During the middle and late 1990s, Wang Yigang et al. first put forward the concept of "Kaijiang-Liangping" trough when studying the sedimentary environment of northeastern Sichuan Basin during Late Permian-Early Triassic period^[1]. It mainly refers to a deep-water siliceous trough in the Guangyuan-Liangping belt of the northern Sichuan Basin during this period. The trough opened northwestward, converged southeastward and disappeared in Liangping area, and was surrounded by shallow carbonate platforms. Previous studies suggested that the formation of the trough began during the late depositional stage of the Wujiaping Formation of the Upper Permian, peaked during the late depositional stage of the Changxing Formation of the Upper Permian, narrowed and shallowed during the late depositional stage of the Feixianguan Formation of the Lower Triassic, and was filled and disappeared during the late depositional stage of the Feixianguan Formation. When Ma Yongsheng et al. studied the sedimentary facies of the Changxing Formation to the Feixianguan Formation in the northeastern Sichuan Basin, they expounded the “Kaijiang-Liangping” trough from the aspects of paleogeographic environment, sedimentary pattern, sedimentary model, sedimentary characteristics and distribution of sedimentary facies, and called the trough "a platform-shelf"^[2]. From the end of the 20th century to the beginning of the 21st century, major breakthroughs have been made in oil and gas exploration around the margin of the trough, and a number of large-medium and super-large gas reservoirs in reef- beach facies of the Changxing Formation-Feixianguan Formation, such as Dukouhe, Luojiazhai, Puguang, Longgang and Yuanba, have been successively discovered^[3-4], with cumulative proved reserves of nearly 10000×10⁸m³ to date^[4]. The existing exploration and research results show that the large-medium gas reservoirs in reef-beach facie in the Sichuan Basin during this period were mainly distributed around the margin of the trough, and controlled by the development position of the trough^{[5⇓⇓⇓⇓-10]}. The determination of the development position and formation period of the trough is one of the key points for oil and gas exploration in this domain.

Based on the field outcrop survey, drilling and geophysical data analysis, we found that the trough already had its rudiment during the late depositional stage of the Maokou Formation of Middle Permian, and its distribution direction and sedimentary characteristics were basically consistent with the "Kaijiang-Liangping" trough during the depositional stage of the Changxing Formation of Upper Permian and Feixianguan Formation of Lower Triassic. In the trough is a set of 10-30 m grayish black-black medium-thin bedded siliceous rock, calcareous siliceous rock, siliceous mudstone and argillaceous shale intercalated with lenticular sandy clastic limestone, which is mainly the product of deep-water low-energy trough (basin) and has stronger hydrocarbon generating capacity. The petrological and paleontological characteristics of this set of deep-water siliceous rocks at the top of the Maokou Formation can be compared with those of the Gufeng Formation in the lower Yangtze Plate. They are deposits in the same period but different facies^[11-12]. In addition, some researchers suggested that this set of deep-water sediments at the top of the Maokou Formation was deposits of "deep platform depression" or "extensional trough"^[13-14]. Other researchers proposed that it was transformed from volcanic tuff. On the south side of the trough, there are thicker medium-thin layered grey-light grey micritic bioclastic limestone and bioclastic micritic limestone, which belong to shallow-water and low-energy products in the platform. The transition zone between the trough and the platform was shallow-water high-energy environment of the platform margin, where thick massive mud-calcsparite biological (clastic) limestone, with a certain reservoir capacity built up. After constructive diagenetic processes, such as dolomitization and dissolution, the limestone can form high-quality reservoirs can be formed. In this work, based on field profiles, drilling core and geophysical data, combined with related analysis and experiments, we examined the sedimentary characteristics inside the trough and the south side of the trough during the late depositional stage of Middle Permian Maokou Formation in detail, and delved into the formation mechanism and source-reservoir configuration, in the hope to determine the characteristics of the trough and its relationship with oil and gas, and open new domains for the exploration and development of the Middle Permian in the Sichuan Basin.

The study area is located in the north of the Sichuan Basin, reaching Longmen Mountain in the west and Micang Mountain in the north, covering Guangyuan, Wangcang, Jiange and other cities and counties (Fig. 1a). Structurally, it is in the northwest margin of the Yangtze Plate (Fig. 1b). During the Middle and Late Permian, the Yangtze Plate and North China Plate had not completely joined together yet, and the tectonic activity was strong^[15]. The formation of Mianlue tectonic belt and the activity of Emei mantle plume on the north side of the Sichuan Basin had a significant impact on the tectonic-sedimentary pattern of the Sichuan Basin. During the early depositional stage of the Middle Permian Maokou Formation, the water body in the northern Sichuan Basin was deeper due to transgression, the rocks deposited were mainly "eyeball-like" limestone with high argillaceous content and rich organisms. During the middle-late depositional stage of the Maokou Formation, along with the drop of sea level, a set of micrite-calcsparite biogenic limestone accumulated in the northern Sichuan Basin. To the late depositional stage of the Maokou Formation, the continental margin of the southern Mianlue Ocean on the northwest edge of the Yangtze Plate subducted to the northern Qinling Microplate^[16-17], the activity of Emei mantle plume intensified^[17-18], the paleo-heat flow in the northern Sichuan Basin increased significantly^[19], large-scale upwelling and eruption of magma occurred in Southwest Sichuan, and the NW-trending basement faults revived, resulting in development of a series of NW-SE trending intra-platform rifts^[13]. Under the same geological background, several similar structural sags developed in Yunnan-Guizhou area^[20]. During the late Middle Permian, the Dongwu Movement uplifted the Sichuan Basin as a whole, the top of the Maokou Formation was exposed to surface and eroded, consequently, the fourth member of the Maokou Formation (shortened as "Mao 4 Member") is absent in some areas. At this time, the topography of the northern Sichuan Basin was basically peneplain. During the early stage of Late Permian, transgression began slowly. During the early depositional stage of the Wujiaping Formation, the northern Sichuan area entered the shore-swamp environment, and the Wangpo shale deposited, which was mainly composed of a set of coal measures. As a result, the Maokou Formation and the Wangpo shale in the Wujiaping Formation are in pseudo conformity contact.

"Trough" refers to a strip-shaped basin in the marine environment, which is deeper than the surrounding water^[9,21]. In the concept of environment, it refers to a deep-water sedimentary area in near linear distribution with water depth under storm wave bottom which includes steeper slopes and flat basin^[7]. During the field outcrop survey of the Middle Permian Maokou Formation in the northern Sichuan Basin, we found that there is a set of typical deep-water sediments of trough facies about 10-30 m at the top of the Maokou Formation at the profiles of Guangyuan Chejiaba, Wangcang Shuanghe and Nanjiang Qiaoting etc. (Fig. 2a). It is mainly composed of medium-thin stratified gray black-black siliceous rocks with lenticular and medium-thin stratified gravity flow limestone (Fig. 2b, 2c). With horizontal beddings, sliding deformation structures at the bottom, weathering crust at the top, this set of sediments is in parallel unconformable contact with coal measures at the bottom of upper Wujiaping Formation (Fig. 2). This set of deep-water sediment has been drilled in Wells K2, ST2, WJ1 and L17 etc. in the northern Sichuan Basin. According to the field outcrops and current drilling results, the trough was mainly distributed in Guangyuan-Wangcang belt, and extended and converged toward the southeast direction (Fig. 3). It extended to the areas of Jiange, Yuanba and Longgang in the southwest, and extended to Nanjiang Qiaoting and Tongjiang Nuoshuihe area in the northeast. It was in northwest to southeast strike on the whole. But there is no drilling and seismic data to confirm where it extended to the southeast. Seismic sections show that there was a steep slope between the south side of the trough and the platform margin, which might be controlled by sedimentation and faults. It is not clear whether there are margin-controlling faults in the east and north sides of the trough. The distribution direction of the trough was basically consistent with that of the “Kaijiang-Liangping” trough developed during the Late Permian-Early Triassic. The siliceous rocks and ancient organisms in the trough are also highly similar to those in the “Kaijiang-Liangping” trough^[22], indicating that the “Kaijiang-Liangping” trough began to develop during the late depositional stage of the Maokou Formation in the northern Sichuan Basin, but was smaller in scale probably.

Before discovery of the Dalong Formation in inner “Kaijiang-Liangping Trough”, the Upper Permian “siliceous trough” in Guangyuan-Wangcang area was called “Guangyuan-Wangcang Trough” while “carbonate trough” in Kaijiang-Liangping area was known as “Kaijiang-Liangping Trough” on the basis of previous studies^[1,7]. Silicalite of the Dalong Formation deposited in the “Kaijiang-Liangping Trough” was found after unremitting exploration, indicating that stratigraphic features of the “Kaijiang-Liangping Trough” are consistent with the “Guangyuan-Wangcang Trough” basically^[7], and confirming that northwest end of “Kaijiang-Liangping Trough” extended to Shangsi surroundings in Guangyuan^[8]. The trough from Guangyuan-Wangcang to Kaijiang-Liangping areas was called “Kaijiang-Liangping Trough” collectively since then. Based on the preferential naming principle in geology, the term “Guangyuan-Wangcang Trough” is followed to refer to the trough developed in Guangyuan-Wangcang area in the late sedimentary period of the Maokou Formation.

The sediment in the trough during the late depositional stage of the Maokou Formation in the northern Sichuan Basin was similar to the upper part of upper Permian Dalong Formation, with lower proportion of pure siliceous rock, and dominated by dark medium-thin stratified siliceous mudstone, siliceous limestone, calcareous silicalite interbedded with light lenticular and medium-thin gravity flow deposits (Fig. 4). The siliceous rocks are black-gray, thin-layered on the whole (Fig. 4a), and tight, contain more siliceous radiolarians, sponge spicules and other deep-water plankton (Fig. 4b). A large number of complete brachiopod fossils with small body and thin shell are distributed on the surface (Fig. 4c), and there are fewer complete ammonoids (Fig. 4d). The lenticular gravity flow deposits are composed of micritic bioclastic limestone and psammitic-psephitic limestone, with bottom in abrupt erosion contact with the underlying strata, and obvious graded beddings with positive grain size upwards (Fig. 4e). The medium-thin stratified gravity flow limestone is mainly composed of biogenic (clastic) limestone containing dust and arene, with graded bedding of positive grain size. It is distal gravity flow deposit, intercalated in large sets of dark siliceous rocks (Fig. 4f). The combination characteristics show that the trough had deep water and low energy, and was intermittently affected by gravity flow.

According to previous studies^[11], the Middle Permian Gufeng Formation between the Qixia Formation and the Wujiaping Formation in the Lower Yangtze Plate is mainly composed of deep gray-gray black thin-layer siliceous rocks with horizontal beddings, and contains ammonites, siliceous radiolarians, sponge spicules, small crustacean brachiopods and other fossils, indicating it was formed in deep-water anoxic hunger basin^[11]. This set of deep-water deposit formed during the late Maokou Formation in the northern Sichuan Basin can be compared with the Gufeng Formation in southern Jiangsu and Anhui, northern Zhejiang and Jiangxi provinces. In fact, the Maokou Formation top and the Gufeng Formation were formed in the same period but different in facies^[12], both of which were formed in deep-water anoxic environment. In addition, the distribution range of the deep-water deposit at the top of the Maokou Formation in the northern Sichuan Basin is basically consistent with that of the overlying Upper Permian Dalong Formation.

From the drilling data of Well ST1, YB7 and ST12 in the south of the trough, it can be seen that the Jiange-Yuanba-Longgang belt in the south of the trough has obvious sedimentary characteristics of platform margin^[13]. The rocks are largely gray thick massive micrite-calcsparite biogenic (clastic) limestone with low argillaceous content. The rocks contain abundant fossils with poor individual integrity, including spines, foraminifera and mesoclasts etc. The rocks contain some intragranular dissolved pores, intergranular dissolved pores and biocoelomic pores, filled with a small amount of fine-grained dolomite and asphalt, with a surface porosity of generally 1%-3% (Fig. 5a). After dolomitization in some zones, the limestone has turned into fine-medium grained dolomite, the dolomite crystals are subhedral-xenomorphic and in mosaic contact, with straight or curved boundaries in between. The dolomite with a small number of unfilled intercrystal pores (Fig. 5b), has some storage capacity.

With no outcrop profile, drilling and seismic data on the north and southeast sides of the trough, it is difficult to define the boundaries of trough in the north and southeast. However, it has been confirmed that the deep-water sediments still exist at the top of the Maokou Formation in Nanjiang Qiaoting-Tongjiang Nuoshuihe region on the northernmost side of the trough. It is speculated that the north side of the trough lies to the north of this region.

In this study, samples were taken from the top of Maokou Formation in Guangyuan Chejiaba profile to conduct geochemical analysis. The sampling location is shown in Fig. 2c, and the sampling interval is about 1 m. Among the samples, are 4 bioclastic limestone samples from the third member of Maokou Formation (Mao 3 Member for short), and 12 siliceous rock samples and 3 gravity flow samples from the Mao 4 Member. The test results of major and trace rare earth elements are shown in Table 1. The Mao 3 Member and the Mao 4 Member are quite different in main lithological formation conditions, and their geochemical characteristics have obvious regularities (Fig. 6).

Referring to the average contents of Sc, Zr, Hf and Th in the upper crust (14.90, 240.00, 5.80 and 2.30 μg/g, respectively)^[23], and according to the stability characteristic (not easily affected by diagenetic alteration) of Th element^[24], we can judge whether the samples are affected by late diagenetic alteration. The data show that the samples CJBd7 and CJBd17 have higher contents of Sc, Zr, Hf, and Th content higher than the average value of the upper crust, while the other samples have contents of Sc, Zr, Hf and Th much lower than the average values of the upper crust, specifically, the content of Sc is less than 2.80 μg/g, Zr is less than 32.00 μg/g, Hf is less than 90 μg/g, and Th is less than 1.76 μg/g. The data shows that the relevant elements of the tested samples are not affected by diagenetic alteration, and the measured values can reflect the real sedimentary environment of the corresponding rocks.

The dispersion and accumulation of elements have a certain correlation with water depth, and are affected by the pH value and Eh value of water body^[25]. The experimental results in this study show that the bioclastic limestone of Mao 3 Member and siliceous rock of Mao 4 Member differ in the contents of Cu, Co, Mo and Ni obviously: the bioclastic limestone is low in these contents, while the siliceous rock is high in these contents (Fig. 7), reflecting that the two kinds of rocks are formed in different water depths. Specifically, the bioclastic limestone of Mao 3 Member was formed in shallower water environment, while the siliceous rock of Mao 4 Member was formed in deeper water environment. In addition, the element contents of gravity flow samples in this test show poor clustering, and no obvious characteristics of sedimentary environment, which may be the result of sediment transportation from shallow-water environment to deep-water environment. The change of sedimentary environment causes element migration, which eventually leads to multiple attributes in geochemical response characteristics.

Some researchers have found that the values of δCe and m_Ni/m_Co in rocks can be used as identification marks of redox conditions in paleoenvironment^{[26⇓⇓⇓⇓-31]}. Li Hongjing et al.^[30] studied the Permian Maokou Formation in Guangyuan area, and considered that according to the calculation formula of Ce anomaly value given by Feng Hongzhen et al.^[31], the sea level of this area during the depositional stage of the Maokou Formation could be better worked out, then the redox environment could be analyzed, and the m_Ni/m_Co value was the best index to judge the paleo-redox environment in this area. According to Bryn Jones' study, the relationship between sediments and redox conditions can be judged according to (m_Cu+m_Mo)/m_Zn value and Uau value^[29]. Based on the cross plot analysis of the test values of bioclastic limestone and siliceous rock samples, the geochemical test results show that the bioclastic limestone samples of the Mao 3 Member were deposited in oxygen-poor environment and relatively shallow water, while the siliceous rock samples of the Mao 4 Member were formed in deep water reducing environment under anoxic condition (Fig. 8).

Based on the genetic triangle chart of Al-Fe-Mn of siliceous rocks proposed by Adachi^[32], most of the samples from the Guangyuan Chejiaba profile fall in the non-hydrothermal origin zone (Fig. 9). According to Bostrom's study on the genesis of siliceous rocks, the value of m_Al/(m_Al+m_Fe+m_Mn) decreases with the increase of hydrothermal participation degree^[33]. Adachi concluded that the m_Al/(m_Al+m_Fe+m_Mn) value of pure hydrothermal origin siliceous rocks is as low as 0.01, and increases with the distance from the hydrothermal center of the ocean ridge, while the m_Al/(m_Al+m_Fe+m_Mn) value of pure biogenic siliceous rocks is about 0.6^[32]. The siliceous samples in this study havem_Al/(m_Al+m_Fe+m_Mn) values between 0.18-0.69 (Table 1), with an average of 0.48, much larger than 0.01. This shows that although the siliceous samples are affected by certain hydrothermal fluid, they are generally more of biological origin, which is consistent with the result that the samples contain a large number of siliceous radiolarians observed under microscope (Fig. 4b). The formation environment of the siliceous samples was judged by using the sedimentary environment identification chart of siliceous rocks proposed by Murray^[34]. After projected on the chart, the test data of the samples are all located in the continental margin zone (Fig. 10), indicating that the tectonic setting of these siliceous rocks is continental margin.

After studying the formation of siliceous rocks, Adachi suggested that the formation of siliceous radiolarians was mainly due to the absorption of terrestrial materials as the main source of marine soil by radiolarians^[32]. Yang Yuqing and Feng Zengzhao studied the formation of Middle Permian stratified siliceous rocks in South China and came to the conclusion that the Sichuan Basin was located near the equator during Middle Permian, when the Paleo-Tethys Ocean was not closed, and the northwest margin of the Yangtze Plate was connected with the Paleo-Tethys Ocean; the silicon-rich nutrients carried by the upwelling currents promoted large-scale reproduction of siliceous organisms^[35]. No matter whether the siliceous radiolarians were formed due to effect of upwelling currents or by absorbing terrigenous materials, it is certain that the siliceous rocks during the late depositional stage of the Maokou Formation were formed on the continental margin, and their formation environment was similar to that of the Dalong Formation, that is, deep-water shelf or trough environment^[2,10].

During the depositional stage of the Maokou Formation, the Sichuan Basin had a low-gentle slope from northwest to southeast and from southwest to northeast, and was platform environment with relatively deep water body^[36]. During the early depositional stage of the Maokou Formation, the whole study area was in deep-water open platform environment intermittently transformed by storm waves, and mainly "eyeball-like" micrite limestone was deposited. During the middle-late depositional stage of the Maokou Formation, due to the drop of sea level, the Changjianggou-Chejiaba belt had a rapid facies change and characteristics of platform margin, but the overall development scale of platform margin beach was small. During the late depositional stage of the Maokou Formation, there was obvious sedimentary difference in the northern part of the basin (Fig. 11). Field outcrops and drilling data show that "trough-platform" differentiation began to appear in Guangyuan-Wangcang belt during this period. The Jiange-Yuanba-Longgang area south of the belt was mainly shallow-water platform environment, where thin-medium stratified micritic limestone and bioclastic limestone with chert nodules deposited, with dolomitization in local parts. Guangyuan-Wangcang area was deep-water trough facies, mainly a set of gray-black sheet siliceous rock intercalated with light-gray lenticular micrite limestone and gravity flow deposits. From the perspective of vertical sedimentary evolution, the Mao 3 Member is shallow-water carbonate deposit, and Mao 4 Member is deep-water siliceous rock deposit. They are in abrupt contact, with "jumping facies" (discontinuous facies sequence) in sedimentary facies. Generally, "jumping facies" has two main reasons: one is the existence of unconformity interface between the two, and weathering and erosion lead to the discontinuity of strata and sedimentary facies; the other is synsedimentary faulting, the downthrown side of a fault could quickly change from shallow-water area to deep-water area, and from shallow-water deposit to deep-water deposit. As the Mao 3 Member and Mao 4 Member in the study area are in conformable contact, with no weathering and denudation discontinuity, it is speculated that the "jumping facies" between the Mao 3 Member and Mao 4 Member may be related to synsedimentary faults, that is, the tensile faults formed by the subduction of the continental margin on the south side of Mianlue Ocean in the northwest margin of Yangtze Plate to the northern Qinling Micro-plate.

The stratum thickness also clearly shows the sedimentary differences between the trough area and the platform area during the late depositional stage of the Maokou Formation. During the middle-late depositional stage of the Maokou Formation, the study area was generally an open platform environment with deep water, and the stratum depositing was stable in thickness (about 130-200 m). To the late depositional stage of the Maokou Formation, the Maokou Formation was thinner in the belt along Changjianggou-Chejiaba-L17 well blocks, while it was thicker in the ST1 well block. It is speculated that the reason may be that the belt along Changjianggou-Chejiaba-L17 well block during the late depositional stage of the Maokou Formation was in the state of under-compensation. As the water body in this belt deepened sharply and the deposition rate was lower than the subsidence rate of the sedimentary basement. In contrast, the ST1 well field south of the belt had normal water deposition, with deposition rate higher than that in deep-water sedimentary environment, so there is obvious difference in thickness of this formation.

On seismic section, the top and bottom boundaries from the Maokou Formation to the Changxing Formation are clear. The top boundary of the Changxing Formation corresponds to a wave crest reflection in the platform, and a wave trough reflection with strong amplitude in the trough. The top boundary of the Wujiaping Formation is a medium-strong wave crest reflection, and the top boundary of the Maokou Formation corresponds to a strong wave crest reflection which can be traced horizontally (Fig. 12a). From the seismic section through Well YB7 and Well L17, it can be seen that the top layer of the Maokou Formation thins obviously from southwest to northeast, and shows clear seismic reflection characteristics of platform margin-slope-trough (Fig. 12b). In L17 well block, the Maokou Formation is thinner and characterized by parallel strong reflections, representing trough facies. From Well L17 to Well YB7, the Maokou Formation becomes thicker gradually. In the vicinity of Well YB7, the Maokou Formation shows slope facies characteristics with low-frequency and inclined strong reflections; and the slope gradient is larger, indicating that there may be sedimentary fault. To the location of Well YB7, the Maokou Formation is thickest, and shows uplift or mound shape, and some onlapping characteristic; and the top of Maokou Formation shows blank or intermittent and disordered reflections, without sedimentary layered structure, which are typical reflection characteristics of platform margin beach facies. In addition, through vertical comparison of the platform margin position between the Maokou Formation and the Changxing Formation on seismic section, it is found that compared with the Maokou Formation, the platform margin of the Changxing Formation has the characteristic of migration from trough to platform margin, indicating that the trough expanded during the sedimentary period of the Changxing Formation in the Middle-Late Permian in the northern Sichuan Basin.

At present, there are still different opinions on the genetic mechanism of the trough^[13-14]. Combined with the regional tectonic setting, through field outcrop observation and analysis of previous research results, we think that the main reason for the formation of the “Guangyuan-Wangcang” trough during the late depositional stage of the Maokou Formation in the northern Sichuan Basin was related to the tectonic extension background of the Sichuan Basin during the late Middle Permian. The northern part of the Sichuan Basin was located in the northwest margin of the Upper Yangtze Plate and the southern continental margin of the Mianlue Ocean. During the late Middle Permian, the Mianlue paleo-ocean basin changed from expansion to closure and subduction^[16-17]. During this process, the southern continental margin of the Mianlue Ocean continued to subduct northward (Fig. 13), providing internal driving force for the formation of the rift trough. At the same time, influenced by the early tectonic action of the Dongwu movement, the NW and NE trending basement faults revived^[13]. Under the action of tensile stress, the "Guangyuan-Wangcang" trough was pulled apart along the direction of the relatively weak basement faults, and finally forming the "Guangyuan-Wangcang" trough trending NW-SE. Moreover, according to the previous studies on Emeishan basalt and diabase intrusions found in the field outcrops in the northern Sichuan Basin, the eruption of the Emeishan basalt probably accelerated the formation of “Guangyuan-Wangcang” trough to a certain extent.

In terms of the formation power or position of the "Guangyuan-Wangcang" trough, it is similar to the "Kaijiang-Liangping" trough, and its formation time was earlier than the "Kaijiang-Liangping" trough, indicating that the "Kaijiang-Liangping" trough already reached a certain scale in the northern Sichuan Basin during the late depositional stage of the Middle Permian Maokou Formation. It can be concluded that the "Guangyuan-Wangcang" trough in the northern Sichuan Basin was the rudiment of the "Kaijiang-Liangping" trough.

Previous studies have confirmed that the best condition for the formation of organic-rich sedimentary layers is anoxic bottom water^[37]. In this work, 14 groups of siliceous samples from typical Guangyuan Chejiaba profile were tested for TOC and R_o. The test results (Table 2) show that the siliceous rocks formed during the depositional stage of Middle Permian Maokou Formation inside the trough in the northern Sichuan Basin have good hydrocarbon generation ability. The samples have TOC values from 3.21% to 8.19%, 5.53% on average. The siliceous rock layer is 10-30 m thick, rich in organic matter, and strong in hydrocarbon generation capacity, suggesting that it is high-quality source rock layer. The samples have measured R_ovalues between 1.009%-1.652% (1.219% on average), indicating that the organic matter in this layer is in the stage of thermal cracking and wet gas generation, and high maturity. The main products of the organic matter in this stage are methane and its gaseous homologues.

During the late depositional stage of the Maokou Formation, reservoirs of platform margin facies and intra-platform point beach facies widely developed along the Jiange-Yuanba-Longgang belt around the trough^[13]. The reservoirs of platform margin beach facies largely developed in the south of the trough, and are dominated by dolomitic limestone and bioclastic limestone, with thicknesses between 15-35 m. This area had high water energy and was frequently exposed above sea level, thus it was subjected to dissolution and dolomitization due to the atmospheric fresh water during syngenetic-quasi syngenetic period. This type of reservoirs has intercrystalline and intergranular dissolved pores, and a certain amount of dissolved fractures and cavities, with a porosity generally between 1%-2%. After later dolomitization and dissolution, this type of reservoir can become high in quality. For example, in Well LG70 and Well YB7, the reservoirs are largely dolomite, dolomitic bioclastic limestone and calcsparite bioclastic limestone, with intercrystalline dissolved pores and biological visceral dissolved pores, and measured porosity of up to 8.37%. Different from platform margin reservoirs, the reservoirs of intra-platform point beach facies were formed in the micro-geomorphic highlands in the platform with lower water energy than platform margin, and sporadically distributed. In places without fresh water dissolution, the reservoir is relatively dense, with fewer pores. While in places suffered fresh water dissolution, the reservoir has significant dissolution and dolomitization, with intercrystalline pores and intergranular dissolved pores formed, thus they can be a kind of relatively favorable reservoir. In the interior of the trough, siliceous rocks are well developed as high-quality source rocks. In the overall spatial distribution, the source rocks are located on the flank or top of the reservoirs of platform margin facies and intra-platform point beach facies, which is conducive to the formation of source rock beside reservoir, and source rock above reservoir combinations.

Based on the deep analysis of the tectonic and sedimentary background of the Sichuan Basin, combined with the data of outcrops, drilling, geochemistry and geophysics, it is confirmed that there was a set of deep-water trough deposits developed during the late depositional stage of the Middle Permian Maokou Formation in the northern Sichuan Basin. This set of deep-water trough deposits is mainly distributed in Guangyuan-Wangcang belt, extending and converging to the southeast, and extending to Yuanba and Longgang areas to the southwest, to Nanjiang Qiaoting and Tongjiang Nuoshuihe belt to the northeast. The trough extended from northwest to southeast, belonging to anoxic reduction environment with deep water and low energy. The rocks in the trough are mainly siliceous rocks rich in organic matter and deep-water biological fossils. This layer of siliceous rocks, 10-30 m thick, with an average TOC value of 5.53%, and an average R_o value of 1.219%, is a better source rock section with stronger gas generation capacity.

Reservoirs of platform margin beach facies and intra-platform point beach facies are widespread around the trough. The platform margin on the south side of the trough is distributed in the Jiange-Yuanba-Longgang belt, and is mainly gray thick massive micrite-calcsparite biogenic (clastic) limestone, with a small amount of intragranular dissolved pores, intergranular dissolved pores and biological visceral pores. If reformed by strong dolomitization and dissolution, these reservoir rocks would have better reservoir properties. In spatial distribution, they and the high-quality source rocks inside the trough form good source rock beside reservoir, and source rock above reservoir combinations, providing favorable conditions for oil and gas accumulation.

The “Guangyuan-Wangcang” trough is basically consistent in development position and sedimentary characteristics and similar in formation driving force with the “Kaijiang-Liangping” trough (proposed by former researchers) during the Late Permian. But the “Guangyuan-Wangcang” trough was formed earlier than the “Kaijiang-Liangping” trough, which indicates that the “Kaijiang-Liangping” trough already had some rudiment during the late depositional stage of the Maokou Formation of the Middle Permian, and there is obvious inheritance between them.