The Sinian System is widely distributed in southern China and contains abundant conventional natural gas and shale gas resources. In 2012, the Anyue giant gas field was discovered in the Gaoshiti-Moxi area of the Sichuan Basin, in which the Sinian Dengying Formation is one of the major gas-bearing strata^[1,2]. In 2017, shale gas was found in the Doushantuo Formation the Yichang area of Hubei Province, that is an important exploration discovery^[3]. The Doushantuo Formation is the first set of sediments in the middle and upper Yangtze region in the stage of stable cratonic depression. Its oil and gas geological conditions haven’t been examined systematically, which has restricted the objective evaluation of the natural gas resource and favorable exploration areas in this formation.

The study of the Sinian Doushantuo Formation in the middle and upper Yangtze region can be traced back to the 1920s. In 1924, Li Siguang and Zhao Yazeng established the Sinian stratigraphic profile, including the stratigraphic units of the Nantuo Formation, Doushantuo Formation and Dengying Formation^[4]. In the following decades, studies on the Sinian System has made great progress. However, the previous researches mainly focused on the stratigraphy^{[5,6,7,8,9,10,11,12]}, biology^[13] and the scientific significance of Neogene Proterozoic Ediacara (Sinian) major life events on the early evolution of the Earth's life. Since 2012, with the discovery of Sinian conventional natural gas and the rapid development of marine shale gas exploration, Doushantuo Formation has gradually become a research hotspot^[14,15,16].

In order to examine the lithofacies paleogeographic characteristics of the Doushantuo Formation, dozens of Sinian outcrop profiles in the Sichuan, Yunnan, Guizhou, southern Shaanxi, and western Hubei and Hunan Provinces, combing with drilling data, logging data, seismic data and other comprehensive research data were investigated. We conducted stratigraphic correlation, sedimentary facies analysis, compiled lithofacies paleogeographic map of the first, second and third members of the Doushantuo Formation, carried out experimental analysis of source rocks in outcrop area, determined the favorable area for source rocks combined with sedimentary facies analysis, to natural gas exploration potential of the Formation.

The Sinian Doushantuo Formation is widely distributed in the middle-upper Yangtze region, it has different formation names in different areas^{[5,6,7,8,9,10,11,12]}. In the eastern and central Yangtze regions, it is called the Doushantuo Formation, in the eastern Yunnan area, it is called the Wangjiawan Formation, in the northern Yunnan-western Sichuan area, it is called the Guanyinya Formation, and in the south Sichuan-north Sichuan- south Shaanxi area, it is called the Labagang Formation.

Doushantuo Formation Profile is located in Doushantuo, west of Liantuo, Yichang, Hubei Province^[9], including grey- black argillaceous dolomite, dolomitic limestone and black shale, with silicophosphorous nodules and lumps, micropaleophytes and macroalgaes. It is in unconformable contact with the celadon tillite of the underlying Nantuo Formation, Nanhua Period, and is divided into four members from bottom to top (Fig. 1). The Member 1 of Doushantuo Formation (Z₂d₁) is only distributed in a small area of western Hubei and northern Guizhou, and is grey dolomite called cap dolomite. The Member 2 of Doushantuo Formation (Z₂d₂) deposited in most areas except the top of archicontinent. This member varies widely in lithology, is made up of black shale and mudstone with grey argillaceous dolomite and dolomite in west Hubei, north Sichuan and north Guizhou, and grey-white and amaranth shale with little grey mudstone and dolomite in southern Sichuan. The Member 3 of Doushantuo Formation (Z₂d₃) is slightly larger in distribution range than Z₂d₂’s, and is composed of grey dolomite, dolomitic limestone and zebra limestone. Member 4 of Doushantuo Formation (Z₂d₄) is similar in distribution range with Z₂d₃, but missing in most areas due to denudation. It is composed of black shale with little marl and limestone in western Hubei.

The strata corresponding to the Doushantuo Formation are called Labagang Formation in central and southern Sichuan, and made up of sandstone, pebbly sandstone, shale with dolomite or dolomitic limestone, gypsum-salt in some areas; called Hujiazhai Fomation in west northern Sichuan, which is a set of thick slate, phyllite mudstone and carbonaceous shale sandstone deposits; named Guanyinya Formation in western Sichuan and central Yunnan, consisting of amaranth sandy mudstone in the lower part, carbonate rock in the upper part, and gypsum salt rock with stromatolites in the Huaping and Yanbian areas of Yunnan Province; called Wangjiawan Formation in Kunming and Jianshui, eastern Yunnan, which is composed of amaranth sandy shale with dolomite and argillaceous limestone of bay lagoon; called Yangshui Formation in Kaiyang, Fuquan and Majiang, central Guizhou, which is mainly made up of celadon sandstone, siltstone and fine conglomerate, with sandy dolomite and phosphorite at the top containing siliceous stromatolites, of 10-50 m thick in general. Doushantuo Formation is a set of grey-black phosphorous shale with siliceous rock and thin dolomite in Songlin profile, Zunyi.

Based on outcrop, drilling data and a small amount of seismic data, thickness map of Doushantuo Formation in Sichuan Basin and its adjacent areas was compiled (Fig. 2). The map shows this formation is thinner in Sichuan Basin and thicker in periphery around Sichuan Basin. Z₂d₁-Z₂d₃ strata are missing in most areas of Sichuan Basin, and its residual thickness is generally 20-60 m. The Doushantuo Formation is thicker in the periphery of Sichuan Basin, generally 120-480 m. It is developed completely in western Hubei, and contains four lithologic members. It overlaps and thins toward the east to Huaiyang Archicontinent, and directly covers the celadon mixed schist of Hongan Group in Xiaochang. In addition, it is missing in the Hannan Archicontinent, Kaijiang Archicontinent and Tianquan Archicontinent (Fig. 3), where the Dengying Formation overlies the Pre-Sinian System directly.

The distribution of each member of Doushantuo Formation and its contact relationship with underlying strata are shown in Fig. 3. Z₂d₁ is only distributed in western Hunan, Hubei and Dabashan-Qinling Region. Most of the Sichuan Basin lacks Z₂d₂ and only Z₂d₃ was deposited. In the aspect of contact relation, Z₂d₁ is in unconformable contact with the underlying tillite of Nantuo Formation in western Hunan-Hubei, Dabashan-Qinling Region. Z₂d₂ is in unconformable contact with underlying Nantuo Formation directly around periphery of Sichuan Basin. Z₂d₃ is in unconformable contact with Mesoproterozoic in the main Sichuan Basin.

The sedimentary paleotectonic pattern of Doushantuo depositional stage consisted of paleouplifts and marginal sags.

Middle-upper Yangtze Region had developed three paleouplifts: Sichuan Paleouplift, Huaiyang Paleouplift and Dianqian Paleouplift before the deposition of Doushantuo Formation^[17,18]. Sichuan Paleouplift covered the present Sichuan Basin and its surrounding areas, where Doushantuo Formation is thin, missing of the lower part, and overlaps from the marginal sags to the Paleouplift.

The Sichuan Paleouplift is about 40×10⁴ km² (Figs. 2 and 3). Pre-Sinian stratum, (Huodiya Group and Ebian Group respectively) is exposed in Micangshan and the west of Emei Mount^[19]. The lower part of Huodiya Group, named Mawozi Formation in Micangshan area is a set of epimetamorphic clastic rock and marble with stromatolite about 3 500 m thick. The upper Huodiya Group referred to as Shangliang Formation, is made up of metasedimentary with marble and volcanic rock more than 1 700 m thick. The intrusive rock emplaced into Huodiya Group was deposited in Mesoproterozoic, and the isotope age of Albite schist (U-Pb) (the primary rock is intermediate-acid volcanic rock), olivine hornblende (K-Ar) and quartz diorite (U-Pb) are 1619.3 Ma^[19], 1065 Ma and 956 Ma, respectively^[20]. On the Yangba profile in Nanjiang, Z₂d₄ and Shangliang Formation are in direct unconformable contact with the absence of Z₂d₁ and Z₂d₃ (Fig. 4). The 6 800 m-thick Ebian Group in the west of Sichuan Basin is a set of pale-white marble with epimetamorphic marine clastic rocks and basic-acid igneous rock, developed in Mesoproterozoic^[19].

Sichuan Paleouplift might be formed from the mid-late Qingbaikou period to the early Nanhua Period due to the large-scale tectonic thermal events. Huodiya Group and Ebian Group contain basic-acid intrusive rock, and the age of the diabase zircon and granite in it are 813.4 + 813.4 Ma^[21] and 750-840 Ma^[22]. Ling Wenli et al.^[23] proposed that the tectonic thermal transformation to the Huodiya Group occurred at the same time with the regional basic-ultrabasic and medium- acidic magmatism in the early Neoproterozoic (about 860 + 12 Ma ago), and the massive rift magmatism in western Sichuan Basin was caused by the mantle plume related with the supercontinent cracking in the middle Neoproterozoic. Yellow-grey granite (794 + 11 Ma ago) and plum dacite were discovered in well Wei117, Gaoshi1 and Nüji in the hinterland of Sichuan Basin. Li et al.^[24] suggested the formation of the super mantle plume was divided into two stages: 795-830 Ma ago and 745-780 Ma ago, respectively. The volcanic-intruded mass in the middle-upper Yangtze region might be invaded when Rodinia Supercontinent broke apart during the former stage. The super mantle plume led to the tectonic uplift in the middle-upper Yangtze region and the erosion in the Mesoproterozoic and Neoproterozoic, to the extent that the stratum of Qingbaikou-Nanhua Period were missing in large area.

Sichuan Paleouplift had strong control on the deposition of Doushantuo Formation. Z₂d₁ is overall missing in the Paleouplift, and only 20-60 m thick mid-upper Doushantuo Formation existed in most area of Sichuan Basin, which is much thinner than that in the periphery of Sichuan Basin.

Huaiyang Paleouplift located in the north of middle Yangtze region is also known as Dabie Archicontinent, with the main area in Dabie Mountain^[25]. The Paleouplift is composed of the pre-Sinian metamorphite in north Hubei, subdivided into Suixian Group (668.00-1 228.03 Ma) and Dabie Group. The Suixian Group is in unconformable contact with the overlying Z₂d₂ in Qixian, and the celadon schist of Dabie Group is in unconformable contact with the overlying Z₂d₂ in east of Xiaochang (named Xiaochang Archicontinent).

In the western margin of upper Yangtze Craton, developed Panxi Rift, a typical continental rift, during Doushantuo depositional stage^{[24, 26]}. The activity of Panxi Rift started first from Mesoproterozoic, and underwent multiple periods of activity and intermittent in Neoproterozoic^[26]. Influenced by Panxi Rift, several marginal sags such as Ningqiang, Qingping, Kangding, Xichang and Panzhihua came up from north to south^[17,18], where the depositional thickness of Doushantuo Formation is about 1 000 meters, and over 1 800 meters in local parts.

The black stratum of Doushantuo Formation is 1 840 m thick in Chengkou Sag (in the north margin of Sichuan Paleouplift). Chengkou Sag was probably a small-scale remnant- ocean basin left in the joining of North China Plate and Yangtze Plate. Manganic stromatolite or halobolite was discovered in Doushantuo Formation by Zhao Dongxu in 1992, which may be of the same origin with the oceanic halobolite nowadays^[27].

Hefeng Sag, located in the eastern margin of Sichuan Paleouplift, is a depression between Sichuan Paleouplift and Huaiyang Paleouplift. Hefeng Sag was the depocenter of the Exi trough during Doushantuo depositional stage, where thick black shale with siliceous and carbonate rocks deposited. Exi trough is a relatively low-lying and narrow-long zone between the upper and middle Yangtze, in nearly north-south strike. It is connected with the Qinling Trough in the north and the Xianggui Basin in the south.

Changning Sag in the southern margin of Sichuan Paleouplift, is sandwiched between Sichuan Paleouplift and Qianzhong Paleouplift. Changning Sag was a semi-restricted to restricted bay, shielded by Tianquan Archicontinent in the west, where gypsum-salt stratum deposited during the mid- late Doushantuo depositional stage.

This paper mainly introduces the sedimentary characteristics of Doushantuo Formation on Yangba Profile of north Sichuan basin, well Wei 117 Profile of central Sichuan basin and well Yidi4 Profile of western Hubei.

3.1.1. Yangba profile in Nanjiang

The profile is located in Yangba town, Nanjiang County, northern Sichuan Basin, and its structural position is in the upper west slope of Hannan Archicontinent (Fig. 2). The formations exposed here are the lower Sinian Labagang Formation (53 m) and the upper Sinian Dengying Formation (836 m), and the Labagang and Doushantuo Formations are synchronous heteropic deposits (Fig. 4). Z₂d₁-Z₂d₃ strata of Doushantuo Formation are missing in partial Yangba Profile. Z₂d₄ is in unconformable contact with the yellow-grey sandy slate of Shangliang Formation, and the yellow-grey medium-thick packsand in the top of Doushantuo Formation is in unconformable contact with the grey dolomite in the overlying Dengying Formation (Fig. 4).

Doushantuo Formation is mainly composed of sand-mudstone deposits of clastic littoral-lagoon facies. The shoal-bar dolomitic pebbly sandstone at the bottom is foreshore beach bar deposits. The ash grey, grey thin-bedded mudstone with little siltstone in the mid-lower part is offshore lagoon deposit. The grey and greyish-green medium-thick intercalated thin- thick siltstone-packsand, and quartz sandstone with argillaceous siltstone, and wedge bedding, cross-bedding, parallel bedding and oblique bedding in the top is foreshore -littoral shoal-bar deposit.

3.1.2. Well Wei117

Well Wei117 is located in Weiyuan Structural Belt of Sichuan Basin of 3 746 m deep drilled through Sinian to the basal granite. The Dengying and Doushantuo Formations revealed in this well are 598 m and 41 m thick, respectively. Sinian was cored completely, which shows the Doushantuo Formation has distinct sedimentary characteristics (Fig. 5), with grey silty dolomite, beige micritic dolomite intercalated by argillaceous dolomite, and algae striation in local part in the upper part, gypsiferous muddy dolomite and white gypsum in the middle, and celadon sandy dolomite, dolomite mudstone with siltstone, argillaceous siltstone, conglomeratic sandstone with ripple bedding in the lower part. It is semi-restricted to restricted platform facies featuring algae dolomite flat and evaporated lagoon. Z₂d₂₊₃ can be correlated across the region, but Z₂d₁₊₄ are missing

3.1.3. Well Yidi4

Well Yidi4 is a shale gas exploration well targeting Doushantuo Formation near Zigui of Yichang city, and is completed at Nantuo Formation. The whole Sinian was cored in this well (Fig. 6). The Doushantuo Formation can be divided into four members: Z₂d₁ is grey ribbon argillaceous dolomite (the light ribbon contains more dolomite, and the dark ribbon is more argillaceous), which represents shallow shelf deposit. The bottom of Z₂d₂ is dark-gray mudstone and calcareous mudstone, the middle-upper part consists of dark- grey shale with phosphorite nodule, representing deep-water shelf deposits. The lower Z₂d₃ is grey ribbon argillaceous dolomite with thin-layer dolomite of shallow shelf facies; the middle is gray dolarenite and the upper is a light-gray ribbon micritic dolostone of shallow shelf and debris flow deposits; the lower Z₂d₄ is black mudstone with thin-layer argillaceous limestone or dolomite of deep-water shelf facies. The middle- upper stratum is grey ribbon mud dolomite, and the top stratum is gray dolomitic mudstone of shallow shelf facies. Doushantuo Formation is in unconformable contact with Nantuo Formation tillite and the overlying Dengying Formation.

The Doushantuo Formation in the middle and upper Yangtze region mainly consists of detrital coastal sediments, carbonate platform deposits, continental shelf and restricted basin sediments, which can be divided into three sedimentary systems, six sedimentary facies, eighteen subfacies, and multiple micro-facies (Table 1). The clastic deposits are mainly concentrated in Z₂d₂ and Z₂d₄, and the carbonate deposits mainly in Z₂d₁ and Z₂d₃. Z₂d₁ and Z₂d₂ are sediments of transgression stage, Z₂d₃ was deposited at the highest sea level, and Z₂d₄ in the regressive sedimentary stage. The deposition of Doushantuo Formation constitutes a relatively complete transgression-high-regression sedimentary cycle.

3.2.1. Sedimentary characteristics of carbonate ramp deposits in Z₂d₁

The Doushantuo Formation is the first transgressive sedimentary system after the Nantuo ice epoch. Therefore, the archicontinent had a great influence on the sedimentation of the Doushantuo Formation.

Sichuan Archicontinent had already existed in Z₂d₁ Period (Fig. 7). The archicontinent had a large distribution area, besides the present Sichuan Basin, it extended to Xichang and Zhaotong in the southwest and Ningqiang in the northwest. The western Hubei, in the east of the archicontinent was shallow water shelf environment with a water depth of about 20 m. Therefore, a set of carbonate rock (cap dolomite) with a small thickness (2 to 10 m) but a wide distribution deposited. There existing large difference in lithology in different areas controlled by sedimentary environment. In areas with shallow waters such as Jingmen to Yueyang, it is mainly dolomite and dolomite with gypsum. In the area from Yichang to Changde and Zunyi to Pan'an, it is mainly zebra muddy dolomite, siliceous dolomite and calcite dolomite with horizontal bedding and stratiform structure. In the areas of Huaihua, Mayang, Fenghuang, Gongxi and Zhijiang, it is shallow water shelf deposits composed of manganese-bearing dolomite, calcite dolomite, pelitic dolostone, with horizontal beddings. In the Qinling Trough from Baokang to Chengkou, it is mainly composed of argillaceous limestone, dolomitic limestone with thin dolomite of deep water shelf or basin sedimentary environment. There is no carbonate deposits of the first member of Doushantuo Formation in the areas of Yangpingguan in the Ningqiang to Mianzhu in the Wangjiaping, west of Sichuan Province, and the Sichuan and Xiaochang archicontinents. In the Pingwu area of the northwestern Sichuan Province, the Doushantuo Formation consists of metamorphic limestone with black argillaceous slate, which cannot be correlated by member with other areas. It is not clear whether there is a phase change stratum equivalent to Z₂d₁ there. Different facies of coastal clastic rock deposit may exist on the periphery of the archicontinent deposited at the same time of Z₂d₁, but this hasn’t been confirmed by profile so far.

According to the sedimentary background, sedimentary thickness and lithology characteristics, sediment of Z₂d₁ appears as carbonate slope deposit on broad and gentle shelf. Due to the thin thickness (2-10 m), it can be considered as the initial stage of carbonate slope, or atypical carbonate slope. For the capped dolomite, Yang et al.^[10] thought it was carbonate slope deposit too. According to the lithological distribution and paleo-tectonic background, the carbonate slope in Z₂d₁ can be divided into several sedimentary environments, including inner gentle slope, medium gentle slope and outer gentle slope-basin. Intertidal-supratidal deposits on gentle slopes are mainly distributed in the Jingmen-Yueyang area. The area of the Yichang-Changde and Zunyi-Weng'an is intertidal-subtidal zone on mid-gentle slopes. The Qinling trough area of the Baokang-Chengkou area is mainly subtidal deposits of outer slope-basin facies.

3.2.2. Sedimentary characteristics of the shore-tidal flat-lagoon-mixed shelf, shallow water shelf, deep water shelf-sea basin in Z₂d₂

The Z₂d₂ is the deposit during the extensive transgression in the middle and upper Yangtze region (Fig. 8). Affected by the transgression, the land area rapidly shrunk. In the late sedimentary period of Z₂d₂, the Sichuan and Yunnan Guizhou Archicontinents widely exposed before, were submerged by the sea. The remaining archicontinents in the higher positions were Hannan, Kaijiang, Tianquan, Huize and Xiaochang Archicontinents. A set of shallow water clastic rock with carbonate and gypsum salt deposited on the archicontinent. A large set of black shale with siliceous rock deposited in the marginal sags on the periphery of the archicontinent. Abundant volcanic ash in Z₂d₂ in the areas of Yichang, Shimen, Yuanling, Xixi, Jiangkou, and Sansui, indicates that massive volcanic eruptions occurred during this period of time.

The sedimentary environment of Z₂d₂ can be divided into shore-tidal flat-lagoon-mixed shelf, shallow water shelf and slope-basin.

(1) Shore-tidal flat-lagoon-mixed shelf. Residual archicontinent was an important source of sedimentary material in the deposition period of the Doushantuo Formation. Sand-mudstone with dolomite and gypsum-salt of shore-tidal flat-lagoon-mixed shelf deposited widely around the archicontinent. Large sets of purple-red sandstone, quartz conglomerate, grey-green and purple-red mudstone were found in the Wangcanggan River in the north of Sichuan and the Weiyuan, Gaoshiti and Longnüsi in the central Sichuan area, indicating that the northern and central Sichuan area was in the sedimentary environment of shore-tidal flat-lagoon-mixed shelf. The bottom of Z₂d₂ is made up of purple-grey feldspar quartz sandstone, argillaceous siltstone and fuchsia silty mudstone (Wangjiaping profile in Mianzhu) in western Sichuan area, is grey-white feldspar-quartz sandstone with siliceous dolomite with cross bedding and slab bedding in the southwestern Sichuan area, and mixed shelf deposits including grey-green mudstone, argillaceous siltstone, grey quartz sandstone in eastern Sichuan area. Z₂d₂ in the area of Jingshanchanghe- Xuejiadian, eastern part of the western Hubei area, is mainly composed of brownish-grey carbonaceous shale with manganese-bearing and phosphorus-bearing shale, grey green silty shale with phosphorus-bearing clay rock, purple sandstone and greyish-brown gravel sandstone, belonging to shore-lagoon or bay deposits. At this time, the Changning Sag was a closed- semi closed bay, depositing a set of gypsum up to 400 m thick.

(2) Shallow water shelf. Z₂d₂ is mainly grey-black carbonaceous shale, grey mudstone with silty mudstone, phosphorus-bearing siltstone and thin dolomite on the Wushan, Shen-nongjia profile, grey carbonaceous shale and black shale with dolomite on the Donghaoping profile, black siliceous shale interbedded with grey-black mudstone and striped argillaceous dolomite in the Rongxi Xiushan, Xixi Luxi and Dongjiahe Xupu profiles. Z₂d₂ in the Songlin area of Zunyi is composed of black shale, siliceous shale with thin argillaceous dolomite or dolomite of shallow water shelf facies. Z₂d₂ in the Yangjiaping Shimen and Taiyangshan Changde is mainly composed of grey argillaceous limestone or argillaceous dolomite with dark grey shale depositing in shallow water shelf.

(3) Deep water shelf-sea basin. The edge of the Sichuan archicontinent became steep quickly, forming a deep water shelf with steep slopes and rapidly transiting into a deep-water basin. Large set of strata dominated by grey-black shale deposited in the Baiguoping profile in Hefeng, Sandouping profile in Zigui, Well Yidi 4, Xiuqi and Gaoguan profiles in Chengkou, Yangpingguan profile in Ningqiang and Wangjiaping profile in Mianzhu, forming several sedimentary centers such as Qingping sag, Ningqiang sag, Chengkou sag and Hefeng sag in the deepwater basin. The thickness of the black shale series in the sedimentary centers is 580-1 840 m.

In the areas of Liuchapo Anhua, Lianhuatai, Songziao and Tianjingshan Taojiang, Z₂d₂ is composed of black carbonaceous shale and siliceous shale interbedded with siliceous rock, representing sedimentary environment of starved basin.

3.2.3. Sedimentary characteristics of the shore-limited semi-limited platform, shallow-water shelf, deep-water shelf and sedimentary basin facies in Z₂d₃

Z₂d₃ is dominated by carbonate rock (Fig. 9) and is the deposit at the peak transgression of Doushantuo depositional stage, when the area of the archicontinent further shrunk and divided. Coastal clastic rocks deposited around the archicontinent. Farther from the archicontinent was limited semi-limited platform, and the marginal sag area formed deep-water basin.

The sediments in Z₂d₃ are mainly carbonate rocks with argillaceous carbonate interbed and gypsum-salt in local parts of only 10-20 m thick in the Sichuan Basin. The Wilson Platform Model cannot explain the contradiction between the wide distribution and the characteristics of surface sea sediments. Therefore, it is regarded as the fusion of the Surface Sea Model and the Wilson Model, which can be called the Atypical Carbonate Platform Model in this study. This carbonate deposit may be the primary stage of the carbonate platform, and due to the short formation time, it has not yet reached the scale of a typical carbonate platform (gentle slope or fringed platform). This type of deposition is referred to as pre- platform deposition in the monograph "The Classification of Stucco Mound Systems and Petroleum Geology"^[18].

(1) Shore. In Z₂d₃ period, there were Hannan, Kaijiang, Tianquan, Xiaochang, Wudang and Huize archicontinents. The Doushantuo Formation is absent in the archicontinents. Near-shore clastic deposits is distributed in the surrounding area of the archicontinents. The yellow-grey and green-grey sandstone at the bottom of the Doushantuo Formation in the Yangba profile of the Nanjiang contains dolomite, which may be the near-shore deposit, and may be phase-change relationship with the platform carbonate far from the archicontinent.

(2) Limited-semi limited platform. The phase belt is mainly distributed in the main part of the paleouplift, forming two independent carbonate platforms in Sichuan and western Hubei, separated by the Western Hubei strait. The upper part of Z₂d₃ in Well Wei-117 in the central Sichuan Basin, is made up of greyish-white powder dolomite and brown-grey micrite dolomite, with algae layers. Its lower part is composed of gypsum-bearing argillaceous dolomite with white gypsum layer, representing limited platform and gypsum salt lake deposits. Z₂d₃ in Zhongxiang of western Hubei Province, Taijiya, Kaiyang in northern Guizhou and Wengan area are mainly grey-white microcrystalline condensate dolomite, phosphorus-bearing dolomite and limemud mound deposits with abundant phosphorus-bearing stromatolite dolomite^[17]. The growth of stromatolites generally happens in restricted sea environment, such as the Shark Bay in Australia and the stromatolite in the Bahamian terraces. Stromatolite in Z₂d₃ must also be formed in the environment of the restricted sea or bay.

(3) Shallow water shelf (or slope). Shallow water sheds or slopes are distributed around the perimeter of the platform, where the deposits are grey muddy dolomite and limestone with black shale. Large phosphate rock can be seen in these places in Z₂d₃, such as the phosphate mines in the Shennongjia area of Hubei and the Dongjiahe area of Huaihua.

(4) Deep water shelf-sedimentary basin. The deposit is dominated by black shale and siliceous shale, with thin layer dolomite, argillaceous dolomite, siliceous dolomite, marl or thin-bedded micrite limestone, mainly distributed in the marginal depression of the paleouplift. The deposits in Z₂d₃ are thin limestone, marl or nodular limestone in the Qinling Trough area, black shale with thin dolomite and argillaceous dolomite interbeds in the Baiguoping Hefeng area, carboniferous argillaceous dolomite, calcite dolomite, siliceous dolomite and a large set of turbidite and debris flow deposit in the Siduping Zhangjiajie area, and black carbonaceous shale representing deep-water basin facies in the Songzi’ao Hunan Province and Tianjingshan Taojiang area.

3.2.4. Sedimentary characteristics of the shore-shelf-sedimentary basin in Z₂d₄

Sediments of Z₂d₄ are absent or not deposited in most areas of the Sichuan paleouplift, and only preserved in the marginal depressions of the paleouplift and in the shelf of the western Hubei Province, due to the denudation. Z₂d₄ in the Yangba area of northern Sichuan is a set of coastal clastic rock, mainly composed of quartz sandstone and siltstone with grey mudstone formed in the shore-lagoon environment. Sediments of Z₂d₄ in the Chengkou-Zhenping area of the Qinling Trough is dark grey mudstone, shale with marl or dolomitic limestone, indicating the sedimentary environment of deep water shelf and sea basin. In comparison, sediments in the western Hubei area is mainly shallow water shelf facies of black shale with argillaceous limestone, thin dolomite, argillaceous dolomite and siliceous dolomite. Z₂d₄ is black siliceous shale with siliceous rock, formed in the deep water basin environment in the areas of Anhua, Taojiang, Yuanling and Xupu in Hunan Province.

The Sinian System is one of strata with abundant sedimentary mineral resources in southern China, containing conventional gas, shale gas, phosphate ore, and manganese ore^[27,28,29]. The giant Anyue gas field, Yichang shale gas rich area and Guizhou-Hunan-Hubei phosphorite belt have been discovered in Sichuan Basin and adjacent areas, and these large and giant deposits are all related to the Doushantuo Formation. Gaoyan manganese deposit, located in Chengkou sag in northern Sichuan Province, is one of the large manganese deposits in China, where manganese is produced mainly from the Doushantuo Formation black shale.

Doushantuo Formation is one of the major source rock strata in the southern China. The study on its distribution will be of great significance for the exploration of Sinian conventional gas, and is also of important guiding significance for the evaluation of minerals such as shale gas, phosphate ore and manganese ore. Systematic analysis of black shale and mudstone of Doushantuo Formation in outcrop profiles of Songlin of Zunyi, Xiushan of Chongqing and Chengkou in northeastern Sichuan on the periphery of Sichuan Basin (Table 2) shows that the hydrocarbon source rocks of Doushantuo Formation are mainly distributed in Z₂d₂, with the TOC value generally greater than 1.0% and up to 13.8% for some individual samples, indicating it is a set of high-quality hydrocarbon source rock. With average ¹³C _kerogen of -31‰ and Ro value of generally over 2.0%, it is in the high post-mature gas generation phase now.

The distribution of hydrocarbon source rock in Doushantuo Formation is closely related to the sedimentary environment (Fig. 10). The areas with thick and high TOC (TOC>2.0%) source rocks are mainly distributed in Chengkou Sag, marine trough in western Hubei and other marginal sags of the paleohighs. Hugely thick black shale is widely developed in the areas such as Qinling and western Hubei marine trough. The black shale in Chengkou Sag and Hefeng Sag can reach more than 500 m thick. The second and fourth members of Doushantuo Formation have abundant black shale, are important shale gas exploration layers. Well Ye1 drilled in Eyang of Yichang area encountered 230 m of grey and black carbonaceous mudstone in Sinian Doushantuo Formation, which had a TOC value of 1.5%-2.5% and gas content tested on site of 4.8 m³/t. The well tested a shale gas output of up to 5 460 m³/d, foretelling promising exploration prospect of Doushantuo Formation in this area. With Doushantuo Formation shale well developed, the areas such as Qinling marine trough, western Hubei, northern Guizhou and eastern Guizhou, are favorable areas for the evaluation of shale gas.

Considering the reservoir conditions, both the clastic rock at the top of Z₂d₂ and the microbial carbonate rock in Z₂d₃ have dissolution vugs and caves. At present, a number of large phosphate ores have been discovered in this member, such as Kaiyang-Weng'an phosphate deposit in Guizhou, Yichang phosphate deposit and Dongjiahe phosphate deposit in Huaihua. From the analysis of sedimentary facies, phosphate deposits are distributed in the shallow water continental shelf and platform margin, which is related to the fact that the shallow water continental shelf and platform edge are activity area and unloading area for upwelling current.

To sum up, Z₂d₂ has high-quality hydrocarbon source rocks and Z₂d₃ has microbial carbonate rock reservoir, which constitute a good source-reservoir combination, and is deemed a new field that should be paid attention to in natural gas exploration. According to the preliminary evaluation, Doushantuo Formation in Qinling marine trough and western Hubei marine trough have better oil and gas reservoir forming conditions, so they are favorable areas for the exploration of mineral resources such as natural gas (including shale gas), phosphate ore and manganese ore.

The Middle-Upper Yangtze region developed structural paleotectonic units of Sichuan Paleouplift, western Hubei continental shelf and Marginal Sag in Sinian, which could control the sedimentary filling sequence and stratigraphic distribution of Doushantuo Formation. The Doushantuo Formation in the paleouplift region was overlayed with thin thickness, including shore facies, mixed continental shelf facies and atypical carbonate platform facies. The marginal sag had complete strata and large thickness, and developed deep water shelf facies and restricted basin facies.

Doushantuo Formation has experienced a complete sedimentary cycle from transgressive systems tract to high stand system tract to regressive system tract, and developed littoral-continental shelf-deep water basin and carbonate platform facies. Z₂d₁ and Z₂d₃ are mainly carbonate platform or gentle slope. Z₂d₂ and Z₂d₄ are mainly littoral - shelf - deep water basin deposits.

Doushantuo Formation is of great accumulation conditions for oil and gas. The Z₂d₂ has high quality hydrocarbon source rock, which is of great thickness, high organic carbon content and great in shale gas generation potential in the sags around Sichuan paleouplift. The carbonate reservoir in Z₂d₃ is rich in dissolution pores and caves, which provide storage space for natural gas, phosphate and other ores. Qinling trough and western Hubei trough are favorable areas for exploration of natural gas (including shale gas) and mineral resources such as phosphate rock and manganese ore.