In recent years, with the improvement of oil and gas exploration technology,a series of volcanic rock reservoirs have been discovered successively in China. Volcanic rock, which acts as an important type of oil and gas reservoir, has gradually attracted the attention of petroleum geologists^[1⇓-3]. Due to the complex rock types and strong heterogeneity, the lithofacies characteristics and distribution laws of volcanic rocks have gradually become a hot topic in the current geology field^[4]. Igneous rocks were widely developed at the end of the Middle Permian in the Sichuan Basin. The oil and gas exploration in volcanic rocks began in 1966. Effusive facies basalt and intrusive diabase porphyrite were drilled at the margins of southwestern and eastern Sichuan Basin, but the exploration progress has been relatively slow for a long time^[5]. Until recent years, research has been conducted on the seismic anomaly zone with weak-amplitude chaotic reflections at the top of the Permian Maokou Formation in the Central Sichuan-Western Sichuan Basin, and it is speculated that eruptive volcanic rocks have been developed in large scale in the western Sichuan area^[6]. The exploratory wells deployed according to the speculation have successively encountered thick volcanic clastic reservoirs in the Permian, and tested industrial gas flows. Since then, important breakthroughs have been made in igneous rock exploration in the Sichuan Basin.

Since then, a series of studies have been carried out on the Permian igneous rocks in western Sichuan. The results suggested that this set of eruptive facies volcanic clastic rocks can form high-quality porous reservoirs with medium to high porosity and large thickness^[5]. In addition, the Permian volcanic rocks in western Sichuan, spatially close to the Cambrian high-quality source rocks in the Deyang-Anyue Rift, have good source-reservoir combination condition^[7], and are currently key targets for gas exploration in the Permian volcanic rocks in the Sichuan Basin. However, the basic lava with strong fluidity is often constrained by paleomorphology in distribution. The Permian Maokou Formation in the Sichuan Basin with elevation differences in paleogeomorphic topography has a significant control effect on the flow direction of Mount Emei basalt lava^[8]. There are few researches on influence and controlling effect of the karst paleogeomorphology of the Maokou Formation on the distribution of Permian volcanic rocks. Moreover, the distribution pattern of volcanic rocks in the Sichuan Basin has remained unclear. The study focuses on the Permian volcanic rocks in the western Sichuan Basin, and explores the relationship between the karst topography at the end of the Maokou Formation and the characteristics of volcanic cycles and lithofacies distribution, thereby to provide support for the overall exploration of volcanic rocks and the Maokou Formation karst reservoirs in the Sichuan Basin.

The Sichuan Basin is a superimposed basin formed on the basis of the Upper Yangtze Craton. It is adjacent to the Qinling orogenic belt in the north and the Yunnan-Guizhou Plateau, which is the western margin of the fold-shaped Upper Yangtze Plate in the south and, transits outward to the Songpan-Ganzi fold belt. It is a nearly rhombic tectonic-sedimentary basin under the influence of strong plate movement especially NW and NE deep fracture zones^[9-10]. Large-scale transgression in the early and middle periods of the Middle Permian left two sets of relatively stable platform carbonate rock strata of Qixia Formation and Maokou Formation on the quasi-plain basement^[11]. At the end of the Middle Permian, the rising of the Mount Emei mantle plume impacted the bottom of the lithosphere^[12-13], and the crust uplifted rapidly and differentially, making the Upper Yangtze area experience a tectonic movement of 1.0-1.5 Ma which manifested as differential uplift and regional extension^[14-15], namely the Dongwu Movement. The Dongwu Movement resulted in the large-scale differential denudation of the carbonate rock strata of the Maokou Formation, giving rise to a widespread unconformity^[16-17] and increasing the paleomorphological height difference. With the Dali-Miyi as the center, the Maokou Formation decreases in denudation^[16] in a ring pattern^[18], and the Sichuan Basin is located outside the denudation zone.

Within 3 Ma after the occurrence of the Dongwu Movement, large-scale volcanic eruptions occurred in Yunnan, Guizhou, and Sichuan^[19-20], forming an ultrabasic-basic volcanic formation which covers ​​more than 2.5×10⁵ km^2[21] and a maximum thickness of 5300 m. It is called the “Mount Emei Basalt” and is the only large igneous rock province recognized by the international academic community in China^[22]. The distribution range of the Mount Emei large igneous rock area is basically the same as the range of the Middle Permian denudation area. With the Dali-Miyi as the center, it thins in a ring pattern, and can be divided into inner, middle and outer zones^[23-24]. The Sichuan Basin, located in the outer zone of the Mount Emei large igneous rock area^[25] has widely distributed basalt in the western part (Fig. 1), with a thickness of 40-500 m^[26]. The overall thickness of the basin is rapidly thinning from southwest to inside.

Under the joint effects of the Caledonian Movement in the Late Silurian and the Yunnan Movement during the Late Carboniferous, the Sichuan Basin was subjected to large-scale weathering and flattening. In the Middle Permian, an open platform environment formed on the whole under the influence of transgression in the basin. With Middle Permian sediments, two sets of carbonate strata with relatively stable thickness of the Qixia Formation and Maokou Formation were formed^[11].

The Maokou Formation in the basin can be divided into 4 lithological sections from bottom to top: the first member of the Maokou Formation (hereinafter referred to as Mao 1 member), mainly dark gray-gray black thin layer-blocky biological limestone, with “eyeball-like limestone” represented by brachiopod Cryptospirifer, showing a mid- -high amplitude serrated funnel shape on the logging response. The second member of the Maokou Formation (hereinafter referred to as Mao 2 member) is dominated by gray-dark gray micritic bioclastic limestone and sparry bioclastic limestone, with the development of high-energy coarse-grained bioclastic shoal sediments on the top, Chusenella and coral Ipciphyllum, and brachiopod Cryptospirifer occasionally; it features medium-low amplitude box shape on log responses. The third member of the Maokou Formation (hereinafter referred to as the Mao 3 member) is dominated by gray-light gray bioclastic limestone, contains sectional ancient organism markers such as Yabeina and Neomisellina, and shows high amplitude tooth shape at top and bottom and low-amplitude tooth shape in the middle on log responses. The fourth member of the Maokou Formation (hereinafter referred to as Mao 4 member) is composed of thick gray-blocky bioclastic limestone with chert strips and brachiopod Cryptospirifer, and features middle-low amplitude finger shape on log curve^[27].

Stratigraphic correlation of drilling wells in the basin confirmed that the top of the Maokou Formation is generally incomplete^[28], with the lowest degree in the southwestern and southeastern Sichuan Basin. With denudation gradually increasing towards the central and northern Sichuan regions, the residual thickness of the Maokou Formation gradually decreases. The residual thickness of the remaining Mao 4 member and its lower strata in wells ZG1 and ZT1 in southwestern Sichuan is 300-345 m (Fig. 2). The residual thickness of the Mao 4 member gradually decreases towards the middle of the basin. In wells MX105 and MT1 in the middle of the basin, the Mao 4 member is completely missing, and only the Mao 3 member and its lower strata remain, with a residual thickness of 180-200 m. In the northeastern part of Sichuan Basin, only the middle and lower parts of Mao 2 member remain, with a minimum remaining thickness of only 150 m, showing the characteristic of zonal denudation. In the Chengdu-Jianyang area, the Maokou Formation thins abnormally. The Maokou Formation actually encountered in Well YT1 is 158 m thick, which is over 100 m thinner than in the adjacent area, with Mao 3 and Mao 4 members missing. The Mao 2 member is in unconformable contact with the overlying Mount Emei basalt formation, which confirms the existence of differential karstification in the basin.

The Yangtze area was located in the low-latitude tropical area near the equator in Permian, characterized by hot and humid climate. There was the development of bauxite weathering residues in the carbonate rock paleo-weathering crust^[29-30] and karst breccia below the interface^[31]. Drilled wells and field sections showed that paleo-weathering crusts containing pyrite and bauxite mudstone 2-10 m thick generally developed on the top of the Maokou Formation in the Sichuan Basin. For example, the core of the top of the Maokou Formation from Well WY17 in the Southern Sichuan Basin showed the occurrence of approximately 2 m thick bauxite mudstone (Fig. 3a), which is the product of weathering residual. Karst breccia can be seen on the top of the Maokou Formation in Well MT1 in Central Sichuan Basin (Fig. 3b), and there is logging curve response characteristic of weathering crust with high natural gamma and low resistivity on the top interface. Volcanic rock overlaid the weathering crust in disconformable contact. In the Mount Huaying field outcrop section, part of the top of the Maokou Formation is missing. The bauxite clay rock, siliceous rock and breccia formed by karst collapse can be identified in the top of the Maokou Formation (Fig. 3c), which is in parallel disconformable contact with the overlying Mount Emei basalt.

In the development zone of eruptive facies volcanic rock in western Sichuan Basin, it also shows characteristics of the weathering crust at the top of the Maokou Formation. The cuttings samples from the top of Maokou Formation in Well YT1 of Jianyang area were tested for the contents of major and trace and rare earth elements, and the tested results were analyzed combined with logging data. Carbonate rock cutting samples of larger grain size were selected under magnifying glass, which were washed by ultra-clean water, dried at low temperature, and crushed to the particle size of 74 μm (200 mesh) in an agate mortar. The inductively coupled plasma mass spectrometry Instrument (ICP-MS) was used to test its trace elements and rare earth elements. The analytical process used the national standards GBW07315, GBW07316 and the US Geological Survey basalt reference material 6BHVO-2 for quality control, and the relative error of the test results is less than 2%. The results show that the top of Maokou Formation in Well YT1 is rich in weakly mobile elements such as silicon, iron, aluminum, and titanium, while depleted in chlorine, sulfur, calcium, magnesium, sodium and manganese, thus forming the bauxite clay on the top of the Maokou Formation. The samples from well depths of 5891, 5893, and 5895 m of this well have evidently positive deviation of terrigenous elements Th, K, Al, and Zr, with high Fe/Sr value, and relatively consistent variation trend (Fig. 4a). The samples from 5891 and 5893 m depths have contents of rare earth elements significantly higher than that of the samples at 5895 m and the underlying depth (Fig. 4b and Table 1), indicating that the top of the Maokou Formation suffered strong weathering which gradually weakened at 5895 m. Within 5 m from the top of the limestone of the Maokou Formation, the content of rare earth element Eu presents a significant negative anomaly, while the rare earth elements below this depth present uniform changes, indicating that Well YT1 experienced strong paleo-weathering before volcanism.

Since the Qixia Formation is a stable carbonate platform stratum depositing after the filling and leveling of the underlying Permian Liangshan Formation^[32], it varies little in thickness within the basin. The Maokou Formation above is in conformable contact with the Qixia Formation and has a set of argillaceous limestone stably distributed in the basin at the bottom with “eyeball-eyelid” structure which can be correlated across the whole region^[33]. It is believed that the paleogeomorphy before the deposition of the Maokou Formation had minor undulations, and its bottom surface, namely the top of the Qixia Formation was regarded as an isochronous interface. On this basis, the residual thickness of the Maokou Formation was compared^[32] (Fig. 2a), thereby to further analyze the paleogeomorphy characteristics before volcanism. The Maokou Formation in the southwest and western margins of the basin is larger in residual thickness, these areas were higher in landform at the end of the Maokou Formation deposition, and were karst highlands. The Maokou Formation in the western Sichuan-north part of southern Sichuan-Central Sichuan Basin gradually thins from the edge to the interior of the basin overall, and is thicker in the south and thinner in the north generally. It was a karst slope with secondary karst paleogeomorphologic units such as karst gullies and residual hills^[31]. The karst valleys originating from the northern part of southern Sichuan and western Sichuan extend northward in dendritic pattern, passing through the central Sichuan region and entering the northern karst basin. However, the top of the Mao 2 member in the Jianyang area suffered evident denudation. The Maokou Formation encountered by Well YT1 in this area is 158 m thick, which is more than 100 m thinner than that of the adjacent area. From the YT1 well area to the southeast and northwest, the karst denudation becomes weaker, the residual thickness of the Maokou Formation gradually increases, and the Mao 4 member or the Mao 3 member and strata below have remained. As mentioned above, the element analysis of Well YT1 reveals that the main reason for the thinning of the Maokou Formation in Jianyang area is the karst erosion at the end of the deposition of the Maokou Formation. Therefore, subjected to strong dissolution before the eruption of the volcanic rock, the YT1 well area was relatively low in paleomorphology. Under the influence of dissolution, the elevation difference of paleomorphology further increased, gradually forming a local karst shallow depression.

The geophysical well-seismic calibration results showed that when the overlying strata of Maokou Formation has different lithologies, the top boundary of the Maokou Formation has different seismic reflection characteristics. When volcanic rocks are not developed, the seismic wave velocity difference between the limestone of Maokou Formation and the mudstone of the overlying Longtan Formation is relatively large, and the seismic reflection appears as a strong wave impedance interface. When the top of the Maokou Formation is in unconformable contact with the effusive facies basalt, the seismic reflections at the top of the Maokou Formation showed weak crests and intermittent reflections due to the similar rock velocity. In the development zone of eruptive facies volcanic rock, there is no evident reflection at the top of the Maokou Formation^[34]. According to the seismic reflection characteristics of the Maokou Formation in the volcanic rock development zone of Jianyang area, the karst paleomorphology of the Maokou Formation in Jianyang area of western Sichuan has been finely described. The results also showed that the development range of Jianyang eruptive facies volcanic rocks is consistent with that of the large karst depression formed at the end of the deposition of the Maokou Formation (Fig. 5).

The Sichuan Basin has basic-ultrabasic alkaline volcanic rocks developed. The volcanic rocks have SiO₂ of 35%-52% in mass fraction^[34], 45.5% on average; and high content of alkaline substances mainly composed of K₂O and Na₂O, with K₂O+Na₂O of 4.3%-8.3% in mass fraction, 6.1% on average. According to rock structure characteristics, the volcanic rocks can be divided into volcanic clastic rock, lava and pyroclastic rock. According to volcanism and lithological characteristics, the lithofacies of Permian volcanic rocks in western Sichuan can be divided into eruptive facies, effusive facies, superficial intrusive facies and volcanic sedimentary facies^{[35⇓⇓⇓-39]}.

The lithology of eruptive facies is dominated by pyroclastic rock, which is mainly developed in the Jianyang area of western Sichuan, and can be further subdivided into narrowly defined pyroclastic rock and pyroclastic lava. The clastics in the pyroclastic rocks, beside the pyroclastic materials such as crystal debris, vitric pyroclast and vitric materials, also consist of a large amount of angular-sub-angular, poorly sorted sparry or micrite bioclastic limestone debris. With typical Mickey algae and Birktulinite of the Maokou Formation, this kind of rock has original structure destroyed by strong recrystallization, and is formed by debris from the Maokou Formation with underlying karst strata carried out through the strong energy of volcanic eruption. Pyroclastic lava is a transitional type between volcanic lava and pyroclastic rock^[38], which is similar to the pyroclastic rock containing limestone breccia, but the pyroclastic is dominated by basaltic breccia, and is formed by relatively weak volcano eruption.

The rock types of effusive facies volcanic lava include cryptocrystalline-microcrystalline basalt, porphyritic basalt, stomata-amygdaloidal basalt^[39] and granular basalt. Among them, the cryptocrystalline-microcrystalline basalt is poorly crystallized and is mostly intergranular-invisible structure. The stomata-amygdaloidal basalt has stomata almond structure, and the stomata are basically filled with chlorite, calcite, zeolite and other minerals, forming amygdala; the rock matrix is mostly cryptocrystalline basalt. The granulite has a higher degree of crystallization, no vitric or cryptocrystalline texture, with intergranular structure and few stomata.

The rock type of superficial intrusive facies is diabase porphyrite, which has a porphyritic structure. The phenocrysts are mainly clinopyroxene and orthopyroxene which are semi-euhedral particles of 0.5 mm×1.0 mm to 1 mm×2 mm, with a content of about 20%. The matrix is mainly plagioclase and clinopyroxene. The plagioclase is in the form of euhedral-semi-euhedral strips with a particle size of about 0.2 mm×0.5 mm. The pyroxene is filled in the pores of plagioclase in the form of xenomorphic particle.

The lithology of volcanic sedimentary facies is dominated by sedimentary tuff in volcanoclastic rock and tuffaceous mudstone in volcanic sedimentary rock. The sedimentary tuff is mainly composed of crystal debris, rock debris and vitric pyroclast of less than 2 mm in grain size in directional arrangement, a certain amount of normal sediment such as mud, and horizontal bedding or graded bedding.

Practical drilling and seismic data showed that the volcanic rocks in the western Sichuan region are locally thick and thinned rapidly toward the edge. The volcanic rocks in Jianyang area are locally thicker with relatively complete volcanic facies. For example, the volcanic rocks in YT1 well area are 100-300 m thick (Fig. 6) and include stacked superficial intrusive facies, eruptive facies and effusive facies rocks. In the Zhongjiang and Santai areas, the volcanic rocks are thinner (20-50 m thick), incompletely in lithofacies, and dominated by effusive facies, with occasionally thin eruptive facies volcanic rock accumulations. Due to the multi-phase volcanic eruption in western Sichuan, short eruptive intermittence and hardly no re-transportation by flowing water, the volcanic rocks in western Sichuan are mostly basalt with thick layers of volcanic clastic rock, with volcanic sedimentary facies underdeveloped, and eruptive facies and effusive facies vertically superimposed.

The interpretation results of basement faults in the study area showed that the distribution of volcanic rocks is related to the development of basement faults. Gravity anomalies showed that the basement faults in this area are mainly in north-east strike, long in extension and large in scale. Moreover, there are many basement faults in northwest strike in the Chengdu-Jianyang area (Fig. 7). The faults largely appear as breaks of seismic events on seismic profiles, with cluttered reflections inside the fractures, and no significant difference in height between the seismic events on two sides of the same fault. The intersection area of the two groups of faults is the primary eruption area of volcanic rocks, where the volcanic rocks are thick and dominated by eruptive facies. The basement faults in Santai area are underdeveloped, so the volcanic rocks are thinner, and dominated by eruptive facies basalt.

The thickness of volcanic rocks in the western Sichuan area and the underlying Maokou Formation and the overlying Longtan Formation have a certain complementary relationship as a whole (Figs. 5 and 6). The volcanic rocks in Jianyang area are locally thicker and have relatively complete volcanic facies. The residual thickness of the Maokou Formation and overlying Longtan Formation were relatively thin. Generally, the residual thickness of the Maokou Formation is 150-160 m, and the thickness of the Longtan Formation is 60-100 m. The volcanic rocks in the Zhongjiang-Santai area are relatively thin, and the residual thickness of the Maokou Formation and overlying Longtan Formation are thicker (270-280 m and more than 100 m, respectively).

From the volcanic lithofacies planar distribution map of Jianyang-Santai area (Fig. 6c) and the karst landform division map of the Maokou Formation (Fig. 5), it can be seen that the place near the shallow karst depressions of the Maokou Formation are mostly dominated by eruptive volcanic rocks, with small distribution range; the effusive facies is in large distribution range, and can extend from the shallow karst depressions to karst residual mounds. In addition, the southwestern Sichuan region is relatively weak in karstification and is located in the structural highs of ancient landform. Although close to the eruption center of the Mount Emei large igneous rock area, the volcanic rocks in this region are dominated by effusive facies. In contrast, the Jianyang area in western Sichuan was far away from the eruption center of the Mount Emei large igneous rock area, but has large-scale eruptive facies volcanic rocks developed along shallow karst depressions. Therefore, it is speculated that the western Sichuan region had an independent eruption center, and the development of shallow karst depressions is an important controlling factor affecting the distribution of volcanic rocks.

According to the distribution characteristics of karst paleo-geomorphology and volcanic rock, combined with seismic data, the evolution of karst paleo-geomorphology and the formation of volcanic rocks in Jianyang-Santai area of western Sichuan can be divided into three phases. In the first phase, at the end of the depositional period of the Maokou Formation, affected by the upwarp of the Emei mantle plume, Sichuan Basin began to stretch and uplift, the western Sichuan area was in the karst slope on the whole. As the uplifting strengthened, multiple sets of small normal faults were formed in Jianyang area, micro-graben structures were gradually formed, and a shallow depression structure with small amplitude was formed (Fig. 8a). In the second phase, under the influence of the micro-graben structural pattern in karst slope area, the downward erosion enhanced, the limestone of the Mao 4 and Mao 3 members in Jianyang area suffered severe dissolution, giving rise to a karst weathering crust, which promoted the formation of local karst shallow depressions; while the Santai area had always been in the relatively high part of the paleo-geomorphology in the slope area (Fig. 8b). In the third phase, volcanic activity was intensified, and molten lava spewed out of the surface along the early faults and weak graben belts, and accumulated in the shallow karst depressions, making thick eruptive facies volcanic rocks build up in Jianyang area; while effusive facies volcanic rocks thinned toward the updip of the karst slope (Fig. 8c).

As different karst paleo-geomorphic units in the Maokou Formation had different fault development degrees, volcanic rocks in different periods and regions of western Sichuan show different eruption characteristics. The volcanic eruption in western Sichuan is mainly divided into six stages. The first stage (pre-magmatism), the Maokou Formation was generally weathered and eroded at the end of its deposition. The Jianyang area provided a favorable paleo-geomorphic environment for karstification under the background of extensional graben structure, where a karst shallow depression was formed at the end of the Maokou Formation deposition (Fig. 9a). In the second stage (the period of intense volcanic eruption), the volcanic rock first rose and erupted strongly along the weak zones with developed faults, and then erupted in a central fissure type with different volcanic structures as units. Due to the strong energy, the magma carried the underlying Maokou Formation limestone clastics and erupted, forming an eruptive facies pyroclastic rock accumulation dominated by lime-bearing breccia pyroclastic rocks in the shallow karst depression (Fig. 9b). In the middle and late stages of the volcanic eruption, as the energy relatively weakened, and the volcanic rocks formed without limestone breccia; instead, an eruptive facies accumulation dominated by pyroclastic lava was formed (Fig. 9c). At the same time, the Zhongjiang- Santai area was still in the stage of weathering exposure. In the third stage (the volcanic effusive period), the energy of volcanic activity further weakened, the western Sichuan Basin was dominated by fractured eruptions, and large-scale effusive facies basalts formed in the karst slope area, covering the whole area (Fig. 9d). In the fourth stage (the weakening of volcanism), the energy of volcanic eruption weakened to the point that the magma could not break through the overlying strata and erupt out of the surface. The volcanic rock was distributed along the weak surface in super shallow layers in Jianyang area, forming lamellar dolerite. Whereas Zhongjiang-Santai area still had effusion (Fig. 9e). In the fifth stage (tephra subsidence period), the volcanism process basically ended and a new stage of transgression began. The part north of Santai was relatively low in terrain, where tephra fell into water, forming sedimentary tuff generally; and Zhongjiang area also had thin beds of sedimentary tuff developed (Fig. 9f). In the sixth stage (the depositional period of the Longtan Formation), seawater gradually covered the whole area, forming an interbedded sedimentary tuff with shale of the Longtan Formation, which covers the volcanic rock (Fig. 9g).

A number of drilling wells in the Sichuan Basin have confirmed that both limestone karst and volcanic rock reservoirs of the Permian Maokou Formation have good storage conditions. But the current exploration of Maokou Formation karst reservoirs is mostly concentrated in the southern Sichuan area. The recent exploratory well drilled in western Sichuan encountered thick karst reservoir of the Maokou Formation, in which a poor gas layer of 21.3 m thick was identified from logging interpretation, indicating that gas-bearing property and distribution of the reservoir still need to be further clarified. The seismic data show that Jianyang area has multiple volcanic eruption centers, hence volcanic clastic rocks in continuous distribution form with the potential for large-scale reservoir exploration. But the volcanic reservoirs have rapid lateral changes, strong heterogeneity, and large differences between wells. However, the western Sichuan region is located in the Deyang-Anyue rift on the whole, where the Lower Cambrian Qiongzhusi Formation has mud shale of 300-450 m thick, with an average TOC of more than 2%, and a cumulative gas generation intensity of up to 180×10⁸ m³/km², showing huge hydrocarbon generation potential and superior accumulation conditions. After the Caledonian Movement, the Middle-Upper Cambrian, Ordovician, and Silurian were completely eroded in this area, so the Permian directly contact with the underlying Lower Cambrian Canglangpu Formation and Qiongzhusi Formation unconformably, and has the advantage of proximity to source rock.

At the end of the Middle Permian, the Maokou Formation in the Sichuan Basin suffered a large-scale weathering and denudation due to the uplift of the crust, giving rise to karst geomorphologies varying with regions. The western Sichuan Basin presented as a karst slope, with the residual thickness of the Maokou Formation ranging from 200 to 300 m. In the Jianyang area, the local shallow karst depressions developed under the slope setting were associated with a large number of small normal faults, forming a micro-graben paleomorphology, where the residual thickness of the Maokou Formation was only 150 m as a result of strong karstification. After the Dongwu Movement, the intense volcanism in western Sichuan Basin created basic volcanic rocks through multi-point eruption along the fissures. These volcanic rocks, mainly basaltic pyroclastic rocks, pyroclastic rocks containing limy breccia, basalt, and dolerite, were concentrated in the Jianyang area, with the thickness up to 300 m, which decreased edge-ward to only 20 m in the Zhongjiang-Santai area. In the late depositional period of the Maokou Formation, in the western Sichuan Basin, karst paleomorphology and fault system jointly controlled the distribution of volcanic rocks within the region. In the Jianyang area, the volcanic rocks of explosive facies reflected a consistent distribution scope with the large karst depressions formed at the end of the Maokou Formation deposition. Especially in the shallow depression area where strong karstification had occurred, the volcanic rocks were thick and contained complete lithofacies association, with thick explosive facies volcanic clastic rock deposits and thick basalt. In the karst slope area with underdeveloped faults, the volcanic rocks were relatively thin and dominated by effusive facies basalt. The western Sichuan Basin is a good region where the Permian volcanic rocks and the Maokou Formation karst reservoirs are stacked, making it a new exploration target. To date, however, the region has been less explored with very few wells drilled. A deeper geological study on clarifying the controlling factors, distribution and hydrocarbon accumulation characteristics of the mentioned two sets of reservoirs will be of great significance to achieving efficient multi-layer exploration in the region.