Marine carbonate rocks play a key role in natural gas production in Sichuan Basin. The marine carbonate rock strata resources account for 85% of the total conventional gas resources in Sichuan Basin, and their proven reserves account for 70%. They are the main strata for effective exploration and development of natural gas in Sichuan Basin, and large and extra-large gas fields in marine strata are the cornerstones of conventional gas production in Sichuan Basin, with a yield ratio of 75%. After more than sixty years of continuous exploration^[1,2], a number of medium- and large-sized gas fields have been discovered in marine carbonate strata, e.g. reef banks of Middle Permian Series and Weiyuan Sinian System in Southern Sichuan Basin (hereinafter referred to as “Southern Sichuan”), Carboniferous high-angle structural belt in Eastern Sichuan Basin (hereinafter referred to as “Eastern Sichuan”) and Permian & Triassic systems in Northern Sichuan Basin (hereinafter referred to as “Northern Sichuan”) as well as Eastern Sichuan. In 2011, a breakthough was made in Sinian Dengying Formation and the Cambrian Longwangmiao Formation in Central Sichuan Basin (hereinafter referred to as “Central Sichuan”)^[3], with the gas production of 102×10⁸ m³ in 2017. At present, large gas provinces with the reserves of trillion cubic meters have been formed, with the existing annual production capacity of natural gas being 120×10⁸ m³. As exploration and development are advancing, the reserves and production of this block will be further increased. Although the basin has been explored for more than half a century, major achievements can also be obtained in exploration, indicating that marine carbonate rocks still have a great exploration potential.

The marine carbonate rocks in Sichuan Basin are widely distributed in Neoproterozoic Sinian-Mesozoic Triassic strata, and have been subject to multiple structural cycles^[4]. In the meantime, lots of medium- and large-sized gas fields in different strata and of different types were discovered in different areas of the basin, and they are rare treasures for oil & gas geologic research of marine carbonate rocks around the world. This paper analyzes the main control factors for the formation of medium- and large-sized marine carbonate gas fields in the Sichuan Basin, and studies the distribution and exploration direction of medium- and large-sized marine carbonate gas fields in order to provide references for oil and gas geologic research and exploration in marine carbonate rocks.

Sichuan Basin is a large superimposed basin developed based on the Upper Yangtze Craton. It experienced the Late Proterozoic-Middle Triassic marine and Late Triassic-Cenozoic continental basin development stages. Among them, the marine stratigraphic depositional stage had been lasting for 4.2×10⁸ a, and the marine strata with a thickness of 4 000- 7000 m, of which, the carbonate stratum is about 3 000-5 000 m thick (Fig. 1). Among 3 major oil- and gas-bearing basins in Western China, Sichuan Basin is obviously characterized by longer development time, more strata and greater thickness of marine carbonate rocks (Table 1).

The marine strata in Sichuan Basin are characterized by development of hydrocarbon source rocks, many types of accumulation combination, and many production zones (Table 1 and Fig. 1). Four marine hydrocarbon source strata of Lower Cambrian, Lower Silurian, Middle Permian and Upper Permian series are widely distributed. The black shale source rocks of Lower Cambrian Qiongzhusi Formation are 100-400 m thick and widely distributed in the basin, with gas generation intensity being (20-140)×10⁸ m³/km². The black shale source rocks of Lower Silurian Longmaxi Formation are 400-700 m thick and widely distributed in Southern and Eastern Sichuan, with gas generation intensity being (60- 260)×10⁸ m³/km². The gray-black marl and mudstone source rocks of Lower Permian Maokou Formation are 100-420 m thick, while the shale, dark gray marl source rocks of Upper Permian Longtan Formation (Wujiaping Formation) are 30-120 m thick. With the gas generation intensity of (10-40)× 10⁸ m³/km², two hydrocarbon source rocks of Upper and Lower Permian series are widely distributed in the basin. There are 20 oil and gas production zones in the marine strata of the basin, including 18 marine carbonate production zones and 2 shale gas production zones. There are 5 major accumulation combinations are formed around 4 hydrocarbon source rocks, and the stratigraphic sequence from the old to the new is: Lower Cambrian-Sinian accumulation combination, Cambrian-Ordovician accumulation combination, Silurian-Carboniferous accumulation combination, Middle and Lower Permian accumulation combination and Upper Permian- Middle and Lower Triassic accumulation combination (Fig. 1).

The marine carbonate reservoirs in Sichuan Basin are characterized by sedimentary facies-controlled dolomite reservoirs^[5]. The Sinian Deng-II and Deng-IV Formations are biotherm-beach facies karst fissure-cave algal dolomite reservoirs, the Lower Cambrian Longwangmiao Formation, Devonian Guanwushan Formation, Carboniferous Huanglong Formation, Permian Xixia Formation, Triassic Feixianguan Formation, Jialingjiang Formation and Leikoupo Formation are grain beach facies pore and cave dolomite reservoirs, and the Permian Changxing Formation is reef-bank dolomite reservoirs. The development and distribution of reservoirs in these strata are mainly controlled by the sedimentary facies, with superimposition of multi-period karst and tectonization in the later stage for further improvement. They are the main reservoirs for marine medium- and large-sized carbonate gas field in Sichuan Basin. Another one is the limestone karst fracture-cave reservoirs, which are mainly developed in the Middle Permian Maokou Formation.

By the end of 2017, marine carbonate strata in Sichuan Basin have been proved to have 23 435×10⁸ m³ geologic reserves. Eight large and extra-large gas fields have been discovered in the marine carbonate strata in Sichuan Basin, with total proven geologic reserves of 17 278×10⁸ m³ accounting for 73.7%, additionally, there are 45 medium-sized gas fields, with proven natural gas reserves 4 962×10⁸ m³, accounting for 21.2%. The total proven natural gas geologic reserves of medium- and large-sized gas fields account for 94.9% of that of all gas fields (Table 2). Large and extra-large gas fields are mainly distributed in the Sinian, Cambrian, Carboniferous, Permian Changxing Formation and Triassic Feixianguan Formation.

One extra-large and one medium-sized gas fields have been respectively discovered in Sinian Dengying Formation. Anyue Deng-IV Formation is a structural-stratigraphic trap gas field, which is of extra-large size with proven geologic reserves of 4084×10⁸ m³. The Weiyuan Deng-II Formation is a structural trap gas field, which is of medium size. Both gas fields have algae dolomite of Dengying Formation as reservoirs, and Lower Cambrian shale as source rock and cap rock.

The extra-large gas field of Anyue Longwangmiao Formation in Central Sichuan had been discovered in Cambrian System. Such gas field has grain beach facies dolomite of Lower Cambrian Longwangmiao Formation as reservoirs, Lower Cambrian shale as source rock, and Middle & Upper Cambrian argillaceous dolomite, marl as cap rocks. The trap is structural-lithologic. The proven geologic reserves are 4404×10⁸ m³.

For Carboniferous System, one large gas field (Datianchi) and 13 medium-sized gas fields such as Dachigan, Qilixia, Wolonghe and Gaofengchang have been discovered in high-steep structural area in Eastern Sichuan. These gas fields have dolomite of Carboniferous Huanglong Formation as the reservoirs, Lower Silurian argillaceous shale as source rock, and Lower Permian marl as cap rock. The structural trap and structural-stratigraphic trap are developed, with proven geologic reserves of 2 351×10⁸ m³.

For Upper Permian Changxing Formation, one large gas field (Yuanba) and 7 medium-sized gas fields such as Yunanchang, Luojiazhai, Tieshan and Longgang have been discovered in Northern Sichuan and Eastern Sichuan. These gas fields have reef-bank dolomite of Changxing Formation as the reservoirs, Permian mudstone (shale), carbonaceous shale and argillaceous shale as source rocks, as well as Triassic marl as cap rock, with the lithologic trap developed, and the proven reserves are 2 976×10⁸ m³.

For Lower Triassic Feixianguan Formation, one large gas field (Puguang), 3 gas fields (Dukouhe, Luojiazhai and Tieshanpo) and 4 medium-sized gas fields (Longgang, Qilibei, Tieshan and Longmen) have been discovered in Northeastern and Northern Sichuan. These gas fields have oolitic dolomite of Feixianguan Formation as the reservoirs, Permian mudstone (shale), carbonaceous shale and argillaceous shale as the main source rocks, as well as Triassic marl & gypsum rock as cap rocks. The structural trap and structural-lithologic trap were developed, with proven geologic reserves of 6316× 10⁸ m³.

The proven geologic reserves of 1 709×10⁸ m³ have been accumulatively obtained from the Middle Permian, Triassic Jialingjiang Formation and Leikoupo Formation, where large and extra-large gas fields have not been discovered presently. For Middle Permian System in Southern Sichuan, 10 medium-sized gas fields, which have limestone fracture-caves of the Qixia Formation and Maokou Formation as the reservoirs, the Lower Permian mudstone (shale) and marl as source rocks, have been discovered, with lithologic trap developed. For Triassic Jialingjiang Formation in Central and Southern Sichuan, 8 medium-sized gas fields, which have grain beach facies dolomite of Jialingjiang Formation as the reservoirs, Permian mudstone (shale) and marl as source rocks, as well as Triassic gypsum rock as cap rocks, have been discovered, with structural trap developed. For Triassic Leikoupo Formation in Central and Western Sichuan, 2 medium-sized gas fields, which have grain beach facies dolomite of as the reservoirs, Leikoupo Formation, Permian mudstone (shale) and marl as main source rocks, as well as Triassic gypsum rock & mudstone as cap rocks, have been discovered, with structural trap developed.

The characteristics of medium- and large-sized gas fields (especially large and extra-large gas fields) discovered in Sichuan Basin are as follows: (1) the source rocks of the scale distribution are developed; (2) layered porous dolomite reservoirs of the scale distribution are developed; (3) various traps, including structural-lithologic, structural-stratigraphic composite traps for large and extra-large gas fields, are developed; (4) natural gas is oil type cracked gas, and it is subject to paleo-oil reservoir accumulation, paleo-oil reservoir pyrolysis into paleo-gas reservoir and finally adjustment and finalization.

The formation of large and medium gas fields demands large area of high quality thick source rock, reservoir stratum spread on a large scale, large entrapment and effective aggregation and storage conditions. The authors believe that paleo-uplifts, paleo-rifts, paleo erosion surfaces and other geologic units formed during the marine cratonic stage played important roles in distribution of source rock, reservoir stratum, entrapment and other key reservoir forming factors as well as scale aggregation of oil gas of large and medium gas fields (Fig. 2). The geologic units controlling reservoir formation (hereinafter referred to as reservoir forming geologic units) can provide clear direction for exploration of large gas fields through targeted exploration, prediction and identification.

Sichuan Basin, located in western Yangtze Craton, was controlled by plate movement. The development of Neoproterozoic-Middle Triassic marine cratonic basin was affected by extension-convergence cycle of two such two super ancient continents-Rodinia and Pangea, undergoing Yangtze, Caledonian, Hercynian, and Indosinian structural cycles^[6,7,8,9,10]. Chinese Plate, mainly comprised of Yangtze, Huabei and Tarim Cratons, was located between Laurasia Oldland and Gondwana Oldland. Many times of dispersion and confluence have resulted in Chinese marine cratons being transformed by many stages of structural movement^{[11,12,13,14]}. Before Neoproterozoic Qingbaikou Period, the base of Sichuan Basin had been formed. Nanhuan Period, featured with strong extension, was developed with rift valleys^[15-17] and filled with glacial period deposit sediments. It uplifted during late Nanhuan Period and then Hannan Oldland and Dazhou-Kaijiang Oldland were developed^[18,19]. Late Sinian Period was featured with weak extension. Tongwan Movement I and II were upward, sea level of Yangtze Platform decreased and thus erosional surfaces were formed on the top of Deng-II and Deng-IV Formation^[20,21]. Middle-Late Dengying Formation deposits were featured with strong extension. The Middle and Western basin was developed with Deyang-Anyue intracratonic rift. The east edge was developed with Exi intracratonic rift. And the rift was deposited with thick layer of Lower Cambrian muddy source rock^[22,23,24]. In the Middle and Late Early Cambrian, regional extension turned into regional uplift, forming underwater paleo-rift in Central Sichuan^[25,26]. The uplift effect increased obviously in Ordovician and lasted till end of Valentian. Leshan-Longnüsi Paleo-rift was formed in Central Sichuan and Western Sichuan uplift^{[9, 26-28]}. At the same time, Caledonian ancient erosional surface was formed. With gradual increase of uplift extent, depression zone was formed around the paleo-rift and Silurian source rock was deposited in Southern Sichuan and Eastern Sichuan Zone.

Sichuan Basin transformed from compression to tension in early Hercynian period, and started marine transgression in the Devonian and expanded in Middle-Late Carboniferous. The east and west of basin were developed with Devonian-Carbonic Carbonate sedimentation. In the Late Carboniferous-Early Permian, Sichuan Basin was controlled by Yunnan Movement and subject to erosion due to large area of lifting^[6,29]. The structure of the basin was stable till Middle Permian. The entire Yangtze Craton was developed with carbonate platform sediments. Now, Longmenshan area on the west of the basin is developed with cratonic edge rift (Bayan Har Basin). Ancient erosion surface was formed in the basin due to lifting by Tungwu movement in the Late-Middle Permian^[4]. Sichuan Basin and its northeast edge were developed with intracratonic rift (Kaijiang-Liangping Trough and Chengkou-Exi Trough) under the extension effect in the Late-Middle Permian and Late Permian^[30,31,32].

In Triassic Period, squeezing background had been developed gradually. Edges of Yangtze Craton were developed with small scale of orogeny. Carbonate evaporation platform sediments were mainly in the Early and Middle Triassic of the basin. In the Middle Triassic, the role of squeezing and uplifting was quite strong. The top of Leikoupo Formation was developed with wide area of erosion surface and Luzhou- Kaijiang Paleo-rifts were formed^[1,6-9]. In Late Triassic, the sedimentary evolution of marine basin ended and continental deposit basin development period had started.

Many reservoir forming geologic units were developed by periodic extension-uplift structure movement during the marine cratonic stage. According to current data, 5 large ancient lifts were developed in the structure extension period: Deyang-Anyue intracratonic rift and Exi intracratonic rift were developed in Tongwan Period; Longmenshan Cratonic edge rift, Kaijiang-Liangping intracratonic rift (Trough) and Chengkou-Exi intracratonic rift (Trough) was developed in Arbuckle Period. Three paleo-rifts were developed in Caledonian Period and Indosinian Period: Leshan-Longnüsi paleo-rift was formed in the Caledonian Period; Luzhou paleo-rift and Dazhou-Kaijiang paleo-rift were developed in Indosinian Period. Five ancient erosion surfaces were developed corresponding to various structures stages: Tongwan Dengying Formation erosion surface, Caledonian Lower Paleozoic top erosion surface, Caledonian Carbonic erosion surface, Middle Permian top erosion surface and Indosinian Early Leikoupo Formation top erosion surface.

2.2.1. Paleo-rift and paleo depression controlling the distribution of large-scale hydrocarbon generation center

The depression area around the paleo-rift and paleo-uplift is the center of large-scale hydrocarbon generation. The paleo- rift and paleo-depression were controlled by the synsedimentary fault or difference of elevation and subsidence, forming a quiet, stagnant, anoxic and reductive sedimentary environment during the rise of sea level, which was favorable for the deposition of organic-rich shale and the formation of regionally distributed hydrocarbon-generating depressions^[33].

The Deyang-Anyue paleo-rift was controlled by synsedimentary faults under the influence of regional tension, and was a intracratonic rift generated during the Sinian Dengying Formation Sedimentary Period-Early Cambrian^[22]. The rift extends from the Western Sichuan Ocean basin to the inner basin in the northwest-southeast direction. The rift developed in the Sedimentary Period of Sinian Dengying Formation was filled up in Lower Cambrian Series with a large formation thickness. The thick layers of high-quality source rocks of the Maidiping and Qiongzhusi Formation were developed. The source rocks were 300-450 m thick. The average organic carbon content was more than 2%. The cumulative gas generation intensity of these two sets of source rocks was (100-180)×10⁸ m³/km², which was more than 4 times as high as the non-rifting area^[23]. Therefore, Deyang-Anyue paleo-rift was the hydrocarbon generation center of Lower Cambrian Series of Sichuan Basin in terms of both source rock thickness and gas generation intensity^[24]. In Western Hubei intracratonic rift of the eastern basin, the source rock thickness of the Qiongzhusi Formation of the Lower Cambrian Series was 100-300 m and TOC value was 1.39%-3.40%; the source rock thickness of the Doushantuo Formation of the Sinian Series was 50-200 m and TOC value was 0.90%-4.70%.

The Kaijiang-Liangping Trough was an intracratonic rift controlled by basement faults under the tension of Dongwu Period. The Dalong Formation of the Upper Permian Series deposited in the trough was a set of deep-water basin deposit stratum series^[30], mainly composed of black mudstone mixed with siliceous rocks, of which the black argillaceous source rocks were about 10-30 m thick and contain a high level of organic carbon, with an average TOC value of 4.49 %, forming a set of high-quality source rocks^[33].

In addition, the underwater paleo-uplift in Central Sichuan continued to uplift during the Caledonian Period, resulting in the absence of Silurian-Carboniferous series in most parts of Southwest and Central Sichuan. The Eastern and Southern Sichuan regions were depression areas generated accompanying the formation of the paleo-uplift with deposited thick layers of Silurian source rocks.

2.2.2. Paleo-rift edge, higher part of paleo-uplift and paleo- erosion surface controlling the development of large-area reservoirs

The edge of the rift was located in the transition zone between the carbonate plant in the shallow water area and deep water sediments, with strong hydrodynamic conditions and high paleogeomorphology. It had a sedimentary environment^[34] for forming coarse and porous carbonate rocks, and was a sedimentary development area for reefs, bioherm and grain banks, which was conducive to large-area contiguous development of carbonate pore-type reservoirs.

The paleo-rift platform margin belt around Kaijiang-Liangping was the main development and distribution area of high-quality dolomite reefs and bank reservoirs in Changxing-Feixianguan Formation ^[35,36,37]. Biotherm-beach facies deposits of platform margin belt of Dengying Formation on both sides of Deyang-Anyue paleo-rift were superimposed and developed in succession. Clot dolomite, algal stromatolite and doloarenite were developed in thick layers in succession. Dissolution pore and karst caves in the reservoir were well developed^[38]. It was the zone where the high quality reservoirs of Dengying Formation were concentrated developed and distributed.

Sedimentary facies belts were distributed in the area controlled by the underwater paleo-uplift during synsedimentary period, and the higher part of the paleo-uplift was a shallow water high-energy sedimentary area, which was favorable for grain bank deposition and pene-sedimentary karst development^[26]. Taking Leshan-Longnüsi paleo-uplift as an example, the paleo-uplift had taken shape in the early Cambrian Canglangpu Formation sedimentary period, and the Longwangmiao Formation sedimentary period developed into a synsedimentary underwater paleo-uplift, with the dolomite reservoir of Longwangmiao Formation widely distributed in the higher part of the paleo-uplift^[28]. However, the development degree of grain banks and dolomitization degree in the slope area around the paleo-uplift decreased and it mainly contained evaporites slope-lagoon facies deposition.

The paleo erosion surface was the development surface of long-term and high-intensity karstification, and the porous carbonate rocks covered in the favorable facies zone were dissolved and reformed, and the karst pores, holes and fractures in the reservoir were more developed. Tongwan movement developed two stages of paleo erosion surface, and karst reformed the reservoirs of Deng-II and Deng-IV Formation^[21]. Caledonian Movement formed a dolomite reservoir with superimposition of karst pores and karst caves formed by karstification in grain beach facies of Longwangmiao Formation in Central Sichuan^[28]. The large-scale uplift of Yunnan Movement resulted in the formation of carboniferous reservoirs^[7,39-40]. The karst transformation of the paleo erosion surface in Dongwu period resulted in the formation of karst fractured-vuggy reservoirs widely developed in the Maokou Formation of the Middle Permianmian^[41,42]. Indosinian Movement caused extensive erosion of Leikoupo Formation and formed bank facies karst reservoir^[7,43].

2.2.3. Paleo-rifts, uplifts and erosion surfaces as well as current structures jointly controlling formation of traps

Many types of traps (trap groups) could be formed by combining paleo-rifts, uplifts and erosion surfaces with the current structures. The formation of lateral shielding from shales deposited in deep water environment in paleo-rifts was the key to the formation of large-scale structural stratigraphic (lithologic) traps. The paleo-uplift was a large positive tectonics unit, which controlled the development of large structure traps in the paleo-uplift area. Large paleo erosion surface was beneficial to the development of large strata or structural stratigraphic complex traps.

For example, Sinian Dengying Formation IV Gas Field in Anyue Gas Field was a large-scale structural stratigraphic trap, containing gas at the platform edge and within the range of 7500 km² the platform^[26]. Deyang-Anyue paleo-rift and paleo erosion surface of Tongwan Movement made Dengying Formation IV missing, and the thick layer argillaceous rocks in Lower Cambrian Series in the rift formed stratigraphic screen, overlapping with the paleo-uplift and the current structure to form a large-scale structural stratigraphic paleo trap. The Weiyuan region in Himalayan period uplifted rapidly, and the Central Sichuan region was located at the lower part of the eastern tilt end of the paleo-uplift. The lack of strata in the updip direction and shale plugging were the key to the preservation of the gas reservoir in Dengying Formation IV.

Another example is the current Longwangmiao Formation gas reservoir, which is a structural lithologic trap gas reservoir, the high part of the Longwangmiao Formation gas reservoir in Moxi-Longnüsi structure contains gas as a whole, and the west side of the gas reservoir is the lithological shielding formed by the pinchout of the bank facies reservoir^[28]. After the formation of the paleo-uplift in the Early Cambrian, it had evolved in an inherited way in Moxi-Longnüsi area in Central Sichuan, and had undergone many structure movements since the Caledonian to Himalayan period, but it had been in the high position of the ancient and modern structures as a whole, with stable structure and small adjustment range. The early ancient reservoirs were cracked into gas reservoirs in situ, forming the present super-large gas reservoirs.

The Permian and Triassic reef bank gas fields distributed around the Kaijiang-Liangping Trough have developed various trap types of gas reservoirs in the high-steep structure in Eastern Sichuan and the gentle structure in Central Sichuan^[37]. The Changxin-Feixianguan Formation in Puguang area is a structural-lithologic trap gas reservoir, which was formed by superimposing a high-steep NE structure with a reservoir in the platform margin belt^[44]. The distribution area of reservoirs at the platform margin of Feixianguan Formation in Luojiazhai and Dukou River areas is larger than the current structure trap range, forming a structure trap gas reservoir^[45,46]. Longgang, Yuanba and Jiange areas on the west side of the trough are now monoclinal structures, with lithologic trap gas reservoirs formed by reservoir heterogeneity of reefs mainly developed^[47,48,49]. The karst dolomite reservoir, stratum residual area and limestone facies change area on the paleo erosion surface of Carboniferous in East Sichuan are superimposed with the current high and steep structures in Eastern Sichuan to form structure or structural-stratum (lithology) trap gas reservoir group^[50].

The natural gas in medium- and large-sized carbonate rock gas fields known in Sichuan Basin is mainly oil-cracking gas^[51]. Most of the paleo reservoirs were cracked into gas mainly during Late Triassic-Jurassic^[52,53,54], during which the basin was extruded and the structure began to adjust. After a large amount of cracking gas was formed, it experienced strong structural compression deformation during Late Yanshanian-Himalayan. The inheritance of paleo and modern traps, the inheritance of structure evolution, and the degree of difference between paleo and modern structure forms have an important influence on the preservation of gas reservoirs and the final scale of reservoir-forming. The inheritance of reservoir forming process is of great significance to the formation of giant gas field, which is embodied in two aspects: stability inheritance and adjustment inheritance. As for the stable structure area, the paleo and modern traps have developed steadily with efficient gas accumulation, which is the most favorable distribution area for giant gas field. In areas with strong structural deformation, the paleo traps are inherited evolved and reformed, and the gas reservoirs are adjusted and accumulated. Since the formation of paleo oil and gas reservoirs is controlled by large-scale reservoir-forming geologic units such as paleo-rifts, uplifts and erosion surfaces, the paleo traps in large-scale reservoir-forming geologic units have undergone structural deformation and adjustment in the later period, and the distribution of current gas reservoirs is controlled.

2.3.1. Inherited development of paleo and modern traps in the structural stability area controlling the efficient accumulation of natural gas

The stable evolution of oil and gas reservoirs is conducive to the efficient enrichment and preservation of natural gas. For the area with stable structure and good inheritance of structural form, cracking in situ is found in the traps of large paleo oil reservoirs, and natural gas is efficiently gathered in situ on a large scale. Taking Leshan-Longnüsi paleo-uplift as an example, the Early Cambrian tectonic uplift formed underwater paleo-uplift, which was widely uplifted and shaped at the end of Silurian, and was developed in an inherited way during Neopaleozoic-Mesozoic, and finally formed the present structural form and distribution pattern after extrusion and adjustment in Himalayan period. In the Gaoshiti-Moxi-Longnüsi area in Central Sichuan at the eastern end of the paleo-uplift, structural deformation adjustment is weak, structural and trap inheritance is good^[9], which controls the efficient accumulation and preservation of paleo oil and gas reservoirs^[26]. With the increase of burial depth, the paleo traps are well maintained and the adjustment of oil and gas reservoirs is small, and the paleo oil reservoirs are cracked and accumulated in situ to form an Anyue extra-large gas reservoir^[55].

2.3.2. Evolution and reconstruction of paleo traps in areas with strong structural deformation controlling the adjustment and accumulation of gas reservoirs

Different from Central Sichuan, the structural deformation in Eastern Sichuan and Southern Sichuan was strong in Himalayan period, and the traps of the paleo oil and gas reservoirs were greatly adjusted and transformed into today's high and steep structural areas. During this process, natural gas was readjusted and accumulated. One way of accumulation is to form medium and large sized gas reservoir groups within the scope of the original paleo oil and gas reservoir after the present structure becomes more complicated. The types of paleo and modern traps in this type of gas reservoir have changed greatly, but they are mainly adjusted within the scope of paleo traps, and the migration distance is relatively close. The paleo and modern traps have good position inheritance, which is beneficial to the accumulation and preservation of oil and gas. The reef bank and carboniferous gas reservoirs in Changxing-Feixianguan Formation of northeast Sichuan within the area of Kaijiang paleo-uplift belong to this category. The other is the formation of gas reservoirs in the late structures outside the trap range of the paleo oil reservoirs, where the location of paleo and modern traps has changed greatly and the distance of migration is relatively far. The Dengying Formation gas reservoir in Weiyuan of Southern Sichuan is a typical example of this kind of late structural gas accumulation^[27].

The three types of large-scale reservoir-forming geologic units of marine carbonate in Sichuan Basin play important roles in controlling the oil and gas reservoir-forming elements, and the superposition relationship between single or multiple reservoir-forming geologic units and the present structure controls the distribution of medium- and large-sized gas fields. Among them, the "three paleo" superimposition areas of paleo-rifts, paleo-uplifts and paleo erosion surfaces developed in an inherited way is the best area for matching key reservoir-forming elements such as high-quality source rocks, large-area high-quality reservoirs and large traps, and is also the most favorable area for forming large and extra-large gas fields. At present, three extra-large gas fields have been discovered in Sichuan Basin, two of which are in the "three paleo" superimposition area.

Exploration practice in Sichuan Basin shows that the periphery of the paleo-rifts is one of the main zones where medium- and large-sized marine carbonate gas fields are distributed. Once the paleo-rifts are discovered, giant gas fields can be discovered at the periphery of the rifts, which is a very important reservoir-forming geologic unit. There are 19 medium- and large-sized gas fields (Fig. 3) discovered around the two paleo-rifts of Deyang-Anyue rift and Kaijiang-Liangping Trough in Sichuan Basin, which have accumulated proven natural gas reserves of 18 370×10⁸ m³, accounting for about 70% of the proven reserves of the whole marine carbonate series in the basin. The Anyue and Puguang extra-large gas fields discovered in the basin are all located at the periphery of paleo-rifts. Paleo-rifts of marine strata is the goal of oil and gas exploration.

In this type of zone, the paleo-rift controls two major reservoir forming units, namely high-quality source rock and large-area high-quality reservoir^[23], forming a lateral docking source-reservoir configuration mode, which has an important control function on the source, reservior and gas reservoir, and is a key reservoir controlling geologic unit. The distribution of medium- and large-sized gas fields is controlled by the superposition of paleo-rifts with other reservoir-controlling geologic units and current structures. If the reef bank and the biotherm-beach in the platform margin belt around the paleo-rift are superimposed with the corrosion transformation of the paleo erosion surface, it will be more conducive to the development of high-quality reservoirs on the platform margin. If the periphery of the paleo-rift is located in the paleo-uplift area during the oil and gas generation period, it will be favorable for the accumulation and forming of paleo oil reservoirs. In the inherited and stable evolution of the paleo-uplift area, the paleo and modern structural deformation adjustment is small, and the paleo oil reservoirs were cracked in situ, thus contributing to the formation of giant gas reservoirs^[55]. In areas with strong structural deformation, gas reservoirs with structural and structural-lithologic traps are formed. The area with gentle structure is dominated by lithology and structural-lithologic trap gas reservoir.

The Sinian Dengying Formation gas field in Anyue is located at the edge of Deyang-Anyue paleo-rift. Tongwan paleo erosion surface further promoted the development of the reservoir in the platform margin belt and forms a lateral reservoir allocation relationship with the Lower Cambrian Series thick layer high-quality source rocks in the rift^[55], which constitutes a "top sealing lateral plugging" large-scale structural-stratigraphic paleo trap. Leshan-Longnüsi paleo-uplift developed steadily in an inherited way in the Central Sichuan region at the late stage of reservoir formation, and forming the large- sized paleo oil reservoirs, cracking to gas eventually and forming extra-large gas fields (Fig. 3a).

Changxing-Feixianguan Formation gas fields of Puguang, Luojiazhai and Tieshanpo in Northeast Sichuan are located at the periphery of the Kaijiang-Liangping Trough^[30] (Fig. 3b). Kaijiang paleo-uplift was developed in Indosinian, and the paleo-uplift overlapped with dolomite reefs and bank reservoirs in the platform margin, forming lithologic paleo traps on the paleo-uplift, and the source rocks in the Middle Jurassic reach the peak of oil generation, controlling accumulation of paleo reservoirs. During the Cretaceous period, liquid hydrocarbons in paleo reservoirs cracked into gas. At this time, high-steep structural zones were gradually formed in Eastern Sichuan, and natural gas generated from cracking of paleo reservoirs was accumulated in the structural zones, eventually forming structural and structural-lithologic trap gas reservoirs. Different from the Eastern Sichuan region, the Central Sichuan and Northwestern Sichuan regions such as Yuanba and Longgang have a gentle structure and are lack of large-scale faults and structural traps, and lithologic, structural-lithologic trap gas reservoirs are mainly developed and are distributed in clusters featuring multiple gas-water interfaces and multiple pressure systems^[37].

Northwest Sichuan is located at the northwest edge of the Upper Yangtze Craton, where multiple sets of platform margin bank facies dolomite reservoirs of Devonian-Middle Permian^[56] are developed. Influenced by Caledonian paleo erosion surface, the Upper Paleozoic in the area directly covers the Cambrian high-quality source rocks in Deyang-Anyue paleo-rift area, and the active faults in the early stage build a good source channel. It is sealed by the thick gypsum beds in the regional cap rocks of Middle and Lower Triassic, forming an effective source-reservoir-seal combination and favorable reservoir-forming conditions.

The paleo-uplift hereby refers to the large-scale paleo-uplift which was formed with underwater paleo-uplift developed in the synsedimentary period as well as uplifts due to multi-period tectonic movement in the late period. Paleo-uplift is the key to gas field formation, which controls two major reservoir-forming factors: reservoir with large area distribution and large trap. The distribution of medium- and large-sized gas fields is controlled by the superposition of such paleo-uplifts with other reservoir forming geologic units and current structures. If superposition with paleo-rifts, hydrocarbon source conditions are excellent. If the reservoir controlled by the paleo-uplift overlaps with the paleo erosion surface, the reservoir conditions are more excellent; if the inheritance evolution of paleo-uplift in the later period is stable and the adjustment of the tectonic deformations are small, the accumulation and preservation of oil and gas are more favorable. In the present tectonic area with larger deformation, the structure and tectonic-lithologic trap gas reservoirs are formed; while in the gentle tectonic area, the lithologic trap gas reservoirs are dominant. In this type of zone, the developed large-scale paleo-uplifts are favorable for reservoir stratum as well as oil and gas accumulation conditions. However, the main concern should be on hydrocarbon source conditions.

Within Sichuan Basin, Anyue Cambrian Longwangmiao Formation gasfield in the area of Central Sichuan, is located in such type of zone. The underwater paleo-uplift in the Central Sichuan is the key to the formation of gas reservoirs and traps (Fig. 4). Before the deposition of the Longwangmiao Formation, the Central Sichuan region began to uplift, and the paleo-uplift with synsedimentary development controlled the extensive distribution of beach facies of the Longwangmiao Formation^[26]. Due to the wide-spread distribution of Lower Cambrian source rocks, the hydrocarbon generation center formed by the Deyang-Anyue paleo-rift made the hydrocarbon source conditions superior. The Longwangmiao Formation in the high position of the Caledonian paleo-uplift was denuded and the karstification reconstructed on the beach facies reservoir, which further optimized the reservoir conditions and formed large lithologic paleo-trap with cap rocks of the Upper Cambrian System and Permian System. The paleo-uplift in the later period was developed stably, and the paleo-oil reservoir in the large-scale lithologic paleo-trap was cracked in situ, thus forming a giant gas field eventually.

In the superimposed area of the hydrocarbon accumulating geologic unit with paleo-rift and paleo-uplift as the core, the paleo erosion surface plays a role in further improving the reservoir performance, but it is not a key hydrocarbon accumulating geologic unit. However, during the marine craton stage, five major erosion surfaces were developed in Sichuan Basin. While one type of paleo erosion surface, which is represented by the large-scale paleo erosion surface formed by the Yunnan Movement, controlled large-area distributed reservoirs and multiple types of trap, playing a key role in controlling the formation of gas fields. The distribution of medium- and large-sized gas fields is controlled by the superposition of paleo erosion surfaces with other reservoir-forming geologic units and current structures. In case of the superposition with paleo-rifts (depression), source conditions are excellent. If the paleo erosion surface overlaps with the paleo-uplift which is formed in the later period, the conditions of oil and gas accumulation are superior. The erosion surface can form the deficiencies in the area and overlap with the current structure to form a variety of traps such as tectonic trap and tectonic-stratigraphic (lithologic) trap. In such zones, the reservoir conditions are favorable, and it is necessary to analyze the hydrocarbon source, reservoir-forming condition and their matching relationship.

The Carboniferous gas field in Eastern Sichuan belongs to this type of zone. The Yunnan Movement has caused the erosion of the top boundary of the Carboniferous system^[1], forming a widely developed karst dolomite reservoir. The distribution zone of Carboniferous residual formation also laid the foundation for the later formation of tectonic-stratigraphic (lithologic) and lithologic traps (Fig. 5). In the Caledonian period, along with the large uplift of the Leshan-Longnüsi paleo-uplift, the paleo-depression was formed in Eastern Sichuan. And a thick layer of black organic-rich mudstone of the Silurian Longmaxi Formation was deposited in the large area^[2]. Episode I of Yunnan Movement caused a large area of the Devonian structure to be lost and made the Carboniferous structure directly cover the Silurian structure. In the Late Carboniferous, the dolomite of restricted estuarine facies for Huanglong Formation was deposited in the Eastern Sichuan region, while the paleo erosion surface caused by Yunnan Movement was transformed into a karst dolomite reservoir. This widely distributed dolomite reservoir and the Silurian source rock form the reservoir-forming combination (lower part for generation and upper part for accumulation), which is conducive to near-source efficient accumulation. In the Indosinian period, paleo-reservoirs began to form and accumulate in the various paleo-traps of the Carboniferous system. Among them, the Indosinian paleo-uplift with an distribution area of about 5 000 km² was developed in the Kaijiang area of the Eastern Sichuan, and the Carboniferous System in the Wubaiti area was in the core part of this paleo-uplift, which is a favorable pointing area for the accumulation of large reservoirs. By the Yanshan Movement and the Himalayan Movement of the Late Mesozoic-Cenozoic, the fold uplift in the Eastern Sichuan region formed a high-steep tectonic belt (the barrier-type fold belt); the paleo-oil reservoirs were adjusted and aggregated at the same time as the cracking, forming a Carboniferous gas reservoir group with a distribution of zones; in Wubaiti area, the Carboniferous stripping line is superimposed with the structural belt, and finally the large-scale tectonic-stratigraphic (lithologic) trap gas reservoir was formed.

In the area of Eastern and Southern Sichuan, due to superposition of the karstification of the Dongwu period on the Maokou Formation, karst fracture-cavern reservoirs were widely developed. In the Indosinian period, the Luzhou paleo-uplift was developed in Southern Sichuan, matching with the mature period of the Middle Permian autogenic hydrocarbon source, controlling the accumulation of oil and gas reservoir, and forming a limestone fracture-cavern gas reservoir in Southern Sichuan (Fig. 5).

The exploration direction and target zone are determined through targeted search for large paleo-rifts, paleo-uplifts and paleo erosion surfaces development zones, and analysis of key reservoir-forming conditions. During the extension period of the marine craton, the focus is to discover large-scale paleo-rifts; while in the tectonic uplift period, the focus is to discover large-scale paleo-uplifts and large-scale paleo erosion surfaces. The superposition zone of “paleo-rifts, paleo- uplifts and paleo erosion surfaces” is the most important exploration direction and favorable target zone.

The Sinian Dengying Formation on the periphery of Deyang-Anyue paleo-rifting and the Cambrian Longwangmiao Formation on the north and south sides of the Anyue gas field are important areas and zones for the continuous exploration of medium- and large-sized gas fields (Fig. 6a). These zones are basically similar to the Anyue gas field in terms of hydrocarbon sources and reservoirs. Two biggest differences are that the process of oil and gas accumulation has experienced the large structural adjustment and the structural trap inheritance of the accumulation process is poor. So the focus is to discover medium- and large-sized lithologic, tectonic-lithologic and tectonic-stratigraphic traps.

In Eastern Sichuan, the Jinzhuping-Heiloumen-Shizhu area is located in the marginal zone of Exi Paleo-rift; while in the Dengying Formation II and IV sections and the upper section of the Longwangmiao Formation, the dolomite reservoirs of biotherm-beach facies are developed. To the east of these areas, the Qiongzhusi Formation and the Doushantuo Formation of the Exi Paleo-rift have hydrocarbon source rocks developed with many current structural traps, which is also an important exploration direction (Fig. 6a).

The Upper Paleozoic in the western margin of the Upper Yangtze Craton is an important exploration direction. In this area, multi-stage platform marginal facies were developed in the Devonian-Permian Systems, and the authigenic Permian source rock developed^[57,58]. The absence of the Ordovician-Silurian Systems in the region is conducive to the hydrocarbon-supply to the Upper Paleozoic from the Cambrian System in the Deyang-Anyue paleo-rift. The area has the characteristics of accumulation of spontaneous generation & spontaneous accumulation and lower part for generation/upper part for accumulation (Fig. 6b). With present tectonic trap development^[59], lithological changes and structural superposition, this area has the geologic condition for the formation of large-scale tectonic-lithologic traps. At present, drilling work is being carried out in the Shuangyushi area, and many high-yield industrial gas wells have been built, controlling over 1 000 km² favorable gas-bearing area.

The Kaijiang-Liangping Trough and the Chengkou-Exi Trough platform margin are important areas for deep exploration of the Upper Permian Changxing Formation organic reef and beach gas accumulation (Fig. 6c). These two areas are developed with reefs and beach reservoirs and have great exploration potential. To the west of the Tieshanpo, for the Wulong 2 Well in the Feixianguan Formation with Fenshuiling structure, the oolitic dolomite reservoir with thickness of 101 m was found. The Fen 2 well and Fen 3 well were encountered the organic reef reservoir, obtaining industrial gas flow. In the Fengjie area on the west platform margin of the Chengkou-Exi Trough, the Changxing Formation reservoir of Fengtan 1 well is 148 m thick.

Within Sichuan Basin, the top boundary of the Permian Maokou Formation is the paleo erosion surface of the Dongwu Movement, and karst fracture-cavern reservoirs were widely developed. The syncline area and gentle tectonic area with low-level exploration are important directions for deepening exploration. In the early stage, gas was drilled out from the syncline area of Yunjin, Southern Sichuan by 9 wells and 7 wells. The average daily output was 21.3×10⁴ m³. Recently, in the wide and gentle syncline area of Wubaiti in Eastern Sichuan, the high-yield gas well with daily output of 80×10⁴ m³ was drilled in Maokou Formation, showing good prospects.

The platform margin of Kaijiang-Liangping Trough and Chengkou-Exi Trough are main directions for the exploration of the Lower Triassic Feixianguan Formation (Fig. 6c). The Leikoupo Formation beach facies dolomite on the margin of the Longmenshan Craton and Lei III-Lei IV paleo erosion surface at the top of Leikoupo Formation in Central and Western Sichuan are the two main exploration directions of the Leikoupo Formation (Fig. 6d). In the Leikoupo Formation in Western Sichuan, dolomite reservoirs are developed; near the front of Longmenshan, tectonic traps in rows and zones are developed; the faults are broken to the Permian and Cambrian Systems; and an effective migration source channel can be formed. Focusing on tectonic traps, the Southwest Oil and Gas Branch of China Petroleum & Chemical Corporation discovered tectonic gas reservoirs such as Yazihe, Xiaoquan and Xinchang. In the area of Lei III-Lei IV of Western and Central Sichuan, a number of denudation surfaces are formed, and grain beach facies karst reservoirs are widely developed. The overlying Xujiahe Formation is the main source rock. Focusing on finding structural-stratigraphic traps, Southwest Oil & Gasfield Company, PetroChina has recently made a breakthrough in the Daxingchang area.

In the background of the periodic extension-uplift tectonic movement in the marine craton stage of Sichuan Basin, five large-scale paleo-rifts were formed in the basin and in the basin margin: the Deyang-Anyue Craton rift and the Exi Craton rift developed during the Tongwan period, the marginal rift of Longmenshan Craton, the inner rift of Kaijiang-Liangping craton (trough) and the inner rift of Chengkou-West Hubei craton (trough) developed during Hercynian period. Three paleouplifts were developed during Caledonian and Indosinian periods: Leshan-Longnüsi paleouplift during Caledonian period, Luzhou paleouplift and Dazhou-Kaijiang paleouplift during Indosinian period. Corresponding to the structure of each period, five paleo erosion surfaces were developed: the Dengying Formation erosion surface during Tongwan period, the top erosion surface of the Lower Paleozoic during the Caledonian period, the Carboniferous erosion surface during Hercynian period, the top erosion surface of the Middle Permian and the top erosion surface of Leikoupo Formation in the early Indosinian period. The above geologic units control the key accumulation factors of marine carbonate rocks in Sichuan Basin.

Large paleo-rifts and paleo-depressions control the distribution of high quality hydrocarbon generation centers. Large paleo-rift margins, high elevations of paleo-uplifts, and paleo erosion surfaces control the development of large-area high-quality reservoirs. Large paleo-rifts, paleo-uplifts, and paleo erosion surfaces together with the current tectonic control the formation of many types of medium- and large-sized traps.

The superposition of single or multiple large-scale reservoir-forming geologic units with current structures controls the distribution of medium- and large-sized gas fields. The superposition area of “paleo-rifts, paleo-uplifts, and paleo- erosion surfaces” is the most favorable. In the inheritance paleo-uplift area with stable structure, large-scale paleo-reservoirs are in-situ cracked, and gas reservoirs are efficiently aggregated; in the area of strong tectonic deformation, the paleo-oil reservoirs are cracked, and the gas reservoirs are adjusted and aggregated.

The main areas and directions for exploration of medium- and large-sized marine carbonate gas fields in Sichuan Basin are Deyang-Anyue paleo-rift, the eastern margin of the Longmenshan paleo-rift, the Kaijiang-Liangping Trough and the margin of the Chengkou-Exi Trough, the high-altitude part of the underwater paleo-uplift around the Central Sichuan, the top paleo erosion surface of Maokou Formation in Eastern and Southern Sichuan, the Leikoupo Formation beach facies dolomite in Western Sichuan, and Lei III-Lei IV paleo erosion surface at the top of Leikoupo Formation in Central and Western Sichuan.