The Sinian System in the Sichuan Basin is one of the oldest hydrocarbon-bearing series in China. After more than 60 years of research and exploration, major research progress and significant exploration discoveries have been made in this system. In 1964, the Weiyuan giant gas field was discovered, which was the largest gas field in China producing from the carbonate rocks of Sinian Dengying Formation^[1,2]. In 2011, the largest monolithic marine carbonate giant gas field in China with the Sinian Dengying Formation and the Cambrian Longwangmiao Formation as the main pay zones was discovered at the core of Gaoshiti-Moxi paleo-uplift^[3,4,5]. As of the end of 2019, the Gaoshiti-Moxi area had proven gas in place of over 1×10¹² m³ and proven, probable and predicted gas reserves of 1.5×10¹² m³. Five giant gas fields with reserves of one-hundred billion cubic meter order, namely Gaoshi-1, Gaoshi-19, Moxi-22, Moxi-109 and Moxi-52 were successively discovered in the mound-shoal complexes at the platform margin of the fourth member of Dengying Formation in (hereinafter referred to as Z₂dn⁴) on the east side of Mianzhu-Changning intracratonic rift. The proven gas in place was about 6000×10⁸ m³, accounting for about 60% of the total proven reserves of the Gaoshiti-Moxi gas field, demonstrating the great exploration potential of the platform margin zone of Z₂dn^{4 [5]}. In recent years, with the breakthroughs in exploration and continuous accumulation of data, major research progress has been made in the aspects of sedimentation, reservoir and tectonic evolution of the Sinian System in the Sichuan Basin^[6,7,8,9,10]. A series of new understandings including in-situ cracked gas accumulation of the paleo-oil reservoirs in Gaoshiti-Moxi paleo-uplift, Mianzhu-Changning intracratonic rift and the platform margin zone in the periphery areas of the rifts as well as the cores of the inherited paleo-uplifts was put forward^{[11,12,13,14,15]}. The Mianzhu-Changning intracratonic rift is also called Deyang-Anyue intracratonic rift by some experts, and there is controversy on whether it is an intracratonic rift or an extensional trough^[8,9], but it is commonly accepted that there developed a platform margin zone east of it^{[4-5, 7]}. It is proposed that the platform margin zone of the Sinian Dengying Formation in the east side of Mianzhu-Changning intracratonic rift is an important area for next step exploration^[16,17]. Through examining massive exploratory wells and seismic data, the platform margin zone of Z₂dn⁴ in the east side of the rift has been clearly delineated^{[4, 8, 16]}. The mound-shoal complex gas reservoirs discovered at the Z₂dn⁴ platform margin were at the core of Gaoshiti-Moxi paleo-uplift and are at the high position of the top surface structure of the Dengying Formation nowadays. The platform margin zones in the south and north of Gaoshiti-Moxi area are located in the slope area of the paleo-uplift and nowadays structural lows positions. Whether the mound-shoal complexes in the platform margin of these areas can form reservoirs and how much the exploration potential of them are remain unclear, restricting our understanding on the accumulation in the platform margin zone and the expansion of the exploration field.

On the basis of previous knowledge, this paper analyzes the development of the mound-shoal complex in the platform margin of Z₂dn⁴ in the east side of the rift by using the latest drilling, seismic, logging and production test data, examines Gaoshiti and Moxi gas reservoirs, analyzes the accumulation conditions and main controlling factors of the mound-shoal complexes in the platform margin and discusses whether the paleo-uplift slope area can form a reservoir on a large scale, in hope of guiding natural gas exploration in the Sinian System in the Sichuan Basin.

The understanding on the lithofacies paleogeography of the Sinian Dengying Formation in the Sichuan Basin is relatively consistent. It is believed that in the depositional period of Z₂dn⁴, the relative sea level fell and the sedimentary water was shallow on the whole, and generally deep in the northeast and shallow in the southwest. Carbonate platform facies dominated inside the basin. Taking Mianzhu-Changning rift as the center, platform marginal facies and open platform facies developed on two sides of the rift respectively^{[3, 16]}. A large number of studies suggested that platform margin zones of Z₂dn² and Z₂dn⁴ developed in the east side of Mianzhu- Changning rift. Although there are some differences in the characterization of the platform margin zones, opinions on the spreading direction and position of the platform margin zone of Z₂dn⁴ in the east side of the rift are basically consistent^{[4, 8, 16]}. In this study, with the support of new seismic and drilling data, the platform margin zone of Z₂dn⁴ in the east side of the rift was systematically delineated, the sedimentary facies of Z₂dn⁴ in the whole basin was studied, and the lithofacies paleographic map of Z₂dn⁴ in the Sichuan Basin was prepared (Fig. 1a). The platform margin zone in Gaoshiti-Moxi area in the east side of the rift is about 10-20 km, and located in the middle of the whole platform margin zone, so it is called "the middle section". The platform margin zone to the north of Gaoshiti-Moxi area becomes wider northward, and extends to the line of Jiange and Well Heshen1, as this section of platform margin zone is located in the north of the whole, so it is called "the northern section". The platform margin zone to the south of Gaoshiti-Moxi area extends to the line of Luzhou and Gulin, located in the south of the whole, so it is called "the southern section" (Fig. 1a). The Sinian Dengying Formation has a thickness of around 200 to 1000 m in the Sichuan Basin, it is mainly controlled by Mianzhu-Changning intracratonic rift and Gaoshiti-Moxi paleo-uplift^{[3, 14]}. It is divided into 4 lithological sections, namely Z₂dn¹, Z₂dn², Z₂dn³ and Z₂dn⁴ from bottom to top, among which Z₂dn⁴, composed of algae and bacteria rich massive dolomite (Fig. 1b), is the main reservoir section and exploration target of the Dengying Formation^[18,19,20].

The top surface of the Sinian Dengying Formation in the Sichuan Basin (i.e., the top surface structure of Z₂dn⁴) is high in the middle and gradually turns lower toward the peripheral areas. It is relatively flat in central and western Sichuan, and has structural traps developed in the east and south of Sichuan. The Gaoshiti-Moxi area and Weiyuan-Ziyang area are uplifts, and there is a relatively low saddle between them, separating the uplifts into two relatively independent large-scale structural traps, namely Weiyuan-Ziyang and Gaoshiti-Moxi traps. The Gaoshiti-Moxi structural trap is an inherited paleo-uplift with an area of about 3×10⁴ km², which has existed for a long time. It was named by Wei Guoqi et al. as "Gaoshiti-Moxi paleo-uplift ". Based on the overlay of the lithofacies paleographic map and the top surface structural map of the Sinian Dengying Formation, it can been seen that the middle section of the platform margin zone in the east side of the rift is located at the core of the Gaoshiti-Moxi paleo-uplift, and the top surface of the Sinian System is generally at about 5000 m deep and is a large structural trap. The south and northern sections of the platform margin zone are located at the slopes of the Gaoshiti-Moxi paleo-uplift, and have been always located in the slope area during the evolution of the paleo-uplift^[3]. Compared with the middle section, they belong to the lower part of the current structure, which is a large slope with no structural traps developed. The burial depth of the top surface of the platform margin zone gradually increases toward the south and the north. The maximum burial depth of the top surface of the Dengying Formation in the northern section of the platform margin zone reaches more than 8000 m in Jiange area; the maximum burial depth of the top surface of the Dengying Formation in the southern section of the platform margin zone also reaches more than 8000 m in the Gulin area.

Different from the Phanerozoic, the Sinian System does not have large-scale reef-building (mound) organisms with wave resistant framework developed, and the reef (mound) shoals at the edge of the platform are dominated by microbial mounds and shoals ^[14]. The mound-shoal complexes at the platform margin of the Dengying Formation in the Sichuan Basin are mainly composed of flourishing benthic microbial community and carbonate rock architecture with wave resistant structures formed by biochemical process of the benthic microbial community. The mound-shoal complexes in the platform margin of Z₂dn⁴ on the east side of the rift vary greatly in morphology and occurrence. They are composed of micro-biogenic clotted limestone dolomite, foamed cotton dolomite, stromatolites and crypt algal laminated dolomite (Figs. 2 and 3), as well as gritty dust, arenite and oolitic dolomite (Fig. 3). The mound-shoal complexes in the platform margin zone of Z₂dn⁴ are similar in lithologic characteristics. For example, the mound-shoal complexes of Gaoshiti area, the Z₂dn⁴ in Well Gaoshi1 is a large mound-shoal complex at the platform margin with a thickness of about 240m, it can be divided into three mound-shoal complexes from the bottom to the top. The first mound-shoal complex is composed of gray and ash gray algal laminated dolomite intercalated with arenite dolomite; the second mound-shoal complex is composed of gray algal agglomerate dolomite intercalated with arenite dolomite; the third mound-shoal complex is composed of gray and ash gray arenite-bearing microcrystalline agglomerate dolomite intercalated with arenite dolomite (Figs. 2a, 2c and 3a). Argillaceous crystalline dolomite develops between the same stage mound-shoal complexes, which is the dolomite tidal flat sediment (Fig. 3a). The mound-shoal complexes in Moxi area have similar lithological association with mound-shoal complexes in Gaoshiti area. For example, the mound-shoal complex in Well Moxi-9 mainly occurs in the middle and upper sections of Z₂dn⁴, and is superposed by two algal mounds and carbonate shoal complexes (Figs. 2b and 3b), and there is a set of argillaceous dolomite tidal flat sediment between the mound-shoal complexes (Fig. 3b). The Z₂dn⁴ drilled in Well Jiaotan1 quite far from Gaoshiti-Moxi area in the northern section of the platform margin zone has a thickness of about 350 m, in which the mound-shoal complex is about 280 m and is composed of two thick layers of algal mound and grain shoal complexes. The lithological association is mainly arenite dolomite, algal agglomerate dolomite, algal laminated dolomite and alga-bonded dolomite (Figs. 2d and 3c). The Z₂dn⁴ drilled in Well Heshen-2 in the southern section of the platform margin zone has a thickness of 380 m (not penetrated). Since there is no coring data, it was inferred from well logging and cuttings data analysis that the mound-shoal complex mainly existed in the middle and lower parts, with a thickness of about 180 m, and was composed of three algal mound and carbonate shoal complexes (Fig. 3d). Through comparison, it can be seen that the mound-shoal complexes along the edge of the platform margin zone of Z₂dn⁴ on the east side of the rift are similar in characteristics of lithological association.

The Yangtze Plate was under extensional background during the Nanhua and Sinian periods. In the Nanhua Period, a large-scale rift developed. After the Sinian rifting, the Yangtze Plate entered the cratonic basin evolutionary stage; from the Sinian Dengying Period to the early Cambrian Meishu Village Period, the Upper Yangtze region was still under extensional environment^[20]. Through the interpretation of a large amount of seismic data, two groups of faults have been mainly identified in the Dengying Formation in central Sichuan Basin. One group in nearly north-south strike is considered to control the boundary of the rift^{[12, 14]}. Faults in this group are normal faults nearly parallel with the platform margin zone, and controlled the formation of the rift and the development of the platform margin zone at the margin of the rift^{[4, 14]}. The other group of faults is in near east-west strike, approximately perpendicular to the platform margin zone. This group of faults is similar in origin with the faults in nearly north-south strike (Fig. 4a) and mainly consists of normal faults. This group of faults affected strata from Nanhua system to Cambrian system in central Sichuan Basin, and some affected the strata prior to the sedimentation of the Permian system. This group of faults was related to the Nanhua rifting in origin^[20]. They were activated frequently in the later stage, resulting in long activity time, which affected many overlying related strata. Under the extensional background, the strata between two faults concaved to form the alternate high and low micro paleogeomorphology (Fig. 4), which affected the deposition of carbonate strata. As the Z₂dn³ in the basin is very thin, generally 10-30 m, and there is a seismic wave impedance interface between Z₂dn³ and Z₂dn², the bottom boundary of Z₂dn³ is generally identified from the seismic data, and the stratum between the bottom boundary of Z₂dn³ and the top boundary of the Dengying Formation is regarded as Z₂dn⁴. By calibrating the horizons of multiple wells such as Well Gaoshi1 and Well Moxi-9, horizons such as the top and bottom of the Sinian System, the bottom of Z₂dn³ and the bottom of Longwangmiao Formation can be identified on seismic sections and the identification reliability of Z₂dn⁴, the target layer, is very high. From the seismic facies of the mound-shoal complexs in Z₂dn⁴ of the area, the mound-shoal complexes mainly turn up in the areas with relatively high paleogeomorphology, while mound-shoal complexes are not developed in the relatively low-lying areas (Fig. 4a). The reef shoal zone at the margin of the carbonate platform is generally composed of multiple platform marginal reef (mounds) shoal complexes, which are separated by waterways and distributed along the margin of the platform^[21,22,23]. Therefore, a series of fault groups can be identified along the platform margin on the east side of the rift (Fig. 4). Through seismic facies identification and drilling data analysis, a series of mound-shoal complexes and inter- shoal depressions controlled by paleo-geomorphology caused by fault actions can be identified. The seismic section shown in Fig. 4b passes through Gaoshiti mound-shoal complex and Hebaochang mound-shoal complex. Similar to mound-shoal complexes in Fig. 4a, due to the action of a group of faults, the paleogeomorphology undulated, resulting in mound-shoal complexes and inter-shoal depressions at the platform margin. Fig. 4c shows a spliced seismic section from the northern section to the middle section of the platform margin with three groups of faults. Similar to Fig. 4a and 4b, the mound-shoal complexes at the platform margin of Z₂dn⁴ mainly occur in the region with higher paleogeomorphology, while the mound- shoal complexes are few in the region with low paleogeomorphology (Fig. 4c).

Through interpretation of massive seismic data and analysis of drilling data of wells in the platform margin zone, it is confirmed that the platform margin zone of Z₂dn⁴ is about 350 km long, with 6 groups of faults trending nearly EW and perpendicular to the platform margin zone identified (Fig. 4). The six sets of faults divided the platform margin zone of Z₂dn⁴ on the east side of the rift into rugged paleogeomorphology, forming 7 large platform margin mound-shoal complexes and 6 inter-shoal depressions. In this paper, the mound-shoal complexes in the platform margin are numbered No.①- No.⑦ mound-shoal complex from north to south respectively. The near EW trending faults separated the platform margin zone into several mound-shoal complexes with high paleogeomorphology and inter-shoal depressions with low paleogeomorphology (Fig. 5a).

Due to the action of the near EW trending faults, the the platform margin zone was divided into alternate high-and-low fault blocks before the deposition of Dengying Formation. The high positions of the paleo-geomorphology had strong water energy and sufficient oxygen and nutrient, which was conducive to the growth of algae and deposition of carbonate shoal facies, so mound-shoal complexes composed of algal mounds and carbonate shoals developed at these places. In contrast, in the paleogeomorphologic low positions with weak water energy, relatively low-lying inter-shoal depressions turned up, where relatively tight rocks such as argillaceous dolomite and micritic limestone, and a small amount of algal mounds and carbonate shoals developed, forming mound- shoal complexes with small thickness in local parts. In areas with high paleogeomorphology, due to well-developed algal mounds and carbonate sediments, the Z₂dn⁴ is thicker, which is reflected to some extent on the seismic sections (Fig. 4). By combining drilling data with seismic facies, statistics show that the Z₂dn⁴ in the region with high paleogeomorphology is generally more than 300 m thick, while the Z₂dn⁴ in the region with relatively low paleogeomorphology is generally less than 300 m thick (Fig. 5b). In Gaoshiti and Moxi areas with abundant drilling data, statistics show that the mound-shoal complexes in the platform margin with high paleogeomorphology account for generally above 60% of the total thickness, while the mound-shoal complexes in the platform margin with low paleogeomorphology generally accounts for less than 40% of the total thickness of the Z₂dn⁴ (Fig. 5c).

Thus, it can be seen that the mound-shoal complexes in the platform margin of the whole four sections of Z₂dn⁴ are similar in sedimentation characteristics and generally divided into multiple periods. Before the sedimentation of the mound- shoal complexes at the platform margin zone of Z₂dn⁴, due to the action of the contemporaneous faults, the sedimentary paleogeomorphology had differentiated, forming the alternate high-and-low paleogeomorphology (Fig. 6a). In one cycle of sea-level rise and fall, one stage of mound-shoal complexes developed in the paleogeomorphologic high positions and one stage of inter-shoal depressions developed in the paleogeomorphologic low positions. After multi-periods of sea level changes, multi-stages of mound-shoal complexes built up, forming large mound-shoal complexes at paleogeomorphologic high positions, while in the inter-shoal depressions located in paleogeomorphologic low positions, due to the low water energy, the mound-shoal complexes developed were relatively thin (Fig. 6b).

The mound-shoal complex reservoirs in the platform margin of Z₂dn⁴ have fairly good physical properties. With dissolution pore, dissolution cavity and fracture as main storage space (Fig. 2), they are fracture-cave type dolomite reservoirs. 436 samples of mound-shoal complexes at the platform margin from Gaoshiti and Moxi area have a porosity range of 2.0%-12.5% (on average 4.0%), and a permeability range of (0.0001-19.4)×10^-3 μm² (on average 0.622×10^-3 μm²). 201 samples of intraplatform mound-shoal dolomite from this area have an average porosity of 3.46% and average permeability of 0.456×10^-3 μm². Apparently, the mound-shoal complexes in the platform margin have better physical properties than the intraplatform mound-shoal dolomite. Well log interpretation results show that most of the reservoirs are in the mound- shoal complexes and are good in continuity (Fig. 7). The reservoir in Well Gaoshi1 is 113m thick with an average porosity of 3.2%, and that in Well Moxi-22 is 134 m thick with an average porosity of 3.4%. The gas test output can reach above 50×10⁴ m³/d in general. The reservoirs in No. ② and No. ⑥ mound-shoal complexes with well drilled have fairly good physical properties. Based on log interpretation, the reservoir in Z₂dn⁴ of Well Jiaotan-1 in No. ② mound- shoal complex is about 166 m thick (not penetrated) and 3.5% in average porosity. The reservoir in Z₂dn⁴ of Well Heshen-2 in No. ⑥ mound-shoal complex has a thickness of about 120 m (not penetrated) and an average porosity of 3.6%. Clearly, the mound-shoal complex reservoirs in the entire platform margin zone have relatively large thicknesses and good physical properties (Fig. 7).

Compared with the mound-shoal complexes in the platform margin zone of Z₂dn⁴ in Gaoshiti-Moxi area, the mound-shoal complexes in the inter-shoal depressions formed by faulting are not well-developed and poorer in reservoir physical properties. It is clear that the main lithologic assemblage of Well Moxi-21 in the inter-shoal depression is argillaceous crystal dolomite, with a set of thin layered algal mound and carbonate shoal assemblage developed. The Z₂dn⁴ drilled in Well Moxi-21 is about 230 m thick. Only one stage of mound-shoal complex occurs at the upper section with a thickness of about 35 m. According to well logging interpretation, the reservoir is 28 m thick (Fig. 7) and 2.68% in average porosity. On the basis of well calibration, the mound-shoal complex reservoirs in Gaoshiti-Moxi were predicted by using seismic data. The results show the mound-shoal complex reservoirs of Z₂dn⁴ are 30-150 m thick totally and about 80 m thick on average. For instance, the reservoirs predicted by seismic data in Well Moxi-18 and Well Gaoshi3 areas are greater than 100 m thick, the mound-shoal complex reservoirs in both platform margin zones are more than 50 m. The reservoirs in the inter-shoal depressions between the mound- shoal complexes are all less than 30 m thick (Fig. 5d).

There have been a large number of studies on the characteristics and genesis of the mound-shoal complex reservoirs in the platform margin zone of Z₂dn⁴. A consensus has been reached on the good physical properties of the mound-shoal complex reservoirs in the platform margin zone, but there is controversy on their genetic mechanism^{[6, 15, 24]}. We hold that there are mainly three reasons: (1) As the water energy at the edge of the rift was high, and the water was connected to the sea, the sea water had rich nutrition and prosperous algal micro-organisms, and abundant carbonate sediments, so mound- shoal complex grew and massively accumulated, forming large scale mound-shoal complexes. In the mound-shoal complexes, massive biological species and their cementation effect produced a large number of lattice holes, providing the foundation for high quality reservoir formation. (2) In the platform margin zones, the mound-shoal complexes in the platform margin were paleogeomorphologic high structures. In the process of frequent sea level fluctuations, the mound- shoal complexes were likely to expose out of the water surface and subject to the dissolution of atmospheric fresh water, forming early dissolution pores, which laid a good foundation for the formation of the mound-shoal complex reservoirs. (3) After the sedimentation of the mound-shoal complexes, Episode II of Tongwan movement made the sea level drop, consequently, the mound-shoal complexes at the platform margin exposed, and subjected to dissolution of the weathering crust, forming large amounts of dissolution pores. The three favorable reservoir forming conditions worked together to give rise to the large-scale mound-shoal complex reservoirs at the platform margin of Z₂dn⁴ (Fig. 6c). In contrast, the mound-shoal complexes in inter-shoal depressions are not well-developed and poorer in physical properties. This is related to the relatively low positions of the depressions in the platform margin zone all the time. Furthermore, in the current structure, the inter-shoal depressions are still relatively low, for instance, the top of Z₂dn⁴ in Well Moxi-21 is at the burial depth of 5042 m, while those in the adjacent Well Gaoshi-3 and Well Moxi-9 are at 4975 m and 4975 m respectively (Figs. 1a and 7)

The mound-shoal complex reservoirs in the platform margin have been analyzed in depth above. On this basis, from the perspective of the joint control of source rock and caprocks, the hydrocarbon source conditions and sealing condition of the caprocks of the mound-shoal complexes in the platform margin were further examined to reveal the main factors affecting gas reservoir accumulation. As the gas pools in mound-shoal complexes at the platform margin are formed after short distance migration independently. The migration conditions are not discussed in this paper. Previous studies had come to a clear understanding on the accumulation conditions of the mound-shoal complexes in the middle section of the platform margin zone of Z₂dn⁴ on the east side of the rift^{[3-5, 7, 16]}. Due to lack of data, the accumulation conditions of the south and northern sections remain murky. Based on the latest data, we tried to make clear the accumulation conditions of the entire platform margin zone. It has been mentioned above that the mound-shoal complex reservoirs in the entire platform margin zone are thicker and better in physical properties. Other accumulation conditions in the south, north and middle sections of the platform margin zone are similar.

On the gas source of the gas reservoir of the Dengying Formation in the Anyue gas field, there is a consistent understanding^{[4, 25]}. Through gas source correlation and analysis of source-reservoir assemblage relationship, it is concluded that the high-quality argillaceous hydrocarbon source rocks of the Qiongzhusi Formation of Lower Cambrian, Z₂dn³ and the Doushantuo Formation of Sinian as well as the argillaceous carbonate source rock of the Dengying Formation have provided sufficient gas source for the mound-shoal complex reservoirs in the platform margin of Z₂dn⁴. In addition, there may be hydrocarbon source rocks of the Pre-Sinian System in the study area^[26]. Among all the source rocks, the Qiongzhusi Formation source rock has made the largest contribution, because: (1) The Qiongzhusi Formation hydrocarbon source rock has good quality. The hydrocarbon source rocks of the Qiongzhusi Formation and Maidiping Formation within the rift have large thickness and good quality, which form a hydrocarbon generating center. The hydrocarbon source rocks within the rift have a thickness of 300-350 m generally. The source rocks in a few wells, such as Well Gaoshi17 inside the rift are over 400 m thick, in Well Tian-1 in Northern Sichuan are 350 m thick, and 450m thick at maximum in Southern Sichuan. (2) This set of source rock has a good configuration with the reservoir. The Qiongzhusi Formation source rock occurs in the sides and top of the mound-shoal complexes in the east side of Z₂dn⁴ and contacts directly with the reservoirs of the mound-shoal complex from both sides, forming favorable configuration with the reservoir allowing hydrocarbon supply in various ways from multiple faces, which is beneficial to the gas accumulation in the mound-shoal complexes at the platform margin.

In addition to the hydrocarbon source rocks of the Qiongzhusi Formation and Maidiping Formation, the hydrocarbons generated by the high-quality argillaceous source rocks of Z₂dn³ and the Doushantuo Formation as well as the argillaceous carbonate source rock of the Dengying Formation could supply the reservoirs in the mound-shoal complexes at the platform margin of Z₂dn⁴ directly upward. In this paper, the source rocks of Sinian and Cambrian are considered as a whole. In the distribution area of the mound-shoal complex reservoirs in the platform margin zone on the east side of the rift of Z₂dn⁴, the Sinian and Cambrian source rocks have hydrocarbon generation intensity of greater than 40×10⁸ m³/km² (Fig. 8a), indicating that the mound-shoal complexes in the whole platform margin zone have the hydrocarbon source conditions for forming large gas fields.

Preservation condition is one of the key factors for natural gas enrichment^[7]. The direct caprock of the gas reservoir of the mound-shoal complex in the platform margin of Z₂dn⁴ is the mudstone of the Lower Cambrian Qiongzhusi Formation, which is both high-quality source rock and good caprock. The caprock of the Qiongzhusi Formation mudstone is widely distributed in the Sichuan Basin and generally 80 to 500 m thick (Fig. 8b). The caprock of the Qiongzhusi Formation mudstone has good sealing capacity. The caprock of the Qiongzhusi Formation mudstone in Well Gaoke-1 is about 117 m, and has an average measured porosity of 1.4%, an average permeability of 0.004×10^-3 μm², an average pore diameter of 4.337 nm, a rock breakthrough pressure of 14.4- 23.1 MPa and an average diffusion coefficient of 4.5×10^-7 cm²/s. The caprock in Gaoshiti-Moxi area in the middle section of the platform margin zone has a smaller thickness of 80-150 m. The caprocks in the south and northern sections of the platform margin zone are more than 200m thick. For instance, the Qiongzhusi Formation mudstone in Well Heshen-2 area in the southern section is 200 m thick, while the Qiongzhusi Formation mudstone in Well Jiaotan1 area in the northern section is approximately 300 m thick (Fig. 8b). The caprock of the Qiongzhusi Formation mudstone is in direct contact with the reservoir of Z₂dn⁴, wrapping the mound-shoal complex reservoir in the platform margin from top and sides. The direct caprock conditions for Z₂dn⁴ in the whole platform margin zone are basically the same, featuring large thickness and good sealing capacity, which has played an important role in preserving gas accumulation in the mound-shoal complexes at the platform margin zone of Z₂dn⁴. The regional caprocks, mainly including gypsum mudstone of the Gaotaizi Formation, mudstone and argillaceous carbonate rocks of Ordovician and Permian, of the mound-shoal complex reservoirs of Z₂dn⁴ also have good sealing capacity. With large thickness and wide distribution, they provide favorable preservation conditions for the Sinian System in the basin.

The gas reservoirs found in the mound-shoal complexes in the platform margin of Z₂dn⁴ in the study area mainly include the mound-shoal complex of Gaoshiti area (No. ⑤) and the mound-shoal complex of Moxi area (No. ④). They have similar reservoir forming conditions but also some differences in characteristics, indicating that the two gas reservoirs may be relatively independent. Through analysis and comparison of the gas reservoirs, it can also be inferred that the gas reservoir in Well Heshen-2 of No. ② mound-shoal complex and the gas reservoir of Well Jiaotan1 of No. ⑥ mound-shoal complex may also be relatively independent from the gas reservoirs of No. ④ and No. ⑤ mound-shoal complexes. Therefore, it can be speculated that the inter-shoal depressions between the mound-shoal complexes have a lateral sealing effect on the gas reservoirs of mound-shoal complexes.

3.3.1. Differences between the mound-shoal complex gas reservoirs at the platform margin in Gaoshiti area and Moxi area

Two gas reservoirs in two mound-shoal complexes in the platform margin of Z₂dn⁴ in the Gaoshiti and Moxi areas have similar characteristics: (1) They are dry gas reservoirs with medium-low sulfur content, medium carbon dioxide and micro-propane, helium and nitrogen contents. (2) The natural gases from them have a relative density range of 0.6079- 0.6336, methane contents of 91.22%-93.77%, hydrogen sulfide contents of 1.00%-1.62%, carbon dioxide contents of 4.83%-7.39%, and trace contents of propane, helium and nitrogen. (3) At burial depths of 5000-5100 m, they have formation pressures of 56.57-56.63 MPa in the middle of the pay zones, and pressure coefficients of 1.06-1.13. (4) They have temperatures of 149.6-61.0℃in the middle.

The differences between the two gas reservoirs include three aspects: (1) Natural gas components: The gas of Gaoshiti gas reservoir has nitrogen, carbon dioxide and hydrogen sulfide contents of 0.57%, 6.19% and 1.06% respectively, while gas of Moxi gas reservoir has nitrogen, carbon dioxide and hydrogen sulfide contents of 1.13%, 7.91% and 1.49%, respectively. (2) Formation pressure: converting the formation pressure of the gas reservoir of Z₂dn⁴ to an elevation of -4 854.8 m, the average converted pressure of Gaoshiti gas reservoir is 56.8 MPa, and the average converted pressure of Moxi gas reservoir is 58.6 MPa, with a difference of 1.8 MPa. (3) Gas- water contact: The gas-water contact of Moxi gas reservoir is -5230 m. There is no water breakthrough in Gaoshiti gas reservoir so far (Table 1 and Fig. 9).

It is inferred from analysis that the reason for the differences is that the two gas reservoirs are relatively independent. The converted pressure of Moxi block is 1.8 Mpa higher than that of Gaoshiti block, indicating that they belong to two different pressure systems. The different gas-water contacts show that the two reservoirs are not unified. The gas from them have little differences in hydrocarbon gas components, but bigger differences in contents of non-hydrocarbon gases such as nitrogen, carbon dioxide and hydrogen sulfide, indicating that the low-lying area formed by faults between the two gas reservoirs is non-permeable, which can basically separate the connection between the two gas reservoirs (Fig. 9).

3.3.2. Differences between gas reservoirs in No. ② and No. ⑥ mound-shoal complexes at the platform margin

Two other gas reservoirs found in the platform margin zone of Z₂dn⁴ on the east side of the rift are Heshen-2 gas reservoir in No. ⑥ mound-shoal complex and Jiaotan 1 gas reservoir in No. ② mound-shoal complex. Though gas test hasn’t been done in Well Jiaotan1 yet, the reservoir has been judged as a gas reservoir based on log interpretation. The main gas reservoir of Well Heshen-2 drilled in the mound-shoal complex in the southern section of platform margin is located in the middle of Z₂dn⁴, with a reservoir thickness of 22 m and an average porosity of 4.9%. The well had a daily gas output of 19×10⁴ m³, gas shows at the depth of 6070 m, and no water produced during testing. The gas reservoir of Well Jiaotan-1 drilled in the mound-shoal complex in the northern section of the platform margin is mainly located in the upper section of Z₂dn⁴. The reservoir is 166 m thick and 3.2% in average porosity. The gas layer is 100 m thick, and a gas-bearing water layer of 50 m thick was interpreted at 7625 m depth, which is about 2000 m lower than the gas-water contact of Moxi gas reservoir. Based on the analysis of available data, the gas reservoirs in the No. ② mound-shoal complex (Jiaotan-1), No. ④ mound-shoal complex (Moxi gas reservoir), No. ⑤mound-shoal complex (Gaoshiti gas reservoir) and No. ⑥ mound-shoal complex (Heshen-2) are all independent (Fig. 9). It can be concluded that the inter-shoal depressions formed by faults in the platform margin have certain lateral sealing effect on the gas reservoirs and can block the up-dip direction of the mound-shoal complexes in the structural lows of the platform margin in the slope of the paleo-uplift, which is conducive to the accumulation and later preservation of gas in the mound- shoal complexes at the platform margin in the slope zone.

The Z₂dn⁴ gas reservoir of Anyue gas field in the middle section of the platform margin zone on the east side of the rift is located in the structural high position, and has always been the direction of oil and gas migration and accumulation. It is formed by "in-situ" accumulation of cracking gas from the ancient oil reservoir^{[4, 7, 25]}. Based on the studies on the thermal evolutionary and hydrocarbon charging histories of the Sinian hydrocarbon source rocks in Well Gaoshi-1^[7], analysis from many aspects such as hydrocarbon generation & evolution of hydrocarbon source rock, paleo-uplift, paleo-structure and paleo-trap evolution of the slope shows that for the sealing effect of the tight rocks in the inter-shoal depressions formed by faulting in the up-dip direction, in-situ cracking gas tectonic & lithologic gas reservoirs as Anyue giant gas field can also be formed in the mound-shoal complexes in the structural lows of the slope of the paleo-uplift. Before the sedimentation of the Cambrian system, there developed a series of relatively independent mound-shoal complexes in the platform margin of Z₂dn⁴. Due to the interlayer karstification and the weathering crust karstification of the Tongwan period, the reservoirs with good physical properties were formed in mound-shoal complexes suffering karstification. Before the sedimentation of the Silurian system, the Z₂dn³ argillaceous source rocks underlying mound-shoal complex reservoirs of Z₂dn⁴ in the platform margin and the argillaceous source rocks of the Qiongzhusi Formation overlying and in lateral side of the Z₂dn⁴ reservoir entered the first peak period of hydrocarbon generation and began to generate oil massively, the oil migrated into the mound-shoal complex reservoirs of Z₂dn⁴ in the platform margin to form reservoirs. The Leshan-Longnüsi paleo-uplift began to form. The paleo-structure was high in the west and low in the east. All the hydrocarbon source rocks in the platform margin zone entered the peak period of hydrocarbon generation and the mound-shoal complexes in the entire platform margin zone could form reservoirs. After the sedimentation of the Silurian System and before the sedimentation of the Permian, for the impact of the Caledonian movement, the formations of the basin were uplifted and denuded as a whole. The structure was high in the west and low in the east, so the denudation degree gradually increased from east to west. In the east section, the Silurian system was retained. In Gaoshiti-Moxi structure of the middle section, the formation was denuded to the Cambrian Xixiangchi Formation. In the west section, the formation was denuded to the Cambrian Longwangmiao Formation at most, and hydrocarbon generation of the source rocks stopped and the oil reservoirs were damaged to some extent. After the sedimentation of the Permian started, with the increase of the burial depth on the whole, the argillaceous source rock of Z₂dn³ and argillaceous source rock of the Qiongzhusi Formation generated oil massively for the second time, and the oil gathered in the mound-shoal complex reservoirs in the platform margin zone continuously. Before the sedimentation of the Triassic, the mound-shoal complexes in the entire platform margin zone were oil reservoirs. For the impact of the Leshan-Longnüsi paleo-uplift, the entire platform margin zone was high in the west and low in the east (Fig. 10a). Before the sedimentation of the Upper Triassic series, for the further increase of the burial depth, the temperature of the reservoirs exceeded 160 ℃. The crude oil in the reservoirs began to crack to form oil and gas reservoirs, and the structure was still high in the west and low in the east (Fig. 10b). From Late Triassic to Cretaceous, the Longmenshan foreland basin began to take shape, and the platform margin zone in the west section increased in burial depth, and the platform margin zone turned high in the middle section and low in the south and northern sections. As the temperature further increased to more than 200 ℃, the oil in the oil and gas reservoirs was cracked completely to form dry gas reservoirs, and the structural features and gas reservoirs have been remained to the present (Fig. 10c). The formation and evolution of the gas reservoirs in the whole platform margin zone of Z₂dn⁴ on the east side of the rift are similar to those of the Z₂dn⁴ gas reservoirs of the mound-shoal complexes in Gaoshiti and Moxi areas which were clearly understood^[4]. They are formed by in situ accumulation of cracking gas from the oil of the ancient oil reservoirs. Their reservoir forming conditions and formation processes are very similar (Fig. 10). The reservoirs in the present structural lows have been preserved to the present thanks to the lateral sealing of the tight layers in the inter-shoal depressions.

Through the analysis of the reservoir forming conditions, reservoir forming process and characteristics of gas reservoirs in the mound-shoal complexes at the platform margin zone of Z₂dn⁴, it can be inferred that 7 mound-shoal complexes in the platform margin zone of Z₂dn⁴ on the east side of the rift may have formed reservoirs and be preserved to the present. The mound-shoal complexes in the platform margin zone of nowadays structural low positions in the slope of the paleo-uplift can form gas reservoirs just the same as the mound-shoal complexes in Anyue gas field in the high positions. Therefore, the mound-shoal complexes in the platform margin zone of the slope area have the same exploration value as those in Gaoshiti and Moxi areas, so they are the directed targets for the next step exploration development. Even if with burial depth reaching 8000-10 000 m, the gas reservoirs formed in the early stage may be preserved to the present due to the sealing of the tight layers of the up-dip direction. Among the 7 mound-shoal complexes found, large scale exploration has been carried out in No. ④ (Moxi block) and No. ⑤ (Gaoshiti block) mound-shoal complexes and major exploration results have been achieved. The two mound-shoal complexes with an area of about 1500 km² in total and a burial depth of top of 5000-5500 m contain gas fully, with proven gas in place of about 6000×10⁸ m³. The No. ①, No. ② and No. ③ mound-shoal complexes in the west section of the platform margin are about 2500 km² in area and at the top burial depth of 6000-8000 m. The No. ⑥ and No. ⑦ mound-shoal complexes in the east section of the platform margin that have been delineated are about 800 km² in total and at the top burial depth of 5500-7000 m. According to the reserves abundance of the No. ④ and No. ⑤ mound-shoal complexes in the platform margin, the mound-shoal complexes in the south and northern sections have the geologic basis for formation of another large gas field with one trillion cubic meter order reserves, and huge exploration potential.

In the east side of Mianzhu-intracratonic rift, the platform margin zone of Z₂dn⁴ is developed, which extends southwards (the southern section) and northwards (the northern section) from Gaoshiti area in the middle section. The middle section is located at the current structural high position of the paleo-uplift core. The southern and northern sections of mound- shoal complexes in the platform margin are located at the slope area of the paleo-uplift and the current structural low positions. The development and reservoir characteristics of the southern and northern sections of mound-shoal complexes in the platform margin are similar to those in the middle section.

The nearly EW-trending faults perpendicular to the mound- shoal zone of the platform margin formed the platform margin zone into multiple paleo-geomorphologic highs and lows. In the high positions, mound-shoal complexes are well-developed and the reservoirs have better physical properties. In the low positions, inter-shoal depressions turned up where mound- shoal complexes are not well-developed and the reservoirs have poor physical properties.

All the mound-shoal complexes in the platform margin have similar favorable reservoir forming conditions and reservoir forming process as the mound-shoal complexes at the platform margin in the Gaoshiti and Moxi areas, so they all can form gas reservoirs. The tight rocks developed in the inter-shoal depressions of the platform margin zone have important up-dip direction sealing function for the mound-shoal complexes during structural evolution, which is conducive to the preservation of the mound-shoal complex gas reservoirs. The mound- shoal complexes at the platform margin in the present structural lows in the slope of the paleo-uplift are favorable exploration targets. Some exploratory wells have confirmed they contain gas, foretelling that they have great exploration prospects.