China’s dependence on imported oil has reached more than 70% in 2018. With the development of national economy, the dependence on imported oil may rise further, so it is urgent to increase the safe supply of domestic oil. However, most of the developed major oilfields in China have gradually entered into a production decline period. In recent years, a large proportion of new reserves has decreased in quality, and thus is low in recovery rate and single well production, and short in stable production period. For PetroChina, it is very difficult to stabilize production, and even more difficult to increase production. It is urgent to find large-scale alternative resources to fundamentally solve the problem of resources security in China^{[1,2,3,4,5,6]}. The continental shale oil in China has huge resource potential. Once the shale oil enrichment mechanisms, distribution characteristics, production methods are clear and breakthroughs are made in shale oil development technologies, shale oil will be an important domain to get stable oil and gas production in a quite long period in the future, and will be the mainstay for the long-term security of China's self-sufficiency of crude oil^{[1, 3]}.

The geological understanding, exploration target selection, evaluation basis, technology and exploration countermeasures for the continental shale oil are very different from traditional oil and gas, and still have high uncertainties. Currently, researchers in the oil industry have differences in the concept and connotation of shale oil, as well as the relationship between shale oil and tight oil. Some viewpoints have overlaps and conflicts, which can mislead and affect the smooth development of the continental shale oil. If the misunderstanding is not cleared up in time, it is possible to make detours due to the misunderstanding. To this end, we looked deep into the basic types, geological characteristics, resource potentials, exploration status, and evaluation criteria of different types of continental shale oil in China, and the relationship and boundaries between shale oil and tight oil, and put forward some ideas and suggestions, in the hope of providing guidance for the upcoming shale oil revolution.

Continental shale oil is a general term for liquid petroleum hydrocarbons and various types of organic matter in continental organic-rich shale strata with a burial depth greater than 300 m and a R_o value greater than 0.5%. It includes petroleum hydrocarbons and various types of bitumen that have formed underground, and solid organic matter that has not been thermally degraded. Shale oil is different from oil shale. The latter refers to shale layers with a burial depth of less than 300 m and an extremely high abundance of organic matter that has not yet been converted into liquid petroleum hydrocarbons, wherein most of the organic matter exists in solid form (Fig. 1).

Maturity of organic matter plays an important role in controlling the formation of continental shale oil^[7]. As the degree of thermal evolution increases, the organic matter in shale will be degraded and gradually converted into oil and gas. The shale enters a liquid "oil window" first, then as the temperature rises, liquid hydrocarbon will crack into natural gas^{[1, 7-10]}. The amount of hydrocarbons generated increases first and then decreases. Traditional hydrocarbon generation model of organic matter is primarily for the exploration and development of conventional oil and gas reservoirs. As the occurrence state of shale oil and gas is very different from that of conventional oil and gas, the maturity stage has been redefined according to the resource types in this paper, which is more suitable for the exploration and development of shale oil and gas. Distribution intervals of different types of shale oil were divided according to the maturity evolution stage: (1) solid organic matter distribution interval with R_o of less than 0.5% is the oil shale occurrence window. (2) The R_o range from 0.5% to 1.0% is the interval in which retained liquid hydrocarbons, multiple types of discharged hydrocarbons from bitumen, and unconverted organic matter coexist, and is the main occurrence window of low-maturity shale oil. In this stage, the amount of liquid hydrocarbons retained in shale layers varies greatly due to different thicknesses of shale layers and different assemblages with surrounding reservoirs, and can reach up to 40% to 60%^{[8-9, 11]}, while the unconverted organic matter can reach up to 40% to 80%. (3) The R_o range from 1.0 % to 1.6% is the stage of massive generation of liquid hydrocarbons, when the oil generated is light and high in gas-oil ratio. This stage is the main occurrence window of medium-high maturity shale oil. (4) The R_o range higher than 1.6% is the stage of large-scale cracking of liquid hydrocarbons and large-scale generation of natural gas, and is the main occurrence stage of shale gas (Fig. 1).

Continental shale oil in China is different from marine shale oil in North American in meaning, development environment, geological characteristic, exploitation method and evaluation criterion. Shale oil in North America largely occurs in tight reservoirs like clastic rocks, carbonate rocks, and mud shale interbedded with organic-rich shale strata of marine facies. The shale oil in North America is mainly developed by horizontal wells and volume fracturing^{[10,11,12,13,14,15,16]}. Marine shale oil in North America has the following characteristics (Table 1): (1) The oil layers have good continuity and larger thickness. (2) The oil layers are higher in thermal maturity (with R_o from 1.0% to 1.7%), and the oil is light oil (with a density of 0.77-0.79 g/cm³) and high in gas-oil ratio (generally 50-300 m³/m³). (3) The oil layers have higher TOC content generally (3%-5% on average), and abnormal high pressure, with a pressure coefficient of 1.3-1.8. (4) The reservoirs have higher average porosity of generally 8% to 10%. (5) The wells have higher initial production (30-60 t/d) and cumulative production (greater than 4×10⁴ t) in general. Continental shale oil in China can be divided into two categories: shale oil of medium- low maturity and shale oil of medium-high maturity. In terms of connotation, exploitation method and technology, and evaluation criterion, the former is not only different from the U.S. shale oil, but also different from shale oil of medium-high maturity in China. Shale oil of middle-high maturity is similar to U.S. shale oil in terms of geological characteristics, exploitation methods, and core technologies, so they can be compared with each other. However, it should be noted that the shale oil discussed in this paper does not include tight oil, so from the perspective of sedimentary lithological combination and environment, it is also very different from shale oil in North American. The shale oil layers of medium-high maturity in China are smaller in thickness; so the shale oil layers in China are mostly in the low to medium maturity window (with R_o from 0.5% to 1.1%, mostly from 0.75% to 1.00%), the oil is slightly heavy (with a density greater than 0.85) and in low gas-oil ratio (less than 100 m³/m³, mostly 20-60 m³/m³); the shale layers vary widely in TOC, and mostly lower in TOC (2%-3%); the shale oil wells in China vary widely in initial production and have lower cumulative production. With short production time to date, the final cumulative production of a single well is still difficult to calculate. In this paper, the Brent oil price was set at \$55/bbl to calculate the minimum cumulative production a single well must obtain to meet commercial development conditions in different testing zones (Table 1). From the current testing of limited wells for one year or longer, the cumulative production of a single well is generally not high, which will be an important factor affecting whether the continental shale oil of medium-high maturity can be exploited on large scale. The marine shale oil layers in North America are thicker, better in continuity, in the light oil-condensate oil window, and higher in gas-oil ratio and stratum energy. Therefore, shale oil wells there can achieve higher initial production and cumulative production by adopting horizontal well and fracturing, and by employing factory-like operations, large-scale production capacity can be built quickly to make good benefits^[14]. Continental shale oil reservoirs in China vary greatly in lateral distribution and have lower thermal maturity. In addition, they have higher wax content and smaller thickness, so they are inferior to those in the North America in stratum energy, and daily production and cumulative production of single well, etc. Consequently, it is difficult to evaluate and select the sweet spot area (interval), and the future development scale of shale oil is still uncertain.

The resource potential of continental shale oil mainly depends on the hydrocarbon generation potential of unconverted organic matter and the amount of liquid hydrocarbons that has not yet been discharged out of the shale strata. Therefore, the continental shale oil is mainly classified according to two parameters, abundance and maturity of organic matter, into two types, namely shale oil of medium-low maturity and shale oil of medium-high maturity (Fig. 1), which are significantly different in geological characteristics (Table 2).

2.1.1. Medium-low maturity shale oil

Medium-low maturity shale oil layers have huge convertible resource potential, thicker retained liquid hydrocarbon, lower proportion of mobile oil, and higher proportion of solid organic matter, and are difficult to be developed commercially by conventional fracturing (Table 3). They have thermal maturity of organic matter not high, with a R_o value of less than 1.0% mostly. Their upper limit of maturity has an overlap with the medium-high maturity shale oil, but the limit should be determined according to the condition of the specific study area. If the shale oil in an exploration area is mainly medium-high maturity oil, the upper limit of maturity can be appropriately set to a lower value. The specific value depends on the crude oil mobility underground and the cumulative production of a single well. The upper limit can be set at 0.9% and should not be too low. Medium-low maturity shale oil is mainly composed of heavy oil, asphalt, and unconverted or-ganic matter, and thus is difficult to reach economic production by horizontal well and fracturing, so underground in-situ heating and conversion technology must be used. Medium-low maturity shale oil layers have TOC content of greater than 6% generally (mostly from 8% to 12%), and the higher the better, to ensure there are enough liquid hydrocarbons and various types of organic matter residues to generate enough hydrocarbons when heated in situ. They should contain mainly type Ⅰ and type Ⅱ₁ organic matter, to ensure easier conversion to and enough amount of liquid hydrocarbons when heated. These shale layers have small storage space, with a porosity of less than 3% mostly, which is made up of largely intracrystalline pores in clay minerals, intergranular pores in detrital minerals, bedding fractures, and micro-fractures, etc. and a few organic matter pores (Fig. 2a₁, Fig. 2b₁, Fig. 2c₁). They have high plasticity and low brittle mineral content, so it is difficult to make effective flow channels in them by artificial fracturing technology.

2.1.2. Medium-high maturity shale oil

Medium-high maturity shale oil dominated by mature liquid petroleum hydrocarbons is lighter in oil quality and higher in movable oil proportion, higher in geological resource potential, highly uncertain in total recoverable resources, and able to be developed by conventional horizontal well and fracturing technology (Table 2). The shale layers of this kind of oil have higher thermal evolution degree of organic matter, with R_o more than 1.0% mostly, and the best R_o from 1.0% to 1.4%. After determining the main development type of shale oil in an exploration area, the lower limit of the R_o of medium-high maturity shale oil can be adjusted to 0.8% to 0.9%. But the lower limit shouldn’t be too low, lest heavy oil and low gas-oil ratio affect the underground mobility of crude oil, directly impairing the daily production and cumulative production of wells. These shale layers have small storage space, with a porosity of mainly 5%-8%, and some can reach the porosity of 10%-15%, but the proportion is relatively small. In these shale layers, liquid hydrocarbons mostly occur in lamellation seams, fractures created by hydrocarbon-generating pressure, structural fractures, and secondary pores formed by constructive diagenesis (Fig. 2a₂, Fig. 2b₂, Fig. 2c₂). These shale layers have a TOC value greater than 2% generally. The oil is medium to light in quality, higher in gas-oil ratio under good preservation conditions, and higher in mobility. These shale layers have a formation pressure coefficient of more than 1.2 largely. With higher content of brittle minerals, they can be developed economically with horizontal well and volume fracturing. The economy of medium-high maturity shale oil reservoir should be evaluated from three aspects: (1) the daily production per well should be higher than the minimum economic daily production limit; (2) under different oil prices, the cumulative production per well should be greater than the minimum cumulative economic production per well; (3) the distribution area and geological reserves must reach a certain scale, to ensure that once put into production, a minimum- scale production can be obtained and maintained for a relatively long time. Clearly, shale oil discoveries in which only limited number of wells can reach economic production but can’t realize scale production over a long period, are generally difficult to put into production.

Continental shale reservoirs discovered in China are concentrated in the Triassic, Cretaceous and Paleogene. Among them, the continental shale oil layers in the Junggar, Ordos, Sichuan, and Santanghu basins in the central and western regions are mainly distributed in the Permian, Triassic and Ju-rassic; the continental shale oil layers in the Songliao Basin in the east are mainly distributed in the Cretaceous; and the continental shale oil layers in Qaidam and Bohai Bay Basin are mainly distributed in the Paleogene^{[17,18,19,20,21,22,23,24,25,26,27,28,29]}.

2.2.1. Cangdong sag in the second member of Paleogene Kongdian Formation

The Cangdong sag is one of the oil-rich sags in the Bohai Bay Basin. The second member of Paleogene Kongdian Formation (simplified as the Kong 2 member) is the main shale oil development interval, and the shale oil in it is mainly medium maturity oil. During the sedimentary period of the Kong 2 member, the Cangdong Sag was in subtropical semiarid- humid environment, where an inland closed saltwater lake basin with a small supply of coarse debris developed. In the middle of the lake basin, an organic-rich shale development area in the semi-deep lake subfacies developed with an area of ​​430 km² and a thickness of 50-300 m. The Kong 2 member can be divided into 4 small layers in the longitudinal direction. The organic-rich shale is mainly distributed in the upper 3 small layers, which are made up of clastic rock, diamictite, and carbonate rock. In these layers, quartz-feldspathic shale, dolomitic-calcareous shale, dolomitic shale and mixed shale can be identified. They have a TOC content from 1.5% to 3.5%, type Ⅱ₁ to Ⅱ₂ kerogens mainly, and R_o from 0.5% to 1.1%. They have lamellation seams, micropores, and many types of fractures as main reservoir space, a porosity range from 3% to 7%, permeability of less than 1×10^-3 μm² in general, and a brittle mineral content between 50% and 80%. They have normal pressure to slight overpressure, and oil saturation of 30% to 70%. The oil, with a density of 0.86 to 0.89 g/cm³ largely, is poor in mobility under the formation conditions. There are already two horizontal wells in this area under production test. The well with the longest test period is Well Guan 1702H. Its production test started on May 28, 2018, and it has produced for more than one year, with a total of 7983 tons of crude oil and 45.5×10⁴ m³ of gas produced^[28,29] (Fig. 3). According to the current production decline rate, the well is expected to have a cumulative oil production of 2.65 × 10⁴ t, and economy near the point of breakeven, so it is necessary to reduce the cost or increase the cumulative production to realize large-scale development.

2.2.2. Jimusar sag in the Permian Lucaogou Formation

The Jimusar sag is located in the Shaqiuhe-Qitai uplift area in the eastern Junggar Basin. It had undergone intense uplift from the end of the Jurassic to the end of the Cretaceous, resulting in severe stratum denudation. Although the shale oil is in the Permian Lucaogou Formation, it is not high in thermal maturity, which may be an important factor restricting the economic development of shale oil in this area. In Middle Permian, the Jimusar sag was an offshore lake, where intermittent seawater encroachment caused the mass death of biota lived in the lake and was conducive to the enrichment and preservation of organic matter. The semi-deep lake subfacies in the center of the sag is a development area of organic-rich shale with a thickness of 50-160 m^[19] and an area of ​​1086 km². The Lucaogou Formation is divided into 6 members in the longitudinal direction. Organic-rich shale mainly occurs in the "upper sweet spot interval" and "lower sweet spot interval", which are made up of mud shale, carbonate and siltstone/fine sandstone, interbedded with marl and tuff. They have a TOC content of more than 3.5%, type II kerogen, and a R_o from 0.6% to 1.1% in general. They have dissolved pores, intercrystalline pores, lamellation seams, and micro-fractures as main reservoir space, a porosity from 6% to 14%, a permeability of less than 0.1×10^-3 μm², an oil saturation of 80% to 90%, a brittle mineral content of above 85%, and normal to slight overpressure. The crude oil, with an average density of 0.88 to 0.92 g/cm³, is poor in mobility under formation conditions. At present, there are 15 production test wells in the area, among which, 9 wells have been tested for more than one year. The wells have a cumulative oil production of 1110-20 343 tons. Among them, Well Ji 172H tested the longest, had an initial daily production of 69.5 tons, and has a cumulative oil production of 2.03×10⁴ tons after more than 5 years of production. According to the input-output relationship, under the Brent oil price of 55 US dollars/bbl, the cumulative production per well in areas with a depth shallower than 3800 m needs to be more than 3.5×10⁴ tons and that in areas with a depth of more than 3800 m needs to reach (3.8-4.2)×10⁴ tons to be economic. Overall, the economic efficiency is the key factor dictating whether the shale oil can be put into large- scale development in the Jimusar sag.

2.2.3. Triassic Yanchang Formation in Ordos Basin

The Triassic Yanchang Formation in the Ordos Basin has primarily low-maturity shale oil, and medium-high maturity shale oil secondarily. In terms of both development conditions and resource scale, it is the most representative area with the highest potential for in situ shale oil underground conversion in China. During the sedimentary period of the Yanchang Formation in the Late Triassic, the lake basin was wide. During the sedimentary period of the 7th member of the Yanchang Formation (shortened as Chang 7 Member), a set of black shale and dark mudstone developed in semi-deep lake-deep lake facies spreading from northwest to southeast, with an area of approximately 5×10⁴ km². Among them, the black shale mainly occurs at the bottom of the Chang 7 Member, with a continuous thickness of 30 to 60 m and a maximum thickness of 130 m^[17,18]. The shale layers have a TOC content from 6% to 38%, 13% on average; a R_o of 0.7% to 1.1%, an average hydrocarbon generation intensity of 560×10⁴ t/km², and a total effective hydrocarbon generation amount of about 1300×10⁸ t. After the expulsion of hydrocarbons, there are still huge amounts of liquid petroleum hydrocarbons left in the shale layers. The shale layers have a porosity of 2% to 3%, an oil saturation of 70% to 90%, and a brittle mineral content of about 40%. The shale is rich in lamellation, sandy lamina, and microscopic lamina, and shale oil is mainly distributed in lamellar bedding planes or microfractures parallel to them. The shale layers have a formation pressure coefficient of 0.6 to 0.8, and the crude oil is 0.84 to 0.86 g/cm³ in density. PetroChina has decided to conduct a pilot test of underground in-situ conversion of medium-low maturity shale oil in the Ordos Basin. A breakthrough will give a huge boost to the commercial development of medium-low maturity shale oil, and will certainly pioneer of China's continental shale oil revolution.

Continental shale oil is a type of unconventional oil and gas resource with the greatest development potential after China Petroleum Exploration entered from "outside source" into "inside source" stage, and is the most strategic alternative onshore resource in China^[7]. Based on the in-situ converted hydrocarbon amount from pyrolysis experiment and the distribution of TOC, R_o, and retained oil amount in shale layers suitable for in-situ conversion in China's major petroliferous basins, the Research Institute of Petroleum Exploration and Development of PetroChina estimated the technical recoverable resources of the medium-low maturity shale oil at around (700-900)×10⁸ t. But as only few wells have been tested for the medium high maturity shale oil for shorter time, it is still difficult to evaluate the total economic recoverable resources of shale oil. Nevertheless, by using horizontal well and volume fracturing, the recoverable geological resources of medium-high maturity shale oil in China are preliminary estimated at about 100×10⁸ t.

Although widely distributed in all China's major oil-bearing basins, the medium-low maturity continental shale oil is mainly distributed in the Ordos Basin, Songliao Basin and Junggar Basin. The available laboratory analysis data and the results of pilot tests conducted abroad show that, through underground in-situ heating, petroleum hydrocarbons, various types of asphalt and solid organic matter in shale can be converted into light oil, condensate and natural gas on a large scale. Meanwhile, fracture network system mainly parallel to laminations and overpressure would be created, forming an artificially effective hydrocarbon displacement system in the shale, and finally high-quality crude oil can be obtained. The conversion of shale oil by in-situ heating can realize the green revolution from high-energy consumption and high-pollution “ground refinery” to high-quality and clean “underground refinery”.

The organic-rich shale suitable for underground in-situ conversion must meet the following conditions: (1) The shale section should have a TOC between 6% and 8% or higher, and a Ro value between 0.5% and 1.0%; (2) the shale section should be more than 15 m thick, less than 3,000 m in buried depth, and greater than 50 km² in area; and (3) the shale section should have roof and floor of good sealing capacity, and water content of less than 5%^{[1, 30]}. At the oil price between \$60 and \$65/bbl, the economically recoverable resources of low-medium maturity continental shale oil were preliminary estimated at about (200-250)×10⁸ t, which is equivalent to the total technical recoverable resources of conventional oil. In addition, the economically recoverable resources of natural gas from the low-medium maturity continental shale were estimated at around (60-65)×10¹² m³ preliminarily, about 3 times the total resources of conventional natural gas in China. The 7th member of the Yanchang Formation (Chang 7 Member) in the Ordos Basin is the most promising stratum for in-situ conversion of shale oil, which has technically recoverable oil resources of about (400-450)×10⁸ t and technically recoverable gas resources of (30-35)×10¹² m³. At the oil price of \$60 to \$65/bbl, its economically recoverable resources are about (150-180)×10⁸ t, 4 to 5 times the technical recoverable resources of conventional oil in the Ordos Basin. The first member of the Cretaceous Nenjiang Formation (shortened as Nen 1 member) in the Songliao Basin, with relatively high organic abundance and low thermal maturity, is also an important interval for in-situ conversion of shale oil. With higher hydrogen index (HI) than the Upper Triassic Chang 7 Member in the Ordos Basin, its potential of underground in-situ shale oil conversion deserves high attention (Figs. 4 and 5). By using the pyrolysis result of the Chang 7 shale from the Ordos Basin as a reference, the resources of the Nen-1 shale in the Songliao Basin were evaluated. The result show that by in-situ conversion, the technical recoverable shale oil resources are (120-150)×10⁸ t, and the technical recoverable natural gas resources are (9-10)×10¹² m³. At the oil price of \$60 to \$65/bbl, the economically recoverable resources are at least (20-25)×10⁸ t^[1].

It should be pointed out that some technological breakthroughs should be made to solve some scientific issues before the low-medium maturity shale oil in China can be produced commercially. (1) Some issues related to the formation and distribution mechanisms of organic-rich shale remain unclear, for examples, the sedimentary paleoenvironmental characteristics of high TOC shale sections and the control factors of excessive biological flourishing are still unclear, the formation mechanism and controlling factors of lamellation in organic-rich shale aren’t confirmed yet, and the controlling factors and the environmental response of the type and heterogeneous distribution of organic matter in organic-rich shale layers are still ambiguous. (2) Kinetic mechanism of in-situ conversion and the best conversion conditions, for examples, the interaction and the relationship of driving and resisting force between organic matter and inorganic minerals under thermal conversion conditions need to be figured out, and the optimal physical and chemical window and conditions of organic matter conversion in shale still need to be studied. (3) Solutions to engineering technical problems need to be further explored, for examples, the control technique of constant and high temperature heating kilometers underground need to be researched and its stability need to be improved, the materials and manufacturing technologies for electric heating pipes need to be explored, and the precise positioning drilling technology and control system for horizontal slim hole and small-spacing boreholes (5-8 meters) need to be field tested, etc. Whether the continental low-medium maturity shale oil can enter the commercial development cycle depends on whether the cumulative production of the well group can reach commercial development scale, whether the production scale of the single well and well group is economical, and whether the durability of the downhole heating system supports the minimum hours of economical production. It is suggested that pilot tests should be conducted to develop key technical equipment and key technologies with independent intellectual property rights. Meanwhile, the criterion for "sweet spot" evaluation should be established, and the technical flow and processes should be optimized to bring about China's continental shale oil revolution as soon as possible.

Medium-high maturity continental shale oil is mainly distributed in the middle and eastern petroliferous basins with high geothermal fields, largely in the Songliao Basin and Bohai Bay Basin, etc. As mentioned above, the medium-high maturity shale oil can be developed at large scale by using horizontal well and volume fracturing. In general, medium- high maturity shale oil in China is small in distribution range, large in burial depth and limited in scale. According to preliminary estimates, its geological resources are about 100 × 10⁸ t, and the economically recoverable resources can’t be estimated before the cumulative production of a single well is confirmed to reach the economic threshold.

To realize commercial exploitation of the continental medium-high maturity shale oil in China, it is necessary to further strictly specify the conditions and evaluation criterion for area selection. First, sweet spots and target areas of test must be selected in shale formations with TOC values of 3% to 5% to ensure sufficient hydrocarbon retention and high formation energy. Second, Ro value higher than 1.0%, too low gas-oil ratio and heavier oil quality are not conducive to the flow of crude oil underground, and will directly affect the cumulative production of single well. Third, to guarantee the scale and economics of the reserves controlled by single well, the oil layers should be 5 to 10 meters thick each, and 25 to 30 meters thick combined. Fourth, both the production and cumulative production of single well must reach the economic threshold. To achieve large-scale development of continental medium-high maturity shale oil in China, several scientific and technological problems still need to be addressed. First, the evaluation and selection criterion of "sweet spot" need to be gradually established during production testing. Second, as the reservoir has strong heterogeneity, the lithological composition characteristics, quantified geochemical indexes and logging response parameters of favorable lithological facies need to be clarified while exploring. Third, the wellbore trajectory and reservoir stimulation design considering the characteristics of China's continental shale oil need to be optimized. Fourth, due to the poor mobility of shale oil and low production of single well, low-cost technologies and development methods need to be innovated to increase single-well production and cumulative production. "Long horizontal section, small well spacing, close cutting, and large-scale volume fracturing" is the current leading technical direction. Meanwhile, in order to increase single well production and single well cumulative production while reducing costs, it is necessary to actively explore the feasibility of “vertical shale oil well, small well spacing, close cutting, and volume fracturing” scheme along the horizontal bedding of shale.

On May 1, 2018, China promulgated the ore specie and definition of tight oil. In it, the tight oil is defined as the oil resource in tight reservoirs such as tight sandstone, tight carbonate, or mixed rock with a permeability of less than or equal to 0.1×10^-3 μm² under overburden pressure condition. Shale oil refers to oil resource produced from organic-rich shale formations, including petroleum hydrocarbons and bitumen that have been formed underground and unconverted organic matter. Shale oil and tight oil have two essential differences^[1]. First, they have different kinds of hydrocarbons, shale oil includes petroleum hydrocarbons and bitumen that have been generated and unconverted solid organic matter, and is generated and stored inside the source rock, while, the tight oil is all produced and discharged from adjacent shale formations, and belongs to near-source accumulation. Second, they have different natural porosity and permeability: shale oil reservoirs usually have lower porosity of less than 3% and lower permeability of less than 1×10^-9 μm²; while tight oil reservoirs have higher porosity of more than 6% and mostly over 10% and higher permeability of less than 1×10^-3 μm² generally.

The continental shale oil and tight oil differ obviously in facies belt, lithological combination and exploitation technology. First, they exist in different sedimentary facies belts. Shale oil mainly develops in the semi-deep to deep lake facies, while tight oil mainly develops in the shallow to semi-deep lake facies of the wide and gentle depression lake basin adjacent to the source shale^[31,32] and the gravity flow deposits of deep to semi-deep lake facies overlapping with shale oil facies in spatial position. Second, the shale oil reservoir has clear lithological boundary. The shale formations formed in lentic water environment are mainly composed of shale, mudstone, and chemical sedimentary rock with more than 70% of organic-rich shale. Tight oil reservoirs are mainly fine-grained clastic rocks and biogenic carbonate rocks formed by traction flow and gravity flow, and have more than 70% of non-source rock. Third, the technical boundaries are clear: medium-low maturity shale oil includes two types of materials: retained hydrocarbons and unconverted organic matter in the shale. The key point in medium-low maturity shale oil development is to reduce viscosity and upgrade the crude and convert the organic matter by heating underground. Therefore, building “underground in-situ refinery factory” is an important technical route for the development of low-medium maturity shale oil from organic-rich shale. As for the development of medium-high maturity shale oil and tight oil, what we concern most is the quantity of moveable liquid, formation energy and oil quality, and the physical properties of the reservoir and the construction of artificial fracture network system are our research focuses. In short, shale oil and tight oil should be considered separately rather than together.

Based on the control of thermal maturity on the evolutionary stages of hydrocarbon generation and expulsion of continental organic-rich shale, two major categories of shale oil, low-medium maturity and medium-high maturity are identified in China. Of them, the low-medium maturity shale oil takes the majority, and has technical recoverable resources of (700-900)×10⁸ t and economical recoverable resources at the oil price of \$60-\$65/bbl of (200-250)×10⁸ t according to preliminary estimates. Once through the pilot production test, the underground in-situ conversion makes breakthrough and can obtain economic benefit, a revolution of continental shale oil will be on the horizon. The medium and high maturity shale oil has quite huge geological resources of about 100×10⁸ t, but its technical and economic recoverable capacity is still difficult to estimate until the production and cumulative production of single well and the reserves scale of the sweet spot are determined.

China's continental medium-low maturity shale oil has its particularities in terms of exploitation methods, applicable techniques, and evaluation standard, therefore, it cannot be compared with medium-high maturity shale oil. Also, the medium-high maturity shale oil is significantly different from the marine shale oil in North America. In general, China’s medium-high maturity shale oil reservoirs are inferior in oil-layer continuity, thickness and distribution scale, source rock maturity, crude oil quality, gas-oil ratio, and formation pressure to marine shale oil in North America. Hence, when looking at the scale and resource economics of medium-high maturity shale oil resources, the economics of both daily production of single well and the cumulative production of single well should be considered. At the same time, enough attention should be paid to the scale of resources to ensure that the resource scale can meet the smallest economically-scaled construction and support stable and long- term production. It is suggested that the area selected should have high organic matter abundance, high thermal evolution (R_o value greater than 1.0%), high single well production, and high cumulative production.

Shale oil and tight oil are significantly different in distribution facies belt, lithological combination, and applicable technology. Tight oil and shale oil can be told apart according to lithological combination and distribution facies belt, and should be considered separately.