Ultra-deep marine carbonate reservoirs have become a key target for onshore oil and gas exploration and development for reserves replacement in China ^[1]. In the past decade, efficient exploration has been carried out in the Sichuan Basin, focusing on the deep Sinian Dengying Formation on both sides of the Deyang-Anyue Rift, and remarkable results have been achieved. During the "12th Five-Year Plan" period, the Anyue Gas Field was discovered in the Gaoshiti-Moxi (referred to as "GM") area in the core of the central Sichuan paleo-uplift to the east of the Deyang-Anyue Rift, with the gas reservoir in the Dengying Formation buried at a depth of 5000-6000 m and the proven reserves exceed one trillion cubic meters^{[2⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-18]}. In 2020, Well Pengtan-1 was tested to produce a high-yield commercial gas flow at over 100 × 10⁴ m³/d, an exploratory well drilled in the platform margin belt of the second member of the Dengying Formation (referred to as the "Deng-2 Member") in the Penglai-Zhongjiang area to the north of the GM area and the east of the Deyang-Anyue Rift ^[8], and made a strategic breakthrough to the exploration of the northern slope in the central Sichuan paleo-uplift. Afterwards, the thick vuggy dolomite reservoir in the Deng-2 Member was revealed in many exploratory wells, with a burial depth of 6000-7000 m and good gas-bearing property, which is expected to discover a new trillion-cubic-meter gas field. In the Mianyang-Jiange area to the east of Deyang-Anyue Rift and the north of the Penglai-Zhongjiang area, the Dengying Formation at 8000-9500 m contains thick mound-shoal complexes developed in the platform margin belt, and close to the Cambrian hydrocarbon generation center (Fig. 1). This paper will focus on whether there are favorable reservoir- forming conditions for a trillion-cubic-meter gas field.

According to the Sinian regional tectonic setting, the Deyang-Anyue Rift is a result of the extensional tectonics at the northwest margin of the Yangtze Craton. There are great differences in structure-sedimentation between the southern and northern sections of the rift depression ^[10]. Based on outcrop, drilling, logging, and seismic data, the reservoir-forming conditions of the Sinian System at the northwest margin of the Yangtze craton region are studied systematically in this paper from the perspectives of tectonic-depositional evolution, source-reservoir combinations and reservoir-forming characteristics, and the exploration potential is also analyzed in order to provide geological theoretical basis for the evaluation of ultra-deep favorable exploration region and the selection of optimal 10 000-meter well drilling sites in this region.

The northwest margin of the Yangtze craton region was part of the Proto-Tethys in the Neoproterozoic-Early Paleozoic Era ^[16]. It is located at the junction of the craton and the western Sichuan-Qinling Ocean Basin, and has been at the passive continental margin for a long term. Affected by the breakup of the Rodinia supercontinent, the Yangtze craton region was in an extensional tectonic setting from the Sinian Period to the Early Cambrian Epoch, with strong intracraton tectonic differentiation. The Chengkou-Exi Rift and Deyang-Anyue Rift distributed in a nearly south-north direction (Fig. 1a) are developed in the central region and northwest margin of the Yangtze craton region, respectively ^[11,17]. The characteristics of Deyang-Anyue Rift are as follows: (1) The width of the northern section of the Rift is 240-320 km, narrowing gradually to the south; the width of the middle section of the Rift in the Anyue area is 150-200 km. (2) The northern section of the Rift has a large boundary fault displacement (200-400 m), with developed syndepositional faults; the southern section of the Rift has a relatively small boundary fault displacement (about 50-200 m), with underdeveloped syndepositional faults. (3) The platform margin belt of the Deng-2 Member in the northern section of the Rift is 600-1000 m thick, and the platform margin belt of the Deng-4 Member is 300-450 m thick. The Lower Cambrian Maidiping Formation-Qiongzhusi Formation in the rift area is 400-900 m thick, in conformable contact with the Dengying Formation. In the southern, the platform margin belt of the Dengying Formation is 300-600 m thick, and the Maidiping Formation-Qiongzhusi Formation in the rifted zone is 300-700 m thick, in unconformable contact with the Dengying Formation^{[6⇓⇓⇓⇓⇓⇓⇓⇓-15]}.

The Sinian System includes the Doushantuo Formation and the Dengying Formation from bottom to top. The Doushantuo Formation is mainly composed of shale and siltstone, intercalated with dolomicrite. The Dengying Formation is divided into four members in terms of the lithology: Deng-1 Member, Deng-2 Member, Deng-3 Member, and Deng-4 Member (Fig. 1b). The Deng-1 Member is mainly composed of dolomicrite with gypsum-salt rock developed locally. The upper part of the Deng-2 Member is dominated by dolomicrite, while grape-lace shaped algal dolomite is developed in the lower part. The Deng-3 Member is mainly composed of mudstone and siltstone, intercalated with thin layers of dolomite and tuff. The Deng-4 Member is mainly composed of algal dolarenite, algal dolomite and stromatolite dolomite, with siliceous bands observed locally. The Lower Cambrian Maidiping Formation is mainly composed of dark siliceous mudstone and siliceous rock, intercalated with dolomicrite, micrite and siltstone, and rich in collophanite. The lower part of the Lower Cambrian Qiongzhusi Formation is dominated by dark carbonaceous shale, while the upper part is dominated by silty mudstone and argillaceous siltstone.

The Early Sinian Doushantuo Formation depositional stage is the first regional transgression period after the end of the Nantuo Glaciation Period. The sedimentary fillings were not only controlled by the paleotopography after the Nantuo Glaciation period, but also by the intracraton rifting ^[15]. During the Doushantuo period, intracraton tectonic differentiation began to appear in the Yangtze craton region, and the Deyang-Anyue Rift in the northwest margin took shape, extending from Jiangyou to Ziyang in a trumpet shape, with the stratum thickness gradually decreasing (Fig. 2a).

The early depositional stage of Late Sinian Dengying Formation, that is, the depositional stage of Deng-1 Member and Deng-2 Member, is the development period of the craton rift. During this period, extensional action and tectonic differentiation continued to increase, and the Deyang-Anyue Rift developed further, extending to Ziyang in Sichuan Basin. The thickness of the Dengying Formation gradually decreases from the platform marginal zone to the inside of the rift ^[19]. According to the drilling data from Well Pengtan-1 drilled at the platform margin belt, the total thickness of the Deng-1 and Deng-2 Members encountered is about 720 m, and the lithology is dominated by algal-laminated dolomite, algal sand- gravel clastic dolomite, algal clotted dolomite, and dolomicrite. According to the drilling data from Well Ziyang-1 drilled in the rift, the total thickness of the Deng-1 and Deng-2 Members encountered is about 63.5 m, and the lithology is dominated by nodular dolomicrite, intercalated with a small amount of algal dolomite. The sedimentary environment changed from shelf sedimentation to shallow carbonate platform sedimentation in Doushantuo Formation, the area of the Hannan paleo-land was narrowed, the terrigenous supply decreased, and the Xuanhan-Kaijiang paleo-uplift was developed in eastern Sichuan ^[20-21]. During the depositional stage of the Deng-2 Member, syndepositional faults were developed, which controlled the development and distribution of the platform margin belt of the Deng-2 Member (Fig. 2b). The palaeogeographic features of the fault block adjacent to the hanging wall were high-lying, and conducive to the development of microbial mound-shoal complexes. The lower part of the fault block was mainly composed of intershoal pelagic deposits, and the lithology was dominated by micritic carbonate rock and argillaceous dolomite. There are obvious differences between the northern and the southern sections of the rift. The Deng-2 Member in the north of the rift is thicker, and mound-shoal complexes are more developed in the platform margin belt. At the end of the deposition of the Deng-2 Member, the Yangtze craton region experienced Episode I Tongwan tectonic movement, and some zones were denuded and a regional weathering crust appeared. The late depositional stage of the Dengying Formation, corresponding to the Deng-3 and Deng-4 Members, is a period of rapid intracratonic rifting, when the platform margin belt was controlled by a number of nearly south-north syndepositional faults, which are distributed in the near north to south direction ^[12,21]. The platform margin belt of the Deng-4 Member is dominated by algal dolarenite, algal-laminated dolomite, algal clotted dolomite, and dolomicrite, with a thickness of 250-600 m. For example, the total thickness of the Deng-3 Member-Deng-4 Member in the Hujiaba section of Ningqiang is about 523 m. During that stage, the rift continued to develop into the basin, extending to the Luzhou-Yibin region (Fig. 2c). The rift was a slope-basin depositional environment, depositing black-dark gray argillaceous dolomite and siliceous dolomite, tens of meters to more than 100 meters thick. For example, the total thickness of the Deng-3 and Deng-4 Members in the Bajiaocun section is about 26 m. At the end of the depositional stage of the Deng-4 Member, the Yangtze craton region experienced Episode II Tongwan tectonic movement, when the Deng-4 Member was denuded in most areas, and a regional weathering crust appeared.

The early Cambrian Maidiping Formation depositional stage-early Qiongzhusi Formation depositional stage was the peak development period of Deyang-Anyue Rift when the fault activity continued to increase. Then the Yangtze craton region entered the craton depression period in the middle and late depositional stages of the Qiongzhusi Formation (Fig. 3). During this period, the rift mainly controlled the sedimentary fillings of the Maidiping Formation and the Qiongzhusi Formation. The Maidiping Formation in the rifting area was in a slope-basin environment, while the Qiongzhusi Formation was deep- water shelf environment, and both sides of the rift were shallow-water shelf environment. Thick organic-rich shale source rocks were developed in the deep water area.

In the middle and late Early Cambrian, the intracraton rifting basically disappeared and the stage of unified carbonate platform development began. The Canglangpu Formation depositional stage was a mixed-deposit (composed of terrigenous clasts and carbonate rocks) shelf stage when rifting entered the extinction stage. During the depositional stage of the Longwangmiao Formation, the system was completely transformed into a carbonate depositional system, and seawater once again inundated the Yangtze craton region. Affected by the previous tectonic-depositional framework, the whole formation is carbonate gentle slope sedimentation under the influence of high evaporation in the west and low evaporation in the east, with a stratum thickness of 50-300 m and weak tectonic differentiation; the rift was completely filled and disappeared. During the Xixiangchi Formation depositional stage-Ordovician, the Caledon paleo-uplift began to develope, and the entire Sichuan Basin was dominated by unified carbonate platform deposition.

According to the drilling data and a large number of outcrop sections, two sets of ancient source rocks were developed in the Sinian System-Cambrian System at the northwest margin of the Yangtze craton region: the Doushantuo Formation and the MaidipingFormation- Qiongzhusi Formation. The gas generating strength of the source rock in the Maidiping Formation-Qiongzhusi Formation is mainly (100-200)×10⁸ m³/km², and that in the Doushantuo Formation is mainly (5-40)×10⁸ m³/km², which provided abundant resources for the formation of large-area oil and gas reservoirs in the Sinian Dengying Formation.

From the Sinian to the Cambrian, the Deyang-Anyue intracratonic rift was developed in the Yangtze craton region. The source rocks there are 50-600 m thick, and they gradually spread from the periphery to the interior of the Basin ^{[22⇓⇓⇓-26]} and became the primary source rocks of the Anyue trillion-cubic-meter gas field in the central Sichuan Basin ^[12]. There are source rocks developed in the Maidiping Formation-Qiongzhusi Formation at the northwest margin of the Yangtze craton region (Fig. 4). The source rocks of the Qiongzhusi Formation are mainly 150-450 m thick (Fig. 1a) while those of the Maidiping Formation are tens of meters to more than 100 meters thick. The Lower Cambrian source rocks of some profiles are as thick as 600 m. The source rocks of the Maidiping Formation-Qiongzhusi Formation are widely distributed, with good quality and conditions. The source rocks (TOC>0.5%) of the Qiongzhusi Formation in Wells Gaoshi-17 and Pengtan-1 located in the rift are 415-418 m thick, and of which the high-quality source rocks (TOC>2.0%) are 200-289 m thick. The source rocks of the Maidiping Formation are 55-130 m thick, of which the high- quality ones are 36-40 m thick. The source rocks of the Maidiping Formation-Qiongzhusi Formation in Wells Pengshen-1, Jiaotan-1, Chuanshen-1 and Mashen-1 located at the rift margin are 238-380 m thick, of which the high-quality ones are 130-229 m thick. Field outcrop profiles reveal that extremely thick high-quality source rocks are developed in the Lower Cambrian at the northwest margin of the Yangtze craton region. The TOC of the Maidingping Formation on the Mianzhu Qingping profile is 0.57%-25.65% with an average of 10.71%. The source rocks there are about 110 m thick, of which the high-quality ones are about 55 m thick ^[25]. Black shale of the Maidiping Formation-Qiongzhusi Formation exposed in the Chenjiaba profile of Beichuan region are more than 400 m thick (top not exposed), with TOC value of 0.84%-4.89%, averaging 3.08%. Among them the high-quality source rocks are more than 250 m thick. The source rocks of the Qiongzhusi Formation exposed in the Dongxihe profile of Guangyuan region are over 195 m thick (top not exposed) and those of high quality are about 80 m thick (Fig. 4 and Table 1).

Generally, the source rocks of the Maidiping Formation-Qiongzhusi Formation at the northwest margin of the Yangtze craton region are the most developed in the northern section of the Deyang-Anyue Rift, with a total thickness of 150-600 m (100-350 m of high-quality source rocks). They are followed by the source rocks of the northern slope of the central Sichuan paleo-uplift at the margin of the rift to northern Sichuan, which have a total thickness of 150-400 m (100-200 m of high-quality source rocks). The intraplatform area of the central Sichuan and the southern Shaanxi region are far away from the rift, with thin source rock 50-150 m thick (high-quality less than 50 m).

The source rocks of the Doushantuo Formation at the northwest margin of the Yangtze craton region are relatively thick ^[15], up to 480 m in the Ningqiang region of northwest Sichuan. The dark shale in the Hujiaba Profile is about 140 m thick. The dark mudstone in the Xiaoyangba region of northern Sichuan is 50-150 m thick. The shale of the Doushantuo Formation in the profiles including Mianzhu Xiaomuling, Qingping, Pingwu Minzu Village, Paoma Mountain (Kangding) and Ebian Xianfeng of Longmen Mountains nappe belt in western Sichuan has an average TOC of over 2.0%. The source rocks of the Doushantuo Formation in the Xixiang-Nanjiang region in northern Sichuan are underdeveloped, which are mainly sandstone and pebbly sandstone. The shale in this region is thin, with low organic matter abundance and TOC less than 1.0%. Notably, in the Jiangyou area in northwest Sichuan, a set of strong continuous seismic events is recorded under the Dengying Formation, which is 50-150 ms in time thickness and has obvious sedimentary filling features. It is expected to be the deposits of the Doushantuo Formation, and a potential favorable zone for the development of Doushantuo Formation source rocks (Fig. 5).

The Dengying Formation at the northwest margin of the Yangtze craton region is close to the open marine shelf facies. At the rift margin, a large microbial mound-shoal complex was formed in the platform margin belt, which provided basic conditions for developing high-quality reservoirs. Seismic data show that the mound-shoal complex in the platform margin belt of the Deng-2 Member in the Suining-Guangyuan region to the north of central Sichuan paleo-uplift is about 10 000 km² and that of the Deng-4 Member is about 5000 km².

The reservoir rocks are mainly composed of algal clotted dolomite, algal stromatolite dolomite and algal dolarenite of the mound-shoal complex, and the reservoir space is mainly dominated by intergranular pores and intergranular dissolved pores and caves, followed by intercrystalline pores and lattice pores (Fig. 6). The mound- shoal complex in the platform margin, located on a relatively high paleogeomorphic structure, was more likely to be exposed out of water with frequent rise and fall of sea level, and subject to syngenetic-quasi syngenetic dissolution, resulting in early dissolved pores. In addition, the Deng 2 and Deng 4 Members were transformed by Episodes I and II Tongwan tectonic movement, respectively, and then two regional large unconformities were developed. Affected by supergene karst, reservoirs with karst pores are widely distributed in the Dengying Formation.

The reservoir of the Deng-4 Member is 47.75-148.23 m thick, and 88.54 m in average, which is mainly composed of algal-laminated dolomite, algal dolarenite and algal clotted dolomite. The reservoir space is dominated by intergranular pores, intergranular dissolved pores, algal lattice pores, caves and dissolution fissures. (Fig. 6a, 6b), and the porosity is 2.10%-8.59%, averaging 4.34%. For example, the Deng-4 Member on the Hujiaba profile discovered on the platform margin belt of the northwest margin is 383.5 m thick, which is mainly composed of algal clotted dolomite, algal-laminated dolomite and dolarenite, with dissolved pores developed (Fig. 6a). Its reservoir is 190 m thick, with total porosity of 2.0%-12.0%, averaging 4.5%. The reservoir of the Deng-4 Member in Well Chuanshen-1 is 71.4 m thick, with total porosity of 2.5%-7.8%, averaging 3.3%. The Deng-4 Member in Well Jiaotan-1 is 348 m thick (drilled but not penetrated), including mound-shoal reservoir of 177.6 m thick, with total porosity of 2.0%-7.7%, averaging 3.3%.

The reservoirs in the Deng-2 Member are 28-340 m thick in total, mainly composed of breccia dolomite, grape lace-shaped dolomite and algal dolarenite. The reservoir space is dominated by intergranular dissolved pores, intragranular dissolved pores, algae lattice dissolved pores, caves and dissolution fissures (Fig. 6c-6f) with total porosity of 2.1%-10.4%, averaging 4.6%. The reservoir of the Deng-2 Member in the Penglai-Zhongjiang area in the northern slope of the central Sichuan paleo-uplift is mainly composed of algal dolomite, the same as that of the reservoirs in the GM area ^[4,8], which are 170-320 m thick in total, averaging about 250 m. The reservoirs of the Deng-2 Member in the Penglai area have total porosity of 2.0%-14.5%, averaging 3.6%, and average permeability of 3.6×10^-3 μm². The reservoirs of the Deng-2 Member in Well Pengtan-1 are mainly composed of dolarenite, grape lace-shaped dolomite and micrite dolomite, and of the 635 m drilled strata, the reservoirs account for 275 m. The reservoir of the Deng-2 Member in Well Pengtan-101 is 300 m thick. The reservoir of the Deng-2 Member in the GM area of central Sichuan is 28-300 m thick, averaging 93 m, with porosity of 2.7%-4.5%, averaging 3.7%.

Generally, the mound-shoal complex in the platform margin belt at the northwest margin of the Yangtze craton region is thick, the Deng-2 Member and Deng-4 Member of which are generally 50-100 m thicker than those of the GM area, and the reservoirs are more developed than those in the GM area, with thickness of 200- 400 m. The reservoirs of the Dengying Formation become poor from north to south, and the reservoirs in the north are better than those in the GM area on the whole.

Large-scale platform margin belts (Fig. 7) are developed in the Deng-4 Member and Deng-2 Member at the northwest margin of the Yangtze craton region, along Gaoshiti-Moxi-Shehong-Yanting-Laoguanmiao-Guangyuan, where several isolated mound-shoal complexes are developed, and between them are inter-shoal depressions with dense lithology and poor physical properties. The Yangtze Plate was in an extensional environment from the Sinian Dengying Formation to the Early Cambrian, and developed two sets of faults at the northwest margin of the Shehong-Guangyuan area: (1) nearly south-north faults, controlling the strike of the Dengying Formation platform margin belt; (2) nearly east-west faults, distributed approximately perpendicular to the northwest margin, with palaeo horsts and grabens in between. The development of dense zones on the platform margin belt of the Dengying Formation may be related to the nearly east- west faults ^[27], corresponding to the mound-shoal complexes in the inter-shoal depressions and the high structures on both sides. Isolated mound-shoal complexes separated by the dense zones developed in a lateral low-energy environment may form large lithologic trap groups which may evolve to independent mound-shoal reservoirs during later hydrocarbon migration and accumulation, and eventually to large lithologic reservoir groups.

Extremely high asphalt content was discovered in the Dengying Formation in Wells Pengtan-1, Pengshen-1 and Gaojiashan profile from the northern slope of the central Sichuan paleo-uplift to northern Sichuan. It is inferred that there were large paleo oil reservoirs in the early stage that were largely cracked into gas in the late stage to form large gas reservoirs.

The source rocks of the Doushantuo Formation and Maidiping-Qiongzhusi Formation in most parts of the Sichuan Basin first generated hydrocarbon in the Late Ordovician-Silurian. However, affected by tectonic uplifting caused by the Hercynian movement, hydrocarbon generation had been stagnant for a long time from the Devonian to the Carboniferous, until in the Permian started the second hydrocarbon generation after deeply buried. Therefore, the maturity of the two sets of source rocks was in "oil window" (R_o less than 1.5%) for a long time from the Late Ordovician to the Early Triassic, then a gas generation stage started from the Late Triassic to the Early Cretaceous. As the two sets of source rocks had Type I-II₁ organic matters, they may generate and accumulate a large amount of crude oil in the mature stage to form paleo oil reservoirs (groups) on slopes and the top of the paleo-uplifts. Bitumen is the direct evidence indicating the existence of paleo oil reservoirs. Bitumen is widely distributed in the reservoirs of the Dengying Formation in Weiyuan, Ziyang-Gaoshiti (in central Sichuan), Moxi-Yanting (in northern Sichuan), Langzhong-Ningqiang (in southern Shaanxi) and Hanzhong, also indicating that a huge paleo oil reservoir group once existed in the Dengying Formation on the northwest margin of the Yangtze craton region. The distribution, bitumen content and plane porosity of the paleo oil reservoir (group) depend on the paleo-uplift and mound-shoal reservoirs, decreasing from the top of the paleo-uplift to the slope area. The bitumen content in the platform margin belt is higher than that within the platform area. The bitumen content of the reservoirs is mainly 1.0%-5.0% in the superimposed area of the top of paleo-uplift and the platform margin belt of the Dengying Formation, as exemplified by 1.0%-3.4% in the Deng-4 Member and 1.0%-3.2% in the Deng-2 Member in the Gaoshiti area, and 1.0%-4.0% in the Deng-4 Member and 1.0%-3.2% in the Deng-2 Member in the Moxi area. The dolomite reservoir bearing bitumen in the Deng-2 Member in Wells Pengtan-1 and Zhongjiang-2 in the Penglai-Zhongjiang area are about 275 m and 101 m thick, respectively, with bitumen content of 1.0%-5.0%. The bitumen-bearing reservoir in the Deng-4 Member in Well Jiaotan-1 on the northern slope of the paleo-uplift is about 180 m thick. The dolomite reservoir bearing bitumen in the Deng-4 Member on the Ningqiang Gaojiashan profile in southern Shaanxi is 387.7 m thick. The dolomite reservoir bearing bitumen in the Deng-4 Member on the Kangjiadong profile is about 80 m thick, with bitumen content of 3.0%-5.0% (Fig. 8).

From the Middle Triassic to the Early Cretaceous, liquid hydrocarbon was entrapped in two sets of source rocks of Maidiping-Qiongzhusi Formation and Doushantuo Formation; semi-dispersed and semi-gathered liquid hydrocarbons present in the paleo oil reservoir of the Dengying Formation and migration pathway were largely cracked into gas, providing sufficient gas for later gas accumulation into reservoir. The natural gas in the gas reservoirs of the Dengying Formation and the Longwangmiao Formation in central Sichuan is mainly oil-cracked gas ^[4]. Late cracking and continuous charging of dispersed liquid hydrocarbon and existing paleo oil reservoirs are essential conditions for the development and preservation of Anyue Gas Field.

According to the research, the development of the Deng-2 Member and Deng-4 Member on the northwest margin of the Yangtze craton region is obviously affected by the syndepositional-fault-controlled sedimentary platform margin belt, on which large mound-shoal reservoirs are developed. The platform margin belt of the Deng-2 Member is 600-1000 m thick and that of the Deng-4 Member is 300-450 m thick and 40-120 km wide. The platform margin belts of the Deng-2 Member and Deng-4 Member are distributed in the Suining-Jiange area (Fig. 1), where mound-shoal complexes spread over 1.5×10⁴ km², and the reservoirs are 200-400 m thick.

Over the mound-shoal complex reservoir is 150-600 m-thick good-quality source rock of the Cambrian Maidiping-Qiongzhusi Formation (i.e., a regional caprock) and under it is 50-150 m-thick good-quality source rock of the Doushantuo Formation. Such a "sandwich" pattern provides good reservoir-forming conditions. The Doushantuo Formation and the Maidiping-Qiongzhusi Formation source rocks provide kerogen to accumulate gas reservoirs in the Deng-2 Member and Deng-4 Member ^[27]. The Doushantuo Formation source rock and the mound-shoal complex reservoir of the Dengying Formation form a favorable upper-source and lower-reservoir assemblage. In addition, the extremely thick and high-quality source rock of the Maidiping - Qiongzhusi Formation in the rift area laterally connects with the mound-shoal complex in the platform margin belt of the Dengying Formation, so it’s possible to form effective reservoirs in the Dengying Formation with near source in a 400-600 m hydrocarbon supply window (Fig. 9).

The exploration progress in the paleo-uplift in central Sichuan suggests that the Deyang-Anyue Rift in the northwest margin of the Yangtze craton region is promising on both sides ^[28⇓-30]. The Mianyang-Jian'ge block located in the northern part of the east side of the rift has the most favorable reservoir forming conditions in the Deng-2 member (Fig. 10), as manifested by the following three points: (1) Large-scale platform margin mound-shoal complexes cover 4000 km², and their seismic reflections are chaotic and discontinuous. (2) Two sets of extremely thick and high-quality source rocks of the Doushantuo Formation and the Maidiping-Qiongzhusi Formation provide excellent sources and contribute to several types of source-reservoir assemblage, such as "lower source and upper reservoir", "side source and side reservoir" and "upper source and lower reservoir", which are conducive to the formation of widespread lithologic oil/gas reservoirs. (3) The Mianyang-Yanting block has been a large, wide and gentle structural platform throughout all geological historical periods. The relatively stable structure provides favorable tectonic setting and favorable reservoir forming conditions. Therefore, in order to realize large-scale, efficient exploration that delivers substantial benefits, taking the Dengying Formation as a major target, high priority should be given to the Sinian System-Lower Paleozoic in the northwest margin of the Yangtze craton region. Notably, the Mianyang-Jian'ge Block has favorable conditions for forming a large lithologic gas reservoir group. There are mound-shoal complexes of 4000 km² in the Dengying Formation. If measured by using the reserve abundance of the Dengying Formation in Anyue Gasfield, there will be reserves of over one trillion cubic meters. This may make it another exploration area with trillion cubic meters of reserves in addition to Anyue Gasfield and Penglai Gasfield with proven reserves of one trillion cubic meters. In particular, the Dengying Formation in the Laoguanmiao structure, which is located at the wide and gentle structural platform in the Mianyang-Yanting Block, is buried at 8500-9500 m deep and has mound-shoal complexes of 1300 km² in the platform margin belt of its second member. The good source-reservoir assemblage is a favorable target for 10 000-meter scientific drilling operations.

The Yangtze craton region experienced craton rifting at its northwest margin from the Sinian to the Early Cambrian. The Deyang-Anyue rift took shape when the Doushantuo Formation deposited in the Early Sinian and then was supplemented with organic-rich shale. The extremely thick mound-shoal complexes developed with the deposition of the Dengying Formation in the Late Sinian provided reservoirs. As the rift area subsided rapidly in the early stage of the Early Cambrian, extremely thick organic-rich shale deposited.

Two sets of high-quality source rocks of the Sinian Doushantuo Formation and the Cambrian Maidiping- Qiongzhusi Formation were developed in the northwest margin of the Yangtze craton region, bearing excellent source conditions. The source rocks were estimated to have gas generation intensity of (100-200)×10⁸ m³/km² in the Maidiping-Qiongzhusi Formation and (5-40)×10⁸ m³/km² in the Doushantuo Formation. That laid a solid material base for massive reservoir formation in the Dengying Formation. During peak oil generation and gas generation, the Mianyang-Jian'ge block was in a relatively high structural position, and reservoirs developed in the platform margin belt of the Deng-2 Member and Deng-4 Member. In addition, an extremely high content of bitumen was found in the block, indicating large-scale paleo oil reservoir existing in the early stage. It is expected that the Mianyang-Jian'ge block is a favorable block with massive oil/gas accumulation.

The Dengying Formation in the northwest margin of the Yangtze craton region developed large-scale microbial mound-shoal complexes in the platform margin belt. Influenced by epigenic karstification that occurred in the syngenetic-quasi syngenetic period and Episode II Tongwan tectonic movement, karst caves and dissolution pores were developed, forming extremely thick porous reservoirs in the Deng-2 Member and Deng-4 Member. Small paleotopographic difference resulted in the development of multiple independent mound-shoal complexes in the platform margin belt of the Dengying Formation. Between the mound-shoal complexes are inter-shoals depressions with dense lithology and acting as lateral seals. The mound-shoal complexes and high-quality source rocks of the Doushantuo Formation and the Maidiping-Qiongzhusi Formation constituted several types of favorable source-reservoir assemblages, such as "lower-source and upper-reservoir", "side source and side reservoir" and "upper-source and lower-reservoir". Near source and valid reservoir are favorable conditions for a large-scale lithologic gas reservoir group.

Containing mound-shoal complexes of 4000 km², the Deng-2 Member in the Mianyang-Jiange Block is of large resource potential and expected to become an ultra-deep large-scale exploration area. The Dengying Formation in the Laoguanmiao structure located in the large-scale wide and gentle structural platform in the Mianyang-Yanting Block, is buried at 8500-9500 m and has mound-shoal complexes of 1300 km² in its second member. Surrounded by extremely thick and high-quality Cambrian source rocks, it bears good reservoir conditions and proven traps, making it a favorable target for 10 000-meter scientific drilling operations.