China’ s continental oil and gas geological theory mainly experienced three stages, proposal, formation and development, and it has strongly supported the birth, establishment and development of China’ s petroleum industry system (Fig. 1).

Fig. 1.

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	Fig. 1. Continental oil and gas discoveries and the development history of geological theory of China.

Stage 1 (1940-1958): the proposal stage. Chinese geologists in this stage, represented by Sun Jianchu, Pan Zhongxiang, Sun Yueqi, Li Siguang, Huang Jiqing, Xie Jiarong, Hou Defeng, Weng Wenbo, Li Desheng, Tian Zaiyi and Qiu Zhongjian et al., did long-term arduous exploration and surveyed the oil and gas geology in the Shaanbei Plateau, the Hexi Corridor, the Sichuan Basin and southern and northern parts of the Mount Tianshan. Sun Jianchu et al. discovered the Laojunmiao Oilfield of Yumen in 1938 and Pan Zhongxiang published the first paper on continental oil generation at AAPG in 1941^{[1, 2]}, proposing the geological cognition that China’ s continental deposits could generate oil, which set China’ s confidence in continental oil and gas exploration and laid solid theoretical foundation for the correct decision of shifting oil exploration area to eastern China.

Stage 2 (1958-1990): the formation stage. A batch of petroleum experts in this stage, represented by Yan Dunshi, Qiu Zhongjian, Min Yu, Zhang Wenzhao, Hu Chaoyuan, Zhai Guangming, Li Desheng, Wu Chongyun, Huang Difan, Hu Jianyi, Dai Jinxing and Qiu Yinan et al., centering on the exploration and development of the Daqing Oilfield and integrative exploitation of the Bohai Bay Basin, formed the systematic continental oil and gas geological theory of China based on continental oil generation, structural sedimentation of continental basins in the Eastern China, the source control theory, complex oil and gas accumulation zone (belt), coal series hydrocarbon generation, continental lake basin deposits and reservoirs, and super-large sandstone oilfield development geology. All these have constantly promoted oil and gas discoveries in continental new series, new field and new basins.

Stage 3 (1990-present): the development stage. Oil experts of the new period, represented by Jia Chengzao, Dai Jinxing, Hu Wenrui, Gong Zaisheng, Sun Longde, Zhao Wenzhi, Jin Zhijun, Zou Caineng, Deng Yunhua, Song Yan and Wang Zhaoming et al, in line with the oil and gas exploration and development in continental formations of basins in the Central and Western China, coastal continental shelf, and exploration of unconventional oil and gas, proposed geological theories such as foreland thrust belts in the basin and range system around the Tibetan Plateau and the Central and Western China, coal-derived gas, large-area accumulation of lower middle-abundance lithologic stratigraphic reservoirs, continental oil and gas accumulation in the coastal continental shelf, unconventional oil and gas geology and low-permeability tight sandstone oil and gas field development, etc., and discovered a batch of large-scale continental oil and gas fields and areas such as the Kela 2 Gas Field and developed a batch of low-permeability tight oil and gas fields such as the Ansai Oilfield and the Sulige Gas Field, which guided and promoted the new march of continental oil and gas exploration and development.

1.2.1. Continental oil generation

After the discovery of continental oilfields such the Laojunmiao Oilfield, the paper published at AAPG in 1941 pointed out that “ oil can also come from freshwater deposits besides marine strata” , which was the first time the viewpoint of “ continental oil generation” was stated officially^[1]. Through years of practice and exploration, a batch of petroleum geologic research pioneers gradually realized the huge oil generation potential of China’ s continental strata^{[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]}. From the late 1950s to the 1960s, the discovery of a batch of large oil and gas fields in Eastern China such as the Songliao Basin and the Bohai Bay Basin proved that continental depositional strata could generate large amount of oil and gave rise to large oilfields, which also accumulated rich oil and gas geochemical analysis data for the establishment of continental oil generation theory. Since the 1970s, experts systematically studied the formation condition for continental source rocks and the hydrocarbon generation mechanism of organic matter, established systematic indicators and parameters for continental source rocks and oil and gas geochemistry series, and set up some important continental hydrocarbon generation theories such as continental kerogen hydrocarbon generation, coal-generated hydrocarbon, immature-low maturity oil, hydrocarbon generation dynamics of overpressure basins^{[12, 13, 14, 15, 16, 17]}. The continental oil generation theory has been developed gradually along with exploration practice, which has played an important role in China’ s oil and gas exploration in continental strata, further developing oil geological theory, making great contributes to world oil industry development.

1.2.2. Structural sedimentation of continental basins in Eastern China

The period before the 1980s was a period when China’ s geotectonics theory flourished and numerous theories with Chinese characteristics were proposed^{[3, 4, 18, 19, 20, 21]}. Based on geomechanics, experts proposed the Neocathaysian structural system of Chinese Mainland and pointed out that the Neocathaysian subsidence zone in Eastern China contained rich oil resources^[3]. Based on the plate tectonics theory, they pointed out the relationship between the Mesozoic-Cenozoic tectonic regime and oil and gas basins in Eastern China^{[22, 23]}. These theories explored the formation mechanism and evolution process of continental basins from the perspectives of structural system, regional geotectonics and plate tectonics, which guided the study on the structural sedimentation characteristics of the depression basins and rift basins in Eastern China as well as the exploration practice of oil and gas reser-voirs^{[3, 4, 13, 22, 24, 25, 26, 27]}.

1.2.3. Source control theory

After the discovery of the Daqing Oilfield in 1959, Chinese petroleum geologists, based on the oil characteristics of the Songliao Basin that the oil-gas migrated a short distance and accumulated locally in and around the favorable zones for oil generation after the formation of source rocks, reached the rule of controlling the distribution of oil and gas fields by oil generation zones. They proposed the geological idea of “ source control theory” in 1963, stressing that “ oil and gas fields are distributed around the oil generation center and strictly controlled by the oil generation zone, and oil and gas reservoirs are distributed around the oil generation center in a zonal pattern” ^[8]. Guided by this theory, China’ s oil and gas exploration always followed the principle of seeking main source rock centers and series and searching favorable targets within the oil and gas generation range. Afterwards, a large batch of oil and gas fields were discovered in China’ s Mesozoic-Cenozoic and the Upper Paleozoic continental series^{[8, 9]}. The “ source control theory” has been showing vigorous vitality, promoting the oil and natural gas production in China to achieve hundred million tons and hundred billion cubic meters respectively.

1.2.4. Complex oil and gas accumulation zone (belt) in graben basins

From 1960 to the 1970s, focusing on the graben basins in the Bohai Bay, experts proposed the complex oil and gas accumulation zone (belt) theory. This theory suggested that faults developed due to the intense block movement in the Bohai Bay Basin, lithology and lithofacies varied greatly, with a lot of stratigraphic overlap, unconformities and hiatuses. Under the background of secondary structural belt, there extensively developed anticline structures of continental sequence, block traps and lithological stratigraphic traps, forming the complex oil and gas accumulation belts (zones) with overlapping of different series and different trap types^{[27, 28, 29]}. This theory enriched the continental oil geological theory, having important significance to the exploration of the Shengli Oilfield and even the Bohai Bay Basin, and guiding the oil and gas exploration in continental graben basins.

1.2.5. Continental lack basin sedimentary system and reservoir system

From the 1950s to the 1960s, experts studied the lake basin sedimentary background and lithofacies paleogeography of the Songliao Basin, Ordos Basin and Junggar Basin, which played an important role in determining favorable oil generation and storage facies belts and providing the exploration direction. From 1970 to the 1980s, China’ s continental sedimentology theory system was established systematically: they established the large-scale lake basin delta system and the complex model in depression basins, and pointed out that multiple sources, multiple sedimentary systems and ring distribution of facies belts were the typical sedimentation patterns of lake basin; they established the fan delta sedimentary system of rift basins and underwater fan sedimentary system pattern, which laid the foundation for the proposal of complex oil and gas accumulation zone theory and progressive development idea, and opened new fields of continental oil detection for gravity flow deposit of lake basins; they successively studied and put forward river sedimentary system, alluvial fan sedimentary system, salt lake sedimentary system, lake carbonate rock sedimentary system, coal-bearing basin sedimentary system and the filling patterns^{[30, 31, 32]}; and they proposed the new view and pattern of secondary pore formation mechanism and the norms to divide clastic rock diagenetic stages^{[33, 34]}.

1.2.6. Oil and gas accumulation in the coastal continental shelf

Since the 1960s, starting from the middle depression basin in the Bohai Sea, experts proposed the theory that the neotectonic movement in the Tancheng-Lujiang fault zone controlled the oil and gas accumulation in the Eastern Bohai Sea, the theory of oil and gas prospecting in two basins in the coastal waters of China and other geological theories^{[35, 36]}, which promoted oil and gas exploration discoveries in the coastal waters of China.

1.2.7. Theory for super-large sandstone oilfields development in continental lake basins

Specific to the strong heterogeneity characteristic of large continental sandstone reservoir in the Songliao Basin, through over 60 years of development, in line with the geological characteristics of continental sedimentary facies and sedimentary microfacies of the Daqing Oilfield, the waterflooding model of tapping remaining oil featuring “ four fines” (fine reservoir description, fine adjustment of injection and production system, fine injection and production structure system and fine production management) has been worked out; a complete set of techniques coring at multidisciplinary reservoir fine description, well pattern optimization and adjustment of multi-layered sandstone oilfield series, and fine layered water injection has been innovated; the chemical flooding techniques represented by polymer flooding and ASP flooding have been developed and promoted at large scale^{[37, 38]}, making Daqing Oilfield lead in the water flooding and chemical flooding field in the world. All these technologies have provided solid theoretical and technical support for Daqing Oilfield to keep the annual crude oil consecutive production of over 50 million tons for 27 years and over 40 million tons for 12 years.

In the late 1980s, with the increasing oil and gas exploration degree in the Eastern China, there was a severe shortage in reserves and the state decided to accelerate the exploration in the basins in the Central and Western China. Guided by the strategic policy of “ stabilizing the east, developing the west” and “ simultaneously developing oil and gas” , the continental series in the basins of Central and Western China became the major fields of China’ s oil and gas exploration. Aiming at searching large oilfields, the exploration intensity was obviously enhanced and more and more reservoirs were discovered, which rapidly increased the reserves and guaranteed the sustainable development of national oil and gas resources (Table 1). Since 1990, 4 major gas areas with reserves of trillion cubic meters and 6 major oil areas with reserves of 1 billion tons have been discovered in continental series in Central and Western China. By the end of 2015, the continental series in these areas had cumulative proved natural gas reserves of 5.75× 10¹²m³ and proved oil reserves of 80.21× 10⁸t, taking 56% and 33% of national natural gas and oil reserves respectively.

Table 1

Table 1

Table 1 Major continental oil and gas discoveries in Central and Western China since the 1990s (by the end of 2016). No. Basin Stratum Structural unit Geological resources Representative oil and gas field Discovery well Proved reserves Annual oil and gas Oil/ 108 t Gas/ 1012m3 Oil/ 108 t Gas/ 1012m3 Oil/ 108 t Gas/ 1012m3 1 Tarim Cretaceous and Paleogene Kuqa depression 7.64 6.99 Kela 2, Keshen, Dabei and Dina Kela 2, Keshen 2, Dabei 1, Dina 2, Yinan 2 0.25 1.03 182 2 Southwest depression in the Tarim Basin 9.40 1.70 Akmomu, Kekeya and Kedong 1 0.32 0.10 3 Ordos Mesozoic Yishan slope 128.50 Jiyuan, Xifeng, Huaqing, Huanjiang 45.22 2500 4 Upper Paleozoic Yishan slope 15.16 Sulige, Yulin, Daniudi, Wushenqi Su 6, etc. 6.23 (including basically proved reserves of 3.13) 246 5 Sichuan Triassic Depression in the Central Sichuan Basin 3.15 Guang’ an, Hechuan and Anyue Guang’ an 2, Hechuan 1 and Anyue 2 0.69 2 6 Junggar Permian and Triassic Mahu sag 46.00 Madong and Maxi Ma 131, Jinlong 2, Ma 18, Bai 25, Aihu 2, Mahu 1 2.10 7 Mesozoic Central depression, Luliang uplift and eastern uplift 35.26 1.18 Luliang, Cainan, Mobei, Shinan Lu 9, Caican 2, Mobei 2, Shinan 4 5.76 0.13 8 Permian Jimusar sag and Mahu sag 20.00 Jimusar and Fengcheng Ji 174 and Fengcheng 2 9 Qaidam Paleogene and Neogene Depression in the Western Qaidam Basin 21.70 Gasikule, Kunbei and Hongliuquan 6.20 223 10 Basement and Jurassic Faulted bock belt in the north rim of the Qaidam Basin 1.18 Dongping Dongping 1, Niu 1 and Jiantan 1 0.05 2 11 Paleogene and Neogene Depression in the Western Qaidam Basin 8.58 Zhahaquan Zha 2, Zha 7, Zhatan 1 0.49 12 Tuha Jurassic and Triassic Taibei sag 10.10 0.75 Shanshan and Lukeqin Taican 1 and Yudong 1 4.00 0.06 13 Santanghu Permian Malang sag and Tiaohu sag 4.50 Beixiaohu and Niujuanhu 1.58

Table 1 Major continental oil and gas discoveries in Central and Western China since the 1990s (by the end of 2016).

Big breakthrough was made in natural gas exploration in the Cenozoic sandstone reservoir of the Kuqa depression in the Tarim Basin. The Kela 2 Gas Field was discovered in 1998, and the Keshen Gas Field was discovered in 2008, giving rise to the major gas area of trillion cubic meters of the Kelasu structural belt. This directly promoted and guaranteed the startup and gas source supply of the project of natural gas transmission from West to East China and changed the setup of China’ s natural gas utilization too. The Kela 2 Gas Field has a gas-bearing area of 47 km², gas column height of 500 m, geological reserves of 2 840.29× 10⁸m³ and recoverable reserves of 2 290× 10⁸m³. In July 8, 2017, the cumulative gas production of the Kela 2 Gas Field achieved over 1 000× 10⁸m³, in which the production of Well Kela 2-7 was 125× 10⁸m³, making it the heroic gas field for the project of natural gas transmission from West to East China; 19 reservoirs were discovered in the Keshen Gas Field, with proved natural gas reserves reaching 8 313× 10⁸m³; the natural gas production of the Kuqa depression in 2016 was 182× 10⁸m³; comprising 77% of that of the Tarim Basin.

Two major gas areas with reserves of trillion cubic meters, namely the Sulige and the eastern Ordos, have been built, where the natural gas in the Upper Paleozoic tight sandstone reservoir is distributed continuously, with proved natural gas reserves of 6.23× 10¹²m³ (including the basically proved reserves of 3.13× 10¹²m³), making it the largest natural gas area in China. The scale in the Mesozoic clastic rock oil reservoir in the south central Ordos Basin has been growing quickly, and two large-scale oil reserve areas at the magnitude of 20× 10⁸t, Jiyuan area and Longdong area, and two at the magnitude of 10× 10⁸t, Zhijing-Ansai area and Huaqing area, have come into being.

A batch of efficient oilfields such as Cainan Oilfield, Shinan Oilfield, Luliang Oilfield and Mosuowan Oilfield were discovered in early stage in the Junggar Basin. And in recent years, big breakthroughs have been made in oil exploration in the Permian and Triassic sandy conglomerate reservoirs in the eastern and western slopes of the Mahu sag, and the annual crude oil production of the basin had maintained at above 10 million tons for 15 consecutive years by 2016.

Several gas fields at the magnitude of 100 billion cubic meters, such as the Guang’ an Gas Field, Hechuan Gas Field and Anyue Gas Field, were successively discovered in the Upper Triassic Xujiahe Formation in the Central Sichuan Basin, forming a tight sandstone gas area at the magnitude of trillion cubic meters, with the three-level reserves of natural gas reaching 1.39× 10¹²m³.

The proved oil reserves in the Tertiary system of the southwestern Qaidam Basin reached 6.2× 10⁸t, and a large- scale oil area with the reserves of 100 million tons has been discovered in the Yingxiongling structural belt and multilayers tight oil bearing blocks have been discovered in the Zhahaquan in recent years; the natural gas reserve scale of 100 billion cubic meters has been ascertained in the piedmont Paleo-uplift basement and the Jurassic system of the Altun Mountains.

Well Taican 1 in the Taibei sag of the Tuha Basin produced 35 m³ of oil a day from the Jurassic Sanjianfang Formation, leading to the discovery of Shanshan Oilfield, and unveiling the oil exploration in the Mesozoic coal series; new breakthroughs have been made in the Malang and Tiaohu sags of the Santanghu Basin, liberating the Permian source reservoirs, and the cumulative proved oil reserves of the continental series of the two basins reached 5.6× 10⁸t.

The continental series in Central and Western China still have huge potential in oil and gas exploration, and a batch of favorable zones and belts with advantageous reservoir forming conditions can sustain the current oil and gas reserve and production level. The potential areas of natural gas exploration are mainly distributed in the northern mountain front and southern slope of the Kuqa depression of the Tarim Basin, the favorable structural belts of the hydrocarbon generation center zone on eastern and western ends of the mountain front of the Southwest Tarim Basin, the Upper Paleozoic erathem of the Southern Ordos Basin, the coal series in the Xujiahe Formation of the Central Sichuan Basin, the gas-bearing sections in Jurassic of the Altun Mountains; the potential areas of oil exploration are mainly distributed in the lower part of the Mesozoic Yanchang Formation and the Chang 7 Member in Ordos Basin, the Permian and the Triassic system of the Mahu-Well Pen 1 of western sag in Junggar Basin, the Middle Permian Lucaogou Formation of the northern Xinjiang in Eastern Junggar Basin, the Jurassic-Cretaceous system in the middle superficial layer in the hinterland of Junggar Basin, oil-bearing series in the deep lake carbonate rock in southwest Qaidam Basin. Unconventional oil such as tight oil, shale oil, oil shale oil and heavy oil will be important supplements or substitute resource types.

Through reviewing the exploration of Mesozoic-Cenozoic continental series in the basins of Central and Western China^{[39, 40, 41, 42]}, the geotectonic background and basin structure characteristics of the continental basins in Central and Western China have been identified, theoretical recognition on regional structures such as the Mesozoic northern Tethys basin group and the Cenozoic basin and range system around the Tibetan Plateau have been proposed and the prototypes and transformation characteristics of continental basins in the Central and Western China have been clarified.

2.2.1. Regional structure theory

2.2.1.1. The northern Tethys basin group

The theory defines the process of expansion and extinction of the Tethys oceanic crust and the collage and accretion of the Eurasia since the Permian and divides the sedimentary basin groups in the Tethys structure domain^{[39, 41]}. It also clearly proposes the concept of northern Tethys basin group and points out that the basins in Central and West, including the Karakum Basin, the Afghanistan-Tajikistan basin and the Tarim Basin, were sedimentary basin groups at the northern continental margin of Paleotethys (Fig. 2). It also points out that the northern Tethys basin group is an important huge natural gas accumulation area in the world, and Tarim, as a part of this area, has broad natural gas exploration prospects.

Fig. 2.

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Fig. 2. Spatial distribution of the Tethys structure domain[44]. 1— Northern Caspian Basin; 2— Kula Basin; 3— Southern Caspian Basin; 4— Persian Gulf Basin; 5— Karakum Basin; 6— Afghanistan-Tajikistan Basin; 7— Syr Darya Basin; 8— Turgay Basin; 9— Sarysu Basin; 10— Tengiz Basin; 11— Fergana Basin; 12— Balkhash Basin; 13— Zaysan Basin; 14— Junggar Basin; 15— Tuha Basin; 16— Tarim Basin; 17— Qaidam Basin; 18— Ordos Basin; 19— Sichuan Basin; 20— Changtang Basin; 21— Helmand Basin (Afghanistan block); 22— Indus Basin; 23— Ganges Basin; IB— Iran Block; CB— Changtang block; LB— Lasa block; TB— Turkey block; YB— Yangzi block.

2.2.1.2. Basin and range system around the Tibetan Plateau

The theory comprehensively divides the Cenozoic Himalayan movement stages in China and points out the structural deformation dynamics mechanism and four structural response regions of the uplift of the Tibetan Plateau, the coupling of basin and orogenic belt and the eastern tension activity when the Chinese Mainland was under the control of dual dynamic systems of the Indian-Tibet collision and the subduction of the Pacific Plate^[43]. It clearly defines the concept of basin and range system around the Tibetan Plateau and the connotation of regional structure^[40]. This theory brings the Paleozoic marine basins and Mesozoic-Cenozoic continental basin groups into Central and Western China in the unified continental dynamics system and points out that the hydrocarbon-bearing basin group inside the basin and range system is China’ s largest natural gas accumulation area, containing over 60% of the total natural gas resources.

2.2.2. Prototype basin types

The central and western Mesozoic-Cenozoic continental basins in China were mainly controlled by the evolution of the paleo-Asian and the Tethys structure domain^{[39, 41, 44]}and formed the sedimentary basin filling featuring foreland basins and down-warped basins.

2.2.2.1. Foreland basins

Since the Mesozoic, prototype foreland basins, such as the Mesozoic early-stage peripheral foreland basins and retroarc foreland basins and the Cenozoic late-stage rejuvenated foreland basins^{[39, 44]} with typical intracontinental structural characteristics, respectively developed in central and western regions. The classification system of “ two stages, three categories and four combinations” of foreland basins in Central and Western China has been established^{[42, 44, 45, 46, 47, 48, 49]}, which has theoretically deepened the geological connotation of foreland basins and comprehensively expanded oil and gas exploration of continental basins in Central and Western China.

2.2.2.2. Large depression basins

Under the control of the evolution of the Tethys structure domain, there mainly developed depression basins from the Jurassic to the Cretaceous period^[41]. During the Jurassic, the northern Tethys basin group included mainly intracontinental large depression basins formed under the background of regional extension, and there developed a set of regional fluvial, lake and swamp facies deposition which can be correlated across the region; while passive continental margin basins of marine facies were created in the Changtang and Lhasa area in the Tethys structural belt^[50]. During the Cretaceous, structural sedimentation of central and western continental basins differentiated a lot, there still developed large spreading depression basins in the Tarim and Junggar areas^[51]; there developed small rift basin groups in the Jiuquan and Ejin Banner areas; while the Qaidam Basin and the Sichuan Basin were compressional depression basins, with molasses sediments of coarse debris developed.

2.2.3. Structural transformation

The basin and range system around the Tibetan Plateau has three basic structural units, namely the small craton basement, the rejuvenated orogenic belt and the foreland basin (thrust belt) group^[41], and experienced the early plate structure and late intracontinental structure processes as a whole, resulting in distinct sedimentation superimposition and structural transformation characteristics.

2.2.3.1. Superimposed basins

The shallow formations of Tarim Basin, Sichuan Basin and Ordos Basin are continental foreland basins or depression basins while the deeper formations are marine craton basin structure, with distinct superimposed basin characteristics^{[47, 48, 52, 53]}. The theory systematically analyzes the concept of superimposed basin and further divided the basic types and structures of China’ s superimposed basins, which enriches basin types and structures^[54]. The systematic analysis on superimposed basins reveals the dynamics process of multi-stage formation, evolution, transformation and maintenance of central and western sedimentary basins, which is of great guiding significance for oil and gas exploration.

2.2.3.2. Thrust fold structure

There developed more than ten foreland thrust belts between basins and orogenic belts in the basin and range system around the Tibetan Plateau (Fig. 3), basically showing the complicated geological structure characteristics of “ vertically layered, horizontally zoned and sectioned along the strike” . On the whole, the western region (southwest Tarim-Kuqa- southern Junggar) represents compression-spreading deformation, the central region (western Qaidam-northern Qaiam- Jiuquan) represents nappe-accretion deformation, and the eastern region (western Sichuan-western Ordos) represents thrust strike-slip-lifting deformation^[40]. The tectonic movements in this area show multi-layer slip, multi-direction strike-slip and multi-stage thrust, and the area experienced various dynamics and kinematics processes such as slanting compression, overthrust nappe, slip thrust and basemen involvement, forming various types of structures, such as compression fault-related fold, slip thrust and salt-related structures^{[55, 56, 57]}.

Fig. 3.

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	Fig. 3. Cenozoic foreland basin and thrust belt distribution diagram in Central and Western China.

In recent years, seismic method, well logging, remote sensing, physical simulation, numerical simulation and nanotechnology are widely applied to the research on sedimentation and reservoir, with research scales spanning six levels from kilometer, to meter, centimeter, millimeter, micrometer and nanometer, consequently, continental basin sedimentation and reservoir research has entered the new industrial application and development stage^[58] and achieved significance progress.

2.3.1. Lake basin sedimentary system

First is the establishment of new models of lake basin multi-type sedimentary systems and lithofacies paleogeography recovery. Combining with modern sedimentation investigation, flume experiment, fabric feature dissection and genetic model research of different sedimentary facies, the theory has perfected the lake basin sedimentary system pattern and proposed new sedimentation models such as sandy debris flow^[59], large shallow-water delta^{[60, 61]}, beach-bar sedimentary system^[62], density flow^[63] and coarse-grained fan^[64], revealing the distribution rules of different types of lake basin sedimentary systems and providing an important basis for objective evaluation in the oil and gas exploration.

Second is source-conduit-sink system research and basin prototype analysis, which focuses on the driving mechanism of sediments in the whole “ source-to-sink” process, the evolution and recovery of Paleo-provenance and the re-establishment of paleo-river system^[65]. It is believed through research that the late Triassic sediments all have the characteristics of “ big prototype basins and small modern basins” . For example, there existed two centers (the western Sichuan and southern Sichuan) in the depositional stage of the Xujiahe Foramtion in the Upper Yangtze platform, and there develop large shallow-water delta sedimentary systems both in the modern Sichuan Basin and the peripheral basins. In the depositional stage of the Upper Triassic Yanchang Formation, the North China platform was shallow in lake water, steep in the south and gentle in the north, deep in the south and shallow in the north and open to the west (Fig. 4), with area far exceeding the distribution range of the modern Ordos Basin.

Fig. 4.

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	Fig. 4. Sedimentary facies of Upper Triassic Chang 6-Chang 7 members in the North China platform.

Third is the establishment of the “ mixed source” fine-grained sedimentation pattern and the distribution prediction of organic-rich shale. The black shale shows strong heterogeneity, cyclic development and sectioned enrichment of organic matter and is found in fresh water basins, salt water basins as well as alkaline lake basins^{[58, 61, 66]}.

2.3.2. Clastic rock reservoirs

First, the diagenetic evolution sequence and the pore development rules in different environments have been studied, and the diagenesis patterns of clastic rock reservoirs in fresh water-brackish water, salt lake alkaline water and acidic water have been worked out^[34]; second is the heterogeneity evaluation research of reservoirs, the effects of lithology, lithofacies and diagenesis on heterogeneity of reservoirs have been examined. For example, sedimentary facies belt, matrix content and diagenesis are the main controlling factors for the Baikouquan Formation conglomerate reservoir of the Mahu sag. Third is that new progress has been achieved in numerical simulation of diagenesis and quantitative evaluation on diagenetic facies, the relationship between core diagenetic facies and logging and seismic attribute has been established, which can help predict development zones of favorable reservoirs^[67].

2.3.3. Unconventional reservoirs

With the development of petroleum industry and the constant deepening of exploration, unconventional oil and gas is becoming an important exploration and development field^[68]. Compared with conventional sandstone reservoirs, unconventional tight reservoirs have such characteristics as small pore throat size, strong heterogeneity and complicated pore struc- ture. The micro-nano pore throat system is an important characteristic of unconventional reservoirs, and the pore types in unconventional reservoirs include primary intergranular pore, intercrystal pore, intergranular corrosion pore, intragranular pore and organic pore^[69]. Field emission scanning electron microscopy, nanometer CT and other analysis means were used to systematically characterize three-dimensional pore structure of shale, tight sandstone and diamictite reservoirs, the nanometer pores of industrial value were found, the lower limits of the filling pore throat diameter of continental shale gas, shale oil and tight oil were demonstrated, and the resource potential of unconventional oil and gas have been expanded greatly^[70].

2.3.4. Deep reservoirs

The geological environment of deep reservoirs is quite different from that of the shallow formations in heat, structure and fluid. Based on the cognition that there developing abnormal pore belts in the deep formations (burial depth above 3500 m) of the Bohai Bay Basin in Eastern China^[71], researches, such as the basin dynamics background of reservoir formation, diagenesis and diagenetic sequence, reservoir space type and genesis, reservoir evaluation and prediction have been carried out specific to deep clastic rock reservoirs in the Kuqa depression of the Tarim Basin and the Junggar Basin^[72]; it is proposed that the long-time shallow burial and late-stage rapid deep burial pattern is favorable for the preservation of pores in the deep clastic rock reservoirs, and the dissolution and fissures improved the physical properties of the reservoirs^{[61, 73]}, which revealed the protective effect of early-stage overpressure and oil and gas filling on the pores in deep sandstone reservoirs, thus the limit depth of oil and gas exploration has been increased significantly^[74].

Since the 1990s, significant progress has been made in research on continental hydrocarbon generation, mainly in aspects such as coal-derived hydrocarbon, hydrocarbon generation evolution of source rocks in continental saline lack basins, hydrocarbon expulsion mechanism of source rock and shale oil resource evaluation^{[75, 76]}, providing resource basis for large-scale natural gas exploration in the coal-measure strata in central and western basins, persistent oil and gas exploration in saline lake basins and continuous unconventional oil exploration.

2.4.1. Coal-derived hydrocarbon geology

In the 1980s-1990s, coal-derived hydrocarbon geochemistry theory was formed preliminarily^{[75, 76, 77, 78]}, which showed that coal-measure source rocks could generate oil and gas but mostly gas^[12]. Since late 1990s, the pattern of coal-derived gas (hydrocarbon), accumulation rules of coal-derived gas, gas reservoir types, natural gas accumulation belts and domains, etc. have been developed and perfected. The main controlling factors of China’ s large gas fields in quantitative and semi-quantitative forms were advanced. For example, large gas fields are mainly developed in the gas generation center where the gas generation intensity is above 20× 10⁸m³/km² and its periphery, the areas with ripening rate of gas source rocks of above the 0.05%/Ma, reservoir-forming period later than 65 Ma, direct cap shale rocks with depth over 20 m, and the favorable reservoir (porosity of dolomite over 3.7%, porosity of sandstone over 5%) are likely to contain large gas fields, which has made great contributions to major gas discoveries in Central and Western China^{[12, 75]}. Besides, natural origin identification chart I was established, developing the identification index and chart, relationship of δ ¹³C-R₀^[79]. The formation and development of coal-derived gas theory explores new field for natural gas exploration and provides resource basis for large-scale gas exploration in coal-measure strata in central and western basins.

2.4.2. Source rock hydrocarbon generation of continental saline lake basins

Source rocks in saline lake basins are important continental oil and gas source and attracted high attention in the “ immature-low mature oil” research in the end of the 20st century^[80]. Progress has been made mainly in two aspects with the exploration in western Qaidam and northwestern Junggar areas. One is that high-quality source rocks can develop in saline lake basins. For example, new well source rock researches show that there exist high-quality source rocks with relatively high abundance and the organic matter is dominated by type I and type II and is high-quality sapropelic organic matter, which improves the oil and gas resource potential in saline lake basins^[81]. For example, the high-quality source rocks discovered in the ancient alkaline lake of the Permian Fengcheng Formation in the Junggar Basin, as a new end member type of continental oil generation, has the unique “ dual peak” efficient oil generation pattern, which provides resource basis for the large-scale exploration of China’ s saline lake basins^[82]. The other is that source rocks of saline lake facies have multiple oil and gas generation stages and multi-stage hydrocarbon generation pattern of source rock in saline lake basins has been established (Fig. 5). For example, both soluble organic matter and insoluble organic matter (kerogen) of saline lake facies have strong oil generation potential, the former could form immature-low maturity oil, while the later has hydrocarbon generation process similar to that of other types of kerogen. For example, organic matter of saline lake facies could generate biogas in the immature stage, which formed industrial accumulation in the Quaternary system of the Qaidam Basin. Based on these cognitions, the hydrocarbon generation mechanism of saline lake facies source rock in the Paleogene of the western Qaidam Basin and the Fengcheng Formation of the northwest Junggar Basin was re-recognized, laying the theoretical foundation for re-evaluating the oil and gas potential in saline lake basins.

Fig. 5.

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	Fig. 5. Multi-stage hydrocarbon generation pattern of source rock of saline lake facies.

2.4.3. Hydrocarbon expulsion of source rock and shale oil potential evaluation

Foreign hydrocarbon expulsion mechanism and efficiency research were started almost simultaneously with hydrocarbon generation dynamics research. Since the 1990s, the hydrocarbon expulsion efficiency plate of source rocks was preliminarily established as an important parameter for evaluation of conventional oil and gas resources^{[83, 84]}. Since the 21st century, unconventional shale oil and gas exploration has developed rapidly, and the hydrocarbon expulsion mechanism and efficiency research has further deepened, and more attention is paid to the essential characteristics and recoverability of residual hydrocarbon in source rocks^[85]. Geochemical analysis indicates that hydrocarbon components in mudstone with high TOC and sandstone interlayers vary a lot, indicating oil and gas expulsion and interlayer lateral migration of oil and gas^[68]. Through hydrocarbon expulsion mechanism research, continental shale gas resources in the Ordos Basin, Junggar Basin, Santanghu Basin and Sichuan Basin have been re-evaluated, revealing that China’ s continental shale oil continuously accumulated in the center of lake basins and the recoverable shale oil quantity is preliminarily estimated at (30-60)× 10⁸t^{[68, 86]}.

The continental series of strata in China’ s central and western basins are widely distributed, with low exploration degree and rich oil and gas resources. To open up this oil and gas strategic replacement field, the key is to study, judge and optimize the reservoir forming and accumulation conditions in key basins and key series of strata. Since the 1990s, as oil and gas exploration has constantly deepened in central and western basins represented by the Tarim Basin and a large amount of first-hand seismic and drilling data has been obtained, the reservoir forming conditions and distribution pattern of continental hydrocarbon-bearing strata are increasingly clear, forming a complete set of geological cognitions on the continental oil and gas accumulation rules in central and western basins, which has effectively guided the large-scale and rapid exploration of foreland thrust belts, lithological stratigraphic reservoirs and unconventional oil and gas (Fig. 6).

Fig. 6.

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	Fig. 6. Distribution of continental oil and gas resources, reserves in the main basins of Central and Western China (including continental conventional and unconventional oil and gas).

(1) There mainly developed two categories of continental hydrocarbon source strata, the Upper Paleozoic-Mesozoic coal-measure and Mesozoic-Cenozoic lake mudstone. The horizontal oil and gas system distribution and vertical favorable reservoir-caprock assemblages in key basins have been pointed out, and the main exploration direction has been clarified. For example, the Kuqa depression has Triassic-Jurassic coal-measure source rocks, and there vertically developed the reservoir-caprock assemblage of Cretaceous-Paleogene sandstone reservoir and Paleogene gypsum-salt layer, and the two large sets of hydrocarbon-bearing systems of the Upper Paleozoic coal-measure source rock and Mesozoic lake mudstone in the Ordos Basin, the large-area favorable source-reservoir assemblage of sandwich structure in the Xujiahe Formation coal measure in the central Sichuan Basin, the two major hydrocarbon-bearing systems of the Permian and the Jurassic systems in the Northern Xinjiang and the three sets of hydrocarbon-bearing systems of the Jurassic, Tertiary and Quaternary systems randomly distributed horizontally in the Qaidam Basin^[87].

(2) The foreland thrust belt is the main structural unit for continental oil and gas accumulation in the central and western foreland basins. Fault-related fold theory, ultrahigh-pressure gas field formation theory etc. have effectively guided the natural gas exploration and discovery in the Kuqa depression, southwest Tarim Basin and the piedmont of Altun Mountains. Source rock, reservoir-caprock assemblage, structural characteristics and preservation conditions are the main controlling factors for petroleum entrapment, and the space-time configuration of thrust belt caprocks, reservoirs and faults linking source rocks control the zoning, sectional characteristics and accumulation rules of natural gas^{[42, 44]}.

(3) Lithological stratigraphic reservoir is a major hydrocarbon accumulation type in slope-depression area in central and western basins. The large-area accumulation of low-middle abundance lithologic stratigraphic reservoirs^[88]has effectively promoted the significant change from exploration of structural reservoirs to lithological stratigraphic reservoirs in the central and western continental oil and gas exploration, largely expanded oil and gas exploration fields and obtained a series of major discoveries, including natural gas in the Upper Paleozoic delta plain-front sandstone in the Ordos Basin, sandy debris flow sandstone oil in the delta front of the Yanchang Formation, the oil in the Mahu delta front conglomerate of the Junggar Basin and natural gas in the delta plain-front sandstone of the Xujiahe Formation in the central Sichuan Basin.

(4) Unconventional oil and gas is an important new field in the continental oil and gas exploration in central and western basins. The theory of coalbed methane in medium-high coal rank supports the establishment and development of China’ s coalbed methane industry^[89]. New theories and new cognitions^{[68, 86]}, such as continuous-type hydrocarbon accumulation, nanoscale pore throat system, “ sweet area (interval)” , “ orderly accumulation of conventional-unconventional oil and gas and “ artificial reservoirs” , have promoted the industrial test and effective recovery of unconventional oil and gas resources of the Chang 7 in the Ordos Basin, the Lucaogou Formation in the Junggar Basin, the Tiaohu Formation in the Santanghu Basin, the Tertiary system of the southwest Qaidam Basin and the Jurassic system of the Sichuan Basin.

Over the past 20 years, centering on the development of oil and gas in low-permeability tight sandstone in central and western basins, the theory and technology series for low-permeability oil and gas fields (tight oil and gas) have been established represented by the Ordos Basin^[90]. Oil and gas fields such as the Ansai and Sulige have been successfully developed and the lower limit for low-permeability oil and gas resource development has been extended from 50.0× 10^-3 μ m² to 0.3× 10^-3 μ m². The annual crude oil and natural gas production of the Changqing Oilfield in 2017 reached 2 372× 10⁴t and 369× 10⁸m³, respectively, realizing the stable oil-gas production of over 50 million tons equivalent for five consecutive years.

(1) Effective development technology for low-permeability oil in the Ordos Basin. Specific to the geological characteristics of the low-permeability reservoirs in the Triassic Yanchang Formation, through the advanced waterflooding, relatively high formation pressure is maintained and relatively displacement pressure system is established, thus reducing the permeability damage caused by formation pressure drop and increasing the final recovery rate. By using this technology, low-permeability oilfields, such as the Ansai, Jing’ an, Xifeng and Jiyuan, have been successfully developed.

(2) Tight gas exploration and development technology for the Ordos Basin. The accumulation area selection and well location deployment technology combining geological and seismic data, enhanced oil recovery technology through well pattern optimization, enhanced single-well production technology through multi-layer and multi-section fracturing and low-cost drilling and production technology with rapid drilling and downhole throttling as the core have been developed, realizing the large-scale profitable development of the Upper Paleozoic tight gas in the Ordos Basin.