The Anyue gas field was discovered in the Gaoshiti-Moxi area of ​​the Sichuan Basin, where the main gas-bearing layers include the Sinian Dengying Formation and the Cambrian Longwangmiao Formation^[1,2]. Gas-bearing layers were found in the Sinian Dengying Formation and shale gas was found in the Doushantuo Formation in Yichang, Hubei province^[3]. These demonstrate great prospects for Sinian natural gas exploration in the middle and upper Yangtze region. The discovery of the Anyue gas field owes to the understanding about the Deyang-Anyue craton internal rift and "four ancient factors" controlled reservoirs^[1-2,4]. However, the origin and evolution of the Deyang-Anyue rift are still highly controversial^[5,6]. To deepen the understanding of the Sinian tectonic lithofacies and paleogeographic features of the Sichuan Basin, focusing on the middle-upper Yangtze region, this study is carried out following the idea of ​​"jumping out of the basin to look at the basin". In the course, tens of Sinian outcrop profiles in the middle-upper Yangtze region are surveyed; a large number of drilling, logging, seismic and other comprehensive research data including the Yichang area are collected and sorted; paleostructure, sedimentary facies, reservoirs and source rock of the Doushantuo and Dengying formations are analyzed; basic maps such as the Doushantuo Formation^[7] and the Dengying Formation tectonic paleogeographic maps are plotted; and the effects of paleo-tectonic pattern on the distribution of lithofacies and paleogeography and the spatial distribution of reservoir-forming assemblages are analyzed, in the hope to provide a reference for expanding new exploration frontiers.

The Jinning-Sibao orogeny formed the unified base of the Yangtze craton^[8]. Under the global tectonic background of Rodinia supercontinent breakup, multi-stage volcanic activities happened in the early and middle Neoproterozoic. The Nanhua rift system was formed in the southeast margin of the Yangtze craton, where a set of volcanic strata were deposited^[9]. In the Nanhua period (635 to 720 Ma), under the background of "Snowball Earth" and extensional structure, passive continental margin clastic rocks were deposited on the edge of the Yangtze craton^[10], and intra-craton rifts were developed inside the craton^[4]. In the Sinian-Early Paleozoic, the craton evolved into a depression, the sedimentary filling was dominated by marine carbonate deposits, and multiple sets of source-reservoir-caprock combinations were developed, laying a material foundation for rich oil and gas resources.

The Sinian system is widely distributed in the middle and upper Yangtze region and is divided into upper and lower series^[11]. The Doushantou Formation in the lower series is the first set of sediments of the Yangtze craton. In the paleo-continent area with the Sichuan Basin as the main part, the Doushantuo Formation is thin, with a thickness of only 20-60 m; in the marginal sags around the paleo-continent, the Doushantuo Formation is complete, with a thickness of 500-1000 m, and dominated by organic mudstone of continental shelf facies^[7,12]. The Dengying Formation in the Upper series is widely distributed, and can be divided into four lithologic members^[13] of first to fourth, with a thickness of 50-1400 m. During this period, affected by structural differentiation within the craton, the Deyang-Anyue craton rift inside platform was developed in the central part of the Sichuan Basin, where the Dengying Formation is 50-150 m thick; in the flank platform margin zone of the rift, the Dengying Formation is 800-1400 m thick, and other areas were platform facies, where the Dengying Formation is generally 600-800 m thick. In the early Early Cambrian, the rift was developed successively. In the middle Early Cambrian (equivalent to the late depositional period of Qiongzhusi Formation), the rift withered, and was filled^[4] and replaced by a large carbonate gentle slope platform high in the west and low in the east. By the late Late Ordovician and Silurian, with the uplift of the Middle Guizhou paleocontinent and Xuefeng paleocontinent, clastic rock deposits began to appear in the foreland basin^[14], ending the evolution of the Sinian-Early Paleozoic carbonate platform.

The middle-upper Yangtze region in the Sinian was in an extensional tectonic environment. It was connected to the western Sichuan sea basin to the west, the southern Qinling passive continental margin basin to the north, and the central and southern Hunan passive continental margin basin to the southeast. Affected by regional stretching, tectonic subsidence differentiation occurred within the craton basin due to co- sedimentary faulting activities^[15]. The middle-upper Yangtze craton was cut apart by the Deyang-Anyuetai rift and Chengkou-West Hubei rift in SN direction, forming a "three uplifts and two sags" structural frame (Fig. 1).

The Deyang-Anyue intra-platform rift is located in the hinterland of the Sichuan Basin, also known as the "Anyue- Deyang craton internal rift"^[4]. It is in a trumpet shape and NS strike, opens to the western Sichuan Basin in the north, and extends to the middle and south of Sichuan, with a width of 50-180 km, a length of 560 km from north to south, and a distribution area of 6×10⁴ km² (Fig. 2). Studies have shown that the rift has syn-sedimentary boundary-control faults and internal secondary faults in mainly NWW strike developed. In the Gaoshiti-Moxi area, the faults have a fault throw of 400-500 m at the bottom of the third member of the Dengying Formation, 300-400 m at the bottom of the Cambrian, and smaller fault throw in the Canglangpu Formation. Except the boundary faults, most of the faults disappear in the Longwangmiao Formation. On the plane, the boundary-control faults have larger fault displacement, and decrease of fault distance from north to south. In terms of genetic mechanism, the rift is the result of the extension of the West Sichuan Basin, and is an extension rift of the West Sichuan sea basin stretching toward the Yangtze craton^[15].

The evolution of the Deyang-Anyue intra-platform rift has gone through three stages: (1) The depositional period of the Doushantuo and Dengying formations was the formation period of the rift. During the depositional period of the Doushantuo Formation, the rift mainly was developed in the area north of Jiange. The seismic section shows that the Doushantuo Formation obviously thickens. During the depositional period of the first and second members of the Dengying Formation, the rift extended southward to the Gaoshiti area, and boundary faults were developed on both sides of the rift, making it a double-fault type rift. During the depositional period of the Deng 4 Member, the rift expanded southward, increasing in scale continuously, meanwhile, the eastern boundary fault increased in activity intensity, forming a graben-shaped rift (Fig. 3). (2) The Early Cambrian was the development period of rift. During the depositional period of the Maidiping Formation, slope-basin facies carbon-silica mudstone, argillaceous nodular dolomite, and carbon-silica mudstone 100-200 m thick were deposited in the rift area, and carbonate platform limestone only 10-30 m thick were developed on the periphery of the rift. In the early depositional period of the Qiongzhusi Formation (equivalent to the depositional period of the Qiong 1-Qiong 2 Member), the rift was filled with deep-water continental shelf facies organic-rich shale, which is the main interval of high-quality source rock of the Lower Cambrian. (3) The middle and late Early Cambrian (equivalent to the depositional period of the third member of the Qiongzhusi Formation) is the demise period of the rift. Drilling data reveals that the third member of the Qiongzhusi Formation is delta-shallow water shelf deposit, with sandstone increasing significantly and organic carbon content of argillaceous rocks decreasing significantly, indicating that the early rift was filled and leveled, entering the stage of depression evolution.

The Chengkou-Western Hubei intra-platform rift is located in the Daba Mountain and the western Hubei region. It has a "Y" shape opening to the passive continental margin sea basin of the Qinling Mountains in the north, and extends toward Enshi and Dayong in the south. It is 80 to 300 km wide and 300 km long from north to south. It may be connected to the continental marginal basin of central and southern Hunan, separating the upper Yangtze craton and the middle Yangtze craton (Fig. 1). Chen Xiaohong et al.^[16] studied the Late Sinian sedimentary environment and genesis of the Dayong-Cili area based on the rock chemical composition and trace element composition, and concluded that the formation of the Sinian black rock system in the area was related to the basin rifting, crustal stretching and thinning, and the upwelling of mantle fluids associated with crustal thinning.

The Chengkou-Western Hubei intra-platform rift began to take shape during the deposition of the Sinian Doushantuo Formation, when shallow water shelf deposits of gray-black carbonaceous shale, gray mudstone interbedded with silty sandy mudstone, phosphorous siltstone, and thin layered dolomite 120-300 m thick developed^[7,12]. During the depositional period of the Dengying Formation, the rift developed successively, and was filled with thin largillaceous dolomite and limestone layers. The Dengying Formation revealed by Well Eshen 1 in the area is only 92.5 m thick, and is mainly composed of thin muddy dolomite and limestone, representing deep-water continental shelf deposits. On both sides of the rift, there are platform margins dominated by mound and shoal facies of large thickness. The Dengying Formation in Well Li 1 at the western platform margin zone, is 833.5 m thick and mainly composed of thrombolites with rich dissolution pores and vugs. The Dengying Formation in Well Eyidi 4 in the eastern platform margin zone is 596 m thick, and made up of thick layers of algal dolomite, granular dolomite, local limestone and siliceous dolomite (Fig. 3). From the analysis of formation thickness and lithofacies changes, there may be normal faults developed there. In the downthrown block of the fault, the Sinian is thin, and the Lower Cambrian Maidiping Formation and 500-600 m thick Qiongzhusi Formation mud shale develop. In the upthrown block of the fault, Dengying Formation is made up of thick layers of microbial mounds and shoals, and the Qiongzhusi Formation significantly reduces in thickness. This feature is comparable to the filling deposits in the Deyang-Anyue rift.

Seismic data in the middle Yangtze block is scarce, and only a few seismic lines are available in Yichang and Zigui areas, making it difficult to characterize the rift boundaries and distribution of faults inside the rift. By interpreting the seismic facies of the seismic profile through Well Yishen 1 (Fig. 4), Well Yishen 1 encountered the platform margin zone of the Dengying Fm. featuring weak amplitude and chaotic reflections in seismic facies, which is similar to the seismic facies of the platform margin of the Gaoshiti-Moxi area. There is a normal fault west of this well, in the downthrown block of the fault, the Dengying Formation shows features of slope-basin facies such as strong amplitude and continuous reflection, and the foresetting phenomenon can be seen from the platform margin to the basin; in the east of the well, a local small rift controlled by the fault can be interpreted, where the reflective layers show good continuity and the formation is obviously smaller than that in the two flanks.

The Sinian Dengying Formation in the middle-upper Yangtze region is characterized by dolomites rich in microbes, including framework dolomite, nucleated dolomite, oolitic dolomite, striated dolomite, gravel-dolomite, micritic dolomite, and argillaceous dolomite, etc. The dolomites have different shapes, including various framework shape (upright, winding, creeping, etc.), thrombolite, spherulite, foam, lamina, layer, and snowflake shapes etc. Generally, the Dengying Formation is a large-scale shallow-water carbonate platform, which can be divided into restricted evaporative platform facies, intra-platform rift facies, platform sag marginal facies, platform marginal facies, and slope-basin facies. The characteristics of sedimentary facies are shown in Table 1 and Fig. 5.

The middle-upper Yangtze region during the deposition of the Dengying Formation basically inherited the paleogeographical frame of the depositional period of the Doushantuo Formation, mainly developed carbonate, and was the first large- scale carbonate platform development period in southern China.

The 30-160 m thick Deng 1 Member is dominated by massive dolomite, with little microbial texture. Deng 2 Member deposited continuously on the Deng 1 Member is dominated by microbial dolomite and botryoidal structures, with a thickness of 350-550 m. Because it is difficult to distinguish the two members from seismic data, and the drilling data is scarce, the first and second members of the Dengying Formation are combined in plotted maps. By combining well log and seismic data, the remaining thickness map of Deng 1 and Deng 2 members is compiled. It can be seen that they are 200-1100 m thick in the upper Yangtze region, much thinner in the area of Jiangyou—Mianyang—Ziyang (less than 200 m), 200-500 m thick in the middle Yangtze region, and less than 100 m thick in Wushan-Padong-Cili area.

According to the dominant facies and changes in stratum thickness, the lithofacies paleogeographic map of Deng 1 and Deng 2 members is compiled (Fig. 6), which shows clearly two independent rimmed platforms. The main body of the upper Yangtze craton is located in the Sichuan Basin, in which the Deyang-Anyue intraplatform rift developed. The main part of the middle Yangtze craton is located in the western Hubei region. The Western Hubei intra-platform rift is between the two platforms. At the periphery of the platform are the platform margin slope and deep water basin, where the sediments are mainly a set of dolomite with micritic calcite or siliceous mudstone and siliceous rock. In the Western Sichuan sea basin to the west of the platform, deep-water dolomitic limestone interbedded with dark gray killas and phyllite occurs in the Pingwu area. In the Chengkou area of the Qinling Basin to the north of the platform developed a hugely thick set of gray-black siliceous rock. The southeastern margin of the platform is the platform margin slope-sea basin, and gray and gray-black phosphorous mud shale interbedded with dolomitic limestone and siliceous dolomite can be seen in Xiushan and Songtao areas, representing the platform margin deposits; in Zhongfang area of Huaihua, Jiangkou, and Sanjiang area of North Guangxi province, thick siliceous rocks are developed, representing deep-water basin sediments.

3.2.1. Upper Yangtze craton

The ancient topography before the deposition of the first and second members of the Dengying Formation, such as the Deyang-Anyuetai rift and sporadic ancient island chains, had a strong control on the distribution of lithofacies and paleogeography. The lithofacies paleogeography of the upper Yangtze craton is mainly composed of carbonate platform and Deyang- Anyue intra-platform rift two paleogeographic units. Both the platform margin and flanks of the rift in platform high-energy environments have large scale mound and beach complexes developed. Distributed around the platform, they constitute the rim of the platform in the upper Yangtze region. Within the platform, there developed small-scale inner platform beaches and inter-tank depressions, appearing in a unique "star-studded" ancient geographical landscape (Fig. 6).

Carbonate platform: The carbonate platform is dominated by restricted platforms, with evaporative lagoons and tidal flats locally. Inside the platform, controlled by the micro-paleotopography, mound-shoal complex composed of algal marl mounds and grain shoals usually developed in the higher parts of the micro-paleotopography. In the Xianfeng section of E’bian, the marl mound of the second member of Dengying Formation can be further divided into mound core, mound top and mound base (Fig. 7). The mound core is micrite dolomite, with a small amount of microbial framework dolomite; the mound top is generally micritic dolomite containing microbial laminae; the base of the mound is sandy beach or early lime mud mound. There were inter-shoal depressions between the mound-shoal complexes, where the water was deep and muddy dolomite deposits developed. In the inter-shoal depressions enclosed by large mounds and shoals, blocked by mounds and shoals, evaporative lagoons and evaporative tidal flats formed, where the sediments are characterized by dolomitic gypsum salt, gypsum salt rock and halite. For example, cores are taken from the whole Dengying Formation in Well Wei 117 in the Weiyuan area, which fully record the sedimentary evolution from evaporative tidal flat of the Deng1 Member, to semi-restricted-open platform of the Deng 2 Member, to detrital rock shelf of the Deng 3 Member, and to the Deng 4 Member platform. Among them, the Deng1 Member of Well Wei 117 has about 60 m thick dolomite containing anhydrite nodules and agglomerates (already replaced by dolomite), with some residual holes after the anhydrite dissolution. The Deng1 Member in Well Ning 2 of the Changning area has a 240 m thick gypsum salt rock layer and a 30 m thick gypsum dolomite layer. On the Yinchangpo section in Huize, Yunnan province, the Deng 2 Member generally contains needle-shaped or columnar anhydrite crystal dolomite.

Platform margin mound shoal complex: developed in high- energy environments such as the margin of the platform and flanks of the rift in the platform. The shallow seawater and warm climate are conducive to the prosperity of fungi and microbes, so a large number of fungus and alga-bearing dolomite marl mound with botryoidal lace structure and microbial mound are developed. The algal mound has certain wave resistance. Under the action of waves, the microbial mound debris and microbial mound form a mound-shoal complex. In the outcrop areas of the Sichuan Basin, such as Yangba in northern Sichuan, Qipangou in Wenchuan of western Sichuan, Huize in Yunnan, Pengshui in Yudong, Lichuan and other places, large algal mounds and mound-shoal complexes up to 20-40 m high can be commonly seen. Wells drilled in the platform margin zone of the Deyang-Anyue intra-platform rift reveal different shapes and occurrences of microbial origin thrombolite, foam sponge layer, stromatolite, lamellar dolomites, and gravel and dolarenite. For example, Well Zi 4 in the western platform margin zone of the rift encountered shoal facies dolomite with a continuous thickness of 120 m in the second member of the Dengying Formation. Well Gaoshi in the eastern platform margin zone of the rift, encountered 430 m thick Deng 2 Member, and the lower part is composed of gray-brown micritic thrombolite, dolarenite interbedded with micritic dolomite, and siliceous dolomite, representing interbedded deposits between mounds and shoals. The upper part is micritic thrombolite, laminar thrombolite interbedded with stromatolite and dolarenite, representing microbial mound-shoal complex deposits.

Intra-platform rift: The Deyang-Anyue intraplatform rift has taken shape during the depositional period of the Doushantuo Formation^[7]. During the depositional periods of the first and second members of the Dengying Formation, it extended to the Gaoshiti area in the craton. The sedimentary facies in the intraplatform rift in the platform include upper slope facies, lower slope facies and trough basin facies. The upper slope facies is characterized by the development of stacked gravel dolomite; the lower slope facies is represented by nodular dolomite and dolomite with argillaceous laminae, and argillaceous strips; the trough basin facies is represented by thin layers of argillaceous dolomite or gravity flow muddy dolomite, muddy (micritic) dolomite and dolomitic mudstone.

Ancient land or islands: During the depositional period of the first and second members of the Dengying Formation, ancient lands or islands were scattered in the southwestern margin of the Sichuan Basin. Studies have shown that the large Xuanhan-Kaijiang ancient land existes in the northeastern Sichuan^[17], with an area of ​​about 1.6×10⁴ km². Well Wutan 1 was drilled in the ancient land. Completed at the depth of 8060.00 m, this well penetrated through the Sinian and entered the pre-Sinian. The Dengying Formation revealed by this well is 303 m thick, much thinner than that in central Sichuan. Stratigraphic correlation shows that the Deng 4 and Deng 3 members are complete and comparable with those in the central Sichuan area in lithology. The Deng 2 Member is only 15 m thick, and is dominated by micritic and silty dolomite and dolarenite (Fig. 8). The Deng 1 Member and Doushantuo Formation at the bottom of the Sinian are missing, and the second member of the Dengying Formation is in direct contact with the thick clastic rocks below. The clastic rocks are interbedded gray-green argillaceous siltstone, siltstone, and tuffaceous mudstone. The main peak age of the zircon in samples taken from 8021 m and 8022 m of the well is 708-754 Ma, the youngest age exceeds 635 Ma, so it is inferred that the formation is Nanhua system. Comprehensive analysis shows that the ancient land already existed in the sedimentary period of the Doushantuo Formation. The ancient land developed successively during ​​the early depositional period of the Dengying Formation. The first member and most of the second member of the Dengying Formation are missing. Carbonate rocks began to deposit in the late depositional period of the Deng 2 Member, and the ancient land disappeared.

3.2.2. Chengkou-Western Hubei intra-platform rift

The Chengkou-Western Hubei intra-platform rift has been formed during the depositional period of the Lower Sinian Doushantuo Formation^[7], and it evolved successively during the depositional period of the Upper Sinian Dengying Formation. The Dengying Formation in it is continental shelf-slope facies micritic-silty dolomite and siliceous dolomite, with parallel bedding and horizontal bedding, and a small number of convolution bedding and sliding structures. It is generally 100-150 m thick, but varies greatly in thickness in the platform margin zone.

The Dengying Formation on the Yangjiaping section of Shimen inside the rift can be divided into four members^[18]. The fourth member of the Dengying Formation is a large set of gray medium thickness-layered micrite dolomite with horizontal bedding and siliceous rock or silicon lenticular bodies between layers. This member has a sandwiched layer of bamboo leaf-like clast dolomite with sliding structures and convolution beddings of about 3 m thick, and is 93 m thick in total. The third member of the Dengying Formation 14.68 m thick is made up of laminated gray-black medium-thick micritic siliceous dolomite and thin argillaceous siliceous dolomite, interbedded with thin carbonaceous shale layers. The Deng 1 and Deng 2 members 68.72 m thick are composed of gray-black medium-thin layered muddy siliceous dolomite, interbedded with thin carbonaceous shale, with micrite-silt structure and horizontal bedding. The Dengying Formation revealed by Well Ecan 1 in Enshi is only 92.5 m thick. The fourth member of the Dengying Formation is gray micrite dolomite, with siliceous and flint clots and a thickness of 56.0 m. The third member of the Dengying Formation, 10.5 m thick, is gray-black-siliceous shale and dolomitic shale. The Deng 1 and Deng 2 members, 26.0 m thick, are made up of gray-green dolomite and dolomitic limestone. The Doushantuo Formation, 65.5 m thick, is dominated by gray-black shale and gray-green mudstone. The Dengying Formation encountered in Well Li 1 at the platform margin on the west side of the rift is 833.5 m thick, and a large set of microbial reefal dolomite, thrombolite framestone dolomite, stromatolite, lamellar dolomite, sand gravel, and dolarenite, etc., showing obvious sedimentary characteristics of high-energy environment.

3.2.3. Isolated platform in the middle Yangtze region

The middle Yangtze region refers to Hubei Province and the large part of Hunan Province, south of the Qinling Trough, north of the Hunan, Guizhou and Guangxi sea basin, and east of the Enshi-Longshan Line in western Hubei. There, the paleogeography during the sedimentary period of the Dengying Formation inherited the main features of the sedimentary period of the Doushantuo Formation, and appeared as an isolated platform surrounded by deeper water around^{[19,20,21,22]}. Platform margin zones developed on the periphery of the platform, and the main deposits were sandy debris shoals and algal dolomitic marl mounds. Carbonate tidal flats and restricted platform facies developed inside the platform. The carbonate tidal flat facies is mainly composed of micritic and silty dolomite, silty dolomite interbedded with algal stratified dolomite, gravel-dolomite, oncolite, thrombolite, and oolitic dolomite, etc., with tidal bedding, trough cross-bedding, pinnate cross-bedding, algal laminae and bird's eye sedimentary structures. The outskirts of the tidal flat area are broad and restricted platform facies zones, where the main sediments are oolitic dolomite, oncolite, thrombolite dolomite and micritic and silty dolomite with horizontal bedding, wave bedding and sand grain bedding, and have shale and silty mudstone interbeds in local parts. There are often shoal and intershoal deposits dominated by sparry oolitic dolomite, oncolite dolomite and micro-powder crystalline dolomite inside the restricted platform zone.

The depositional period of the Deng 3 Member is an important period of lithofacies paleogeographic change. At the end of the depositional period of the second member of the Dengying Formation, EpisodeⅠof the Tongwan movement happened in the upper Yangtze region. Outcrop and drilling sections show that the mud-rich clastic rocks and carbonate rocks of the third member of the Dengying Formation are in disconformable contact with the microbial dolomite of the Deng 2 Member. The Deng3 Member has a stable thickness of 30-50 m largely and big changes in lithology. Due to the small thickness of the Deng 3 Member, except that the bottom interface of the Deng 3 Member shows strong continuity and strong reflection, there is little seismic information within the stratum that can reflect lithological changes. Therefore, lithofacies paleogeography map of the Deng 3 Member here wasn’t compiled in this study.

During the depositional period of the Deng 3 Member, as the sea level rose, the middle and upper Yangtze region entered the transgression period. Due to the input of a large amount of terrestrial materials, the development of carbonate rocks was inhibited, making the sedimentary environment in the middle and upper Yangtze region change into a restricted sea environment dominated by terrestrial debris. On the Yangba section of the Nanjiang River, the third member of the Dengying Formation is a set of near-source gravel-bearing feldspar quartz sandstone in disconformable contact with the underlying weathered crust-type dolomite of the second member of Dengying Formation. In the western Sichuan-East Yunnan area, the Deng 3 Member is purple-red limy mudstone and limy sandstone; the Deng 3 Member in the southern Sichuan-Qianbei region is blue-gray mudstone, and the Deng 3 Member in the Moxi—Gaoshiti—Longnüsi area is gray-black mudstone and sandy mudstone. The Deng 3 member in the middle Yangtze region is siliceous dolomite depositing continuously on the Deng 2 member. It can be seen that an uneven uplifting and subsiding movement occurred in the upper Yangtze region at the end of the depositional period of the second member of Dengying Formation, consequently, the area was high in the west and low in the east, and the west part suffered erosion and the east part had continuous deposition.

The Deng 4 Member deposited continuously after Deng 3 Member, is the product of highstand systems tract after the transgression, and also corresponds to another important period of large-scale carbonate platform in the middle and upper Yangtze region. The remaining stratum of Deng 4 Member is 50-600 m thick. Its lithofacies and paleogeographic features are characterized by three carbonate platforms divided by two intra-platform rifts (Figs. 9 and 10).

The paleo-tectonic frame during the depositional period of the Deng 4 Member inherited that during the depositional period of the Deng 2 member on the whole, but the tectonic activity in this period was stronger, so the lithofacies paleogeography of the Deng 4 member show some particularities from the Deng 2 Member.

(1) In terms of sedimentary scope, during the depositional period of the Deng 4 Member, as the transgression continued to expand, the early ancient land gradually disappeared, and the platform covered the entire middle-upper Yangtze craton. However, in Well Zeng1 and Hui1 of northern Sichuan, 12.5 m and 23.5 m of evaporative lagoon-evaporated tidal flat gypsum-salt rock, dolomite gypsum salt, and gypsum dolomite were encountered. The formation of these rocks might be related to the poor circulation of seawater caused by the blockage of huge mound shoals in the north craton platform margin and platform in the west of the craton.

(2) The water in the depositional period of the Deng 4 Member was relatively deep, not conducive to the prosperity of fungi and microbes, so this member has few fungi and microbial laminae, and is mainly composed of micritic-powder crystalline dolomite and a small amount of dolarenite, with siliceous bands or siliceous agglomerates universally. The edge of the platform was small in scale, and the intraplatform rift developed successively, where muddy rocks deposited, with a thickness of 50-100 m.

(3) Tectonic activity became stronger, consequently, the big carbonate platform in the early stage was separated into multiple isolated platforms. The Deyang-Anyue intra-platform rift continued to extend toward the hinterland of the platform, and the deposition range of the intra-platform rift expanded constantly, further dividing the upper Yangtze craton. The isolated platforms in the middle Yangtze region formed in the early stage develop successively, and characteristics of the platform margin zone in the northwest of the platform became more obvious. On the other hand, the siliceous thermal fluid activity brought by the deep tectonic movement was relatively strong, resulting in common siliceous bands or siliceous agglomerates in the thick micritic- microcrystalline dolomite in the Deng 4 Member.

The intra-craton structural differentiation occurred in the middle-upper Yangtze region in the Late Sinian, giving rise to the ancient structural frame of three platforms and two sags, which not only controlled the lithofacies paleogeographic distribution of the Sinian Dengying Formation, Cambrian Maidiping Formation and Qiongzhusi Formation, but also controlled the spatial and temporal distribution of the combination of oil and gas accumulation factors^[23].

Drilling and seismic data have confirmed that there develop four sets of high-quality source rocks in the Sinian Doushantuo Formation and Dengying Formation, Lower Cambrian Maidiping Formation and Qiongzhusi Formation in the Deyang-Anyue intra-platform rift.

The Sinian Doushantuo Formation source rock is encountered by only a few wells, but 35 samples of the Doushantuo Formation mudstone in the Songlin section of Zunyi have TOC values ​​of 0.11% to 4.64%, average TOC of 1.51%; kerogen isotope composition of -31.5‰ to -30.3‰, -30.8‰ on average, representing sapropelic type; and equivalent vitrinite reflectivity of 2.08%-2.34%, indicating over-mature stage. The Douzantuo Formation in the seismic profiles of northwest Sichuan features continuous strong reflection, which is the seismic response of argillaceous rock, and is up to 150 m thick. The source rock of the third member of the Dengying Formation is 10-30 m thick black shale. 67 samples of the black shale from Well Gaoke 1 have TOC values from 0.50% to 4.73% and an average TOC of 0.87%, and samples with TOC greater than 0.5% account for 59.8%. Their isotopic compositions of kerogen range from -33.4‰ to -28.5‰, with an average value of -32.0‰, indicating that the organic matter is sapropelic type (type I). And their equivalent R_o values range from 3.16% to 3.21%. The source rocks of the Lower Cambrian Maidiping Formation are mainly 30-50 m thick siliceous shale and carbonaceous mudstone, etc. The source rocks have high organic matter abundance, with TOC from 0.52% to 4.00%, and 1.68% on average. Their kerogen isotopic compositions are from -36.4‰ to -32.0‰, with an average of -34.3‰, indicating that the source rocks are typical saprolitic source rock. Their equivalent R_o values range from 2.23% to 2.42%, indicating that the source rocks are in high-over mature stage.

The source rocks of the Lower Cambrian Qiongzhusi Formation are mainly black and gray-black mud shale and carbonaceous mudstone, with a thickness of 300-450 m^[1,2], which is a set of regionally distributed high-quality source rocks. Studies have shown that the Qiongzhusi Formation can be divided into 3 members^[24], and the first member of Qiongzhusi Formation at the bottom is mainly distributed in the Deyang- Anyue intraplatform rift and is the product of the early transgression. It is dominated by black-dark gray mudstone and shale, with a thickness of 50-300 m. On seismic profiles, it appears as strong continuous reflection overlaping toward the rift flanks. With high TOC values from 0.5% to 4.8%, and an average TOC of 1.98%, it is the main section of the source rock of the Qiongzhusi Formation. The second member of the Qiongzhusi Formation is distributed across the basin and is the product of the largest transgression period. It is mainly composed of black-dark gray carbonecous shale and mudstone, and is 50-200 m thick. The source rock of the Qiongzhusi Formation in the rift is thicker, ranging from 100 to 200 m; the source rock of the Qiongzhusi Formation in the central Sichuan platform is 50 to 100 m thick, 0.4% to 3.1% in TOC (1.68% on average). The third member of the Qiong Formation is the sedimentary product of the highstand systems tract. Influenced by the material supply from the western Sichuan paleo- uplift, it increases in proportions of silty mudstone and argillaceous siltstone obviously, and has lower organic carbon content of generally less than 1.0%, indicating it is poorer in quality.

The formation and distribution of reservoirs in the Dengying Formation show obvious characteristics of "facies control" + "karstification".

During the depositional period of the Dengying Formation, the middle-upper Yangtze region was epicontinental sea with shallow water body, and microbial mounds and shoals developed in the platform margin zone and in the platform, laying a material foundation for the formation of large-scale reservoirs. In the high energy platform margin, microbial mounds and shoals accumulate and grow to a larger thickness, for example, Well Moxi 108 (Fig. 11a). In the platform, the microbial mounds and shoals are mainly controlled by micro-paleogeomorphology, and the microbial rocks are dominated by dolomite with algal laminae, thrombolite, dendrite and homogeneous dolomite, for example Well Moxi 51 (Fig. 11b). The reservoir rock with the best storage performance is microbial skeleton dolomite, with an average porosity of 5.41%; followed by algal thrombolite and dolarenite, with an average porosity of 3.85%; micritic and powder-crystalline dolomite reservoirs have the worst physical properties, with an average porosity of only 1.41%.

The superimposed karstification of microbial carbonate rocks is the key factor for the large area distribution of the Dengying Formation reservoirs. Episodes I and II of the Tongwan Movement resulted in two regional erosion unconformities in the Deng 2 and Deng 4 members respectively, which facilitated the formation of large-scale karst reservoirs^{[23,24,25,26,27]}. According to the wells drilled in both the platform margin and the platform, the Deng 2 and Deng 4 members generally have dissolution pore-vuggy reservoirs, but the reservoirs at the platform margin zone are thicker, and only the Deng 4 member can reach 130 m thick. Inside the platform, the reservoirs are mainly concentrated in the upper 100 m of the Deng 4 Member, and are 30-70 m thick. The main factors that cause the thickness difference of the Dengying Formation reservoirs in the platform margin and the platform are the exposed surface of the formation and the degree of karstification. The platform margin shoal bodies on the top surface of the Dengying Formation and the flanking slope zone adjacent to the rift zone were subjected to karstification, especially the slope zone karstification was more favorable, which ultimately leads to a larger karstification depth and larger reservoir thickness in the platform margin zone. Mound and shoal bodies inside the platform only suffered karstification from the top surface, so the karstification depth is much smaller than that in the platform margin zone.

Platform evolution controls large-scale source-reservoir combinations, which can be divided into two types. The first type is the intra-platform rift-platform margin source-reservoir combination, which is mainly composed of the thick high- quality source rock in the intra-platform rift and the large scale reservoir in the platform margin, constituting the most favorable near-source combination, for example, the source- reservoir combination in the hinterland of the Sichuan Basin. There are two sets of large-scale reservoirs in the second and fourth members of the Dengying Formation on both sides of the rift, and the source-reservoir combinations are superior, which is the main factor for the formation of the Anyue gas field. The second type of source-reservoir combination is the intra-platform source-reservoir combination, which is mainly the “source above reservoir” combination composed of the mound and shoal reservoir of the underlying Dengying Formation and the overlying Qiongzhusi Formation source rock, with wide distribution. At the same time, connected by large faults reaching the source rock, the Cambrian grain shoal reservoirs distributed in large area can also form large-scale reservoirs, for example, the Longwangmiao Formation and the Xixiangchi Formation.

The above analysis of accumulation conditions shows that the Dengying Formation has favorable conditions for large scale oil accumulation, which determines its important position in future oil and gas exploration. At present, the exploration is mainly concentrated on the Deng 4 member around the axis of the central Sichuan paleo-uplift. In the future, the exploration of favorable areas such as the platform margin zone, the Deng 2 Member, the Chuanzhong shoal in platform, and eastern Sichuan should be strengthened.

The regional extensional tectonic environment resulted in structural differentiation in the middle-upper Yangtze craton in the Late Sinian. Controlled by synsedimentary fault activities, the Deyang-Anyue intra-platform rift developed in the central Sichuan Basin, and the Chengkou-Western Hubei intra-platform rift developed between the middle and upper Yangtze regions, forming the structure-paleogeographic frame of “three platforms and two sags” during the depositional period of the Dengying Formation. The structural differentiation controls the carbonate sedimentary differentiation of the Dengying Formation. The intra-platform rift was deep-water continental shelf sedimentary environment, where thin layers of micrite dolomite and limestone deposited. The platform margin zone on the periphery of the rift was high-energy environment of the water body, where accretionary microbial mounds and shoals developed to a larger thickness. The platform was epicontinental sea environment, where medium-thickness microbial mound and shoal bodies developed extensively. The evolution of the platform of the Dengying Formation has gone through three stages: during the depositional period of the first and second members of the Dengying Formation, the platform coexisted with the rift, and the upper Yangtze-rimmed platform, the intra-platform rift and the middle Yangtze isolated platform developed. The depositional period of the Deng 3 Member is a period of structural transformation, with stable structure and weak sedimentary differentiation, and this member represents shallow water shelf sediments on the background of the eroded ancient geomorphology of the Tongwan Movement Ⅰ. During the depositional period of the Deng 4 Member, the Deyang-Anyue intra-platform rift expanded and the middle Yangtze craton evolved into two isolated platforms. The tectonic-sedimentary differentiation controls the distribution of the source-reservoir combinations in the Dengying Formation. The intra-platform rift controls the distribution of high-quality source rocks, and the platform margin zone controls the high-quality reservoirs. Both the rift-platform margin zone and the intraplatform source-reservoir combination types have favorable conditions for large-scale oil accumulation. The area the Central Sichuan paleo-uplift and the Dengying Formation platform margin zone overlap, the second member of Dengying Formation inside the rift, and the intraplatform shoal of the Central Sichuan are the key areas for large-scale exploration.