China's dependence on imported oil exceeded 70% in 2019. Domestic conventional crude oil production is too low to meet the needs of the economic development of China. It has become an urgent need to find a replacement for conventional oil resources to ensure national energy security. Inspired by the shale oil revolution in the United States, since 2008, major oil field companies in China have begun to explore the potential of shale oil resources step by step. Exploration activities have confirmed that there are several sets of organic-rich shale developed in China's continental basins, including the Cretaceous System in the Songliao Basin, Permian System in the Junggar Basin and Santanghu Basin, Triassic System in the Ordos Basin, Jurassic System in the Sichuan Basin, and Paleogene in fault basins in east China, such as the Bohai Bay and Jianghan basins^{[1,2,3,4,5,6,7,8,9,10]}. Dozens of wells targeting Permian Lucaogou Formation in Jimsar sag, Paleogene Kongdian Formation in Cangdong sag of the Bohai Bay Basin, and Upper Triassic Yanchang Formation in the Ordos Basin have obtained industrial oil flow, foretelling the great exploration potential of shale oil in China.

To facilitate technical research, researchers in China classified the continental shale oil reservoirs into three types according to lithologic combination and sand-formation-ratio (SFR), namely, type I with separated source and reservoir layers, type II with source layer as reservoir layer, and type III of pure shale^[9,10]. Among them, the pure shale oil reservoir has the widest distribution area and largest resource potential but is also most difficult to exploit, with no industrial breakthrough made yet.

The shale oil in the Upper Cretaceous Qingshankou Formation (shortened as Qing 1 Member) in the Songliao Basin can also be divided into three types, among which the type I has achieved large-scale benefit development, type II has made important progress, while the type III of pure shales, deposited in a semi-deep or deep lacustrine environment, characterized with SFR less than 10%, and single sand body thickness less than 2 m, is still in the exploration stage. The pure shale oils in Qing 1 Member are widely distributed and large in resources. But they are special in geological conditions for shale oil accumulation^{[11,12,13,14,15,16]}. In particular, the shale of Qing 1 Member in Changling sag, southern Songliao Basin is different from the marine shale of North America and the continental shale of other basins in China. The shale is deposited in the semi-deep and deep lacustrine environment, with high clay mineral content, generally 40%-50%, 46.7% on average, which is much higher than shale in other basins in China and abroad. The formation mechanisms of high-quality shale and pure shale oil accumulation, and enrichment law of the shale oil remain unclear. There are no effective evaluation and prediction methods of the sweet spot in this kind of pure shale oil reservoir, and no effective stimulation technologies for the pure shale reservoir with high clay mineral content, hindering down its exploration and development. So, it is urgent to study the geologic conditions and engineering technologies to realize the effective development of this kind of reservoir. Given the major issues in the exploration and development of this kind of shale reservoir with high clay content, taking shale oil layer of Changling sag in the south of the Songliao Basin as study object, we have carried out pertinent geologic and technological research, reached some geologic understandings, and worked out some engineering technologies, in the hope to guide the exploration and development of continental shale oil reservoirs with high clay content in China.

The Songliao Basin is divided into the north and south parts with the Songhua River as the boundary. According to the characteristics of the basement and the regional geological characteristics of caprock, the southern Songliao Basin is divided into four primary structural units: central depression, western slope, southeast uplift, and southwest uplift. The central depression located in the center of the basin is the main hydrocarbon generation and enrichment area in the southern Songliao Basin, and also the potential area for shale oil enrichment. It can be further divided into four secondary structural units, Changling sag, Fuxin uplift zone, Honggang terrace, and Huazijing terrace (Fig. 1a). The Changling sag is located in the south of the central sag. It is a large sag in the NE strike with an area of about 6500 km². It is connected with Gulong sag in the north, Honggang terrace and Huazijing terrace in the northwest and southeast, and adjacent to the Fuxin uplift belt in the East^[17,18]. The Changling sag is characterized by rifting and depression superimposed deposition. In the rifting period, the Upper Jurassic to Lower Cretaceous Huoshiling Formation (J₃h), Shahezi Formation (K₁sh), Yingcheng Formation (K₁yc) and Denglouku Formation (K₁d) with a maximum thickness of 5000-7000 m deposited, which are mainly composed of continental clastic rock, volcanic rock, and pyroclastic rock. In the depression period, the Lower Cretaceous Quantou Formation (K₁q) and the Qingshankou Formation (K₂qn), Yaojia Formation (K₂y), Nenjiang Formation (K₂n), Sifangtai Formation (K₂s) and Mingshui Formation (K₂m) deposited in Upper Cretaceous (Fig. 1b), these formations are largely continental clastic rock, and the Qingshankou Formation is the main source rock^[19,20]. The Songliao Basin has experienced four tectonic evolution stages: (1) the stage of volcanic eruption and fracturing (T-J₃); (2) the stage of syn-rifting (J₃-K₁); (3) the stage of depression (K₁-K₂); and (4) the stage of structural inversion (K₂-Q)^[21,22].

The Qingshankou Formation can be further divided into 3 members, Qing 1, Qing 2, and Qing 3. The Qing 2 and Qing 3 members are widely distributed in the basin, 250-550 m thick in general, and thinner in the Keshan-Lindian area in the north and Jiangqiao-Baicheng area in the west. They consist of mainly dark gray, gray and grayish-green mudstone, a small amount of purplish-red mudstone, gray and grayish-white argillaceous siltstone, siltstone and fine sandstone interbeds, and thin calcareous siltstone interlayers. This member is in conformable contact with the underlying Qing 1 Member and the overlying Yaojia Formation in most part, and parallel unconformable contact with them in local parts. The target layer in this study is Qing 1 Member. This member is composed of mainly thick layers of grayish-black shale and gray argillaceous siltstone in local parts and is stably distributed in the region. This member was deposited in the 1st large-scale lacustrine transgression period of the Songliao Basin when the lacustrine basin was large in area and deep in water, abundant in algal organisms, and largely reducing environment. The member is mainly semi-deep and deep lacustrine facies^{[23,24,25,26]}. According to the change of paleoenvironment, the Qing 1 Member is subdivided into three units. This set of dark organic-rich shale is the main source rock and caprock in the south part of Songliao Basin. It has favorable conditions of self-generation, self-storage, and accumulation in source rock conditions^{[27,28,29,30,31,32]}. The depocenters of Qing 1 Member in Changling sag are Tahucheng sub-sag and Qian’an sub-sag, where the organic-rich shale of semi-deep and deep lacustrine facies is widespread, making them the main areas of the type Ⅲ pure shale oil reservoir.

The Qing 1 Member in the study area is mainly shale of semi-deep and deep lacustrine facies, with a wide distribution range and large sedimentary thickness of generally 30-100 m and 70 m on average. On the plane, the sedimentary center is in the Da’an and Qian’an areas, and the Qing 1 Member shale occurs in the southeast of Honggang terrace, the north of Changling sag, the west of Fuxin uplift and the north of Huazijing terrace (Fig. 2). Statistics show that the Qing 1 shale over 50 m thick in the study area is about 7265 km², laying a solid material foundation for shale oil enrichment.

Six hundred and eighty-eight (688) samples of dark shale from the south part of the Songliao Basin were tested on geochemical properties, and the results show that they have high abundance and good types of organic matter. To be specific, they have a TOC range from 0.5% to 4.5%, and an average TOC of 2.15%; 80% of them have TOC of more than 1%, and 38.9% of them have TOC of more than 2%. In the Da’an and Qian’an areas, the shale samples have TOC of more than 2.0% in general, and the shale with TOC of over 2.0% is 4750 km². The study and analysis show that the shale samples from Qing 1 Member in the south of the Songliao Basin have mainly typed I and type II₁ organic matters (Fig. 3), and a few samples have type II₂ organic matter. Especially in the deep lacustrine sedimentary area in the center of the sag, the parent material is mainly algae and sapropelic parent material, which has high oil generation potential.

The shale samples from the Qing 1 Member in the south part of the Songliao Basin have a R_o range from 0.5% to 1.0%, and the shale layer with R_o of over 0.7% is about 6975 km². The shale areas with higher maturity are mainly concentrated in the Da’an, Qian’an, and Heidimiao areas of Changling sag. The shale samples of the Qing 1 Member in the south part of the Songliao Basin have a higher amount of retained hydrocarbon. They have a chloroform asphalt "A" content of more than 0.2% mostly, and 0.35% on average, and 2% at maximum. The samples have a pyrolysis analysis parameter S₁ of more than 1.0 mg/g in general (Fig. 4), 4 mg/g at maximum, and 1.5 mg/g on average. The areas where the S₁ values of shale samples are over 2.0 mg/g and are mainly distributed in the Qian’an area and the Da’an-Tahucheng area of Changling sag. The comprehensive evolution profile of organic geochemistry (Fig. 4) shows that at the depth of 1600 m, organic matter is transformed into hydrocarbons in large quantities, corresponding to the R_o value of about 0.8%; and at the depth of 2100- 2500 m, the retained hydrocarbon in shale reaches the peak, corresponding to the R_o value of about 1.0%-1.2%. At the R_o value of less than 1.2%, the Qing 1 shale of Changling sag has a hydrocarbon expulsion efficiency of less than 20%. Hence, a high proportion of hydrocarbons would be retained in the pores of the shale, forming the shale layer rich in oil. The Qing 1 Member at 2100-2500 m depth is preliminarily confirmed to be the favorable exploration interval of shale oil in the south part of the Songliao Basin.

In the south of the Songliao Basin, the shale of Qing 1 Member is mainly composed of clay minerals, quartz, and feldspar, and a small amount of calcite and dolomite. In the shale samples from the Qing 1 Member, brittle minerals, mainly quartz and feldspar, account for 33%-70%, and 53% on average (Fig. 5a); clay minerals, mainly illite, and illite/ smectite mixed layer, account for 23%-68%, and 47% on average, of which illite makes up 19%-68%, 33% on average, illite/smectite mixed-layer makes up 30%-76%, 58% on average, chlorite occupies 2%-14%, 6% on average, kaolinite has a relatively low content of less than 3% in general (Fig. 5b).

The mineral composition of the shale in the Qing 1 Member is affected by the sedimentary environment, consequently, the Qing 1 Member shale in different areas differs somewhat in mineral composition. The closer to the semi-deep and deep lacustrine facies, the higher the clay mineral content of the shale is. The shale samples from Qing 1 Member in Well JYY- HF1 located in the Qian’an depocenter in the middle of Changling sag have a clay mineral content of 38.0% to 57.0%, and 46.7% on average. Shale samples from the Qing 1 Member in Well Qian262 and Hei197 located in the outer front facies of the delta of Changling sag have lower clay mineral contents than shale samples from Well JYY-HF1. Shale samples from Well Qian 262 have a clay mineral content of 23.5%-53.0%, 42.0% on average. In comparison, shale samples from Well Hei197 have an average clay mineral content of 19.0%. The shale samples from the Qing 1 Member of Well Ta25 located in the center of Da’an-Tahucheng depocenter in the north of Changling sag have a clay mineral content of 28.0%-59.4%, 53.6% on average. Shale samples from the Qing 1 Member of Well N380 in the Xinbei area in the semi-deep and deep lacustrine facies have an average clay mineral content of 50%.

According to the measured data of the State Key Laboratory of Oil and Gas Resources and Exploration of China University of Petroleum (Beijing), the shale samples from Qing 1 Member have a porosity from 2.0% to 11.8%, 4.5% on average (Fig. 6a); and permeability of (0.0014-0.3300)×10^-3 μm², 0.07×10^-3 μm² on average (Fig. 6b). The Qing 1 shale has an effective porosity of 0.8%-9.0%, 5.1% on average, and an average permeability of 0.15×10^-3 μm² from MRI log interpretation.

Fractures are found in cores from the Qing 1 Member of several wells. The fractures are mostly high angle tectonic fractures caused by the stress field. Some fractures have a dip angle of 80°-90°, and the fractures are about 20-80 cm long. The core surfaces may have 1-10 fractures, most of which are oil-bearing, and some are filled with calcite (Fig. 7). The Qing 1 Member shale is also rich in bedding fractures. The bedding fractures are higher in density and 1-10 cm in spacing, and are effective and not filled by carbonate minerals. The bedding fractures have oil film on the surface, and are connected with the structural fractures to form a complex fracture network favorable for shale oil storage.

The Qing 1 shale samples have mainly inorganic pores and sporadic organic pores. The inorganic pores are mainly intergranular, dissolution, intercrystalline, intragranular pores, and microcracks (Fig. 8). Among them, the intergranular pores, relatively large, have a diameter of 30 nm-5 μm, dissolution pores, and intergranular pores have a diameter of 10 nm-2.5 μm and 50 nm-1 μm respectively; organic pores, relatively small, have a diameter of 10 nm-50 nm. The microcracks vary greatly in width. The microcracks filled with calcite are 1-3 μm wide, the bedding cracks are 2-5 μm wide, and the interlaminar cracks between lamina illite are only 8-50 nm wide.

Through core observation, thin section identification, scanning electron microscopy, X-ray diffraction (XRD), and mineral composition analysis, the shale samples of the Qing 1 Member in the south of Songliao Basin are classified into different types. Firstly, according to mineral composition, the shale samples in the Qing 1 Member are divided into three types: clayey shale, mixed shale, and felsic shale (Fig. 9). According to the characteristics of the sedimentary textures, there are three types of sedimentary textures: bedded, lamina, and interbedded. The bedded shale samples have no obvious change in color, grain size, and mineral composition, and have bedding fractures; the lamina shale samples have obvious changes in color, grain size, and mineral composition; and the interbedded shale samples have thin felsic interlayers and various sedimentary beddings. In conclusion, the shale of the Qing 1 Member in the south of Songliao Basin can be further divided into three major lithofacies: layered clayey shale, laminated mixed shale, and interbedded felsic shale (Fig. 10).

Layered clayey shale (Fig. 10a) has horizontal beddings and rich bedding joints and is uniformly gray-black and black. In its mineral components, clay minerals take a majority, with an average content of 63.7%; quartz and feldspar account for a lower proportion of 22.4% and 7.5% respectively; pyrite appears in the form of raspberry crystal aggregate, higher content of pyrite indicates stronger reduction condition, and carbonate minerals are low in content. This kind of shale mainly occurs in the upper part of the Qing 1 Member (No. 1 layer), and contains largely type I organic matter (Fig. 3).

The laminated mixed shale, gray-black (Fig. 10b), has laminae less than 2 mm thick, and clay mineral content (45% on average) much lower than that of layered shale. The laminae are mainly composed of quartz and feldspar (with contents of 26.6% and 16.3% respectively), and contain a large number of Ostracoda composed of calcite in some parts, so the calcium content is slightly higher, up to 14.8%, and the average pyrite content is 6.5%. In addition to strawberry-like crystal aggregates, the pyrite also replaces the Ostracoda shell, indicating that this type of shale is formed in a strongly reducing environment. This type of shale comes up mostly in the middle of the Qing 1 Member (No. 2 layer) and has type I and II₁ organic matters.

Interbedded felsic shale (Fig. 10c) has sandy laminae of more than 1 cm thick, gray-white sandy laminae, and gray argillaceous laminae of nearly equal thickness interbedded with each other. The sandy laminae are in discontinuous lenticular shape and lower in clay mineral content (43.9% on average). There is a small amount of calcareous cement in the sandy laminae, making the calcite content rise. This kind of shale mainly appears in the lower part of the Qing 1 Member (No. 3 layer) and contains mostly type Ⅱ₁ and Ⅱ₂ organic matters.

The controlling factors of paleo-environment during shale deposition include paleoclimate, paleo-depth, paleo-redox, and paleo-salinity. Based on the analysis of the elements reflecting the paleoclimate of 172 shale samples, the vertical paleo-environment of shale formation was simulated inversely. The analysis results show that the Qing 1 Member of Changling sag in the south part of Songliao Basin has clear paleoenvironment interfaces, indicating it has experienced three different sedimentary environments, namely, semi-humid shallow water and weak reduction environment, semi-humid semi-deep water reduction environment, and humid deep water strong reduction environment from bottom-up (Fig. 11).

The early stage of the deposition of the Qing 1 Member was in lowstand systems tract, the water body began to deepen, but had periodic oscillation, and was dominated by shallow water and semi-deep water; the climate was semi-arid and semi-humid, the environment was weak reduction and weak oxidation, and more continental materials fed in. Shale depositing during this period is dominated by interbedded felsic shale. In the middle stage of the deposition of the Qing 1 Member, the water body gradually deepened, and was dominated by semi-deep lacustrine, the climate was more humid, the environment was mainly reducing, and continental sediments fed in periodically, so the shale deposited in this period is largely laminated mixed shale. In the later depositional stage of Qing 1 Member, the water body deepened steadily, the environment turned into a deep lacustrine, the sedimentation rate was slow, the water salinity increased, and the climate was relatively humid, giving rise to a strongly reducing environment with high salinity and rich sulfur in the deep lacustrine. Hence the shale depositing during this period is mainly laminated clayey shale.

In addition to the differences in situ, mineral composition, organic matter content, and organic matter type, the shales of different lithofacies have significant differences in pore type, pore throat structure, and movable oil content, and these factors are the main ones controlling shale oil enrichment^{[33,34,35,36,37,38]}. The fluorescence characteristics, pore type, pore throat structure, and movable oil content of the three lithofacies of shale samples from the south of Songliao Basin were characterized using scanning electron microscopy, fluorescence thin section, nitrogen adsorption, pyrolysis in different stages and extraction in steps to find out the lithofacies most favorable for oil enrichment.

4.1.1. Layered clayey shale

Layered clayey shale contains mainly small pores, and it also contains a large number of bedding fractures and structural fractures, which effectively improve the shale's storage and seepage capacity. At the same time, this type of shale is characterized by higher total oil content and higher ratios of free oil and movable oil, making it one of the most favorable shale oil enrichment lithofacies in the Qing 1 Member.

Pores in layered clayey shale are mainly illite intercrystalline pores in a slit shape. The nitrogen adsorption-desorption curve of this kind of shale has obvious desorption and small hysteresis loop, reflecting that the slit-like (parallel wall-like) pores parallel with a mineral wall with smaller diameter take a majority (Fig. 12). The samples after dichloromethane extraction mainly have pores smaller than 32 nm, while pores of more than 32 nm in size take a very small proportion (Fig. 13). This type of shale has a higher total oil content, and fluorescence mainly distributed in bedding fractures and microfractures scattered and rather than the continuous pattern (Fig. 11). The results of step pyrolysis and step extraction show that this kind of shale has a high total oil content (Fig. 11); with the increase of polarity of the extraction solvent, the pore space increases gradually, and the pore space increment of the first extraction is smaller than that of the second extraction (Fig. 13), which show that this type of shale contains both light components easy to move and heavy components hard to move, but free oil is at a slightly lower proportion of 45%. Because of the small pore diameter, both the light and heavy components are mainly in pores less than 32 nm in size.

4.1.2. Laminated mixed shale

The laminated mixed shale has pores of larger size and good morphology, and a higher proportion of weak polarity and movable components, so it is another favorable lithofacies for shale oil enrichment in the Qing 1 Member. This kind of shale has intergranular pores between quartz, feldspar, and Ostracoda shell grains and dissolution pores in the Ostracoda shell. Nitrogen adsorption experiment shows that ink bottle shape pores and open parallel wall slit-like pores account for the majority of pores in this kind of shale (Fig. 12). The pores in this kind of shale are significantly larger than those in layered clayey shale, with a large proportion of pores (more than 50%) being larger than 32 nm (Fig. 13). With the increase of the polarity of the extraction solvent, the pore space in this shale increases gradually. The increment of pore space in the first extraction is larger than that in the second extraction (Fig. 13), indicating that the light components easy to move in this type of shale oil take a higher proportion than the heavy components hard to move. In this kind of shale, free oil accounts for 53% on average; the fluorescence is mainly light blue and appears in breaking bands, indicating that the oil is mainly composed of light components with weak polarity easy to move (Fig. 11). The results of step pyrolysis and extraction show this kind of shale has a higher total oil content and a higher proportion of movable hydrocarbon (Fig. 11).

4.1.3. Interbedded felsic shale

Because of lower oil content, the interbedded felsic shale has no obvious advantage in shale oil enrichment. This type of shale has intergranular pores surrounded by quartz and feldspar grains, and the pores present mostly in ink bottle shape with fine neck and wide-body (Fig. 12). In fluorescence thin sections of this kind of shale, the fluorescence parts are relatively scattered and significantly smaller in area than those in the former two types of shales (Fig. 11). The experimental results of pyrolysis and step extraction show that this type of shale has the lowest total oil content among the three lithofacies (Fig. 11). This type of shale is formed in the turbulent water environment of shallow lacustrine, so it has lower content and poor types of organic matter, and thus lower potential of hydrocarbon generation and lowest oil content.

It is believed from the above comprehensive analysis that the layered clayey shale and laminar mixed shale are the most favorable lithofacies for shale oil enrichment in the Qing 1 Member of the Songliao Basin.

The Oil enrichment models in the layered clayey shale of deep-lake facies with high TOC and the laminar mixed shale of semi-deep lake facies with medium-high TOC in the Qing 1 Member of Songliao Basin are shown in Fig. 14.

In the continental deep lacustrine bedded shale with high TOC, oil occurs in the matrix pores and fractures, generally formed in the semi-deep and deep lacustrine anoxic reduction environment. The flow behavior of crude oil in shale with high porosity (mainly contributed by clay mineral intergranular pores) is characterized by high retention hydrocarbon and high content of free oil. In the shale, rich bedding fractures, and local structural fractures make up fracture network system, improving the reservoir quality of the shale greatly improved and providing space for the accumulation of movable oil.

In the laminated mixed shale of semi-deep lacustrine facies with medium-high TOC, oil mainly occurs in intergranular pores in Ostracoda and sandy laminae. This type of shale is generally formed in the reduction environment of semi-deep lacustrine, medium-high in organic matter abundance, and high in oil content. The oil in this type of shale is characterized by a high content of total retained hydrocarbon, high content of light components, and good mobility.

The sweet spot of shale oil refers to the high-yield target area (target interval) of shale oil that can be preferentially explored and developed in a shale formation, mainly including geological sweet spot and engineering sweet spot. The evaluation parameters of geological sweet spots include oil content, reservoir conditions, crude oil mobility, natural fractures, and formation energy, etc. The evaluation parameters of engineering sweet spot include the rock compressibility, and in-situ stress anisotropy, etc. Based on the evaluation of favorable lithofacies, fluid mobility, and rock friability, a sweet spot evaluation method combining geological and engineering parameters have been established through this study, and also a corresponding prediction technology has been worked out to sort out sweet spots on the plane (Fig. 15) and sweet layers on the vertical section (Fig. 16). These technologies have guided the deployment of Well JYY-1HF and the optimization of favorable target layers in horizontal wells.

5.1.1. Optimization of favorable area

At present, the prediction of shale oil sweet spots on the plane is facing some problems like little data available and low prediction accuracy. To solve these issues, we have come up with a sweet spot evaluation and prediction technology combining geology and geophysics, including superposition evaluation of semi-quantitative key parameters to define shale oil sweet spots^[39,40].

The favorable lithofacies thickness, TOC, free hydrocarbon S₁, thermal evolution maturity R_o, and mobility index S₁/TOC value were selected for superposition evaluation to delineate favorable areas, and finally combined with seismic inversion and sweet spot prediction with seismic attribute data, the distribution of sweet spots on the plane was confirmed. In this study, two favorable shale oil accumulation areas were sorted out in the No.1 layer of Qing 1 Member in the south part of Songliao Basin, i.e., the Qian’an favorable area and the Da’an favorable area, of which, the Qian’an favorable area is located in the middle of Changling sag (Fig. 15), with an area of 1300 km², the Da’an area is in the north of Changling sag with an area of 1580 km². The total favorable area of Qing 1 Member in the south part of the Songliao Basin is 2880 km².

5.1.2. Optimization of sweet spot section in vertical direction

To avoid the influence of human factors in qualitative evaluation, the calculation method of shale oil sweet spot index S₁ was established to realize the quantitative evaluation of the shale oil sweet spot section. The evaluation process is based on the selection of the main parameters. According to the distribution ranges of the main geological parameters in the study area, four geological parameters, including the movable free oil content (pyrolysis analysis parameter S₁), S₁/TOC reflecting oil mobility, the effective porosity characterizing physical property, and the rock mechanical brittleness index indicating fracability were normalized and evaluated comprehensively to obtain the shale oil sweet spot index SI.

According to the production capacity of shale oil, the continental shale oil of Qing 1 Member in the study area is divided into three types of sweet spot sections: type I that can obtain industrial oil flow under the existing fracturing conditions with SI ≥ 50%; type II, that can obtain low yield oil flow under the existing test production conditions with 30% ≤ SI < 50%; type III that can produce only a small amount or no oil under the existing test production conditions, with SI < 30% (Table 1). According to this evaluation method of shale oil sweet spots, the vertical section of the No. 1 layer of Qing 1 Member in the pilot hole section of Well JYY-1HF was evaluated. It is found that the No. 1 and No. 2 layers of the Qing 1 Member are 55% and 52% in SI value respectively, ranking them type I shale oil sweet spot sections; the No. 3 layer is 40% in SI value and evaluated as type II shale oil sweet spot section. Thus, No. 1 and No. 2 layers are selected as the preferred target layers for shale oil exploitation in the Qing 1 Member of Well JYY-1HF (Fig. 16).

The Qing 1 Member shale in the Songliao Basin has high clay mineral content, strong plasticity, strong heterogeneity, and multiple stress separation interlayers. Drawing on experience abroad, the supercritical CO₂ dry fracturing and large-scale hydraulic fracturing are combined to form the supercritical CO₂ composite fracturing technology, which has been successfully applied in shale oil reservoir fracturing to realize the large volume fracturing of continental shale formation with high clay content.

5.2.1. The functioning mechanism of supercritical CO₂

Supercritical CO₂ refers to the CO₂ fluid with a phase temperature greater than 31.26 °C and a phase pressure greater than 7.43 MPa. It has the characteristics of low viscosity, low surface tension, and strong diffusion^[41]. Compared with pure water-based fracturing fluid, it has three obvious advantages:

(1) It can significantly reduce shale fracturing pressure. Previous studies have shown that supercritical CO₂ can increase rock brittleness and reduce rock compressive strength, which is conducive to crack initiation of the shale formation. The results of fracturing simulation experiments on the Qing 1 Member shale core from the south part of Songliao Basin also show that supercritical CO₂ can effectively reduce the fracturing pressure of the shale, which reaches 35 MPa under the condition of water-based fracturing fluid and drops to 19 MPa in supercritical CO₂ fracturing, that is a drop of 45.7%, indicating that supercritical CO₂ can reduce reservoir fracturing pressure significantly (Fig. 17).

(2) It has strong penetration and fractures inducing ability. In a supercritical state, CO₂ molecules can enter pores with very small throat radius and weak surface with a small opening. Under the condition of large displacement injection, it can reduce the guiding effect of rock heterogeneity on the flow direction of fracturing fluid, and thus is more likely to fully break the reservoir in a large range to achieve matrix seepage. Comparing the spatial distribution of fractures after water fracturing and supercritical CO₂ fracturing, it can be seen that there were only two longitudinal fractures in the Qing 1 Member shale sample after water fracturing, which were not penetrated laterally and thus limited in the fracture connected range. In contrast, at least five fractures were generated in the sample after supercritical CO₂ fracturing, and these fractures penetrated through the whole sample, resulting in a large fracture connected range. This fully proves the supercritical CO₂ can increase the complexity of the fracture system and increase the effective range of fracturing (Fig. 17).

(3) It can improve the reservoir conditions and increase the percolation capacity of the reservoir. The supercritical CO₂ soaking experiment of the Qing 1 Member shale sample shows that the shale samples changed significantly in micromorphology, mineral composition, and pore structure with soaking time. With the increase of soaking time, the shale samples had two main changes. On one hand, the carbonate minerals dissolved, the pores in the samples increased in diameter continuously, and the contents of calcite and dolomite decreased remarkably; on the other hand, intercrystalline pores in clay minerals increased in scale, and even enlarged into microcracks (Fig. 18), proving that dissolution and contract of clay minerals caused by supercritical CO₂ can give rise to a large number of dissolution vugs and clay mineral contraction joints. Mercury injection experiments were carried out before and after the supercritical CO₂ soaking of shale samples of different lithofacies. Before the supercritical CO₂ soaking, none of the samples of three lithofacies were washed to remove oil. The mercury injection data shows that the samples had hardly any pores larger than 100 nm, which was consistent with the data of nitrogen adsorption before oil washing. After the supercritical CO₂ soaking, all the samples in the three lithofacies had a large number of pores larger than 100-10000 nm appearing, while the samples of different lithofacies differed somewhat in the increased number of pores less than 100 nm. This is because a large number of smaller dissolution pores would be formed along with a large number of large dissolution pores when Ostracoda shells in the laminated mixed shale are dissolved. In contrast, layered clayey shale and interbedded felsic shale have lower contents of carbonate minerals, so they mainly have newly formed large-scale shrinkage joints of clay minerals, and lower increment of pores less than 100 nm in size (Fig. 19). In conclusion, the dissolution of carbonate minerals and the shrinkage of clay minerals caused by supercritical CO₂ effectively improve the seepage channels of the shale reservoir. Also, because there is no water in the supercritical CO₂, the harm of pore throat plugging caused by clay expansion, rock wetting reversal, and water sensitivity of shale can be avoided, and thus the conductivity of ultra-low permeability shale reservoirs can be improved significantly.

5.2.2. Evaluation of volume fracturing effect

To systematically evaluate the effect of supercritical CO₂ composite volume fracturing, artificial fracture inversion, transient well testing, and oil source correlation were used to evaluate the artificial fracture penetration range and fracturing effect in Well JYY-1HF. The inversion results after fracturing show that the artificial fractures are 170-220 m in half-length and 50.5-68.9 m in height. The calculation results with transient good test data show that the artificial fractures are about 190 m in half-length and 56 m in height. The results of the two means are consistent.

In oil-source correlation, two crude oil samples were collected at the initial production stage (September 5, 2019) and stable production stage each (November 5, 2019) and tested. The results show that the samples from the two different stages have biomarkers comparable with the No. 1 and No. 2 layer shale of Well JYY-1HF, and biomarkers different from the No. 3 layer shale. Thus the crude oil is confirmed to be produced by No. 1 and No. 2 layers of Qing 1 Member (Fig. 20), and the vertical communication range of the fracture network is greater than 40 m. Based on these evidence, it is believed that the artificial fractures of the horizontal well JYY-1HF have penetrated the highly heterogeneous formations vertically, and connected the No. 1 and No. 2 layers, realizing the large-scale fracturing of continental shale formation with high clay content.

Based on the established semi-deep and deep lacustrine facies shale oil enrichment model, the Qian’an area in Changling sag was selected as the favorable shale oil exploration area, and Well JYY-1HF was drilled in this area. The laminated and bedded shale were selected as two favorable lithofacies of Qing 1 Member. The horizontal well, JYY-1HF well, was completed at the depth of 3943 m and the horizontal section length of 1252 m. For the first time in the field of continental shale oil, supercritical CO₂ composite fracturing was used to achieve large-volume fracturing of shale formation with high clay mineral content and strong heterogeneity. The fracturing section of Well JYY-1HF oil is 1431 m long, and fractured in 82 clusters of 21 stages, with an average cluster spacing of 15.8 m. A total of 37 315 m³ of fracturing fluid and 1978 m³ of sand were injected. A total of 3265 m³ supercritical CO₂ was injected. The main displacement was 12-18 m³/min, and the CO₂ displacement reached 4 m³/min.

The pressure coefficient of the Qing 1 Member in the Qian’an area of Changling sag in the south part of the Songliao Basin is about 1.0, indicating that the Qing 1 Member belongs to a normal pressure system with low formation energy. In Well JYY-1HF, blowoff and drainage regime was controlled to induce fractures in the early stage by controlling drainage amount and to stabilize production by controlling the pressure drop. A total of 7078.9 m³ fracturing fluid was flown back with a flow back rate of 17.4% at 2412-2500 m in Qing1. The well had a maximum daily oil production of 36 m³ in the stage of natural flow, and daily stable oil production of 16.4 m³ in the stage of hydraulic pump drainage (Fig. 21). The oil density is 0.85 g/cm³ at 20 ℃, with kinematic viscosity of 15.7 mPa·s, wax content of 35.27% and colloid and asphaltene content of 18.2%. Thus, it set the highest record of shale oil production from China's continental normal pressure shale formation with high clay mineral content, making a breakthrough in the strategic investigation of continental shale oil.

Continental semi-deep and deep lacustrine shales cover a large area in the south part of Songliao Basin and have favorable geological conditions for shale oil enrichment. Although the pure shale oil has huge resource potential, the high content of clay minerals of continental semi-deep and deep lacustrine shale adds difficulty to the exploration of this type of continental shale oil. The Qing 1 Member shale in the south of the Songliao Basin can be divided into three major lithofacies: layered clayey shale, laminated mixed shale, and interbedded felsic shale. In the three major lithofacies, TOC value is positively correlated with the total oil content of shale. With the increase of TOC value, the total oil content increases. There are a large number of large intergranular pores in the laminae composed of brittle minerals, which provide storage space for movable shale oil.

The two continental shale oil accumulation models, characterized by different TOC values and different continental lamina, are established to explore the sweet shale oils. Among them, the continental deep lacustrine and high TOC value layered shale oil accumulation model is characterized by high hydrocarbon generation capacity, large overall oil content, but a slightly low proportion of movable oil. This type of shale has mainly small clay mineral intergranular pores and some horizontal bedding fractures and high angle tectonic fractures which improve the shale reservoir storage capacity and the mobility of shale oil to some extent. The medium-high TOC laminated shale of semi-deep lacustrine facies has higher hydrocarbon generating capacity, higher overall oil content, large intergranular pores between brittle mineral grains, and high movable oil content, proving that the semi-deep deep lacustrine shale can be effective shale oil reservoirs and pure shale oil has development potential. A quantitative evaluation method of shale oil sweet spot has been proposed through our work, based on lithofacies, mobility, and fracability evaluation. Two sweet spots of shale oil, i.e., the Qian’an and Da’an zones, were sorted out in the Qing 1 Member in the south part of Songliao Basin, which has a predicted area of 2880 km².

Because of the difficulties in the fracturing of shale reservoirs with high clay mineral content, the supercritical CO₂ large-scale composite fracturing technology was developed to reduce the rock fracturing pressure, create complex fracture network, and reconstruct the reservoir by dissolution, to realize large-scale volume fracturing of continental shale system with high clay content and strong heterogeneity.

Through the application of the newly established shale oil enrichment models, sweet spot evaluation method, and supercritical CO₂ composite fracturing technology, etc, the Well JYY-1HF reached a stable oil production of 16.4 m³ per day in the initial production test stage and set a record of highest shale oil production of China's continental shale with high clay mineral content. This fracturing technology provides a solution to the development of oil with low mobility in semi-deep and deep lacustrine shale with high clay mineral content. A new replacement field of China's continental shale oil has been opened. This study will provide a reference for the exploration of deep and semi-deep lacustrine pure shale oil in other areas of China.

HI—hydrogen index, mg/g;

S₁—free hydrocarbon content, mg/g;

S₂—pyrolytic hydrocarbon content, mg/g;

SI—shale oil sweet spot index, %;

T_max—maximum pyrolysis temperature, °C.