Unconventional oil and gas will be the most important alterative resources in the middle and late 21st century, and source rock oil and gas, such as shale gas, shale oil and coalbed methane (CBM), are extremely important contributors^[1]. General “exploring petroleum inside source kitchen” involves exploring tight oil and gas “near sources”, exploring shale oil and gas, and CMB “inside sources”, and developing shale oil, oil shale oil “in full sources”. This paper only covers two aspects: exploring petroleum “inside sources” and exploiting petroleum “in full sources”. To exploring oil and gas “inside sources” means to identify sweet areas and sections by entering the layer generating oil and gas. To develop oil and gas “in full sources” means to develop retention hydrocarbons and converted organic matter and try to utilize all potential oil and gas resources. The continuous and targeted research on the theoretical technology will have important theoretical and strategic significance at present and in the future. In the past 10 years, supported by the National 973 Program and the National Science and Technology Major Projects, based on the experimental bases such as the National Energy Tight Oil and Gas R&D Center, and taking the source rocks in major basins such as Ordos, Junggar, Songliao, Sichuan, and the Western Coast of the United States, experimental analysis, geological evaluation and industrial practice have been carried out on the forming and distribution and industrial evaluation of source rock oil and gas in a multi-disciplinary mode involving geology, geochemistry and geophysics; the geological connotation of source rock oil and gas has been analyzed; and the classification and comparison, the forming mechanism of “sweet sections”, the distribution law of “sweet areas” and the development prospects of source rock oil and gas resources have been investigated.

Responding to climate change and accelerating the transformation of low-carbon energy has become a global consensus. The main means is to control and reduce carbon emissions from fossil energy, and vigorously develop non-fossil energy sources such as renewable energy^[1,2].

However, objectively, fossil energy will keep increasing as a main energy source in a long time. The successful development of unconventional oil and gas further strengthens the foundation of oil and gas resources, it is predicted that oil and gas will continue to play a major role in the future, and will play an irreplaceable role in a certain period of time. Natural gas will become the simple fossil energy with the highest percent after 2030, and it will serve as a reliable bridge for the transformation of fossil energy into renewable energy^{[1, 3]}.

For China, the continuous increase of oil and gas, decrease of coal, and development of new energy in a long period of time is the key to optimizing the energy structure. The rapid development of unconventional oil and gas will be an important guarantee for the long-term stable development of oil and gas, and the important weight of national basic energy security.

Unconventional oil and gas refers to continuously distributed oil and gas resources that can be economically exploited by new technologies to improve reservoir permeability or fluid viscosity, rather than traditional techniques to obtain natural industrial production^[4] (Fig. 1). The unconventional oil and gas revolution refers to subverting the traditional oil and gas production mode, breaking the conventional permeable reservoir and the classical oil and gas trap accumulation concept, discovering industrial-grade nano-scale pores from tight sandstone and shale, innovating the theory of large-scale continuous oil and gas accumulation and the factory technology for volume fracturing of horizontal wells, transforming the traditional trap “reservoir” exploration and development mode, breaking through the technical theory of vertical-well seepage development, and achieving a major energy revolution in the industrialized production area of traditional oil and gas. According to the characteristics of migration and accumulation, unconventional oil and gas can be divided into three types: source rock oil and gas, tight oil and gas, and retained and accumulated oil and gas. Source rock oil and gas are unconventional resources that are generated and preserved in the same formation. They include shale gas, shale oil, CBM and oil shale oil, this paper discusses the first three types mainly. Tight oil and gas are the resources that are generated in source rocks and then migrate and accumulate in the reservoir near the source rock. They mainly include tight oil and tight gas. Retained and accumulated oil and gas refer to the resources accumulating in reservoirs such as near-surface layers, seabed sediments, and tundra after transporting over a long distance and experiencing water washing, degradation and thickening, and crystallization at the stable temperature and pressure zone (Fig. 1, Table 1), primarily including hydrates, oil sands and heavy oil.

In the 21st century, represented by American shale oil and gas, Canadian oil sands and Venezuelan heavy oil, a series of breakthroughs have been made in exploration and development of unconventional oil and gas which have become an important part of global oil and gas production, and deeply reshaped the global energy chart and the geopolitical pattern^[5,6,7]. Tight gas, CBM, heavy oil, asphalt sandstone, etc. are targets of exploration and development of unconventional oil and gas in the world, shale gas is a hot spot, and tight oil is a highlighted spot. With the rapid growth of unconventional oil and gas production worldwide, unconventional oil and gas are more and more prominent in the global energy supply and have become important components of oil and gas production. In 2017, global oil production was 43.9×10⁸ tons, of which unconventional oil accounted for 14%; global natural gas production was 3.7×1012 m³, of which unconventional natural gas accounted for 25%^{[1, 3, 6]}. In the past 10 years, the United States has made a “golden decade of revolutionary development” of shale gas and tight oil. Relying on three major experiences in the accumulation theory of continuous oil and gas, volume fracturing for horizontal wells and support of national policy, the United States has successively realized the revolution of shale gas and tight oil. In 2017, the US natural gas production was 7 345×10⁸ m³, of which unconventional gas accounted for 85%, including shale gas of 4 746×10⁸ m³, tight gas of 1 200×10⁸ m³, and CBM of 302×10⁸ m³. Thanks for the rapid development of unconventional gas, the natural gas production in the United States hit a record high, and made it almost sufficient by itself. In 2017, the US tight oil production was 2.36×10⁸ tons, accounting for 37% of total oil production^{[1, 3, 6]}. In general, the US unconventional oil and gas production has accounted for 70%, become the leading contributor to the production increase, and will continue promoting the implementation of the US strategy in “energy independence”.

In China, after more than 10 years, industrial development has been basically made to unconventional oil and gas. First, production of unconventional natural gas has been at industrial scale. By the end of 2016, the proven reserves of unconventional natural gas reached 4.7×10¹² m³, accounting for 36%. In 2017, the production was 499×10⁸ m³, accounting for 34%^[1]. Tight gas covers a number of gas provinces at scales of trillion m³ and even 100 billion m³ represented by the Upper Paleozoic in the Ordos Basin and the Xujiahe Formation in the Sichuan Basin. The total national production in 2017 was 360×10⁸ m³; the proven geological reserves of shale gas were up to 9 209×10⁸ m³; and three marine shale gas fields at the scale of 100 billion m³ were built in Fuling, Changning and Weiyuan in the Sichuan Basin, which produced shale gas of 90×10⁸ m³ in 2017. The development of CBM is effective, and two production bases were built in Qinshui and Ordos, with the production of 49×10⁸ m³ in 2017^[1]. Second, the development of unconventional tight oil has been initially industrialized. Tight oil has entered the reserve stage, and three billion-ton-scale tight oil fields have been built, including Xin'anbian in the Ordos Basin, Fuyu in the Songliao Basin, and Jimusal in the Junggar Basin. The production in 2017 was about 150×10⁴ tons. In addition, significant progress has been made in the exploration of natural gas hydrates. In 2017, natural gas was produced from the Miocene and Pliocene formations, 203-277 m thick, below the seabed at 1 266 m deep in the Shenhu area of ​​the Pearl River Mouth Basin. More than 23.5×10⁴ m³ of natural gas was produced in 42 days^[8]. The potential of shale oil resources is huge, and the “sweet areas (sections)” are almost clear. Pilot test feasibility demonstration on in-situ heating conversion, non-water fracturing and other scientific exploration are being carried out on the Chang 7 shale of the Triassic Yanchang Formation in the Ordos Basin. It is expected to take the lead in China to achieve the continental “shale oil revolution”^[9,10]. By the end of 2017, the production of unconventional oil and gas reached 6 600× 10⁴ tons of oil and gas equivalent, accounting for 20% of total oil and gas production, and achieved a “strategic breakthrough” in China. This provides basic conditions for revolutionary development.

Unconventional source rock oil and gas resources such as shale gas, shale oil and CBM (referred to as “source rock oil and gas”) are strategic fields for global oil and gas supply. The global unconventional oil recoverable resources are 6 200×10⁸ tons, which is equivalent to conventional oil, and of which source rock oil accounts for 50%. The global unconventional natural gas recoverable resources are 3 922×10¹² m³, which is 8 times that of conventional natural gas, and much of which source rock natural gas accounts for 77% (excluding gas hydrate)^[1,7,11-14]. In general, source rock oil and gas have huge resource scales, but the overall proven percent is extremely low. It is a pivotal component of the second 150-year life cycle of the petroleum industry and has an extremely important strategic position^[4].

The mainland of China is located at the intersection of three major plates, the Pacific Ocean, the Siberia and the India. The formation and development of sedimentary basins are diverse. They have experienced two stages of the Paleozoic marine and Mesozoic and Cenozoic continental evolutions, and formed more than 10 sets of organic-rich shale and coal series. China’s source rock oil and gas has a good resource base. Compared with the source rock oil and gas in North American, the formation and distribution of China's source rock oil and gas are more complex, and more affected by tectonic activities; the reservoir is more heterogeneous in vertical and horizontal distribution; the relationship among oil, gas and water is relatively more variable; and the evaluation on sweet sections/ areas is more difficult. The shale gas discovered is dominated by highly-to-over mature marine facies; the CBM discovered is dominated by highly mature marine and transitional facies; the shale oil discovered is dominated by middle-to-low mature continental facies (Fig. 2). According to the geological conditions, it is important for large-scale development of source rock oil and gas by strengthening the research on the distribution law, evaluation and production technologies of source rock oil and gas in China.

The study on the geological theory and method of source rock oil and gas will be the focus of future petroleum geology. Compared with conventional resources, the source rock con-taining oil and gas is characterized by nano-pore and -throat systems, continuous hydrocarbon distribution, tight reservoirs, multi-phase coexistence, inside-source accumulation, and special reservoir-forming mechanisms (Fig. 1, Table 1). These break through many limitations in petroleum geology and bring major challenges to traditional petroleum geology. To deep the research on the geological theory of source rock oil and gas and to develop the theoretical system of reconstructed petroleum geology have become the frontier and are urgent requirements for the future oil and gas industry^[4].

In the past 10 years, many significant achievements have been made to micro-characterization of nano-pore and -throat system, pore evolution and hydrocarbon occurrence in tight reservoirs, resource evaluation based on production performance, and evaluation on “sweet areas” based on the concept of continuous oil and gas accumulation^{[4, 14-31]}. These promoted the development of source rock oil and gas. As a new discipline of petroleum geology, source rock oil and gas will face many theoretical and methodological challenges such as pore development mechanism, oil and gas migration and accumulation law, and sweet evaluation and prediction. How to effectively break through the bottlenecks in exploration and promote large-scale development is the important part of geological evaluation and prediction of source rock oil and gas.

Source rock oil and gas refers to the resources generated in the source rock, and then retained or accumulated inside or near the source rock. As one of the important types of unconventional oil and gas resources, they are continuously distributed and can be produced industrially using new technologies to generate “man-made permeability” and construct “man- made reservoirs”. New technologies include horizontal-well volume fracturing, in-situ heating conversion/modification, and other industrial measures to improve permeability, reduce fluid viscosity, and increase formation energy. The geological connotation of source rock oil and gas refers to a geological discipline that studies the hydrocarbon generation model, forming mechanism, distribution law, development geology, engineering geology and development strategy of unconventional source rock oil and gas, covers shale gas, CBM, shale oil, etc., and include exploration geology and development geology. The exploration geology takes favorable resource zones with hydrocarbon occurrence as objectives to evaluate six types of properties like “lithology, hydrocarbon source, reservoir, hydrocarbon-bearing, compressibility and anisotropy” of the source rock, with the core to clarify the scope of continuous oil and gas accumulation and reserves scale. The development geology takes “sweet sections” and “sweet areas” as objectives to induce “man-made permeability” or “man-made energy field” and construct “man-made oil and gas reservoirs”, with the core to realize the large-scale and effective production of oil and gas resources.

The organic matters of source rock oil and gas are generally classified into type I and type II₁ sapropel-type lacustrine black shale, type II sapropel-type marine black shale and type III humus-type black coal series. According to the hydrocarbon generation characteristics, the source rock oil and gas can be divided into oil shale oil, shale oil with low-to-medium maturity, shale oil with medium-to-high maturity, favorable shale gas, and favorable coal-formed gas. They are produced in different windows. Based on hydrocarbon generation simulation experiments on lacustrine shale, marine shale, coal seam samples^{[10, 31-36]}, the generation, migration and accumulation models have been established (Fig. 3). The oil shale oil section is an unconverted organic-rich shale section, currently which is produced by artificial refinement from surface or near-surface layers. The shale oil section with low to medium maturity contains potential hydrocarbon from unconverted organic matter and hydrocarbon generated but not discharged. The favorable maturity window falls in the Ro of 0.5% to 0.9%, and the amount of liquid hydrocarbon retained in the window and that of unconverted organic matter are large. The shale oil with medium to high maturity contains mainly the hydrocarbon generated but not discharged. The favorable maturity window has the R_o of 0.9% to 1.3%, and the amount of liquid hydrocarbons retained in the window reaches the maximum. The favorable shale gas is mainly the gas generated but not discharged, the R_o greater than 1.3% means a favorable maturity window, and the amount of retained gaseous hydrocarbons in the window reaches the maximum. The favorable coal-formed gas is mainly adsorbed gas generated but not discharged, the R_o greater than 0.8% means a favorable maturity window, the amount of gaseous hydrocarbon adsorbed in the window gradually reaches the maximum, and the amount of coal-formed gas greatly increases.

The actual resource potential of source rock oil and gas depends on the amount of hydrocarbons that have been generated and retained in the source rock, and the amount of hydrocarbons that can be affected and produced by manual stimulation. The staged volumetric fracturing technology of horizontal wells has basically solved the large-scale development and utilization of shale oil with medium to high maturity, favorable shale gas and favorable coal-formed gas. For oil shale oil and shale oil with low to medium maturity, underground in-situ heating conversion technology may be effective in the future, but now is still under research^[9].

The coupling mechanism of rock (organic matter)-fluid- pore/fracture system in shale is a new topic of petroleum geology theory. The mechanism of near-source secondary migration and accumulation of oil and gas in the tight reservoir near the source rock is basically clear, but the mechanism of primary migration and accumulation inside the source rock is still a major scientific problem that plagues petroleum geologists. The shale burial and thermal evolution process is a complex coupling process of the solid-fluid-pore/fracture ternary system in a temperature and pressure evolution environment^[18,36]. It is often difficult to recover actual evolution of shale by conventional experimental methods and consideration of single factors^[35], and accordingly it is difficult to grasp the critical time-depth node of pore and fracture changes and hydrocarbon occurrence. Therefore, systematic research on the three elements of solid, fluid and pores/fractures at different stages of maturity is an effective way to recover the history of hydrocarbon generation and discharge of shale oil and gas and the development mechanism of pores/fractures in shale.

Compared with domestic and international shale oil, shale gas and CBM, their common characteristics are the organic- rich source rocks widely distributed and thick, with high abundance and hydrocarbon potential, dominant nano-pores, strong adsorption to hydrocarbon, high fullness in the reservoir space, continuous distribution and large resources (Fig. 4, Tables 2-4).

The “sweet section” refers to the source rock section with relatively rich or potential oil and gas accumulation and that can provide high production after stimulation. The study on “sweet section” is one of core tasks to study the geology of source rock oil and gas. The formation of “sweet section” is the result of long-term coupling between the hydrocarbon generation inside the source rock and the reservoir space or medium after long geological evolution (Fig. 5). The formation of the “sweet section” of shale oil is a process that at the peak of oil production, and after the kerogen adsorption is saturated, oil/gas migrates from the oil generation section with rich organic matter and poor permeability to the shale section with poor organic matter and good permeability at a large scale. The “sweet section” generally has better oil/gas-bearing properties and is easier to fracture than the oil generation section. The formation of the “sweet segment” of CBM is a process in which the coal-formed gas is adsorbed, in place, on the surface of the coal seam and forms a gas-rich section with high gas abundance, high formation pressure, and good preservation conditions. The sweet section and the gas generation section are the same. The formation of shale gas “sweet section” is a process of natural gas self-generating and self-storing in the black organic shale. The “sweet section” generally has high TOC, abundant joints and pores, high content of free gas, overpressure developed natural fractures, and better top and bottom plates sealing conditions (Fig. 5). The “sweet section” is generally thin, for example the shale gas “sweet section” from the Ordovician Wufeng Formation to the Silurian Longmaxi Formation is a graptolite-rich shale section 20-40 m thick and formed within 6.85 Ma. The identification of “sweet sections” plays a crucial role in targeting the source rock oil and gas layer, the selection of geophysical methods, the evaluation of the resource scale in the “sweet area”, the horizontal section and the determination of stimulated intervals.

The source rock “sweet area” refers to the relatively high- yield and enrichment area distributed in the favorable source rock, and that can be industrially improved by artificial stimulation. The “sweet area” of source rock oil and gas is generally distributed continuously in the stable structures in basin centers and slopes. The “sweet area” of shale gas is generally located in the organic-rich shale area in a semi-deep, deep, isolated and anoxic environment. For example, three “sweet areas”—Jiaoshiba, Changning-Zhaotong, and Weiyuan—have been identified in the Wufeng Formation-Longmaxi Formation in the middle and upper Yangtze Region in South China. The “sweet area” of CBM is generally located in the syncline of the coal-bearing basin at medium to high ranks, such as the San Juan Basin in the United States and the Qinshui Basin in China. The “sweet area” of shale oil is generally located in the semi-deep, deep, organic-rich shelf shale area, and semi-deep, deep, lacustrine, organic-rich shale area. In China, the shale oil is mainly rich in organic matter with low to medium maturity, at lake basin centers. The selection of “sweet area” is a core part of geological evaluation and development plan of source rock oil and gas.

In addition, it should be noted that source rock “sweet areas (sections)” have rich connotations, mainly including three aspects. “Geological sweet areas (sections)” focus on comprehensive evaluation involving the characteristics of oil and gas occurrence, the distribution of organic-rich source rocks, the distribution of top and bottom plates, natural fractures and formation energy. “Engineering sweet areas (sections)” focus on comprehensive evaluation involving the lateral continuity of target intervals, formation fractureability, and geostress anisotropy. “Economic sweet areas (sections)” pay attention to comprehensive evaluation involving the buried depth of target intervals, the scale of recoverable oil and gas resources, surface conditions and infrastructure. “Geological sweet areas (sections)” refer to the target zones (sections) where unconventional source rock oil and gas are relatively rich and high in production in the developed source rock, which can be explored and developed first under the current economic and technical conditions. Using available horizontal well volume fracturing, platform “factory” operation and other technologies, the “higher resource abundance zones (sections)” of shale gas, CBM and shale oil with medium to high maturity are generally deemed favorable zones (sections) which are thick, stable and distributed continuously, with relatively high porosity and permeability, good preservation conditions and tectonic background. These are geological properties for evaluating “sweet areas (sections)”. Under potential technical conditions, using underground in-situ heating conversion, the "sweet areas (sections)" of oil shale oil and shale oil with medium to low maturity are organic-rich shale sections which have large thickness, high organic matter abundance, stable and continuous distribution, shallow burial, and top and bottom plates with good sealing abilities. “Engineering sweet areas (sections)” mean “man-made permeability zones (sections)”, generally favorable zones (sections) with developed fractures, high content of brittle minerals and small horizontal stress difference. These are "engineering properties for evaluating “sweet areas” (sections).

The “sweet sections” of source rock oil and gas include “geological, engineering and economic sweet areas (sections)”. And only these three “sweet areas (sections)” match well and superimpose with each other, the resources can be developed effectively. “Geological sweet areas (sections)” focus on the quality of source rock (for volume fracturing on shale oil and gas, the R_o should be 0.9% to 3.5%, the TOC should be greater than 2%; for in-situ heating conversion on shale oil, the R_o should be 0.5% to 0.9%, the TOC should be greater than 6%), reservoir capacity (the porosity of shale oil and gas should be greater than 3%), seepage capacity (formation pressure, permeability, natural fractures, oil and gas quality, etc.), resource abundance (the oil/gas saturation should be greater than 50%; the resource abundance should be greater than 2 × 10⁵ t∙km^-2), resource scale (the resources should be greater than 1 × 10⁸ tons, and two years of cumulative production per well should be greater than 2.0 × 10⁴ tons of oil/gas equivalent). “Engineering sweet areas (sections) focus on rock brittleness (the brittle minerals in shale should be greater than 40%), anisotropy (the horizontal stress difference should be less than 10 MPa), burial depth (less than 3500 m), surface conditions (infrastructure, hydroelectric power supply, transportation). “Economic sweet areas (sections)” focus on changes in oil prices, market mechanisms (engineering services companies, pipeline companies, sales companies, etc.), management methods (seamless chains of development, operation, transportation, sales, etc.), policies (finance and fund for R&D of new technology), environmental protection (Environmental Protection Act, green operation).

In short, source rock oil and gas have two basic characteristics: extensive and continuous distribution, and low resource abundance; no natural industrial production, but production to be obtained by artificial stimulation. Therefore, “to find oil/gas inside the source rock” means to find “higher resource abundance zones (sections)” and those easy to induce “man-made permeability”. Based on favorable resources areas, and available and potential economic and technical conditions, evaluation and selection of “sweet areas (sections)” is the core task of “ to find oil/gas inside the source rock”, and should be carried out throughout the whole process of exploration and development.

Source rock oil and gas resources are quite abundant. Source rock oil accounts for 25% of the total recoverable oil resources all over the world, including 2 080×10⁸ tons for shale oil and 1 051×10⁸ tons for oil shale oil. Source rock natural gas is 1.5 times the total conventional natural gas recoverable resources, including shale gas of 456 × 10¹² m³, and CBM 256 × 10¹² m^{3[7, 11-14]}. The formation and development of sedimentary basins in China have undergone two evolutionary stages—the Paleozoic marine facies and the Mesozoic and Cenozoic continental facies, and the geological conditions for the formation of source rocks are better (Fig. 6). China's oil shale oil recoverable resources are 131.80×10⁸ tons, shale gas recoverable resources are 12.85×10¹² m³, and CBM recoverable resources are 12.51×10¹² m³. It is predicted that underground in-situ heating conversion may bring recoverable resources of (700-900)×10⁸ tons and natural gas of about (60-65)×10¹² m³ from the organic-rich shale with medium to low maturity^[10], and recoverable resources more than 200 × 10⁸ tons from the shale oil with medium to high maturity by volume fracturing in horizontal wells.

After the discovery of Daqing Oilfield in 1959, according to the practical experience from the Songliao Basin, Chinese petroleum ancestors summarized the theory that oil and gas migrated in a short distance, and the oil source area controlled the distribution of oil and gas fields, referred to as the “source control theory”, and proposed the exploration idea of “selecting favorable belts in defined sags”^[37]. In the past 60 years, adhering to the “source control theory”, oil and gas exploration in China has always followed the principle of looking for favorable conventional oil and gas fields in the hydrocarbon supply range based on major hydrocarbon generation sags, and many oil and gas fields have been discovered in eastern Mesozoic and Cenozoic continental and marine fault basins, western Mesozoic cratonic and foreland basins, and even foreign oil and gas basins^[38,39].

The geology of source rock oil and gas is a new development of the “source control theory” in the unconventional oil and gas stage. It is an important part of the unconventional oil and gas geology, and an emerging discipline that is booming and needs to be continuously summarized and improved. Following the principle of “exploring petroleum inside source kitchen”, the development of source rock oil and gas in North American is getting better and better. The development of source rock oil and gas in China and other several countries is just beginning, but that in many regions all over the world is still slow. The changing geological conditions and harsh technical requirements determine the theoretical and technological progress and overall development of source rock oil and gas in the world is a long-term process. Large conventional-unconventional oil and gas “coexisting basin” is the major area for developing source rock oil and gas, which has rich resource, high exploration degree, deep geology understanding and geophysics exploration data, etc. As scientists working on the development of oil and gas, and focusing on the actual geological conditions in China, it is our task to keep thinking and summarizing the geology theory of source rock oil and gas in China with long-term persistence and continuous innovation and develop foreign source rock oil and gas. The geology of source rock oil and gas will provide a new theoretical basis for following and promoting the new journey in the upstream field of the post-industry era.

Source rock oil and gas still faces challenges in geological recognition and innovation, effective evaluation on sweet areas/sections, drilling and production technologies, cost reduction and profit increase. Geological research includes characterization of source rock oil and gas occurrence, recovery of fluid migration and accumulation process, and revealing oil and gas distribution law. Evaluation on sweet areas/sections includes the description of the nano-pores and fractures system, the establishment of a reservoir evaluation system, and the search for enrichment sweet areas/sections. Drilling and production technologies include the development of rotary-steering drilling technology, and breaking the bottlenecks in core technologies. Economic evaluation involves cost reduction and profit increase, large-scale production, and strategic resource replacement plan. Development technologies for inducing “man-made fractures” or building “man- made energy field” and constructing “man-made oil and gas reservoirs” are important for the large-scale and economic development of source rock oil and gas (Fig. 7).

Source rock oil and gas development in China faces many problems. For the development of shale gas, problems lie in complex surface conditions, deep burial, poor water source conditions, imperfect pipeline network and facilities, difficult engineering technology below 3 500 m, high thermal evolution of Cambrian shale, no breakthrough in normal shale gas, and no breakthrough in transitional shale gas. Problems in the development of CBM include complex gas and water distri- bution, unclear enrichment law, unavailable technology below 800 m, and difficult reservoir stimulation. Problems in the development of shale oil include strong heterogeneity of mud shale, high content of clay mineral, low maturity of organic matter, poor fluidity of crude oil, and unsuitable application of horizontal well fracturing technology.

Recommendations on the development of source rock oil and gas in China: (1) Continue innovative research on the geology theory of source rock oil and gas. At present, the formation mechanism and distribution law of shale oil are still in the exploratory stage. The “sweet areas” and “sweet sections” with continuously distributed oil and gas still need to be verified repeatedly. The innovative research on the geological theory is still the means to guide the future development of source rock oil and gas. (2) Establish three types of development test areas. A technological breakthrough test area will study key development technologies such as for shale oil in the 7^th member of the Triassic Yanchang Formation in the Ordos Basin. An enhanced oil recovery test area will make breakthrough in key production technologies such as for CBM. A cost reduction and profit increase test area will implement integrated low-cost and large-scale development of shallow shale gas in southern Sichuan Basin. (3) Invest on bottleneck technology. Efforts will be made to strengthen the technical research on three types of bottlenecks that restrict the development of source rock oil and gas. Core technologies will be strengthened, including precise underground drilling, fine seismic modeling, geo-steering vertical and fast drilling, rotatory-steering drilling in horizontal wells. Deep shale oil and gas reservoirs will be stimulated, especially “well factory” fracturing in the shale interval 3 500-4 500 m deep. Pilot tests on in-situ heating conversion and non-water fracturing stimulation will be carried out to develop shale oil, and “shale oil revolution” will be realized in continental shale in China. (4) Promote national policy support for source rock oil and gas. The sustainable development of unconventional source rock oil and gas in the United States depends on the long-term stable national policy support, which is worth learning.

Source rock oil and gas resources have a great potential and have broad development prospects. Just as the conventional-unconventional “coexistent basins” in North America such as Appalachian, Permian, Gulf Coast, Williston where rapid development of the source rock oil and gas is going on, in the large conventional-unconventional “coexistent basins” in another more than 10 mature exploration areas, such as Central Arabia, West Siberian and Zagros in Middle East, Central Asia-Russia, Africa, South America and Australia, the source rock oil and gas will see a “golden period” of development in the future. In China, source rock oil will be the main contributor to the stable development of oil in the future; source rock gas will be the growth point of natural gas production; and it is estimated that the production of source rock oil and source rock gas will account for 15% and 30% respectively in 2030, “coexistent basins” like Ordos Basin and Sichuan Basin will make great contribution. It should be further increase the development of source rock oil and gas, promote and accelerate China's source rock oil and gas revolution, and lead source rock oil and gas to become important weights in the energy transformation stage and ensure China's basic energy security.

Source rock oil and gas are continuous resources that are generated in source rocks, and then retained or accumulated inside or near the source rocks, and can be produced by new technology. The primary task of “exploring petroleum inside source kitchen” means to find “sweat areas (sections)”, and developing retained hydrocarbon and transformed organic matter. The geology of source rock oil and gas is a new development of the “source control theory” in the unconventional oil and gas stage, and is an important part of the geology of unconventional oil and gas.

As a strategic field of global oil and gas supply, source rock oil and gas has seen four advances. (1) The geology connotation of source rock oil and gas was proposed as a geological discipline for studying the hydrocarbon generation model, formation mechanism, distribution law, production mechanism and development strategy of source rock oil and gas; the models of hydrocarbon generation, drainage and retention were established for source rock oil and gas; it was pointed out that five source rock oil and gas generation sections determined the actual resource potentials under available technical conditions. (2) The formation mechanism of source rock “sweet sections” was analyzed; it’s proposed that “sweet areas” should be considered in terms of geological, engineering and economic aspects; and the evaluation and selection of “sweet areas (sections)” is the core part of the principle of “exploring oil and gas inside source rocks”. (3) Conventional and unconventional oil and gas “coexistent basins” all over the world are major fields for the development of source rock oil and gas, and have broad development prospects. It is expected that China’s source rock oil and gas production will account for 15% and 30% respectively in 2030. (4) To promote the development of source rock oil and gas, recommendations were proposed, involving theoretical innovation, technical research, development pilot areas, and policy support.

At present, the development of source rock oil and gas also faces a series of basic theoretical and technical problems such as individual types of source rock oil and gas. At the same time, solar energy, energy storage, hydrogen energy and other new energy development, geopolitical economy, etc. have profoundly affected the fossil energy development process and production. In the future, the production of source rock oil and gas will change with the reform of fossil and non-fossil energy. This is an irreversible development trend of energy revolution.