness of sweet spot, high gas oil ratio, oil saturation and formation pressure. The current standard system is not adaptive to the evaluation of this kind of shale oil and it is difficult to evaluate objectively. Therefore, it is urgent to establish a new index system.

At present, the evaluation of "sweet spot section/area" of shale oil generally focuses on the matching relationship evaluation among the characteristics of source rock, lithology, physical properties, brittleness, oil and gas bearing property and stress anisotropy ^{[1, 4-5, 11, 17, 32]}. The TOC value of favorable shale series in North America is greater than 4%, which is laminated shale or marl, with the porosity greater than 7%, brittle mineral content greater than 50%, and oil saturation of 50%-80%. When the relative density (ρ_r) is less than 0.825 and pressure coefficient is greater than 1.30, natural fractures are developed ^{[10, 33]}. Favorable shale series in China are characterized with TOC value greater than 2% (S₁ greater than 2 mg/g), lithologies of laminar shale, tight sandstone or tight carbonate rock, high porosity, brittle mineral content greater than 40%, oil saturation of 60%-90%, low crude oil viscosity or high formation pressure, and natural fractures ^[2].

Recently, it is found that the enrichment of retained movable hydrocarbons in lacustrine shale is controlled by five factors: shale organic carbon content (TOC), organic matter thermal evolution (R_o), diagenetic evolution, natural fracture development degree and the ratio of lake basin size to source input distance (B/A value). Each factor has generally moderate indicators, that is, TOC value of 2%-4%, R_o value of 0.7%-1.0% (buried depth of 3200- 4300 m). Diagenetic evolution stage is in middle diagenetic stage A. Natural fractures are developed, but it did not damage the top/bottom caprock formations of shale oil. B/A value is 40%-60%. It is not conducive for shale oil enrichment when the indicators are too high or too low ^[34]. The R_o value of shale controls not only the oil-gas type and gas/oil ratio, but also the shale oil "sweet spot" ^{[31, 35]}. Therefore, R_o is undoubtedly an important parameter for sweet spot evaluation. In addition, the TOC values of the continental shale needs to be objectively analyzed according to the difference of hydrocarbon generation capacity of organic carbon in the freshwater or saline lake basin, which should be treated differently according to the water environment of lake basin ^[23]. In addition, at present, the international community mainly carries out prestack seismic data analysis based on the constraints of logging and rock measured data to obtain the key parameters of rock properties (acoustic impedance, elastic modulus and Poisson's ratio, etc.). Based on this, the “sweet spot” was optimized and the single well production was predicted relying on basin simulation technology and seismic data, combined with the lithology, porosity, total organic carbon, mineral composition and existing production data.

Different from other basins where the shale series is a stable monocycle deposition, the Paleogene shale deposition in Qaidam was affected by the high-frequency oscillation of lake water and changed frequently in vertical and horizontal directions, showing the characteristics of "thin, many and miscellaneous". In addition, the Yingxiongling area is different from the typical shale oil in other areas of China in terms of sedimentary environment, source rock characteristics, sweet spot reservoir and mobility (Table 2). Although Jarvie et al. proposed that oil saturation index (OSI) greater than 100 mg/g can be regarded as sweet spot section ^[31], for shale type, shale oil recoverability may be poor and productivity decreases rapidly due to low carbonate content, high TOC value and low permeability. Therefore, when applying specifically the evaluation index system above, the indexes should be selected by classification and the evaluation system should be constructed on the premise of determining the type of shale oil reservoir.

Referring to the single well evaluation, the star map of relevant parameters is usually used to predict shale oil risk. For block prediction, the idea of hierarchical evaluation of shale oil development potential can be obtained by overlay superposition or weighted sum of the important parameters of target layers ^[36]. Combined with the latest parameters used in North America for evaluating the oil-bearing grade and exploitability of shale ^[37], according to the key parameters of oil enrichment and engineering transformation parameters of the Yingxiongling shale, eight evaluation parameters including organic carbon content, effective porosity, oil saturation, brittle mineral content, thermal evolution degree of organic matter, pressure coefficient, lamellation density, and burial depth are proposed, and the evaluation criteria are given (Table 3). Therein, three parameters including effective porosity, oil saturation and brittle mineral content are used in the “sweet spot section” evaluation of a single well vertically. The “sweet spot” on the plane is controlled by total organic carbon content, effective porosity, brittle mineral content, oil saturation and thermal evolution degree of the organic matter. Brittle mineral content not only controls lithology, but also has a positive correlation with reservoir properties and oil content, which is an important reference index. The fracturability index is determined according to the brittleness index and stress difference index, and the standard for fracturability identification and classification is established through the effect of oil test and production test.

From the Middle Eocene to the Early Miocene, the sedimentary center of the Chaixi Depression migrated from the west to the east. The effective source rocks distributed throughout the region, with an area of 3650 km² and a maximum thickness of nearly 2000 m (Fig. 11). The saline lacustrine source rocks have great hydrocarbon generation potential. The chloroform bitumen "A" of this set of shale is 0.05%-1.00%. The organic matter type is mainly Type I-II₁, and the effective source rock thickness accounts for 51%. Compared with the source rocks in other continental basins, although the TOC value in the Chaixi Depression is mostly less than 1%, it has a high hydrogen index. In addition, the saline environment is good for the preservation of soluble organic matter. Large-scale hydrocarbon can be generated in the low maturity stage and the contribution of liquid hydrocarbon yield of the soluble organic matter is as high as 60%. It is characterized by high hydrocarbon conversion rate, high intensity of hydrocarbon generation, and great retained resources potential in the source. With the increase of burial depth, temperature and pressure of this shale from the west to the east, the thermal evolution degree of organic matter increases greatly.

Currently, the well-controlled shale oil area in the Ganchaigou area is 42 km², and the estimated geological reserves are over 3×10⁸ t, with an abundance of up to 880×10⁴ t/km². Based on the analysis of drilling results in the Yingxi-Yingzhong and Chaishen areas, the favorable exploration area of the Yingxiongling shale oil with a buried depth of less than 5500 m outside the well-controlled area is nearly 800 km² (Fig. 12). According to the parameters (Table 4) including the thickness and distribution area of the source rock, the vertical distribution of sweet spot and the content of free hydrocarbon (S₁) in the upper member of the Lower Ganchaigou Formation, the shale oil resources in the Yingxiongling area are estimated as 21×10⁸ t using volume calculation method, showing that the shale oil in this area is a realistic field of a large-scale reserve increase and effective production construction in the "14th Five-Year Plan" of the Qaidam Basin.

The Chaixi Depression was an intact lake basin in the Paleogene, including three sub-depressions, namely the Yingxiongling Depression, Xiaoliangshan Depression and Zhahaquan Depression. Their tectonic and sedimentary backgrounds were similar, and the Paleogene saline lacustrine effective source rocks were developed in all of them. The study shows that the Paleogene in the Chaixi Depression has the conditions for large-scale development of shale oil. According to the free hydrocarbon content, TOC value and the source rock area and thickness, it is preliminarily estimated that the shale oil resources with the buried depth less than 6000 m in the Chaixi Depression are about 44.5×10⁸ t.

4.2.1. Vertical distribution of sweet spot section

The thickness of shale in the Yingxiongling area is 1000-2000 m, with good oil bearing property vertically, long well section span, large cumulative thickness of oil layer and buried depth of 2500-5500 m. According to the sedimentary cycle characteristics and combined with the height of reservoir reconstruction fractures (50 m) and the sweet spot distribution, the 1200 m thick shale section of IV-VI oil formation in the upper member of the Lower Ganchaigou Formation is divided into 23 boxes vertically (Fig. 13), and the thickness of each box is 42-55 m. The boxes have good comparability horizontally, and the shale formation is stably developed. According to the core analysis, oil test and production test data, it is preliminarily considered that two shale oil sweet spot sections are developed in IV, V, VI oil formations in the upper member of the Lower Ganchaigou Formation in the Yingxiongling area, covering 10 boxes. The oil test was carried out on 8 boxes, and the industrial oil and gas flow was obtained. After the vertical well fracturing, the daily oil production was 12.7-44.95 m³, and the long-term stable production can be achieved. According to the grading evaluation standard of the Yingxiongling shale oil sweet spot, the sweet spot section is divided into three categories. Type I has the thickness of 150-280 m, accounting for an average of 18% of the total; the thickness of Type II is 250-300 m, accounting for 23% on average. The pressure coefficient is 1.73-2.16, which is the basis for high and stable production of shale oil in the area.

4.2.2. Horizontal distribution of favorable areas

The drilling revealed that the sweet spot section between the Yingxiongling shale oil wells has good comparability and stable horizontal distribution. At present, the well-controlled shale oil area in the Ganchaigou area has been proved as 42 km². According to the regional drilling data and in-depth study of sedimentary and hydrocarbon generation characteristics, it is found that two types of favorable carbonate reservoirs, mainly limy dolomite in the shore-shallow lacustrine basin and laminar limestone in the semi-deep lacustrine basin, are widely distributed in the whole region. According to the distribution area of the high-quality mature source rocks (TOC>0.8%) in the saline lacustrine facies in the Chaixi Depression, combined with source-reservoir configuration combination, structural evolution, buried depth (less than 5500 m) and drilling effect, it is clear that the stable structural area is the main exploration direction of shale oil, and the most realistic favorable exploration area of the Yingxiongling shale oil is 800 km² recently (Fig. 12). However, due to the influence of late tectonic movement, the shale oil plane heterogeneity and continuity are the further research focus.

The Yingxiongling shale oil developed in the saline environment of the upper member of the Paleogene Lower Ganchaigou Formation. It is mainly the source-reservoir integration type, which is composed of mixed sedimentation and shale type. It has six geological characteristics: large amounts of retained hydrocarbon, good reservoir property, large thickness of sweet spot section, high formation pressure coefficient, multiple light components of crude oil and high content of brittle minerals. Compared with shale oil in other regions in China and abroad, it has particular geological characteristics and is favorable for exploration and development.

Referring to the evaluation standards of shale oil sweet spot in China and abroad, the key parameter indexes for the evaluation of the Yingxiongling shale oil sweet spot are proposed, and an evaluation system based on 8 evaluation parameters is established, including organic carbon content, effective porosity, oil saturation, brittle mineral content, thermal evolution degree of organic matter, pressure coefficient, lamellation density and burial depth. By applying the volume calculation method, the shale oil resources in the Yingxiongling area at buried depth less than 5500 m are estimated amount to 21×10⁸ t. It is preliminarily estimated that the shale oil resources with buried depth less than 6000 m in the Chaixi Depression are about 44.5×10⁸ t.

The thickness of the Yingxiongling shale is 1000-2000 m, with good oil-bearing property and large cumulative thickness of oil layers. It can be divided into 23 boxes vertically, and the thickness of each box is 42-55 m. It is clear that the stable structural area is the main exploration direction of shale oil, and the most realistic favorable exploration area in the near future is 800 km².

HI—hydrogen index, mg/g;

S₁—free hydrocarbon content in the rock, mg/g;

T_max—maximum peak temperature of rock pyrolysis, °C.