The Sinian system is the oldest sedimentary stratum exposed and drilled in the Sichuan Basin, and it is the first set of regional sedimentary cap rocks above the Neoproterozoic basement of the Upper Yangtze Block ^[26]. Since the Sinian, the Sichuan Basin has undergone multiple tectonic movements, resulting in multiple stages of unconformities with different scales. For example, the Tongwan Movement formed an unconformity inside the Sinian system, and another between the Sinian system and the Cambrian system ^[20,22]. The Caledonian Movement induced different degrees of denudation of the Cambrian, Ordovician, and Silurian systems, forming the unconformities between Cambrian and Ordovician, between Lower Paleozoic and Upper Paleozoic ^[16]. The Yunnan Movement made the Devonian missing in Sichuan Basin, forming an unconformity between Silurian and Carboniferous. The Dongwu Movement caused the eruption of Emei basalt and regional uplift, which resulted in different degrees of denudation of the Maokou Formation, forming an unconformity between Middle and Upper Permian (Maokou Formation and Longtan Formation) ^[23]. The Indosinian Movement led to the end of marine sedimentation in the Upper Yangtze block, with the basin gradually transitioning from a marine sedimentary basin to a continental sedimentary basin and forming an unconformity between Middle Triassic and Upper Triassic, and an unconformity between Triassic and Jurassic ^[24-25]. The Yanshan Movement resulted in three unconformities by the end of the Middle Jurassic, from the end of the Late Jurassic to the beginning of the Early Cretaceous, and from the end of the Early Cretaceous to the beginning of the Late Cretaceous, respectively. These unconformities vary in distribution and impact scope in the Sichuan Basin. Among them there are six regional unconformities distributed throughout the basin, which are between pre-Sinian and Sinian, Sinian and Cambrian, pre-Permian and Permian, Middle and Upper Permian, Middle and Upper Triassic, Triassic and Jurassic, respectively from bottom to the top (Fig. 1 and Table 1).

By the end of 2021, 21 conventional (including tight) large gas fields have been found in the Sichuan Basin, with total proven geological reserves of about 3.65×10¹² m³. Statistics show that 16 large gas fields are related to regional unconformities with proven geological reserves of about 3.09×10¹² m³, accounting for 85% of the total proven reserves in the conventional gas fields (Table 1 and Fig. 1). It can be seen that the 6 stages of regional unconformities have an important relationship with the development and distribution of large oil and gas fields. By analyzing the source and storage conditions, accumulation and preservation factors of these 16 gas fields, it is concluded that they are closely related to four unconformities, namely, the unconformities between Sinian and Cambrian, between pre-Permian and Permian, between Middle and Upper Permian, between Middle and Upper Triassic. So far, few large gas fields have been found related to the unconformity between pre-Sinian and Sinian and that between Triassic and Jurassic.

There are two large gas fields related to the unconformity between Sinian and Cambrian and that between pre-Permian and Permian, namely, Weiyuan gas field and Anyue Sinian-Cambrian extra-large gas field, with proven geological reserves of 1.215×10¹² m³, and accounting for 39.34% of the total proven reserves. The reservoirs are mainly the Sinian Dengying Formation and the Cambrian Longwangmiao Formation. The two unconformities play an important role in the formation of karst reservoirs. The natural gas mainly comes from the high-quality source rocks in the Cambrian Madiping Formation and Qiangzhusi Formation ^[29]. The regional unconformity between Sinian and Cambrian provides a channel for lateral migration of hydrocarbons into reservoirs.

There are three gas fields related to the pre-Permian/ Permian unconformity, namely Dachiganjing, Datianchi and Wolonghe gas fields, with proven reserves of 1.820× 10⁸ m³, and accounting for 5.89% of the total proven reserves. The reservoirs are mainly the Carboniferous Huanglong Formation. The unconformity is important for the formation of the Carboniferous karst reservoirs. The natural gas mainly came from the Silurian Longmaxi Formation and the overlying Liangshan Formation. The unconformity is the transport channel through which hydrocarbons could migrate into the Carboniferous reservoirs.

Six large gas fields are related to the regional unconformity between Middle and Upper Permian, namely, Dukouhe, Longgang, Luojiazhai, Puguang, Tieshanpo and Yuanba gas fields, with proven reserves of 9373×10⁸ m³, and accounting for 30.35% of the total proven reserves. The reservoirs are mainly reef-shoals of the Changxing Formation of the Upper Permian and the Feixianguan Formation of the Lower Triassic. The unconformity provides a paleogeomorphic basis for the platform margin reef-shoal deposits around the Kaijiang-Liangping "trough". The natural gas mainly comes from the Middle Permian, Lower Triassic and Silurian Longmaxi Formation. The unconformity acts as a lateral channel through which hydrocarbons generated by the Middle and Upper Permian source rocks migrates into the reef-shore reservoirs of the Changxing Formation-Feixianguan Formation.

Five large gas fields are related to the unconformity between Upper and Middle Triassic, which are Anyue (Xujiahe Formation), West Sichuan, Hechuan, Qiongxi and Xinchang gas fields with proven reserves of 7544×10⁸ m³, and accounting for 24.42% of the total proven reserves. The reservoirs are mainly the second member of the Upper Triassic Xujiahe Formation, the Middle Triassic Leikoupo Formation and the Jurassic System. The unconformity controls the formation of the weathered crust reservoirs at the top of the Leikoupo Formation. The natural gas mainly comes from the Upper Triassic Xujiahe Formation in the west and middle of the basin. The unconformity acts as a channel for long-distance migration of hydrocarbons.

Unconformity-related weathered crust karstification is critical to the formation of underlying large karst reservoirs ^[30-31]. Regional unconformities control the formation of karst reservoirs from deposition and diagenesis. On deposition, the sediments beneath the unconformity occur at the end of the high-level system tract when the sea level stops dropping. The sedimentary water becomes shallow and the water energy within the platform area increases. Large-scale platform margin mound (reef and shoal) reservoirs begin to deposit on the platform margin. Intra-platform mound reservoirs (reef and shoal) or micritic or crystal powder dolomite reservoirs deposit on the high points of the platform in a large area. Regional unconformities are the results from large-scale and long-term exposure of strata. For instance, the pre-Permian was exposed tens of millions of years, which made the strata in the southwestern Sichuan Basin denuded to the second member of the Sinian Dengying Formation, such as those in Well Hanshen-1 (Fig. 4). Reservoirs under the unconformity may undergo atmospheric freshwater karstification when they are exposed to the surface, and those not exposed to the surface experience bedding dissolution. Atmospheric freshwater could go down hundreds of meters along oblique permeable layers and induce large-scale karst reservoirs. Long-term dissolution by atmospheric freshwater results in a large number of dissolution pores and fractures beneath the unconformity. At the same time, tectonic movements produce a large number of tectonic fissures to communicate with the dissolution pores, further improving the physical properties of the reservoirs (Fig. 4).

The regional unconformity between Sinian and Cambrian has a strong control on the fourth member of the Sinian Dengying Formation. In Deyang-Anyue rift, Deng 4 Member and Deng 3 Member have been denuded, which also affected the formation of the Deng 2 reservoirs. For example, a large number of dissolution pores have been developed in the upper part of the Deng 4 Member in the cores taken from Well GS-1, Well MX-8, Ziyang-Weiyuan areas (Fig. 5a and Table 2). More than 400 small core samples were analyzed. The porosity is from 2.0% to 12.5%, with an average of 4.0%; the permeability is around (0.01-19.40)×10⁻³ µm² with an average of 0.62×10⁻³ µm² (Table 2). The average porosity of more than 100 full diameter samples is 5.2%, indicating that the dissolution pores have taken up a large proportion of the reservoir space. The regional unconformity between Permian and pre-Permian had an important control on the formation of the karst reservoirs in the Sinian Dengying Formation, Cambrian Longwangmiao Formation, Xixiangchi Group and Carboniferous Huanglong Formation (Fig. 5 and Table 2). The Caledonian tectonic movement formed the Leshan-Longnüsi paleouplift which made the southwest of the basin uplifted and denuded, and the Dengying Formation, Cambrian Longwangmiao Formation, Xixiangchi Group and Carboniferous Huanglong Formation denuded to varying degrees. Multiple intervals of reservoirs were formed under the unconformity. With long-term karstification, bedding karstification also improved the properties of the reservoirs far from the unconformity, forming a large number of karst pores, caves, and fractures. Dissolution pores are well developed in the outcrops and cores. A large number of macro-pores and caves were found (Figs. 4, 5b, 5c). When drilling in the Moxi-Gaoshiti area, a large number of karst caves were discovered in the cored sections of Deng 2, Deng 4 members and Cambrian Longwangmiao Formation, with a maximum diameter of up to 10 cm × 15 cm ^[32-33]. The Carboniferous Huanglong Formation was developed in the slope of the Leshan-Longnüsi paleo-uplift. After sedimentation, it was uplifted as a whole, resulting in weathered crust karstification with a large number of karst pores, caves, and fractures (Fig. 5c). The reservoir porosity is 2.0% to 20.4%, with an average of 5.2%. The permeability is (0.01-77.30)×10⁻³ μm², with an average of 0.80×10⁻³ μm² (Table 2). The karst reservoir covers a large area in eastern and northern Sichuan (Fig. 4).

The denudation of the Maokou Formation under the unconformity between Middle Permian and Upper Permian gradually increases from southwest to northeast. The northeast denudation reaches the second member of the Maokou Formation at most. Karst reservoirs are mainly developed at the top of the Maokou Formation. The reservoir in areas where granular limestone (dolomite) is well developed ^[34], has porosity generally ranging from 2% to 8%, average permeability generally less than 0.8×10⁻³ µm², and cumulative reservoir thickness of 10-50 m (Table 2). The unconformity between Middle and Upper Triassic affected the formation of karst reservoirs at the top of the Leikoupo Formation in almost the entire basin, and even affected the formation of reservoirs in the Jialingjiang Formation in the core of the Luzhou paleouplift. The karst reservoirs at the top of the Leikoupo Formation are mainly composed of fine powder crystal dolomite and breccia dolomite. The reservoir is fractured-vuggy type (Fig. 5d), with porosity of 3% to 6%, permeability of (0.01-10.00)×10⁻³ µm², and cumulative thickness of 10-80 m (Table 2), with strong heterogeneity. The quality of the shoal reservoirs in the fourth member of the Leikoupo Formation in the western Sichuan Basin is good. Due to the Indosinian Movement, the core of Luzhou paleouplift was denuded to the second member of the Lower Triassic Jialingjiang Formation and weathered crust karstification occurred in the intraplatform granular shoal from the first member to the fifth member of the Jialingjiang Formation, forming a large number of dissolved pores, caves and fractures. The reservoir has porosity of 2.0% to 22.0% (averaged 3.9%), and permeability of (0.01-56.70)×10⁻³ µm² (generally less than 0.10×10⁻³ µm², Table 2).

Regional unconformities are usually generated at the end of the eustatic cycles of first, second or third order global sea level. The overlying strata were the beginning of another sea level eustatic cycle. When the sea level rose rapidly, the accommodation space was increased rapidly, the water body was deepened, and the sediments were mainly fine-grained sediments. The sedimentary environment was mostly deep/shallow water shelf facies or deep/shallow water lacustrine facies. A large area of fine-grained sediments overlay the karst reservoirs below the unconformity, which served as both source rocks and cap rocks. A good source-reservoir-cap assemblage was formed centered on the unconformity (Figs. 1b and 4).

Six regional unconformities in the Sichuan Basin have formed five sets of large-scale source-reservoir-cap assemblages from the bottom to the top: (1) Pre-Sinian weathered crust reservoirs and Sinian Doushantuo Formation argillaceous hydrocarbon source rocks ^[29]. So far, there are few wells drilled into the Doushantuo Formation source rocks in the basin, resulting in unclear distribution. Many outcrops found in the periphery of the basin revealed that the argillaceous hydrocarbon source rock of the Doushantuo Formation is thick and has a high TOC. (2) Weathered crust reservoirs of the Sinian Dengying Formation, argillaceous hydrocarbon source rock of the Cambrian Maidiping Formation and Qiongzhusi Formation, and mud shale source rock of the Maidiping Formation and Qiongzhusi Formation with a large thickness and good quality in Sichuan Basin, which are distributed almost all over the basin, and have a gas generation intensity greater than 20×10⁸ m³/km^{2 [32]}. (3) Carboniferous weathered crust reservoirs or Cambrian and Sinian Dengying Formation weathered crust reservoirs, and argillaceous source rocks of the Middle Permian Liangshan Formation ^[35]. The Middle Permian Liangshan Formation is mainly composed of grayish black shale, gray siltstone intercalated with gray bauxite mudstone and coal series. The source rocks are relatively thin in most areas, generally less than 10 m, but over 100 m in the southeast of the Sichuan Basin. The average TOC revealed by Well JT-1 is 1.94% ^[35]. (4) Weathered crust reservoirs of the Middle Permian Maokou Formation and coal source rocks of the Longtan Formation of the Upper Permian. The Longtan Formation is coal source rock of transitional facies, generally 60 to 150 m thick, and mainly distributed in eastern and central Sichuan. (5) Weathered crust reservoirs of the Leikoupo Formation or Jialingjiang Formation and coal source rocks of the Upper Triassic Xujiahe Formation. The coal source rocks of the first member of the Xujiahe Formation is generally 10-90 m thick, and mainly distributed in the western-central Sichuan Basin (Fig. 6). On the unconformity between the Triassic and the Jurassic, it is a Jurassic self-source and self-reservoir assemblage. The source rock of the Jurassic Zhuzhuchong Formation is semi-deep to deep lacustrine deposits, 14-200 m thick and averaged 100 m ^[36]. Hydrocarbon could migrate through the unconformity to the sandstone reservoirs at the top of the Xujiahe Formation and accumulate there (Figs. 1b and 4).

Unconformities are the result of tectonic movements. Regional unconformities are usually accompanied by paleo-uplifts and large-scale denudation, forming large- scale pinch-out. The six regional unconformities in the Sichuan Basin have resulted in multiple pinch-outs of varying degrees and scales. For instance, the unconformity between the pre-Permian and the Permian was accompanied by Leshan-Longnüsi ancient uplift and multiple pinch-outs of the Sinian, Cambrian, Ordovician, Silurian and Carboniferous from west to east (Fig. 4). In the pinch-out zones, the Deng 2 Member, Deng 4 Member, Cambrian Longwangmiao Formation, Xixiangchi Formation and Carboniferous Huanglong Formation became karst reservoirs after weathered crust karstification (Fig. 4). Other regional unconformities also induced large- scale stratigraphic pinch-outs in Maokou, Jialingjiang, and Leikoupo formations.

The large-scale reservoirs in the stratigraphic pinch- out zone are covered by thick and high-quality source rocks which are also good cap rocks and may form large-area stratigraphic traps. For example, the Deng 2 and Deng 4 members in Penglai gas play are stratigraphic traps. Their reservoirs are mound shoals of the Sinian Deng 2 and Deng 4 members, formed after constructive diagenesis such as dolomitization, interlayer karstification and weathered crust karstification. The Tongwan Movement caused large-scale denudation of the Dengying Formation. The denudation reached the Deng 2 Member in wells PS-3 and PT-1, and the top of the Deng 4 Member in Well MX-111 to form stratigraphic pinch-out. The Maidiping Formation and Qiongzhusi Formation deposited above the unconformity are both high-quality source rocks and high-quality caprocks, forming two stratigraphic traps in the Deng 2 and Deng 4 members (Fig. 7a). Drilling data show that gas production from the Deng 2 Member in Well PS-3 and Well PT-1 is 22.06×10⁴, 121.00×10⁴ m³/d, respectively, and 30.21×10⁴ m³/d from the Deng 4 Member in Well MX-111, indicating that stratigraphic traps formed by unconformity can have reservoirs in a large scale.

Atmospheric freshwater karstification occurs when formation exposed. Due to the different lithology of the exposed strata, the dissolution rate varied greatly, forming rugged ancient landform. The structural high is prone to dissolution, forming large-scale reservoirs with pores, caves and fractures. The structural low is easy to catch water, forming tight reservoirs with poor porosity and permeability. The hydrocarbon source rocks above the unconformity completely cover the reservoirs formed by weathered crust karstification, forming a lithologic trap group with the support from the tight zone between the reservoirs. For example, due to different degrees of karstification occurred at the top of the Maokou Formation in central Sichuan, the structural high zone at the top became reservoirs and the overlying argillaceous hydrocarbon source rock of the Longtan Formation served as high-quality caprock, which constituted a lithologic trap group (Fig. 7b). Karst reservoirs in such lithologic trap groups have good quality, and large hydrocarbon facing surfaces, so it is easy to form a large-scale lithologic reservoir group with a good exploration potential.

Unconformities are good channels for oil and gas migration ^[37-38]. The lithology above and below an unconformity differs greatly. Generally fine-grained sediments are dominant above, which are mainly mud shale source rocks, while below are dolomite or coarse clastic rocks. The permeable zone caused by karstification could become a good channel for lateral migration of oil and gas. In addition, unconformities are often accompanied by paleo-uplifts which bring large paleotopographic elevation difference to drive oil and gas migration.

The unconformity between the Cambrian and Sinian provided an advantageous pathway for the accumulation and lateral migration of oil and gas, as well as for the adjustment and transformation of oil and gas reservoirs in the Sinian system. According to the analysis of natural gas sources in the Dengying Formation of the Sinian System, it is believed that the natural gas mainly came from the source rocks of the Cambrian Maidiping Formation-Qiongzhusi Formation. The hydrocarbon generation center is located in the Deyang-Anyue Rift ^[32]. Hydrocarbons mainly entered the karst reservoirs of the Dengying Formation through lateral source-reservoir connection or the unconformity.

The regional unconformity between the Middle and Upper Triassic provided a lateral and long-distance pathway for the formation of the weathered crust karst reservoirs at the top of the Leikoupo Formation and sandstone reservoirs of the Xu 2 Member in central Sichuan foreland uplift. The primary source rocks of the natural gas reservoirs in the upper Leikoupo Formation are mudstones of the Upper Triassic Xujiahe Formation ^[39]. When the Xu 1 Member deposited, the primary sedimentary center and hydrocarbon generation center are located in the western Sichuan depression, and they were thick and high-quality hydrocarbon source rocks. The Xu 1 Member hydrocarbon source rock was undeveloped or thin. Hydrocarbon generated by the Xu 1 Member source rock migrated from west to east along the unconformity, and entered the weathered crust karst reservoirs at the top of the Leikoupo Formation and the Xu 2 Member sandstone reservoirs in the foreland uplift zone, forming large-scale oil and gas reservoirs, such as the oil and gas reservoir of the Leikoupo Formation in the Longgang area (Fig. 6) and the gas reservoir of the Xu 2 Member in Hechuan area.

A good source-reservoir-cap assemblage centered on an unconformity is conducive to large-scale accumulation of oil and gas, especially the constructive transformation of reservoirs under the unconformity by weathered crust karstification, forming high-quality large-scale reservoir bodies. When source rocks are sufficient, it may accumulate hydrocarbons on a large scale (Fig. 8a). If the stratum is flat and the paleogeomorphic fluctuation is small, an "inverted penetration" accumulation pattern with reservoirs lying below the source rocks would be formed. However, the most common geological conditions are stratigraphic dips or undulating paleo-geomorphology (Figs. 4 and 8b). For instance, hydrocarbons migrate from bottom upward along the unconformity, enter adjacent large-scale reservoirs, and accumulate with the pattern of "side source and side reservoir play" (Fig. 8b). The denuded reservoirs below the unconformity and the source rocks above the unconformity form a good lithologic trap group. If there is mudstone as cap rock above the reservoirs and tight rock as a lateral seal, good sealing conditions can be formed. Although undergone multiple tectonic movements, tight rock bodies such as mudstone cap rocks can effectively protect oil and gas reservoirs, and the oil and gas reservoirs formed around the regional unconformity can be preserved during later tectonic movements (Fig. 8c).

Regional unconformities played an important role in controlling oil and gas accumulation in the Sichuan Basin. Based on the analysis on the geological characteristics and resource potential of six regional unconformities, it is believed that the fields related to the unconformities are still the primary areas for increasing reserves and production in the Sichuan Basin. For example, many large gas fields have been discovered in areas related to the unconformity between Sinian and Cambrian, and that between pre-Permian and Permian. These large gas fields are mainly distributed in the east of the Deyang-Anyue rift and the core of the Gaoshiti-Moxi paleouplift. At present, significant discoveries and breakthroughs have been made in the northern slope of the paleo-uplift, and there are good reservoir forming conditions in the northwest, south and east of the Sichuan Basin. Favorable accumulation conditions as well as the basis for forming large gas fields are available, so they are important fields for the discovery of large gas fields ^[32]. Other exploration fields related to regional unconformities also have great exploration potentials. "The fourth round of national oil and gas resource evaluation" for six exploration fields related to regional unconformities suggest that the natural gas resources are about 13.0×10¹² m³, the proven geological reserves are about 3.1×10¹² m³, and the overall proven rate is about 23.8%, indicating a great potential for further exploration. The areas with large remaining resources are Sinian, Cambrian, Carboniferous, Permian Qixia, Maokou, and Changxing formations as well as the Triassic Feixianguan and Xujiahe formations which will be the focus of exploration for large gas fields in the Sichuan Basin. Statistically, no large gas fields have been discovered in the areas related to the unconformity between pre-Sinian and Sinian, and that between Triassic and Jurassic (Table 1). Above the unconformity between Triassic and Jurassic, thick and high-quality source rocks of the Lower Jurassic are developed, and multiple sets of thick sandstone reservoirs are developed in the peripheral areas. For example, the large-scale delta and channel sand body in the overlying Shaximiao Formation have a good relationship with source rocks, and they constitute a lower-source and upper-reservoir assemblage with a great exploration potential. In the Jurassic system buried at about 2000 m or shallower in the central Sichuan Basin, large-area oil reservoirs have been discovered in the Da´anzhai Formation. In addition, large-scale gas reservoirs have been discovered in the transitional Tianfu and Qiulin zones from central to western Sichuan Basin, and shale oil reservoirs have been developed in central to eastern Sichuan Basin, indicating that fields related to unconformities have great exploration potentials.

The pre-Sinian system is deeply buried in the Sichuan Basin, except for local zones such as the Weiyuan structure. The depth of the pre-Sinian system is generally deeper than 6000 m. Without sufficient drilling data, the geological characteristics of oil and gas reservoirs are still unclear, so resources in the pre-Sinian system were not taken into account in the "fourth round of national oil and gas resource evaluation". Near the unconformity between pre-Sinian and Sinian, hydrocarbon source rocks of the Doushantuo Formation and various types of karst reservoirs are developed, and forming a good assemblage (Fig. 9), which has a great exploration potential and is deserved to attention.

The hydrocarbon source rocks of the Doushantuo Formation are a set of high-quality hydrocarbon source rocks. Due to the lack of drilling data, their distribution characteristics within the basin are not very clear. According to the available data, the thickness of the source rocks in northwest and central Sichuan is 10 m to 30 m, greater than 50 m in southeast Sichuan, and greater than 50 m outside the eastern edge of the basin. For instance, the thickness of the Doushantuo Formation in Chengkou section is about 80 m, and that of effective source rock is about 50 m. The TOC is around 0.21% to 10.88%, with an average of 3.39%. The thickness of the source rock in Songlin section of Zunyi is about 65 m, and the TOC of more than 30 samples is 0.5% to 4.6%, with an average of 1.5%, indicating a large hydrocarbon generation potential. Based on the analysis of outcrops in the margin of the basin and drilling data in the basin, the pre-Sinian reservoirs were mainly formed through karstification of bedrock, volcanic rock and moraine glutenite of the Nantuo Formation under the unconformity. After long-term karstification, it's possible to form large-scale reservoirs. For example, the Nantuo Formation glutenite in Chengkou section has a large number of dissolution pores and caves visible in outcrops (Fig. 5e), and a large number of intergranular and intragranular dissolution pores visible on thin sections (Fig. 5f). The porosity is 3% to 5%. Large-scale reservoirs may be available. In addition, the bedrock weathered crust encountered in Well W-117 and Well NJ may be potential reservoirs. At the same time, high-quality and large-scale source rocks developed in the outcrops of the Datangpo Formation of the Nanhua System can provide hydrocarbon for the karst reservoirs under the unconformity ^[40], forming a good source-reservoir configuration relationship. The source rocks of the Doushantuo Formation can serve as direct caprocks to form large gas fields of bedrock type, moraine glutenite type, and volcanic rock type (Fig. 9). By now, there are very few wells drilled in the pre-Sinian system. Buried at 6000 m to 7000 m and with long-term stable structures, the pre-Sinian system is conducive to the preservation of gas reservoirs, and new breakthroughs are expected to make to exploration.