As a key target in the Sichuan Basin, the Middle Permian Maokou Formation has been explored for more than 60 years, but most researches and drilling activities on this formation are concentrated in the central and southern Sichuan Basin^[1,2,3,4]. Recently, exploratory wells ST1, L16, and YB7 in the northwestern Sichuan Basin tapped high-production industrial gas flows, proving that the Maokou Formation in northwest Sichuan has huge exploration potentials too^{[5,6,7,8,9,10]}. The sedimentary environment of the Maokou Formation in the northwestern Sichuan Basin has not been studied in depth due to the limited and concentrated drilling data. It is urgent to make clear the distribution of sedimentary facies and differentiation features of the platform from trough, and reconstruct the paleokarst landforms of the Maokou Formation in the northwestern Sichuan Basin after the understanding that the deposition of the deep-water Gufeng Member in the Jiange-Jiufengshan area has been confirmed^[11,12].

It is confirmed by the drilling data that the main type of reservoir rocks in the Maokou Formation of the northwestern Sichuan Basin is limestone with fractures and vugs^{[13,14,15,16]}. The Maokou Formation have many pieces of evidence of multiple episodes of exposure^[17], suggesting that favorable sedimentary facies and eogenetic karstification of the weathering crust are the key factors affecting the formation of high-quality reservoirs^{[18,19,20,21]}. Available drilling, logging, and seismic data are integrated to reconstruct the sequence-lithographic paleogeography and paleokarst geomorphology of the northwestern Sichuan Basin in different periods based on the tectonic background and the high-resolution sequence stratigraphy to find out the responsive features of the Maokou Formation deposition to the episodic Dongwu Movement and their petroleum geological significance. The results of this study are expected to provide effective support for the comprehensive exploration of deep to ultra-deep marine carbonate successions in the western Sichuan Basin.

Located on the northwestern margin of the Upper Yangtze Plate, the study area is in the transitional zone of the low-gentle fault fold belt in northern Sichuan Basin, the Longmenshan fault fold belt, and the Micangshan uplift belt. The collision and collage of the North China Plate and the Yangtze Plate resulted in the extensive thrust-nappe structures in the southern margin of the North China Plate and the large-scale nappe detachment structures in the northern margin of the Yangtze Plate^[22]. The Sichuan Basin witnessed tectonic differentiation in the late Middle Permian (Maokou Fromation sedimentary period) under the influence of the Dongwu tectonic movement, when NWW- and NE-striking intra-craton extensional rift basins developed. The tectonic stresses during the Maokou Period were mainly originated from the extensional rifting process of the passive continental margin of the Mianlue Ocean in the northern margin of the Upper Yangtze Block and the uplift of the Emeishan mantle plume accompanied by basalt eruption^[12]. At the end of the Middle Permian, the main episode of the Dongwu Movement caused differential uplift of structures and large-scale regression, consequently, the Maokou Formation suffered long-term exposure and denudation of up to 1 to 3 Ma^[23]. Rock stratigraphic units that are commonly used in previous studies to classify the internal stratigraphy of the Maokou Formation are vulnerable to the issue of stratum diachronism when their applications are extended to the regional scale. In this paper, the Maokou Formation in the study area is divided into three long-term cycles, denoted as LSC1, LSC2, and LSC3, which correspond to the first, second, and third members of the Maokou Formation, respectively (Fig. 1). This is based on the concept of the isochronous sequence division and will ensure the objective reconstruction of the stratum deposition and filling conditions. Specifically, LSC1 is 90-190 m thick and it is mainly located in the platform facies, with main lithologies of marl, bioclastic micrite limestone, and micrite bioclastic limestone, which often appear as "eyelid-eyeball" structures on the macroscopic view. With a thickness of 20-90 m, LSC2 consists of micrite bioclastic limestone, mud-sparry bioclastic limestone, and a small amount of bioclastic micrite limestone in the platform and platform margin facies, and bioclastic micrite limestone and micrite bioclastic limestone in the slope facies. LSC3, a few meters to more than 100 meters thick, is thicker and mainly composed of micrite bioclastic limestone and micrite-sparry bioclastic limestone in the platform and platform margin facies, while it primarily consists of siliceous mudstone and carbonaceous shale in the slope and shelf facies.

2.1.1. Sequence division and boundaries

Guided by the theory of high-resolution sequence stratigraphy, the Maokou Formation is divided into three long-term base-level cycles by comprehensive analysis of thin sections, logging, imaging, and seismic data, namely LSC1 (the first member of the Maokou Formation), LSC2 (the second member), and LSC3 (the third member) from bottom to top.

2.1.1.1. The sequence boundary between the Qixia Formation and LSC1

This sequence boundary is easy to identify in both the shallow water area represented by the platform or platform margin and deep-water area represented by the shelf. Moreover, it is a typical lithologic and lithofacies transitional surface, below which lie the micrite-sparry bioclast limestones of the Qixia Formation, while above which are the sparry micrite limestones of the Maokou Formation. It is represented by the large-scale positive excursion in the natural gamma-ray logging curve, which indicates the waning depositional energy (Fig. 2a). As seen in the imaging data of Well YB6, this sequence boundary is an obvious unconformity surface (Fig. 2b), below which are dissolution-related karrens and gravels, while above which are layered deposits (Fig. 2c). Therefore, it reflects the change of base level from falling to rising. This boundary is demonstrated to have the peak reflection features in both shallow and deep water areas through the calibration of the seismic data (Fig. 3).

2.1.1.2. The sequence boundary between LSC1 and LSC2

The characteristics of this sequence boundary in shallow and deep water areas are basically the same. Moreover, it is a typical lithologic and lithofacies transitional surface, below which are micrite-sparry bioclastic limestones while above which are bioclastic micrite limestones (Fig. 4a). It is represented by the transition of the natural gamma-ray from falling to rising (Fig. 4a). In the imaging data of Well YB6, this sequence boundary is an exposed surface, with granophyric dissolution characteristics visible below and thin layered deposits above (Fig. 4b). Obvious karst features below this boundary are observed on the Changjianggou Section, where karst breccias and karrens are developed (Fig. 4c). It can be inferred that the base level changed from falling to rising. The boundary is demonstrated to have the peak reflection features in the shallow water area, while it has zero-phase reflection features in the deep-water area (Fig. 3).

2.1.1.3. The sequence boundary between LSC2 and LSC3

This sequence boundary has different characteristics in shallow and deep water areas. In the shallow water area, it separates the sparry bioclastic limestone below from the bioclast micrite limestone above (Fig. 5a). In contrast, in the deep-water area, they are respectively sparry bioclast limestones or bioclastic micrite limestones below the boundary, and bioclastic micrite limestones or dark siliceous mudstone above the boundary (Fig. 5b). It is represented by the transition of natural gamma from falling to slow rising in the shallow water area (Fig. 5a), while large-scale positive excursion of natural gamma in the deep-water area (Fig. 5b). In the imaging data of Well YB224 drilled in the shallow water area, this sequence boundary is a typical exposed surface, with karrens, caves, and breccias below and uniform lithology above (Fig. 5c). In contrast, in the imaging data of Well YB3 drilled in the deep-water area, this sequence boundary is an exposed surface, with many dissolution caves and vugs below and thin layered normal deposits above (Fig. 5d). On the Changjianggou Section, it is a typical exposure surface, with dissolution breccias and karrens below (Fig. 5e). Therefore, it can be inferred that the base level changed from falling to rising. The boundary is demonstrated to have the peak reflection features in both the shallow and deep water areas (Fig. 3).

2.1.1.4. The sequence boundary between LSC3 and Wujiaping Formation

This sequence boundary has different characteristics in shallow and deep water areas. In the shallow water area, it separates the below micrite sparry limestones from the above Wujiaping Formation mudstones (Fig. 6a). In contrast, in the deep-water area, dark siliceous mudstone is below the boundary and Wujiaping Formation bauxitic mudstones and mudstones are above the boundary (Fig. 6b). Multiple logging curves are needed to identify this boundary according to the actual situation. In shallow water area, the natural gamma value is relatively low below this boundary, but turns high after sharp positive excursion above this boundary in general; but due to long time exposure and weathering at the end of Maokou Formation deposition, a large amount of mud may fill between the weathered residual breccias at the top, which leads to relatively high natural gamma and acoustic time difference at the top of LSC3 (Fig. 6a). In contrast, it is difficult to recognize this boundary in the deep-water area since the dark siliceous mudstone below and the bauxitic mudstone above it both show high gamma values. In this context, the natural gamma-ray spectroscopy log must be combined with the natural gamma log to discern this boundary. The stratum below it is characterized by high-U, low-Th, and low-K that indicate reduction environment, while the stratum above it is featured by low-U, high-Th, and high-K that suggest oxidation environment (Fig. 6b). As shown in the imaging data of Well YB224, the boundary appears as a typical exposed surface in the shallow water area, high-angle karst karrens and caves can be seen below it, while normal thin-layer deposits are above it (Fig. 6c). In comparison, this boundary is hard to recognize from imaging logging in the deep-water area represented by Well Yuanba3 as the formations below and above the boundary are both thin-layer mudstones (Fig. 6d). From various data, this boundary is also the position where the base level changed from falling to rising. Through seismic data calibration, this boundary is demonstrated to have the peak reflection features in the shallow water area, while it has zero-phase reflection features in the deep-water area (Fig. 3).

2.1.2. Division and comparison of the sequences through wells

As shown on the Shuangyushi-Kuangshangliang well-tie section, the three long-term cyclic sequences have strong lateral continuity and comparability (Fig. 7). LSC1 is characterized by overall high GR values and large thickness, and it is thicker in the Shuangyushi Area than in the Changjianggou and Kuangshanliang areas. LSC2 is relatively thin and lower in GR values, and thins from the Shuangyushi area towards the northeast. LSC3 has a big variation in thickness, it is thicker and lower in GR in the Shuangyushi area, and turns thinner and higher in GR quickly in the northeast.

2.1.3. Mapping of key horizons based on the seismic time difference

Except for LSC3 bottom, all the other sequence boundaries (LSC1, LSC2 and the bottom of Wujiaping Formation) are continuous and traceable (Fig. 8). Twenty-one high-quality seismic survey lines (including two- dimensional and three-dimensional seismic data and merged 2D and 3D) were taken to trace and compare these boundaries (Fig. 1). Accordingly, it is feasible to calculate the time-difference thickness of LSC1 and LSC2+LSC3 bounded by sequence boundaries (Fig. 9). In general, LSC1 is thickest in the Cangxi-Langzhong area, while LSC2 and LSC3 are thickest in the Zitong-Majiaoba- Jiange area and become thinner towards the Jiange Area in the northeast.

2.1.4. Planar distribution of the sequences

The locations of the outcrops and wells in the hanging wall of the Longmenshan nappe were roughly reconstructed by translation by a distance of nearly 20 km to the northwest according to the internal results of the Exploration and Development Research Institute of Southwest Oil and Gas Field Company to reflect the thickness distributions of the sequences during their deposition to the best extent. Although the time-difference thicknesses of LSC2 and LSC3 weren’t described separately, thicknesses of LSC2 and LSC3 are found to be positively correlated with the total thickness of LSC2+ LSC3 from statistics. The planar thickness variations can be roughly represented by the planar variation of the total thickness of LSC2+LSC3.

During the early depositional period of the Maokou Formation, the Sichuan Basin experienced extensive transgression^[3]. According to statistics on drilling data of more than 40 wells and data of field outcrops, it is found that there is a positive correlation between the thickness of LSC1 and that of underlying Qi2 member in the northwestern Sichuan Basin, indicating they had similar sedimentary landforms. Therefore, the planar thickness distribution of LSC1 was mapped (Fig. 10a) based on the time-difference thickness of LSCI and previous study results of the Qixia Formation^[24,25]. In general, a thicker band of LSC1 in NE-SW appears in the south of the Qingchuan and Shuangyushi areas, and LSC1 is also thick in the northeast and southeast parts of the study area due to the controls of the Hannan paleo-uplift and the northern Sichuan paleo-uplift, and thickest in the Cangxi-Langzhong area (Fig. 10a).

The thickness of LSC2 is differentiated. Specifically, it is thinner than 10 m in the north of Qingchuan, Guangyuan, and Zhengyuan areas. It is also thin in the south of the Guangyuan-Wangcang area and the north of the Guangyuan-Majiaoba area, with a thickness range of 10-20 m. In contrast, it is thicker in the Shuangyushi-Jiange area, the south of Zitong area, and the Longgangxi-Yuanba area, at over 80 m on average (Fig. 10b).

The LSC3 further differentiates in thickness. It is generally thinner than 20 m in the north of the Majiaoba area and the Qingchuan, Guangyuan, and Zhengyuan areas, while it is thicker in the Shuangyushi-Jiange area, the north of Jiangyou area, the south of Zitong area, and the Longgangxi-Yuanba area, at an average of over 80 m and a maximum of 100 m (Fig. 10c).

2.2.1. Main types and characteristics of sedimentary facies

Several types of sedimentary facies have been identified in the Maokou Formation based on the outcrops, thin sections, and drilling data, including open platform, platform margin, slope, and shelf.

The open platform had relatively open sedimentary water body, and included intra-platform high-energy shoal, intra-platform low-energy shoal, inter-shoal sea, and open sea subfacies. The intra-platform high-energy shoal formed in the environment above the normal wave base has many broken biological debris and particles cemented by sparry calcite (Fig. 11a). The intra-platform high-energy shoal formed in the environment near and below the wave base with relatively low energy is mainly micrite bioclastic limestone (Fig. 11b). The inter-shoal sea and open sea were both low energy environments below the wave base, and thus the deposits formed in them have generally low contents of bioclastics, and are dominated by bioclastic micrite limestone and micrite limestone respectively (Figs. 11c, d).

The platform margin refers to the transition zone between the open platform and the slope, which had stronger wave disturbance to the wide sea side. The deposits in the platform margin are mainly high energy shoal subfacies deposits with reef-building organism debris (Fig. 11e) and crystalline dolomite (Fig. 11f). Regarding the genesis of the crystalline dolomite, previous researches showed its diagenetic fluids were seawater, hydrothermal fluid, or mixed water^[26,27,28]. However, regardless of the nature of the diagenetic fluid, it is believed that the early formed porous layers during the deposition of the shoal facies were the main channels for the dolomitization fluid^[29,30], and the original rock of the dolomite was mainly sparry bioclastic limestone with some pore space.

The slope is located in the transition zone from the platform margin to the deep-water shelf with deeper water body. The deposits in the slope facies are mainly micrite bioclastic limestone and bioclastic micrite limestone (Fig. 11g, h), and they are often adjacent to the shelf deposits.

The shelf is located in the deep-water area on the side of the slope towards the open sea with extremely low water energy. Correspondingly the deposits are mainly interbedded siliceous shale and carbonaceous shale, with a large number of organisms reflecting deep-water genesis, such as radiolarians (Fig. 11i).

2.2.2. Vertical and lateral characteristics of sedimentary facies in different facies belts

Vertical and lateral characteristics of sedimentary facies in different facies belts have been analyzed based on the actual sedimentary background and the identified sedimentary facies. There are mainly four sedimentary facies belts, namely, open platform, platform margin, slope, and shelf. The platform/platform margin belts have different sedimentary features from the slope/shelf belts. Well ST1 representing the platform/platform margin facies and the Changjianggou Section representing the slope/shelf facies (Fig. 12) are taken as examples to illustrate. During the deposition of LSC1, there was no major differentiation of the sedimentary environment, and the main deposits were intra-platform high-energy shoal, intra-platform low-energy shoal, inter-shoal sea, and open sea subfacies. During the depositional period of LSC2, the Well ST1 area was open platform and platform margin, while the Changjianggou Section was low-energy slope, and the deposits had an obvious thinning trend. During the depositional period of LSC3, the Well ST1 area was still open platform and platform margin, whereas the Changjianggou Section area transited from slope to deep-water shelf with siliceous mudstone deposited. The vertical and lateral changes of sedimentary facies generally reflect the continuous differentiation of the sedimentary environments in different regions of the study area.

Due to the uplift of the Dongwu Movement, the Maokou Formation in the Sichuan Basin experienced various degrees of denudation at the end of its deposition, and the LSC3 now loses some thickness than when it deposited. The northwestern Sichuan Basin was characterized by erosion-dominated karstification in the low areas at the end of the Maokou Formation deposition^[18]. It is found through correlation of the sequences through wells (Fig. 3) that the areas with thick residual LSC3 now are generally low in GR values and high in shoal facies proportions. In contrast, the areas with thin residual LSC3 now are generally high in GR value and low in shoal facies proportions. The residual thickness and lithofacies proportion of LSC3 now can still roughly reflect its characteristics during the depositional period. The exposed surfaces within the Maokou Formation are smaller in exposure degree and scope than the exposed surface at the end of the Maokou Period^{[3, 17]}, and have little effect on the trend of the overall formation thickness. Therefore, the residual thickness of this sequence was used as the main single factor^[30] to reconstruct the lithofacies paleogeography of LSC1, LSC2, and LSC3 (Fig. 13).

During the deposition of LSC1, the northwest Sichuan area basically inherited the sedimentary landform characteristics in the late depositional period of Qixia Formation. But as the transgression in this period was large, the shoal facies reduced in proportion and area than in the late depositional period of Qixia Formation. Open-sea shelf deposits developed west and north of the Qingchuan area, NE-SW-striking stripe-shaped shoal deposits developed in the south of the Qingchuan and Shuangyushi areas, large-scale shoal deposits turned up in the northeast and southeast of the study area due to the control of the Hannan and Chuangbei paleo-uplifts, while inter-shoal and open-sea low-energy deposits developed in the other areas (Fig. 13a).

During the deposition of LSC2, the extensional structure and basement faults formed by the continuous extension of the Mianlue Oceanic Basin on the northern margin of the Upper Yangtze Platform^[31,32] gave rise to NEE- and NW-striking tectonic stresses^[12] and the sedimentary basement subsided episodically from north to south. In this period, the northernmost margin of the study area had submerged under the sea, receiving open-sea shelf deposits; the south of Qingchuan and Wangcang areas also had various degrees of subsidence, forming small-scale outer platform margin belt; the North Majiaoba-Guangyuan-South Wangcang area subsided into intra-platform depression, hence the inner platform margin belt turned up in the Jiangyou-Shuangyushi-Jiange area and Longgangxi-Yuanba-Longgang area; a large intra-platform shoal developed to the south of the Zitong area; and the other areas were dominated by inter-shoal seas, open seas, and a small number of low-energy intra-platform shoals (Fig. 13b).

During the depositional period of LSC3, as tectonic activities further strengthened, the northern part of the study area all sunk into an open-sea shelf, and the outer platform margin disappeared. The platform margin belts during this period were basically consistent with that during LSC2, including the NE-striking Jiangyou- Shuangyushi-Jiange platform margin belt and NW-striking Longgangxi-Yuanba platform margin belt; intra-platform high-energy shoal deposits developed in the Zitong area and to the south of the Langzhong area; the other areas were dominated by inter-shoal sea and open-sea deposits; affected by tectonic component stresses, the southwestern Cangxi area and the Langzhong area had platform depressions with deep water in local parts (Fig. 13c).

The whole process from the deposition to the final exposure of the Maokou Formation must be accompanied by continuous differential tectonic subsidence, and the sedimentary characteristics of each stage are responses to the extension of the Mianlue ancient ocean basin and the uplift of the Emei mantle plume^[12].

Paleokarst geomorphology has an important effect on the development of karst reservoirs^[33]. Previous studies on the paleokarst geomorphology of the Maokou Formation primarily focused on the top of the Maokou Formation, and generally used the entire Maokou Formation thickness to reconstruct the paleokarst geomorphology on the top of the Maokou Formation. In this study, the sequence distribution characteristics of the Maokou Formation in each depositional period have been made clear, and the corresponding lithofacies paleogeographic evolution has been figured out. Moreover, it has been evidenced by the field outcrops and imaging data that the top of LSC1, LSC2, and LSC3 all had various degrees of exposure. All these results make it possible to analyze the ancient karst landform in more detail.

Taking the top boundary of LSC1 on Changjianggou Section and in Well YB6 as an example, it is found through this study that there are obvious karstification characteristics below this sequence boundary, including breccias and karrens (Fig. 4), but the overall depth affected by the karstification is small. Since the Changjianggou Section and Well YB6 were located in the intra-platform low-energy shoal facies during the deposition of LSC1, it is speculated that there might be a small-scale sea level fall at the end of LSC1. Consequently, only a short pene-contemporaneous exposure happened to the shoal bodies in the high part of the topography, whereas the low-lying inter-shoal and open sea areas didn’t expose, and thus had no conditions to form large-scale karst landform.

Exposure characteristics have been found on the top boundary of LSC2 in the Changjianggou Section and multiple wells. On the Changjianggou Section, there are breccias and karrens below this boundary; there is a 20 m thick karst belt with breccias, karrens, and caves below the boundary in the Well YB3 (Fig. 5); there is a 10 m thick karst belt with high-angle karrens and plastic breccias below this boundary in Well YB224. According to the facies belt positions and the strength and characteristics of karst development of the Changjianggou Section, Well YB3, and Well YB224 during the LSC2 deposition period, it is inferred that the regression that occurred after the deposition of LSC2 was larger in magnitude and range than that happened at the end of LSC1, the sea level dropped to at least below the platform margin slope, and the entire platform, platform margin, and slope were exposed and modified by karstification, providing conditions for the development of paleokarst geomorphology. Moreover, as the areas with large LSC2 thicknesses generally had high depositional energy, while the areas with small LSC2 thickness had low depositional energy during its sedimentation (Figs. 10b and 13b), it is safe to say that there is a positive correlation between the residual thickness and the topography of the paleokarst geomorphology. The residual thickness of LSC2 was used to reconstruct the paleokarst landform at the end of LSC2 deposition (Fig. 14a). The results show the residual thickness of LSC2 is relatively large in the karst highland (more than 55 m), which is mainly distributed in the Jiangyou-Shuangyushi-Jiangge-Cangxi area and area south of this line, and secondarily in the Changjianggou and northern Wangcang areas in isolation and smallscale. The residual thickness of LSC2 ranges between 25 and 55 m in the karst slopes, and it is less than 25 m in the karst depressions.

The seawater withdrew from the Sichuan Basin due to the regional uplift at the end of the Maokou Period, which caused the largest exposure event at the end of the Middle Permian. The entire study area suffered a long-term intense karst erosion process. Taking Well YB224 as an example, the karstification impacted stratum below the top boundary of LSC3 is about 30-40 m thick, featuring high-angle karrens, caves, and karst breccias (Fig. 6), with intensity decreasing from top to bottom. Since the karstification at the end of the Maokou Period appeared mainly as erosion in the low-lying areas^[18], it is also feasible to use the residual thickness method to reconstruct the paleokarst geomorphology. The residual thickness of LSC3 was used to reconstruct the paleokarst geomorphology at the end of the Maokou Period (Fig. 14b). It is found that LSC3 is thickest in the karst karst high (greater than 60 m) mainly distributed in Jiangyou-Shuangyushi-Jiange-Cangxi area and its south, thinnest in the karst depression (less than 25 m) north of the Jiangyou-Shuangyushi-Jiange-Cangxi area, and in between in thickness in the karst slope (25 to 55 m).

The geological significances of sedimentary facies and paleokarst geomorphology of different periods of the Maokou Formation deposition in the northwestern Sichuan Basin for oil and gas accumulation have been discussed based on the reconstructed lithofacies paleogeography and paleokarst geomorphology as well as the oil and gas test data (Table 1). It is found that the karst monadnocks and slopes are the favorable units for gas accumulation, and most high-yield wells (including gas or water wells) are mainly concentrated in the karst monadnocks of the karst highland and the karst slopes close to the karrens, and platform margin and intra-platform high-energy shoal deposits are the main favorable reservoirs. There are mainly two types of high-quality reservoir belts, namely (i) the high-energy shoal and karst monadnock and (ii) the platform-margin slope and karst slope. The first type is formed in the high-energy shoal deposits with better pore-throat structure above the wave base^[34]. After the fall of sea level, the shoal deposits would often be exposed and evolve into the karst monadnocks in the karst highland. Subsequently, leached by the meteoric fresh water, they would increase in storage space, as dissolution products were easily carried away, and eventually, they have more dissolution pores and caves remained. The second type has different forming mechanisms, and its sedimentary facies is not the most favorable type. The platform margin slope was located in the transition zone between the platform margin and the deep-water shelf, and had some relatively high-energy deposits developed at the top near the platform margin shoal. These platform margin deposits generally had poor original reservoir properties on the whole. The tectonic activities at the end of each stage of sedimentation resulted in the fall of sea level and then the karstification. In general, the landform of the sedimentary period largely determined the terrain of the paleokarst geomorphology. Therefore, the platform margin slopes in the sedimentary period basically developed successively into karst slopes where flow runoff dominated and strong dissolution consequently happened^{[18, 35-37]}. In this context, even with limited original reservoir space, the deposits can be modified into pore-vug reservoirs by dissolution.

Based on the idea that the favorable reservoir zones are where the high-quality reservoir facies belts overlap with the favorable paleokarst geomorphic units, the most favorable reservoir zones of the Maokou Formation in the northwestern Sichuan Basin were predicted by superimposing favorable facies with paleokarst geomorphic units in each period (Fig. 15). The overlapping areas of high-energy shoal facies + karst monadnock are mainly distributed in the Shuangyushi, Longgangxi, and Yuanba areas. The platform margin and intra-platform high-energy shoal facies deposits in these areas are likely to form pore-type and small cave-type reservoirs when subjected to karstification at the same time. The north part of the study area mainly has karst slopes of late LSC2 period, these karst slopes are mostly distributed in the peripheries of the Changjianggou-Kuangshanliang area and the Wangcang-Jiulongshan area. The multiple-stage superimposed karst slopes are mainly distributed in the Shuangyushi-Longgangxi-northern Yuanba area in ring belts, where the formation of reservoir space is mostly ascribed to the late karstification, but not the original sedimentary facies. With strong heterogeneity, the deposits in these areas can form large- scale pore-vug type reservoirs due to large scale dissolution. To conclude, these two types of reservoir zones in the Maokou Formation are favorable targets in the future exploration of the marine carbonate strata in the Western Sichuan Basin.

There are three long-term cycles in the Maokou Formation of the northwestern Sichuan Basin, denoted as LSC1, LSC2, and LSC3, which correspond to the first, second and third members of the Maokou Formation, respectively. These strata continuously differentiate in thickness, generally thin northward in the study area, and are thicker in the Shuangyushi-Jiange area, north of the Jiangyou area, south of the Zitong area, and the Longgangxi-Yuanba area. LSC1 was mainly a carbonate platform sedimentary system with relatively deep water body. Since the deposition of LSC2, tectonic differentiation occurred in NWW and NE directions, due to the extensional structure formed by the continuous extension of the Mianlue Ocean Basin in the northern margin of the Upper Yangtze Platform and the basement faults produced by the uplift of the mantle plume. In this context, the sedimentary basement subsided episodically from north to south, so the sedimentary system consisted of shelf-slope-platform margin-platform from north to south during the depositional period of LS2 and LSC3.

The Maokou Formation has three main exposure surfaces located on the tops of LSC1, LSC2, and LSC3, respectively. The tops of LSC2 and LSC3 are extensively exposed, contributing to favorable conditions for the development of paleokarst geomorphology, including karst highland, karst slope, and karst depression. Moreover, these landform units have a certain inheritance to sedimentary landform units. For instance, the shoal deposits in the high part of the landform during the sedimentary period developed into karst monadnock areas in the karst highland, whereas low parts such as inter-shoal seas and open seas developed into shallow depression areas, and platform margin slopes often developed sucessively into karst slopes.

It is concluded through analysis that sedimentary facies and paleokarst geomorphology are of great significance for oil and gas accumulation. High-energy shoal + karst monadnock as well as platform-margin slope + karst slope are two types of high-quality reservoir assemblages that have been verified by existing high-yield wells. By overlapping the multi-stage high-quality facies belts and paleokarst geomorphological units in the northwestern Sichuan Basin, favorable reservoir areas have been predicted: (i) the areas where high-energy shoal and karst monadnock overlap, including Shuangyushi, Longgangxi, and Yuanba areas; (ii) the karst slopes in the periphery of the Changjianggou-Kuangshanliang Sections and the Wangcang-Jiulongshan areas at the end of LSC2 deposition as well as the areas where multi-stage karst slopes superimpose in a ring-like pattern, including the Shuangyushi, west Longgang, and north of Yuanba areas.