The petroleum industry of the United States has achieved leapfrog development through the transformation of exploration idea from conventional oil outside the source to shale oil inside the source kitchen and the technological innovation for medium- high maturity marine shale series petroleum resources^{[1,2,3,4,5,6,7]}. In 2018, the oil production from shale series was 3.29×10⁸ t in the U.S., accounting for 49% of its total oil production^[8,9]. A new peak of oil production is coming, improving oil self- sufficiency rate of the U.S. significantly. Drawing on the successful experience in the development and utilization of oil resources inside the source kitchen in the United States and innovating petroleum geological theories and key technologies for continental shale series with Chinese characteristics are important ways to ensure national oil security^{[1-2, 10-13]}.

On the basis that maturity controls the evolution model of oil generation and expulsion in organic-rich shale series, the research team predicted the future development prospects of continental shale oil by classifying continental shale oil and analyzing the geological conditions for large-scale development of continental shale oil. According to the possible challenges in shale oil exploitation, specific technical measures were proposed, in the hope to provide references for the development of China's continental shale oil.

In this paper, continental shale oil is a general term for liquid hydrocarbons and many kinds of organics in continental organic-rich shale series with vitrinite reflectance of more than 0.5% at buried depth of more than 300 m. The organics include petroleum hydrocarbons and asphalt that have been generated and various unconverted-organic substances in the formations^[1].

Maturity controls the formation of continental shale oil resources^[14]. As the degree of thermal evolution increases, the solid organic matter in the shale series would gradually convert into hydrocarbons, and the amount of retained liquid hydrocarbons would increase first and then decrease. (1) R_o value of less than 0.5% is the stage when the solid organic matter hasn’t been converted, with little retained liquid hydrocarbons, and is the oil-shale oil occurrence window. (2) R_o value of 0.5%-1.0% is the stage when solid organic matter and retained liquid hydrocarbons coexist. The unconverted organic matter accounts for 40%-90%, liquid hydrocarbons retained in shale account for 5%-60%. It is one of the shale oil occurrence windows. (3) R_o value of 1.0%-1.5% is the stage when liquid hydrocarbons and gaseous hydrocarbons coexist. In this stage, the oil quality is generally light, with high gas-oil ratio, and the formation energy is sufficient. Therefore, this stage is the best window for the occurrence of continental shale oil and gas. (4) R_o value of greater than 1.5% is the stage when natural gas generates massively, and the primary window for shale gas occurrence (Fig. 1).

1.2.1. Division according to thermal evolution degree of shale series

According to the maturity difference of shale organic matter, China’s continental shale oil can be divided into two types: medium-high maturity (with R_o value of generally greater than 1.0%) and medium-low maturity (with R_o value of generally 0.5%-1.0%). Shale oils of different maturity intervals have different material composition and occurrence characteristics, and their development methods are also different.

The medium-high maturity shale oil has the characteristics of large quantity of generated liquid hydrocarbons, light oil quality and high proportion of recoverable oil due to the high thermal evolution degree. (1) This kind of shale is generally greater than 1.0% in R_o value and in the window of coexistence of liquid hydrocarbons and gaseous hydrocarbons. It contains mainly low density crude oil and small amount of unconverted organic matter. (2) Affected by the degree of thermal evolution, this kind of shale series has more organic pores, better porosity and permeability conditions, and a porosity of mostly 3%-8%. In this kind of shale, secondary pores formed by constructive diagenesis processes, micro fractures, horizontal shale bedding fractures act as the primary space for liquid hydrocarbon storage. As the oil in this shale is light, high in gas-oil ratio and mobility, this shale oil can be developed economically by horizontal well and volume fracturing technology. (3) Continental diamictite and carbonate- bearing shale formations with high brittle mineral content can be stimulated by artificial fracturing to form effective artificial fracture network systems, to reach high production in single well and cumulative EUR (single well ultimate recovery). Therefore, medium-high maturity shale oil has a high probability of achieving commercial exploitation by horizontal well volume fracturing technology.

With low thermal evolution degree, the low-medium maturity shale oil has the characteristics of large transformable resources potential, heavy oil quality and low proportion of recoverable oil. (1) This kind of shale series isn’t high in thermal evolution degree, with a R_o value of generally less than 1.0%, and mainly contains high density crude oil and solid organic matter that has not been converted. (2) This kind of shale series has low porosity and permeability, few organic pores, and mainly primary pores such as intercrystalline pores of clay minerals, intergranular pores of clastic minerals, bedding fractures, and micro-fractures. (3) The retained liquid hydrocarbon in this kind of shale is more viscous, lower in gas-oil ratio, and poorer in mobility. (4) This kind of shale series has higher plasticity and lower brittle mineral content, so it is difficult to create effective artificial flow channels in the shale by artificial fracturing. And with low single well prodution, it is unable to reach commercial flow. The new development methods must be explored for the medium-low maturity shale oil to achieve large-scale economic development. After years of collaborative research, our research team proposed that in-situ heating conversion underground may be the first choice for the large-scale development and utilization of medium-low maturity organic-rich shale oil resources (Table 1).

1.2.2. Classification according to the type of source- reservoir assemblage of shale series

Based on the differences in source rock, reservoir and the type of source-reservoir assemblage of shale series, China's continental shale oil is divided into three types, i.e. source-reservoir in one type, separated source and reservoir type and pure shale type (Table 1).

(1) Source-reservoir in one type: shale series have different types of rocks, which can act as both source and reservoir layers. The distribution of shale oil has the characteristics of rapid lithology changes and frequent interbedding of source- reservoir, and sweet sections of small thickness on the profile, but wide distribution on the plane. Hydrocarbon generation pressurization is the primary driving force for shale oil accumulation. Represented by the Middle Permian Lucaogou Formation in the northern Xinjiang area and the Paleogene Kongdian Formation in the Bohai Bay Basin, affected by the comprehensive influence of climate rhythm change, hydrodynamic condition change, peripheral volcano, basin bottom hydrothermal solution, seawater intrusion etc., this kind of shale series often has superimposed laminae, frequent interbedding and multiple sweet sections. For example, the shale section of Well Ji 174 drilled in Jimusaer sag in the northern Xinjiang area, with a thickness of 60 m, has 6 sweet sections of sandy and dolomitic reservoirs identified. Of them, 3 sweet sections of Class I are preferentially developed currently.

(2) Separated source and reservoir type: In this kind of shale series, the source and reservoir layers are interstratified, and the pressure difference between source and reservoir controls the accumulation of shale oil. Represented by the middle and upper parts of the seventh member of Upper Triassic Yanchang Formation (hereinafter referred to as “Chang 7 Member”) in Ordos Basin and the middle-upper member of Cretaceous Qingshankou Formation in Songliao Basin, this kind of shale series features sufficient supply of terrigenous clast, sand-shale interbeds, less sand and more shale layers, and separated source and reservoir layers. The thin sandstone layers sandwiched in the shale series have better physical properties and are the primary reservoirs for shale oil enrichment. For example, thin sandstone reservoirs in Chang 7 Member shale series in Well Yangping 7 of Ordos Basin, have a porosity of 6%-12% and cumulative oil production of 3.75×10⁴ t.

(3) Pure shale type: In shale oil reservoir of this type, shale acts as source rock and reservoir rock; and the main resource types include unconverted organic matter and retained liquid hydrocarbons. The medium-high maturity shale oil reservoir of this type is represented by the Da'anzhai Member of the Lower Jurassic Ziliujing Formation in the Sichuan Basin, the lower part of Chang 7 Member in the Ordos Basin, and the lower member of the Qingshankou Formation in the Songliao Basin. Dominated by semi-deep lacustrine-deep lacustrine facies fine-grained sediments, this kind of shale series ischaracterized by high abundance of organic matter, rich laminae, high clay content, and low porosity. Recently, the medium- high maturity pure shale oil is being explored. For example, in the Ordos Basin, mixed water volume fracturing technology has been adopted to complete 29 oil test wells, of which 13 wells have obtained industrial oil flow. However, it is difficult to keep the production of these wells stable, and technologies for economic development of this kind of shale oil still need to be explored. The medium-low maturity pure shale oil is mainly composed of unconverted organic matter, and needs a technological revolution to be effectively exploited.

China's continental shale series have a large span of geological ages. The continental shale series are mainly distributed in the Cretaceous and Paleogene in basins of eastern China, such as Songliao and Bohai Bay, in the Triassic and Jurassic in the central basins such as Ordos and Sichuan, in the Permian, Paleogene and Jurassic in the western basins such as Junggar, Santanghu and Qaidam (Fig. 2)^[15]. So far, a large number of key research and industrial experiments have been carried out in the major continental oil-generating series in basins such as Ordos, Junggar, Songliao, Santanghu, and Bohai Bay etc., and important achievements have been made^{[15,16,17,18,19,20,21,22,23,24,25,26,27,28]}. In general, the continental shale series in China have favorable geological conditions and the geological foundation for the development of continental shale oil resources on a large scale.

1.3.1. The source rocks

Good source rock development environment lays the geological foundation for shale oil formation. Recent studies show that China's continental lake basins have two types of source rock development environments, freshwater and saltwater. Both freshwater and saltwater environments can develop high-abundance shale (Table 2). Shale formations deposited in freshwater environment have TOC values of 3%-32%, S₁ values (free hydrocarbon content) of 0.2-7.1 mg/g, and S₂ values (pyrolysis hydrocarbon content) of 0.3-46.1 mg/g in general. In contrast, shale formations deposited in saltwater environment often have TOC values of 2%-14%, S₁ values of 0.01-3.00 mg/g, S₂ values of 0.06-110 mg/g.

The freshwater environmental source rock is represented by the Chang 7 Member shale in the Ordos Basin, which features segmented enrichment of organic matter. There are 4 organic matter enrichment sections in the Chang 7 Member shale on Yishicun profile. The massive mudstone has a lower TOC of 3.74% on average. The laminated shale has a higher TOC of 13.75% on average, nearly 4 times that of massive mudstone. Organic matter enrichment is mainly controlled by two factors. (1) Lake eutrophication and algae bloom caused by volcanic activity and hydrothermal action. The study of the Tongchuan outcrop profile shows that suitable volcanic activities can provide abundant nutrients conducive to biological bloom. The shale in the section with a tuff content of 5%-7% in the Tongchuan profile has the highest TOC value of generally greater than 20%. Statistical results show that the mineral content of hydrothermal fluid is obviously positively correlated with the abundance of organic matter in the Chang 7 Member source rock, indicating that deep hydrothermal activity can provide nutrients such as Fe, Mo, P, and thus promote booming of organisms. (2) Low sedimentary rate and anoxic quiet reduction environment conducive to the preservation of organic matter. Zircon dating and Milan kovitch cycle analysis show that the organic-rich shale section in the Chang 7 Member deposited in 0.5 Ma at an average sedimentary rate of 5 cm/1000 a. The sedimentary rate and terrigenous clastics supply rate were relatively low, reducing the dilution of organic matter. The pyrite grains are mostly less than 10 μm, indicating that the Chang 7 Member source rock was deposited in an anoxic or hypoxic quiet reduction environment. On the whole, the Chang 7 Member source rock has the characteristics of large thickness and wide distribution range. The black shale has a superposition area of 4.3×10⁴ km², an average thickness of 16 m and a maximum thickness of 60 m. The dark mudstone has a superposition area of 6.2×10⁴ km², an average thickness of 17 m and a maximum thickness of 124 m. The high-quality source rock is widespread, laying a sound material foundation for the large-scale development of shale oil.

Represented by the Lucaogou Formation in the Jimusaer sag, Junggar Basin, the saline environmental source rock features highly heterogeneous distribution of organic matter, big variations of parent materials in longitudinal direction, obvious stratification, and aquatic parent material in high abundance section. The massive mudstone has an average TOC of 3.2%; the laminated shale has an average TOC of 6.1%, about two times that of massive mudstone. Two major factors control the enrichment of organic matter in this kind of shale series^[18]. (1) Early pyroclastic materials provided sufficient nutrients for organism prosperity and promoted organic matter enrichment. It is found through study that the rapid hydrolysis of volcanic ash can promote the enrichment of elements such as P, Fe, Mo, V in the water. Interbedded tuff and algae laminae and highly enriched algae in layers on the lithologic profile indicate that the addition of volcanic materials is conducive to algal bloom. (2) Saltwater body promotes organic matter flocculation and improves capture efficiency of organic matter. The physical simulation experiment of fine-grained sediments and organic matter enrichment in the saline lake basin revealed that when the salinity increased from 1% to 3%, the capture efficiency of organic matter increased by 3 times; when the sediment concentration increased from 2% to 4%, the capture efficiency of organic matter increased by 1 time. Therefore, it is speculated that the flocculation of organic matter in saltwater environment may be one of the important reasons for the high enrichment of organic matter. Saline environmental source rocks are widely developed in the Middle and Lower Permian of the Junggar Basin. The high-abundance source rocks have a thickness of 50-450 m and an area of approximately 7×10⁴ km².

1.3.2. The reservoirs

In general, continental shale series have multiple types of reservoirs in extensive distribution, such as terrigenous clastic rock, diamictite, carbonate rock, and mud shale, which provide ample accumulation space for shale oil enrichment.

(1) Shale series have two types of sedimentary models, terrigenous and endogenous. Lake basins dominated by terrigenous sediments, represented by the Chang 7 Member in the Ordos Basin and the Qingshankou Formation in the Songliao Basin, have semi-deep to deep lacustrine sandy clastic flow, slump, and turbidity flow clastic reservoirs etc. Lake basins dominated by endogenous sediments, represented by the Lucaogou Formation in northern Xinjiang and the second member of the Kongdian Formation in the Cangdong sag of Dagang Basin, have shallow lacustrine limestone, dolomite and semi-deep to deep lacustrine diamictite, tuff reservoirs etc.^[18]. Wells drilled reveal that both the clastic reservoirs dominated by terrigenous sediments and the diamictite reservoirs dominated by endogenous sediments have good reservoir physical properties and high initial production^[18].

(2) Shale series have lamination structure. Affected by climate rhythm change, hydrodynamic condition change, mixing stack of sediments from different sources, and organic matter flocculation jointly, the continental shale series have extensively developed lamination structures, creating conditions for the formation and enrichment of shale oil in large areas^[18]. First, microscopic observation shows that shale samples of different lithologies in the continental lake basin all have lamination structure. For example, there may be a positive correlation between the development degree of the lamination and organic matter abundance of the Yanchang Formation shale in the Ordos Basin (Fig. 3). Second, sample analysis shows that both shale samples with wavy and horizontal lamination have good reservoir properties and rich micro-nano pores. But shale layers of different structural types differ somewhat in reservoir properties, become worse successively from laminated shale to layered shale and then to massive shale, and the laminated shale is the most favorable (Fig. 4).

1.3.3. Formation and distribution

Continental shale oil reservoirs feature near-source accumulation or source-reservoir integration, multiple types of source-reservoir assemblages and multiple sweet sections in the longitudinal direction. (1) Hydrocarbon generation pressurization is the primary driving force for shale oil accumulation. Hydrocarbon generation simulation experiments reveal that the hydrocarbon generation of source rock can make the pressure increase by 50-60 MPa and the source-reservoir pressure difference in shale series reach 7-8 MPa. (2) Laminations and micro-fractures play an important role in controlling hydrocarbon expulsion, migration and accumulation of organic-rich shale. First, simulation experiments of hydrocarbon generation and expulsion reveals that the source rock texture determines the efficiency of hydrocarbon expulsion. Shales of different textures have a large difference in hydrocarbon expulsion efficiency. When the R_o value is greater than 0.9%, the laminated organic-rich shale has a higher hydrocarbon expulsion efficiency. The laminated shale with high TOC and low clay content has hydrocarbon expulsion efficiency of greater than 45%, the quasi-laminated shale has an average hydrocarbon expulsion efficiency of 30%-40%, bedding clastic mineral rich shale has the lowest hydrocarbon expulsion efficiency of less than 20% in general^[18]. Second, observation of thin sections shows that hydrocarbon fluorescence is mostly distributed in fractures and laminae, and the fluorescence intensity increases first and then decreases, indicating that micro-fractures and laminaeare are effective channels for shale oil migration and accumulation. (3) In the shale series, source and reservoir layers interbed frequently longitudinally, and the reservoir layers have higher oil content generally. For example, the high-resistance shale section of the second member of the Kongdian Formation in the Cangdong sag of the Dagang Oilfield with a thickness of 400 m, has 21 reservoir sections identified. Of them, 7 high-quality sweet sections have been selected as the first choice for exploration.

Comparison of the continental shale oil reservoirs discovered in typical basins show the shale oil series have "sweet sections" with small thickness, but wide distribution of "sweet areas" on the plane. The test and analysis of horizontal permeability and vertical permeability of the shale series show that the horizontal permeability of shale is tens to hundreds of times of its vertical permeability. The much higher horizontal permeability than the vertical permeability is conducive to the large-scale lateral migration and accumulation of shale oil inside the source. This is also why the continental shale oil features thin "sweet sections" vertically, but large oil-bearing area on the plane (Fig. 5). On the whole, the distribution range of organic-rich shale developed in China determines the macro-distribution of shale oil-rich areas.

Continental shale oil is the strategic onshore oil replacement resource with the highest potential in China. It is estimated that the favorable area of continental shale oil in China is about 8.5×10⁴ km² (Table 3). Medium-low maturity shale oil is a major strategic replacement resource of the petroleum industry. If breakthrough is made in the medium-low maturity shale oil, it will bring a true revolution in continental shale oil. Medium-high maturity shale oil is an important replacement field to ensure that the China's oil production will reach 2×10⁸ t again in the near future^[1,2].

Medium-high maturity continental shale oil resources are abundant. In recent years, the Ministry of Natural Resources, China National Petroleum Corporation (hereinafter referred to as "CNPC") and China Petrochemical Corporation (hereinafter referred to as "Sinopec") have initiated shale oil exploration research and resource evaluation,consecutively. In 2015, EIA estimated technical recoverable resources of shale oil in China at 43.7×10⁸ t. In 2019, Sinopec preliminarily estimated China’s shale oil technical recoverable resources at (74- 372)×10⁸ t. In recent years, CNPC assessed China’s shale oil technical recoverable resources at 145×10⁸ t (including oil shale)^[29,30]. According to the evaluation results of various institutions, the medium-high maturity continental shale oil resources are more abundant, and more concentrated mainly in large basins such as Songliao, Ordos, Bohai Bay, Sichuan, and Junggar.

Recent exploration revealed that different types of shale oils all have good development prospects. Since 2010, PetroChina Changqing, Daqing, Dagang, Xinjiang, Tuha and Sinopec Shengli, Jianghan Oilfield companies have been continuously working on "sweet area (section)" prediction of shale oil, low cost drilling and completion, and production increase technologies. They have also actively carried out development experiments of medium-high maturity shale oil and made important progress. First, for the “source-reservoir in one type” shale oil, large-scale production areas have been built. Based on the Permian Lucaogou Formation shale oil in Jimusaer sag, Xinjiang Oilfield planned to build a production capcity of 1 million tons in recent years. Based on the second member shale oil of Kongdian Formationin Cangdong sag, Dagang Oilfield planned a new productivity of 100 000 t in recent years. Second, for the “separated source and reservoir type” shale oil, large-scale reserves areas have been discovered. Based on the enhanced genetic mechanism research and key technical research of shale oil in the middle and upper parts of the seventh member of Yanchang Formation in the lake basin center, Changqing Oilfield has confirmed a reserve scale of 20×10⁸ t and built productivity of 200×10⁴ t in the development test areas, and produced about 60×10⁴ t of oil in 2018. Third, the exploration of shale oil in pure shale, has achieved initial results. Changqing Oilfield conducted mixed water volume fracturing experiments in the middle and lower shale of the Chang 7 Member, and many wells obtained industrial oil flow after fracturing. Daqing Oilfield tapped industrial oil flow from the middle and lower shale of the Qingshankou Formation. After nearly 10 years of continuous exploration, PetroChina has confirmed that shale oil inside source kitchen is expected to become an important replacement field of conventional oil.

The continental organic-rich shale with medium-low maturity has huge oil production potential. According to the analysis of the thermal evolution model of source rock, when the R_o value of the high abundance shale is 0.5% to 1.0%, the unconverted organic matter accounts for 40%-90%^[1]. In the in-situ conversion laboratory of Shell Petroleum Company, PetroChina Research Institute of Petroleum Exploration and Development has completed 2 sets of simulation experiments on Chang 7 Member shale samples from the Ordos Basin. The results show that the Chang 7 Member shale samples can produce oil at 68 kg/t and gas of 26 m³/t by converting organic matter, and have retained liquid oil content of 68 kg/t^[1]. The Chang 7 Member black shale in the Ordos Basin with R_o value of 0.7%-0.9% and the burial depth of 1200-2700 m has mainly type II₁ and type I organic matters, an average TOC value of 13.8% and a maximum TOC value of 38%, an average thickness of 16 m and a maximum thickness of 60 m, and a distribution area of about 4.3×10⁴ km², showing huge potential of in-situ conversion. The shale in the Nenjiang Formation in the Songliao Basin has a R_o value of 0.4%-0.7%, buried depth of less than 2000 m, continuous thickness of 6 to 22 m, and an average TOC of 5.5%-9.0%, and a distribution area with TOC value of more than 6.0% of 3.02×10⁴ km².

The in-situ heating pilot test of China's continental organic-rich shale oil with medium-low maturity is about to start. Medium-low maturity shale oil is a general term for organic matter and retained oil and gas in shale series, which is still difficult to achieve commercial development by using mature technologies such as horizontal wells and volume fracturing. At present, in-situ conversion technology is expected to be the key technology for achieving efficient development and utilization of medium-low maturity shale oil^[31,32]. The development potential of medium-low maturity shale oils has drawn wide attention of the industry. Many oil companies such as Shell, ExxonMobil, and Total are conducting research on in-situ conversion technology and pilot tests. In cooperation with Shell, CNPC has carried out study on in-situ heating conversion potential and favorable area selection of medium-low maturity (R_o value less than 1.0%) shale oil, based on the simulation experiments completed by Shell Laboratory and the analysis and thermal simulation experiments of sealed core samples from 2 wells recently drilled in organic-rich shale in the Chang 7 member of the Ordos Basin^[31]. The proposed technical scheme of on-site test has passed the examination of experts, and the in-situ conversion technical test of shale oil will soon enter on-site implementation. If the technology succeeds in the test, it can convert a huge amount of shale oil resources into actual crude oil reserves and production, and will play an important role in stabilizing and even increasing oil production in China in long term.

Compared with marine shale oil in North America, China's continental shale oil is significantly different. The marine shale oil reservoirs in North America have large thickness and good continuity. In light oil-condensate window with high oil-gas ratios, they have higher formation energy, so wells in them can achieve higher initial production and higher cumulative production. At the same time, the ground conditions of those oilfields support the factory operations on well pad, so these reservoirs can build production capacity rapidly and achieve good development effect. In contrast, the continental shale oil reservoirs in China generally have big lateral changes, mostly lower thermal evolution, higher wax content, and smaller thickness, so they lag behind inherently in formation energy, single well daily production and single well cumulative estimated ultimate recovery (EUR). In general, it is difficult to evaluate and select “sweet areas (sections)” of shale oil in China, and supporting and applicable technologies have not been formed yet, and the future development of shale oil in China will face more challenges and risks^[33].

Challenge in figuring out the potential: the research foundation is weak and the understanding of genetic mechanism is unclear. Based on conventional methods and technologies, the shale and mudstone with TOC value greater than 0.5% are evaluated together. The evaluation criterion for shale oil is unclear, and the resource potential of shale oil is yet to be evaluated. The shale series have poor physical properties, and mainly micro-nano pore-throat systems. The coupling relationship between hydrocarbon fluids and pore-fracture media is not clear under the evolution conditions of temperature, pressure and stress fields. The "sweet spot" prediction of shale oil lacks the support of pertinent methods and technologies, so it is difficult to sort out sweet spots.

Challenge in technical equipment: China's continental organic-rich shale strata have a large age span, frequent facies changes, complex organic matter types, and low thermal evolution degree. Different types of shale oil in different basins have problems such as high clay content, low brittleness index, low pressure coefficient, and insufficient driving energy. The development of medium-high maturity shale oil faces challenges such as "ineffective fracturing, inactive proppant, low flowback, and difficulty in stabilizing production". In-situ conversion technology for medium-low maturity shale oil is being explored internationally, and the core technology and equipment for controlling and monitoring the dynamic evolution of in-situ generation and expulsion of hydrocarbons, heat field and heat transfer, porosity and permeability, fractures, catalysis and anti-corrosion, ecological geological environment, etc. are still blank. Hence, the development of medium- low maturity shale oil faces challenges of lagging technological breakthroughs and high-end equipment manufacturing.

Challenge in economic cost: shale oil resource development has the characteristics of large investment scale and long investment recovery period. National policy support and enterprise cost reduction and efficiency increase are the keys to achieving economic development. Under the premise of strong support of national policies, the scale economic development of shale oil in the United States has experienced more than 10 years of arduous explorations of cost reduction and efficiency increase. Only after that has the current cost of shale oil in the U.S.A. been reduced basically equivalent to that of conventional oil. Shale oil reservoirs in China are more complex in "ground" and "underground" conditions, market competition and management, fiscal and tax incentives, and national subsidy policies are imperfect. Whether or not the development is economic is another challenge for the development and utilization of shale oil resources.

Resources and favorable areas should be sorted out, demonstration areas and test areas should be built, and demonstration guidance should be strengthened. It is necessary to set up major research projects, carry out basic geological research and potential evaluation of shale oil, deepen the study of shale oil enrichment mechanisms, find out resource base, select favorable enrichment areas, and evaluate "dessert areas (sections)". Through risk exploration, exploration of pilot areas and construction of demonstration areas, the applicable technical specifications should be tested and established, and the scale economic exploration and development template of shale oil should be set to support the industrial process of continental shale oil.

Scientific and technological research should be strengthened, striving to break through the key technologies. It is necessary to establish an integrated technical research team for industry-university-research-application of shale oil, accelerate the construction of shale oil laboratories, speed up the development and application of core technologies, revolutionary technologies, and major equipments. Key technologies suitable for the economic development of medium-high maturity shale oil and the core technologies and equipments for underground in-situ conversion of medium-low maturity shale oil should be developed with all our efforts, so as to solve the core technologies and equipment problems faced by shale oil development.

Development strategies and supporting policy should be made to gather all forces. It is necessary to strengthen the top-level design at the national level, formulate mid- and long-term development strategic plans, introduce fiscal incentive policies, made tax support policies and financial support policies to gradually improve supporting policy and promote the orderly and rapid development of the shale oil industry. Oil companies should develop theories and technologies vigorously, reduce cost and enhance profits by making full use of market mechanisms, to promote the scale and economic development of shale oil.

Based on the idea that maturity controls the evolution model of oil generation and expulsion in organic-rich shale series, shale oil can be divided into two types, medium-low maturity and medium-high maturity. China onshore has a large-scale continental shale oil resource base, two types of high-quality source rocks, freshwater and saltwater, and multiple types of reservoirs, including clastic rock, diamictite and tuff, carbonate rock, and shale, etc., forming a number of "sweet sections" and "sweet areas" with continuous distribution near source rocks. It is pointed out through evaluation that medium-high maturity shale oil is an important replacement field for petroleum exploration, and medium-low maturity shale oil is a major strategic replacement resource for the future development.

The development of continental shale oil industry in China is still facing a series of major scientific and technological problems. It is imperative to make breakthroughs in the core technologies such as "sweet spot" evaluation and selection, horizontal well volume fracturing, well layout and production, and in-situ heating conversion equipment to drive the continental "shale oil revolution."