In the Sichuan Basin, the large-scale Deyang-Anyue “trough basin” has a significant control on the formation and distribution of the Sinian-Cambrian giant natural gas reservoirs, as demonstrated by exploration practices. As early as in the 1960s, PetroChina conducted exploration in the periphery of the large-scale Deyang-Anyue “trough basin” and the paleo-uplift in the Sichuan Basin, and discovered a vast number of gas fields in the Sinian Dengying Formation, such as Weiyuan, Ziyang, and Gaoshiti-Moxi gas fields, of which the Gaoshiti-Moxi gas field has over trillions of cubic meters of proved gas reserves. In 2020, Well PT1 was drilled on the northern slope of the Central Sichuan paleo-uplift, penetrating a 127 m thick gas layer in the second member of the Sinian Dengying Formation (hereafter referred to as the Deng 2 Member) and testing at 122×10⁴ m³ of gas production per day. It is of great significance, as it demonstrated that the Dengying Formation platform margin at the east side in northern segment of the Deyang-Anyue “trough basin” has huge potential as a gas province. It is expected that another giant gas field can be discovered, with trillions of cubic meters of gas resources ^[1].

Continuous effort in oil and gas exploration for the Sinian-Cambrian strata in the Sichuan Basin has reached an agreement on the existent of the Deyang-Anyue “trough basin”. In light of the limitations of study, the understandings of the formation mechanism and evolution process of the “trough basin” remain controversial. A variety of terms were proposed by scholars based on their understandings of the property of the “trough basin”, such as extensional trough ^[2-3], erosional valley ^[4-5], rift trough ^{[6⇓⇓⇓⇓-11]}, platform basin ^[12], and extension-erosion groove ^[13]. The concepts of “extensional trough” and “rift trough” emphasize the control of the extension regime on the formation of the “trough basin”, but the views on the extension mechanism and time of development differ between them. Extensional trough highlights the effect of the Xingkai taphrogeny, and considered that the “trough basin” was formed at the early Dengying period before it was greatly uplifted and eroded and its evolution commenced since Cambrian ^[2-3]. “Rift trough” highlights the rifting effect and the important role of “activated” basement fracture in the formation of the “trough basin”. The Central Sichuan paleo-uplift appears to be depositional in nature, and began to develop as early as in the Cambrian (along with the Deng 2 Member) ^[6⇓-8,11]. “Erosional valley” stresses that the “trough basin” is essentially the valley formed by the erosion of karst and surface flow, which was later filled with the Lower Cambrian deposits ^[4-5]. Both the “platform basin” and the “extension-erosion groove” concepts acknowledge the joint effect of extension and erosion, but highlight the predominance of erosion ^[12-13].

Although there are various opinions regarding the origin of the Deyang-Anyue “trough basin”, the main focus of the debates is on the depositional facies change of the Dengying Formation in the “trough basin” and the platform zone, and whether the Dengying Formation is absent in the “trough basin”. In addition, these conclusions are limited to the studies focusing on the localized parts of the “trough basin” and lack a correlation of depositional characteristics and genetic evolution across the entire “trough basin”. Recent studies by the authors suggested that the Dengying Formation varies largely in lateral depositional distribution and depositional model over the Deyang-Anyue “trough basin”. This illustrates that the formation and evolution of the “trough basin” are likely segmented. It is therefore unwise to conclude its origin as extensional rifting or karst erosion. It becomes a topic to be discussed segment by segment. In this study, based on the previous studies, a correlative analysis has been conducted on the depositional distribution characteristics of the Dengying Formation across the Deyang-Anyue “trough basin” and its periphery using a variety of data (e.g., drilling, field outcrop sample, core, and seismic) and a segment-by-segment discussion has been made on the formation of the “trough basin” by combining the geomorphologic features with development characteristics. Lastly, a genetic evolution model has been built in order to point the direction for expanding the exploration areas of the Dengying Formation in the Sichuan Basin.

Studies show that the final break-up of the Rodinia supercontinent and the initial amalgamation of the Gondwana continent occurred during the Sinian-Early Cambrian ^[14]. During that period, the Yangtze Plate was under the paleo-oceanic drifting setting. Hence, it was controlled by the break-up effect of the Rodinia supercontinent without being amalgamated with the Gondwana continent. In the Sichuan Basin, which located on the Upper Yangtze Plate, an extensional tectonic regime was dominated, with weak compressional stress subordinated. The volcanic magmatic events indicated that multi-period of rifting occurred at the northern and western margins of the Basin ^[15-16], and then controlled the distribution and formation of the Pre-Sinian rifts by extending into the Upper Yangtze block. Hence, the rifting had a control on the depositional and geomorphologic framework of the Sichuan Basin during the Sinian-Early Cambrian ^[17]. The multi-episodic Tongwan movement in the Sinian caused the Upper Yangtze Region to uplift. As a result, the Dengying Formation in the Sichuan Basin was eroded at varying degrees, resulting in parallel conformities in the Dengying Formation and between the Dengying Formation and the Cambrian^[18]. The Deyang-Anyue “trough basin” was re-mapped with the latest drilling and seismic data to determine its distribtution. It reaches the Mianyang to Deyang areas (Fig. 1) in the north and the Yibin to Luzhou areas in the south, being centered on the Zizhong to Anyue areas. Its deposition involves two periods, corresponding to the Deng 2 and Deng 4 members, respectively. The Deng 2 Member “trough basin” is located in the central and northern parts of the Sichuan Basin (hereafter referred to as the Central and Northern Sichuan, respectively) and appears to be “U”-shaped, while the Deng 4 Member “trough basin” extends over the entire basin, striking N-S. Studies show that, Deng 2 and Deng 4 members controlled the matching of reservoir-forming elements and the distribution of oil and gas in the Sinian-Cambrian strata at their periphery ^[18].

By building the depositional framework of the Dengying Formation across the Deyang-Anyue “trough basin” and its periphery, the depositional characteristics of Dengying Formation has been correlated, which is significant for revealing its origin.

Based on the lithologic and lithofacies analyses for four typical field outcrops of the Dengying Formation in the Northern Sichuan Region (Fig. 2 and Fig. 3), it is considered that the formation is composed of deep-water deposits. Extending from the “trough basin” to the platform, a complete depositional model comprising the basin, slope, platform margin, and restricted platform was developed.

The basin facies can be observed on the Pingwu Jinfeng profile. It is located within the intra-basinal slope deep-water zone, conformably overlying the Doushantuo Formation mudstone and underlying the Maidiping Formation thin-bedded silicalite, with faint traces of erosion. The Deng 1 to Deng 2 members consist mainly of dark grey-greyish black micritic dolomite and siliceous dolomite, interbedded with greyish black silicalite and dark grey micritic limestone (Figs. 2a and 3). The Deng 3 Member is composed mainly of dark grey thin- bedded argillaceous dolomite and greyish black silicalite. The Deng 4 Member comprises mainly dark grey-greyish black micritic dolomite, silicalite, and micritic dolomite (Figs. 2b and 3). This profile was below the wave base during the Dengying period, where the energy was too low to permit the presence of grain and algal deposits. Also, the sedimentation ratio was too low to permit the strata as thick as those in the shallow-water zone. For example, the Dengying Formation is measured at only 44 m thick on the profile.

The Dengying Formation in the slope facies is measured at 183 m thick (not outcropped at base) on the Lin’ansi profile. It is underlay conformably with the Maidiping Formation. The Deng 2 Member consists mainly of dark grey, thin- to moderate-bedded micritic dolomitic limestone and moderate- to thick-bedded limy dolomite, with local lamina structures (Fig. 2c). The bottom of Deng 4 Member consists mainly of interbeds of thin-bedded micritic limestone and micritic dolomite, and the top of Deng 4 Member consists mainly of the black thin-bedded silicalite. The slope sedimentary environment enabled the development of rich structural deformations, such as the slump structure formed by intermittent landslide (Fig. 2d). Silicalite deposits are mingled with siliceous dolomite slump bodies. In the transition zone between the platform and the basin, the water was relatively deep, and the temperature was low, below the compensation depth of carbonate. The water body remained stagnant and less euphotic, not favorable for the build-up of carbonates.

The Dengying Formation of the platform margin facies is measured at 852 m thick (not outcropped at base) on the Gaojiashan profile (Fig. 4). The layered psammitic dolomite on the top of the Deng 2 Member is unconformably overlain by the thin-bedded yellowish grey sandy mudstone of the Deng 3 Member. The thin-bedded yellowish grey quartz sandstone of the Deng 3 Member is conformably overlain by the thin-bedded grey sandy dolomite of the Deng 4 Member. The top of Dengying Formation is a weathering crust, and unconformably overlain by the basal Cambrian argillaceous limestone. With high water energy, the platform margin was favorable for the development of the algae-bonded beach and algal mound. Major deposits include the sparry granular dolomite, algae-bonded dolomite, and algal stromatolithic dolomite. On the Gaojiashan profile, the Deng 4 Member often contains the microbial mound-shoal bodies showing moundy, positive geomorphologic features. It consists of the superimposed grain beach, algae-bonded beach, and algal mound facies, with considerable size and broad lateral extension.

The Dengying Formation of the restricted platform facies is measured at 824 m thick on the Yangba profile (Fig. 4). It is conformably underlain by the Guanyinya Formation (Doushantuo Formation) and unconformably overlain by the Lower Cambrian Qiongzhusi Formation. An unconformity exists between the Deng 2 and Deng 3 members. Continuous deposits extend from the Deng 3 Member to the Deng 4 Member. Deposits of the restricted platform facies are widespread within the platform, with a relatively gentle and flat topography. Intra-platform mound-shoal bodies might occur on local geomorphic highs, with smaller vertical and lateral scale than the platform marginal mound-shoal. Micritic dolomite prevailed the majority part of the intra-platform zone, since the shielding of the uplifts significantly limited the water circulation by weakening the water energy. The Deng 2 Member shows multiple thin-bedded mound-shoal bodies on the Yangba profile. The Deng 4 Member also contains thin-bedded mound-shoal bodies, with the lithology dominated by micritic dolomite, followed by psammitic dolomite. Typical depositional structures include horizontal beddings, with occasional wavy beddings.

As stated above, it is considered that in the northern part of Sichuan, the Dengying Formation was deposited in the deep-water, low-energy setting. The Dengying Formation of the platform facies, however, consists of high- energy, shallow-water deposits, indicating a dramatic regional change in lithology and lithofacies. During the Late Sinian, intensive extensional rifting occurred at the northern margin of the Sichuan Basin. As a result of the differences in depositional paleo-geomorphology and environment, a dramatic change in lithology and lithofacies of the Dengying Formation deposits between the rift and platform settings came out. During the Tongwan episodes I and II, the Northern Sichuan Region was in low-lying depositional paleo-geomorphology, the Dengying Formation was less eroded and recorded more deep-water deposits.

The Central Sichuan region differs from the Northern Sichuan region in depositional characteristics of the Dengying Formation. There is no lithologic and lithofacies difference between the “trough basin” and the platform. For example, Well ZY1 penetrated the Dengying Formation with relatively thin thickness of the “trough basin” in the Central Sichuan region, before encountering the Doushantuo Formation. The top of the Dengying Formation is algal stromatolithic dolomite, which is overlain by phosphorus-rich argillaceous dolomite and dolomitic mudstone, which is in the Maidiping Formation. Therefore, it is speculated that the Deng 3 and Deng 4 members are absent and the Deng 2 Member is unconformably overlain by the Cambrian strata (Fig. 5). The top of Deng 2 Member is the algal laminar dolomite, with abundant dissolved pores and vugs (Figs. 2e, 2f and 5), with depositional characteristics similar to the shallow-water platform facies in Dengying Formation but different from that of the deep-water deposits observed on the Jinfeng profile in the Northern Sichuan region.

The Deng 4 Member penetrated by Well GS131 on a steep ridge at the eastern side of the “trough basin” contains algal laminar dolomite (Figs. 2g and 6), indicating a relatively shallow-water, high-energy depositional environment that appears favorable for the build-up of microbolite. However, it is inferred that the middle and upper parts of this member were eroded out, given its thickness of as low as 55 m in the well. In addition, it differs greatly from the slope facies deep-water deposits observed on the Lin’ansi profile in the Northern Sichuan region in depositional characteristics.

In the Central Sichuan region, the platform margin belt of the Dengying Formation has a small-scale of deposition. And there is no deep-water deposit observed in the “trough basin”, and no dramatic change in depositional characteristics of the Dengying Formation between “trough basin” and platform (Fig. 6). This suggests that the “trough basin” either was yet to form or just began to take its initial shape in the Central Sichuan region. The water level was too shallow to permit the accumulation of the basin and slope facies deep-water deposits. It is, speculated that the present-day “trough basin” and platform framework in the Central Sichuan region was formed as a result of intensive erosion during the Tongwan movement.

The water environment in the Southern Sichuan region was limited compared with that in the Central Sichuan region. There was no depositional facies change between “trough basin” and platform. In the Deng 1 Member and the lower part of the Deng 2 Member, 240 m thick gypsum salt rock and 30 m thick gypsum dolomite encountered by Well N2 are deemed to be the deposits from the evaporative lagoon setting. The upper part of the Deng 2 Member consists mainly of algae-clotted dolomite, algal stromatolithic dolomite and micritic dolomite deposits of the intra-platform mound-shoal facies and platform flat facies, with abundant grape lace structures and dissolved pores and vugs (Fig. 2i). Due to the absence of the Deng 3 and Deng 4 members, the Deng 2 Member is overlain unconformably by the Maidiping Formation silicalite and siliceous dolomite (Fig. 7). This suggests that in the Southern Sichuan region, the carbonate restricted platform with a relatively shallow-water environment when Deng 1 and Deng 2 members were deposited ^[19]. The Deng 2 Member penetrated by Well TT1 is composed of algal dolomite and psammitic dolomite, and the Deng 4 Member is only 96 m thick and consists mainly of algal psammitic crystal powder dolomite and fine-crystalline dolomite (Figs. 2h and 8), which are deemed to be the post-erosion residual stratigraphic deposits. A notable variation in thickness of the Deng 3 and Deng 4 members exists between “trough basin” and platform. The Deng 2 Member encountered by Well Y1 is 287 m thick (not penetrated), and consists mainly of algal dolomite and crystalline dolomite. It penetrated the Deng 4 Member with a thickness of 374 m, much thicker than that in Well TT1. The lithology includes the algal laminar dolomite, argillaceous dolomite and crystalline dolomite. All of them were deposited in the shallow-water platform setting (Fig. 8). This suggests that there is no geomorphologic differentiation of “trough basin” and platform during the depositional period of Deng 4 Member in the Southern Sichuan region. The present-day geomorphology of “trough basin” and platform were formed primarily by the Tongwan Episode II erosion.

Studies show that the Dengying Formation in the Sichuan Basin generally inherited the Doushantuo Formation in terms of the paleo-geographic framework ^[20]. The “trough basin” in the Dengying Formation not only is segmented but also varies greatly in geomorphology across regions, due to the joint control of the Late Sinian rifting extension and the multi-episodic Tongwan erosions.

The Dengying Formation “trough basin” is divided into the northern, middle, and southern segments depending on its depth and width (Fig. 1). The northern segment extends over the Shehong, Mianyang and Guangyuan areas in the Northern Sichuan region. The “trough basin” is 20-120 km wide in the Deng 2 Member, and 140-270 km wide in the Deng 4 Member. The maximum burial depth exceeds 1000 m. The platform marginal belt of the Deng 2 Member with the thickness of 650-1000 m and the width of 40-130 km is separated from that of the Deng 4 member with the thickness of 350-450 m and the width of 20-70 km. The middle segment extends over the Suining, Anyue and Ziyang areas in the Central Sichuan region. The “trough basin” is 0-80 km wide in the Deng 2 Member, and 140-150 km wide in the Deng 4 Member. On the eastern side, the platform margins in the Deng 2 and Deng 4 members are superimposed, and are much smaller in size than that in the Northern Sichuan region. The Deng 2 Member platform margin belt ranges from 15 km to 20 km wide and 260 m to 300 m thick, and the Deng 4 Member ranges from 10 km to 15 km wide and 450 m to 500 m in thickness. The southern segment extends over the Yibin, Longchang and Yongchuan areas in the Southern Sichuan region, with the Deng 4 Member “trough basin” only, the Deng 2 Member “trough basin” is absent. This “trough basin” ranges from 140 km to 200 km in width. On the eastern side, the size of the Deng 4 Member platform margin became further limited compared to that in the Central Sichuan region, with the thickness of 260-300 m and the width of 10-15 km. The “trough basin” is wide and steep in the north, but narrow and gentle in the south, as evidenced by its distribution characteristics.

The northern segment of the “trough basin”: the seismic data clearly shows two periods of retrogradational platform margins within the Deng 2 and Deng 4 members around the northern segment of the “trough basin” (Fig. 9). This illustrates the effect of the multi-period extensional tectonic settings on the Dengying Formation in the Northern Sichuan region. In addition, the Deng 4 Member underwent a more intensive rifting extensional stress than the Deng 2 Member. The platform margin is thicker, wider, and better preserved in the Northern Sichuan region than that in the Central Sichuan region. Field outcrop data demonstrates that the Dengying Formation and the sedimentary records are well kept in the northern segment of the “trough basin”, where the deep-water deposits of the Deng 2 and Deng 4 members are present. In areas proximal to the eastern side of the “trough basin”, the Dengying Formation platform margin belt corresponds to clear progradational reflection characteristics (Fig. 10), indicating that deposits move outward from the platform into the basin to form the depositional platform margin belt. In the Northern Sichuan region, the Dengying Formation was deposited on a relatively low-lying area and hence less eroded by the Tongwan Movement, and with more complete sedimentary records. Thus, it is considered that the present-day “trough basin”-platform framework in the Northern Sichuan region is a result of the extension regime, and related to the extension activity that occurred at the northern margin of the Upper Yangtze Plate ^[21-22].

The middle segment of the “trough basin”: with the available drilling and seismic data, a fine tracking has been conducted on the Sinian Dengying Formation strata in the middle segment of the rift trough and its periphery. The base of the Dengying Formation, the top of the Deng 2 Member, and the base of the Cambrian are calibrated with the data from Well GS1, while the bases of the Maidiping and Qiongzhusi formations are calibrated with the data from Well ZY1. Seismic data shows that the Deng 3 and Deng 4 members are eroded out in the middle segment of the “trough basin”, resulting in direct contact between the Lower Cambrian and the Deng 2 Member. The Deng 2 and Deng 1 members thicken gradually while extending from the “trough basin” towards the platform. Seismic events corresponding to the top of the Deng 2 Member vanish in the platform margin belt. The Deng 3 and Deng 4 members appear to thin while extending from the platform towards the basin and pinch out gradually at the outer boundary of the “trough basin” (Fig. 11). The truncated Dengying Formation on the seismic section illustrates that the middle segment of the “trough basin” was intensively eroded. As a result, the Deng 2 Member platform margin is vertically superimposed with the Deng 4 Member platform margin to produce unique geomorphology (Fig. 12).

The southern segment of the “trough basin”: it is smaller in scale compared with the central and northern Sichuan regions (Fig. 13), as there is no significant change in thickness of the Dengying Formation. The Deng 2 Member “trough basin” extends from the Northern Sichuan region into the Gaoshiti area in the Central Sichuan region but has yet to reach the Southern Sichuan region. Seismic data shows that the Deng 1 and Deng 2 members are thick-bedded and distribute stably, with no “trough basin”-platform geomorphology observed. The lithology and lithofacies are predominately carbonate platform deposits ^[19]. The Deng 3 and Deng 4 members deposited on the platform are thin-bedded, gentle and flat, thinning towards the trough. Furthermore, strata deposited on the platform in the periphery of the southern segment of the “trough basin” are truncated greatly (Fig. 14), indicating that the present-day “trough basin”-platform geomorphology is a result of the erosion of the Tongwan movement.

Based on the study of the depositional characteristics and the analysis results of the geomorphological features, it is considered that the northern segment of the Deyang-Anyue “trough basin” shows a distinctively different depositional mode of the Dengying Formation from that in the middle-southern segments. The “trough basin” is clearly segmented. Tectonically, the northern segment was controlled predominately by the extensional rifting, with relatively weak erosion, and has retained more sedimentary records of the Dengying Formation. In contrast, the middle-southern segments of the “trough basin” were formed as a result of the erosional reworking. Thus, it is considered that the Deyang-Anyue “trough basin” is a result of the joint effect of “the deposition in the north and the erosion in the south”, and should be named as the “erosion rift trough” (in the following sections, the term “trough basin” will be replaced by the “erosion rift trough”). The implication of the “erosion rift trough” highlights the segmented evolution of its origin in the space rather than a unified evolution (e.g., the extensional erosion trough ^[13]), which makes it different from previous nomenclatures.

During the Sinian-Early Cambrian, the Northern Sichuan region differs from the Central-Southern Sichuan regions not only in the dynamic tectonic setting, but also in the intensity of karst erosion. From the perspective of geodynamics, it is believed that intensive intra-platform extensional rifting occurred at the margin of the Yangtze platform under the joint effects of the tectonic weak zone in the Sinian basement, the intrusion of the uplifted mantle along the Southern Qinling Mountain, and the dragging of the Proto-Tethys Ocean crust plate, extensive extensional rifting occurred at the northern margin of the Yangtze platform and even inside the platform ^{[15⇓-17,22]}. It was the most intensive at the northern margin of the basin and gradually weakened basinward, producing the uplift-depression depositional framework, in a pattern similar to graben-horst. Thus, the extensional rifting remained active mainly in the Northern Sichuan region. That is the main reason why the Deyang-Anyue erosion rift trough emerges with a nature of extensional rift. The intensity of erosion differs from the Central and Southern Sichuan regions to the Northern Sichuan region due to the differential erosional geomorphology. Hence, the temporal and spatial evolution of the Deyang-Anyue erosion rift trough differs from part to part and should be analyzed and studied segment by segment. Based on drilling, field, and seismic data, it is necessary to discuss the formation and evolution of the Deyang-Anyue erosion rift trough segment by segment in the following sections.

During the depositional period of Deng 1 and Deng 2 members, due to intensive extensional rifting, an extensional rift trough was formed in the Northern Sichuan region. With the deep water environment in the trough, the build-up rate of the microbial carbonate in the Dengying Formation was slow, forming low-energy deep- water deposits with thin-bedded stratum. On the contrary, the water level was relatively shallow on the platform, which was favorable for the formation of the microbial carbonate build-up, resulting in the deposition of thick-bedded Dengying Formation and well-developed mound-shoal facies. The extension of the extensional rift trough from the Northern Sichuan region into the Craton was limited, mainly extended to the north of the Suining area in the Central Sichuan region (Fig. 15a). The extensional rifting that controlled the Central and Southern Sichuan regions were too weak to produce the “trough basin”-platform geomorphology. Instead, carbonate platform shallow-water high-energy deposits were developed (Fig. 16a).

During the Tongwan Episode I, the negative geomorphology occurred on the top of the Deng 2 Member due to the erosion. The depositional-dominated Deng 2 Member platform margin belt was formed in the Northern Sichuan region despite the effect of erosion, due to its low-lying landform and low level of erosion (Fig. 15b). In contrast, the Central-Southern Sichuan regions were relatively high-lying and intensively eroded to produce the erosion trough (Fig. 16b).

During the depositional period of Deng 3 and Deng 4 members, the Hannan archicontinent was uplifted to supply the basin with abundant terrigenous materials during Deng 3 Member deposition. Continental shelf and tidal flat deposits were dominated within Craton. Regression occurred at the early stage of depositional period of Deng 4 Member. Meantime, carbonate platform deposits emerged and then prograded towards the deep- water shelf to produce the “early-stage platform margin” belt that stretches northeast. Within it, the high-energy mound-shoal deposits were controlled by the geomorphologic highlands ^[23-24]. During the late depositional period of the Deng 4 Member, regression occurred across the Central and Northern Sichuan regions. Carbonate platform and platform margin facies further prograded and expanded northwestward, towards the deep-water shelf. Sedimentary differentiation occurred between “trough basin” and platform in the Northern Sichuan region, and high-energy mound-shoal facies deposits were formed in the proximity of the platform margin belt (Fig. 15c). Central-Southern Sichuan regions developed carbonate platform deposits and did not produce the “trough basin”-platform geomorphology (Fig. 16c).

During the Tongwan Episode II, the Central Sichuan region was rested on the Archean-Lower Proterozoic rigid crystalline basement, which was stable and highly resistant to structural deformation. In contrast, the Northwestern Sichuan region was rested on the folded basement, which was readily structurally deformed ^[25]. During the Sinian-Early Cambrian, the South China Plate was situated in the proximity of the equator ^[26] under the hot climate setting ^[27-28], with weathering and erosion occurred to carbonate rocks. During Tongwan Episode II, greater level of regional regression and more intensive erosion than Episode I occurred. Under the uplifting setting in the Tongwan Episode II, the Sinian Dengying Formation platform margin belt subsided due to the control of basement differentiation. The subsidence resulted in a low-lying karst basin with relatively deep level of water inside in Northern Sichuan region. Thus, the deposited strata were less eroded during the Tongwan Episode II. The platform margin belt appears to be well preserved with more sedimentary records in the Dengying Formation (Fig. 15d). The Central-Southern Sichuan regions presented the karst slope and depression geomorphology, located in the drainage zone and catchment zone of the meteoric freshwater, with the most intensive erosion ^[29]. Within the trough, the majority of the Deng 3 and Deng 4 members were eroded. Residual mounds were present in local uneroded areas, and the erosion trough with erosion origin was developed finally (Fig. 16d).

The periphery of the erosion rift trough is divided into three major exploration domains in terms of its segmented formation and evaluation (Fig. 17).

Firstly, the Dengying Formation developed a large-scale depositional-type platform margin belt that controlled the development of the lithologic mound-shoal bodies, which was caused by northern segment of the erosion rift trough formed by the extensional regime. In addition, due to the relatively weak erosion, the unconformity at the top of the Dengying Formation could not act as an effective migration pathway, which hindered the migration of the generated oil and gas into the structural high parts. Instead, hydrocarbon accumulated into the lithologic mound-shoal bodies. Hence, the exploration for the Dengying Formation platform margin belt on the slope in the Northern Sichuan region should be focused primarily on large-scale lithologic reservoirs. On the slope in the Northern Sichuan region, Well PT1 penetrated a 119.26 m gas layer in the Deng 2 Member platform margin belt, with daily gas production of 122×10⁴ m³ during production testing ^[1]. The gas-water contact is at -5550 m, about 400 m lower than that in the Deng 2 Member in Moxi area. Within the platform margin belt of the Deng 4 Member, Well JT1 penetrated a gas layer with thickness of 101 m. The gas-water contact was about 2000 m lower than that in the Gaoshiti-Moxi area, which demonstrated the existence of the large-scale lithologic trap in the depositional-type platform margin of the Dengying Formation in the Northern Sichuan region ^[30-31]. Initial study shows that in the Northern Sichuan, the total area of the lithologic traps in the Deng 2 Member platform margin belt is 5732 km² in Penglai, Santai and Jiange areas, and the total area of the lithologic traps in the Deng 4 Member is 3840 km² in Shehong, Yanting and Langzhong areas. The estimated gas resources are over trillions of cubic meters. The Northern Sichuan is a new area for increasing gas resources in the Sichuan Basin.

Secondly, the erosion rift trough in the Central Sichuan region is mainly of erosion origin. Subject to a widespread erosional karstification, extensive karst reservoirs were developed in the Dengying Formation on the platform. A giant gas reservoir has been discovered in the Dengying Formation platform margin belt in the Gaoshiti- Moxi area, and has entered into the high-efficiency exploration and development phase. Proved reserves are estimated at over 4000×10⁸ m^{3 [32]}. In the Central Sichuan region, intensive erosion occurred during the Tongwan Episode II, resulting in extensive development of karst reservoirs at the uppermost 50 m of the intra-platform Deng 4 Member mound-shoal bodies. Through stimulation with horizontal wells, the drilling ratio of the high-quality reservoir has been increased. The MX129H horizontal well was drilled in the intra-platform Longnüsi area, with daily gas production of 141×10⁴ m³ from the Deng 4 Member. As of now, the prospect area of the intra-platform Deng 4 Member in the Central Sichuan paleo-uplift is over 1950 km², mainly in Tongnan and Hechuan areas, with 1590×10⁸ m³ of proved gas reserves, being considered as a new area for natural gas exploration in the Sichuan Basin.

Moreover, the erosion rift trough in the Central and Southern Sichuan regions, carbonate platform deposits are dominated in the Dengying Formation, mainly comprising well-developed microbial mound-shoal bodies, which would form the karst residual mounds. They are surrounded by the Lower Cambrian high-quality thick- bedded source rocks within the erosion rift trough to produce the lithologic-stratigraphic composite trap, with favorable hydrocarbon accumulation conditions. The Deng 2 Member karst residual mounds extend over a total area of 290 km² in the Zizhong-Lezhi area within the erosion rift trough, with 1500×10⁸ m³ of estimated gas resources in total. The Deng 4 Member karst residual mounds extend over a total area of 1650 km², with 11 500×10⁸ m³ of estimated gas resources in total. They are considered as a new domain for making strategic breakthrough in gas exploration for the Sichuan Basin.

The depositional characteristics of the Dengying Formation differ widely across the Deyang-Anyue erosion trough and its periphery. In the northern segment of the erosion rift trough, deep-water deposits are present in the Dengying Formation. In the Northern Sichuan region, the Dengying Formation presents the depositional model comprising basin, slope, platform margin, and restricted platform. In the Central and Southern Sichuan regions, there are no the deep-water deposits in the Dengying Formation, and carbonate rock platform deposits are dominated.

The Deyang-Anyue erosion rift trough was formed under the joint action of the sedimentary differentiation and the karst erosion. The extensional rifting regime dominated in the northern segment of the erosion rift trough to form the depositional-type platform margin, while the erosion dominated in the middle and southern segments to form the erosional-type “trough basin”-platform geomorphology.

According to the segmented evolution and origin, the Dengying Formation in the erosion rift trough and its periphery is divided into three exploration domains: i.e., the platform marginal lithologic mound-shoal bodies in the Northern Sichuan region, the intra-platform karst mound- shoal bodies in the Central Sichuan region, and the intra-trough karst residual mounds in the Central and Southern Sichuan regions, which have great potential of gas resources. It is noteworthy that the intra-trough karst residual mounds in the Central and Southern Sichuan regions are considered as the new domain for making strategic breakthrough in gas exploration in the Sichuan Basin. The platform marginal lithologic mound-shoal bodies in the Northern Sichuan region are deemed to be new frontier for increasing the gas resources to a level of trillions of cubic meters.