The “pyroclastic flow” was first proposed by Shigeo Aramaki in 1961. It refers to the high-density clastic flow composed of red-hot clastics and high-temperature gas generated by volcanic eruption. Formed after the collapse of Pliny eruption column, the high-density fluids flowed, migrated and then accumulated along volcanic slope or low-lying zones ^[1]. By researches on the characteristics of accumulative facies of pyroclastic flow in the Tianchi, Changbai Mountain, Yi et al. ^[2] once again put forward that the pyroclastic flow generated by the collapse of Plinian volcanic eruption column is a kind of gravity flow which is characterized by joint migration and accumulation of coarse clastics such as volcanic rock and breccia, and volcanic ash.

The Dongwu Movement during the Middle-Late Permian resulted in powerful volcanic eruption events in southwest China, leaving ultra-thick “Emeishan volcanic rock” (Emeishan traps) with an area of over 50×10⁴ km^{2 [3]} in Yunnan, Guizhou and Sichuan regions. The volcanic rock affected the continental and marine changes, sedimentation and hydrocarbon generation in the corresponding period in southwest China, and so it has been widely concerned by scholars in China and abroad ^[4]. Volcanic reservoir exploration in the Sichuan Basin started from the Weiyuan area in 1966. By 2020, more than 500 volcanic wells had been drilled. On December 16, 2018, a high-yield gas flow of about 20×10⁴ m³ was obtained from Well YT1 in the Jianyang area of Western Sichuan Basin. It was the first time that the pyroclastic flow volcanic porous reservoir of explosive facies had been found in the widely distributed basic volcanic rock. The Permian volcanic gas reservoir found in Well YT1, and featured by great thickness, good physical properties and high pressure, is roughly predicted to have a gas-bearing area of more than 500 km². It can be described as an ultra-deep, sulfur-free and large gas reservoir with high abundance and medium production, and shows an excellent gas exploration prospect ^[5,6].

Focusing on the Western Sichuan Basin, utilizing the data from outcrops in the Mugan Town and Sayutuo of Yanji County and southern Gongquan Town, and data from wells YT1, TF2 and TF8 wells, the petrologic and reservoir characteristics of the pyroclastic flow volcanic rock are studied through the analysis of rock thin sections, mineral components, scanning electron microscopy and high pressure physical properties. The accumulation process of pyroclastic flow from the near to the far crater is analyzed, and the migration and accumulation model is established according to the spatial variation of accumulation sequence. Finally, based on geological and geophysical methods, the distribution, development characteristics and exploration potential of the pyroclastic flow volcanic rocks in Western Sichuan Basin are predicted and evaluated.

“Emeishan volcanic rocks”, with complex geological structures, is located in the western margin of the Yangtze Plate, close to the Sanjiang tectonic belt, and generally at the junction of the Pacific and the Tethys tectonic domains ^[7]. It was named by Mr. Zhao Yazeng in 1929, especially indicating the basalt strata exposed above the Middle Permian Maokou Formation in the Mount Emei area ^[8,9]. He et al. divided the “Emeishan volcanic rock” into inner, middle and outer zones according to the surface uplift (Fig. 1a) ^[10]. The “inner zone”, centered from Yongren in Yunnan to Panzhihua of Sichuan, refers to the acting area of the mantle plume head. In the inner zone, the volcanic rock from early eruption is very thick, with multiple stages and long period of erupting activities ^[10]. The “middle zone” along the Kangding-Kunming line and the “outer zone” along the Chengdu-Guiyang line are the middle and outer areas of the “Emeishan volcanic rock”, respectively, representing the outward spreading wings of the mantle plume along its head part. The volcanic rocks in these zones are characterized by non-uniform thickness and late-stage eruption, and experienced two eruption periods. In the inner zone with the strongest eruption, the accumulated basalt in great thickness has been in a denudation environment for a long time, so the volcanic rock is directly covered by unconformable continental Triassic sediments at the top instead of the Upper Permian ones ^[11]. In the middle and outer zones, from west to east, the overlying strata contain successively the Upper Permian Xuanwei Formation and Shawan Formation from continental sedimentation, the Longtan Formation from marine-continental sedimentation, and the Wujiaping Formation from the shallow-sea carbonate sedimentation ^[10] (Fig. 1b). It indicates that the “Emeishan volcanic rocks” evolved gradually from the inner to outer zones. Volcanoes in the inner zone erupted early in a large scale with long duration, whereas those in the outer zone erupted late in a small scale with short duration. Evidences show that a large number of radial fissures caused by the upwelling of mantle plume in the inner zone spread gradually from the Kangding-Yunnan rift valley into the Sichuan Basin, and resulted in the reactivation of old basement faults. The forming mechanism may be the creep of continent which brings a viscous flow of continental crust. The process is similar to pulling the accordion, and it enables the outer edge of continental crust to extend to marine. As a result, the crust is pulled and becoming thinner, even apart ^[12]. Previous studies show that the tensional fault system located in the outer zone split from west to east, and the development degree of the rift valley decreased successively. The eruption environment of “Emeishan volcanic rocks” was similar to that of the Panzhihua-Xichang rift valley. The eruption model is fissure-type or multi-point center type ^[12], which belongs to inner continental eruption in general ^[13,14,15]. The lava flow erupted early formed large area of effusive facies basalts, whereas that erupted late formed pyroclastic rock and pyroclastic lava (Fig. 1c). In this paper, the pyroclastic rock and pyroclastic lava formed by high-density hot pyroclastics, mixed up with pieces of surrounding rocks and accumulated from the eruption center along slope and shallow depression are collectively called pyroclastic flow volcanic rocks.

Located in the outer zone of “Emeishan volcanic rocks” and developed in the early Upper Permian, the volcanic rocks in Western Sichuan Basin are in unconformable contact with the underlying Middle Permian Maokou Formation and the overlying Upper Permian Longtan Formation. The Maokou Formation consists of marine bioclastic limestone, and the Longtan Formation contains coal-bearing deposits of marine-continental sandstone and mudstone interlaced with thin limestone intervals. Regionally, due to inconsistent eruption periods and duration of “Emeishan volcanic rocks”, and karstification of the Maokou Formation, the top and bottom contact are not isochronous. But their characteristics of lithologic abrupt contact with the underlying and overlying strata are obvious. There experienced two erupting periods in this area. Two different rock assemblage units are present in the upper and lower intervals, both of which are in unconformable contact (Fig. 2). According to the outcrop profile from old to new, in the lower unit, there developed thick basalt with columnar joints, and weathering crust at the top, which is in unconformable contact with the upper unit. In the overlying unit, there exist coarse- and fine-grained tuffs which belong to continental empty stacked accumulations. At the top, there developed thin-layered basalt with small columnar joints. The downhole profile shows that there are two lithologic units from old to new. The lower unit is dominated by basic-ultrabasic volcanic rock which has diabase porphyrite and basalt from bottom to top, and locally late intrusive rock. The upper unit is mainly composed of basic to intermediate-basic pyroclastic flow volcanic rocks, including pyroclastic rock, pyroclastic lava and tuff from old to new. The boundary between the upper and the lower units is unclear due to broken cores. However, obvious lithologic and electrical properties abrupt changes suggest the existence of an eruption discontinuity (Fig. 2).

Based on the observation of core data, and the comparison of cuttings and logging data of several wells in Western Sichuan Basin, six types of main lithology have been identified, with characteristics as follows.

(1) Diabase porphyrite (Fig. 3a, 3e). This gray rock is thick-layered, clumpy and tight, with undeveloped fractures. In the rock, medium-fine hypidiomorphic plagioclase alternates with pyroxene, and some idiomorphic magnetite, ilmenite, and locally olivine. In the cores, no vugs and pores are observed, but late structural fractures have been fully filled with mud (locally sericitization).

(2) Dolerite (Fig. 3b, 3f). This gray rock is thick-layered, with fractures undeveloped. It shows the tight structure and uniform color in the cored interval. The dominant minerals include fine crystal plagioclase and clinopyroxene with coarse crystals, which are quite different from basalt. There exist idiomorphic ilmenite and magnetite, and locally a small amount of olivine, but no hyaline. Dissolution phenomenon is not observed. Vugs and pores are not found in cores. Most late high-angle structural fractures have been fully filled, and a few small fractures have been half filled with mud (locally sericitization).

(3) Basalt (Fig. 3c, 3g). This clumpy rock is thick-layered and tight, contains porphyritic texture locally and well-developed fractures. Its main color is gray black, followed by green gray. Formed by condensation and consolidation of molten magma, the rock displays a low degree of crystallization. Microcrystalline and hyalopilitic texture were found. With the increase of crystallization level, basalt transforms to dolerite. In Well TF8 and other wells and outcrops far from the crater, only basalt was found, but no dolerite. In Well YT1, dolerite transitions to basalt, and microcrystalline and hyalopilitic texture were found under a microscope. The grain size of feldspar phenocryst is 0.1-1.0 mm (Fig. 3d, 3h). According to the shape of crystals, it is considered that Well YT1 is close to the crater and at high temperature, so it’s assumed that basalt in the well condensed slowly, and coarse crystal grains were formed after rapid accumulation and cooling down. Far from the crater, basalt was accumulated with a small volume, and condensed fast, leaving small crystal grains. No vugs or pores are found in the cores. Most late high-angle structural fractures have been fully filled, or half filled with mud (locally sericitization). No hydrocarbon show has been observed.

(4) Pyroclastic rock (Fig. 3i, 3m). The rock is tight and hard. The color is mainly dark gray, followed by gray. It contains primarily volcanic breccia with grain size larger than 2 mm, and less limestone. The breccia is angular and sub-rounded, unevenly distributed, unsortable (1 mm × 2 mm to 40 mm × 60 mm), and locally accumulated, but the boundary is clear. The clastics are formed by the accumulation and fusion or compaction of volcanic clastics, such as debris, vitroclastics, crystal pyroclasts and hyalines, with the content of more than 75%. In addition, there is a large amount of limestone clasts, most of which are angular and sub-angular, ranging from 2 mm to 60 mm, with poor sorting. On the core section from Well YT1, there are obvious limestone clumps, and pyroclastics whose grains are bigger in the lower part, and gradually become smaller toward the upper part, in a positive sequence. No vugs developed and no hydrocarbon shows have been found in the cores. Cylindrical dissolution pores are unevenly distributed and locally enriched, and the surface porosity is 2%-3%.

(5) Pyroclastic lava (Fig. 3j-3l, 3n-3p, 3t-3w). The clumpy rock is thick-layered, tight and hard, in green- gray and dark-green-gray. The broken surface is smooth, but no fractures have been found. The texture is tuffaceous and highly permeable (quickly seeping water). Clastics account for 10% to 75%, in 2 mm to 60 mm in particle size, and cemented by lava. The clastics generally contain tuff and breccia, featured by obvious edge angles and disordered distribution. The compositions are generally the same or similar to that of cemented lava. A small amount of basalt clast, magma clast or hyaline can be observed. The magma clast is mostly elongated or in a chicken-bone shape. The clasts experienced chloritization and carbonate metasomatism (Fig. 3u), so presenting angular and irregular shapes (Fig. 3n). Most magma clasts are in irregularly flowing and elongated shapes, indicating that they were slowly solidified during the flow and elongated along the flowing direction (Fig. 3o). Dissolved pores and vugs are unevenly distributed and locally enriched in the rocks, but mostly isolated. Cylindrical dissolved pores in cores are unfilled. The vugs are 1 mm×1 mm to 2 mm×4 mm in size, some larger than 3 mm×5 mm. The surface porosity is generally 4% to 5% and locally more than 10%. Under a microscope, fine pores disperse in tuff or tuffaceous breccia. No bubbles were found in the core gas-bearing property experiment. Gas invasion was observed when drilling cores. The maximum total hydrocarbon is up to 51.5%.

(6) Tuff (Fig. 3q-3s). This clumpy rock is thick-layered and tight, in gray and dark-gray, and contains a small amount of tuffaceous breccia, with horizontal beddings and local microfractures. The tuff is mainly composed of debris, crystal pyroclasts and vitroclastics less than 2 mm in size, and more than 70% in content. In addition, there found a certain number of normal sediments such as mud. The cements are mainly carbonates or clay minerals. The clastic grains have a certain directional characteristics, and showing graded bedding. With the muddy content higher than 50%, the tuff may change into tuffite or tuffaceous mudstone. No vugs or pores were observed in cores, or hydrocarbon shows.

Volcanic lithofacies refers to the summation of the environment of volcanic activity and specific volcanic rock types formed in this environment. “Volcanic activity environment” includes the geomorphic characteristics when volcanic eruption occurs, accumulating modes of volcanic rock, the distance to the crater and natures of magma ^[15].

Based on the previous classification of volcanic edifice and volcanic facies, referring to Wang Pujun’s scheme ^[16], the volcanic facies in the Western Sichuan Basin can be classified into effusive, explosive and intrusive facies according to the characteristics of fissure and multi-point central eruptions, rock types and accumulation features (Fig. 4).

Effusive facies refers to the rock assemblage formed by magma erupting from the crater, extending and flowing in bands and finally condensing on the surface. It was generated in the early eruptive period. By magma erupting, a wide rock cover was formed. The rock type is mainly gray-black and green-gray basalt, which is thick- layered, clumpy and tight, with porphyritic texture locally and well-developed fractures.

Explosive facies refers to various pyroclastics, such as volcanic rock, volcanic gravel and volcanic ash, which were mixed in different proportions and accumulated in situ, or transported by atmosphere, gravity and pneumato hydatogenesis and compacted, finally forming different types of pyroclastic accumulations. Two primary subfacies, i.e. pyroclastic flow and empty stacked subfacies, were identified in Western Sichuan Basin. The rock types include pyroclastic rock, pyroclastic lava and tuff.

Intrusive facies, or sub-volcanic subfacies, refers to shallow-super-shallow intrusive body formed by magma consolidated underground. This intrusive body was formed in the middle-late period of volcanic activity, by magma intruding along the tensional fractures near the volcanic edifice. The rock type is mainly gray diabase porphyrite, which is thick-layered, clumpy and tight, with undeveloped fractures.

Outcrops, drilling and seismic data show that the Permian volcanic rock covers an area of 11×10⁴ km² in Sichuan Basin. From southwest to northeast, the rock thickness decreases sharply from the eruption center of the inner zone to the outer zone and finally reaches 10-300 m (Fig. 1a). Vertically, the rocks are composed of two lithologic units (Figs. 2 and 4). The lower unit is effusive facies basalt, characterized by a widely covered and large-area flood basalt ^[13] lava platform with thick columnar joints. No strong explosion was observed. And the ejecta is mostly basic-ultrabasic molten magma. The upper unit is explosive pyroclastic flow volcanic rock, i.e., pyroclastic rock and pyroclastic lava. Breccia in different sizes and psephicities can be seen on the core cylinder (Fig. 3n). The magma clasts are oriented as long strips and have a flow-like structure (Fig. 3o). With magmatic eruption weakening in the late period, effusive facies basalt of 10-20 m (in wells YS1 and YT1) directly accumulated on the top of the pyroclastic flow volcanic rock. In the area far from the crater (Well ZJ2), the lack of accumulation of explosive facies pyroclastic flow volcanic rocks led to the tuff overlying directly effusive facies basalt. On the outcrops found in the Mugan Town in Yanjin County, similar features were found, namely early columnar flooding basalt (without a bottom) with a weathering-denudation top, and pyroclastic rock overlaid by small columnar basalts (without a top). This indicates that there was a long interval after early volcanic activity. The rock formed in the early period were seriously dissolved and broken due to long-time weathering and leaching. Subsequently, there accumulated pyroclastic rock, and small columnar basalts from late eruption (Fig. 2). In conclusion, magma activities in Western Sichuan Basin show weak, strong and weak activities in its early, middle and late periods, respectively.

(1) Early magmatic activity. The geological setting, i.e. “checkerboard” fault combination ^[13] composed of NE trending major faults and the NW trending secondary faults, which created conditions for large-scale upwelling of deep magma from the upper mantle in the fault system. The early magmatic activity was not strong, instead magma just effused to the surface along the fractures in the crust. Although with strong upwelling energy, due to the relatively large resistance during the upwelling, the super-large scale flood basalt of effusive facies was formed. This is called fissure type eruption. In addition, there accumulated dolerite near the crater (Well YT1) and basalt far from the crater. Between them, there is a transition zone of dolerite (wells YT2 and TF8). Because of high temperature near the crater, and continual upwelling and accumulation of magma, the rocks were crystallized slowly under HT/HP conditions. According to the conclusion and seismic data, the crater location can be identified. If magma supply gradually decreases at the edge of effusive facies, the eruption discontinuities may can be formed. Weathered crusts can be seen at the top of basalt on many outcrop sections in Longshengtang of Leshan and Gongquan of Gong County.

(2) Middle period of magmatic activity. In this period, the frequent tectonic activities created more basement faults which interlaced in different directions, and provided upwelling channels for deep magma, and created favorable conditions for central eruption ^[13]. With energy accumulation, gas in the upwelling magma expanded highly and exploded strongly at the intersection of faults, destroyed the original basalts and limestone of the Maokou Formation near the crater. As a result, a large amount of basalt and lime breccia was erupted. Depending on gravity differentiation, the erupted rocks accumulated and transported along slopes, forming pyroclastic flows. Finally, pyroclastic rock and pyroclastic lava of explosive facies were formed through compaction and condensation. Volcanic ash drifting downwind far from the crater gradually deposited as tuff or sedimentary tuff.

(3) Late magmatic activity. After strong volcanic eruption and energy release, magma eruption slowed down, and developed into small-scale effusive basalt around the crater. The basalt is relatively tight and less weathered, with small columnar joints. As energy continued to decrease in the late period, molten magma gradually flew into volcanic conduits, or along rock surface, joints and tensional fractures near the volcanic edifice, and finally forming diabase porphyrite.

The karst palaeogeomorphology and fracture system in the late sedimentary period of the Maokou Formation controlled jointly the distribution of the volcanic rock. The area where explosive volcanic rocks developed is consistent with the large-scale karst depression formed in the late sedimentary period of the Maokou Formation ^[14]. The volcanic rock in the shallow karst depression zone is thick. There accumulate thick-layered effusive basalt and explosive pyroclastic rock, with relatively complete volcanic facies assemblage. Whereas in the karst slope with under-developed fractures, the volcanic rock is relatively thin. The lithology mainly contains the effusive basalt of the lower unit ^[14], and the overlying thin-layered pyroclastic deposits of empty stacked facies.

In general, volcanic rocks in Western Sichuan Basin have the characteristics of interlacing different lithologies in plane and superposing pinch out in longitudinal direction. In the slope zone of the volcanic edifice and karst depression of the Maokou Formation, there accumulated effusive and explosive volcanic rocks with a relatively complete cycle, i.e. pyroclastic flow volcanic rocks, including pyroclastic rock and pyroclastic lava.

The necessary conditions under which the pyroclastic flow can be formed include ^[1]: continental pyroclastic-steam magmatic eruption, a certain elevation difference between the crater and its peripheral area, the gravity flow generated by clastics erupted from crater, unconsolidated magma clasts keeping plastic and slowly condensing and forming welded texture.

Firstly, a large amount of pyroclastic breccia associated by tuffaceous texture was found in the upper lithologic unit in drilled wells. The breccia in the lower part of the lithologic unit is abundant and large in size, while that in the upper part is smaller, showing a positive graded bed sequence. In addition, the dark lava clasts are elongated/ lacerated, and directional, and locally magma clasts are in oblique arrangement and deflected around large debris, indicating the flow direction. The eruption is characterized by continental pyroclastic-steam magmatic eruption, rather than effusion of other continental facies ^[17,18]. This lithologic feature is similar to that of the pyroclastic flow accumulation of explosive facies in Keramaly Gasfield ^[5,6]. Secondly, the upper rock unit among wells YT1, TF2 and TF8 is different in thickness. Seismic data show that there are elevation differences between the wells. The high-density clastic flow geological body, composed of hot clastics and high temperature gas produced by volcanic eruption, flowed and accumulated along the eruption center to slope or depression zone. As a result, high-density clastic flow volcanic rocks were formed.

In conclusion, the “Emeishan volcanic rocks” were generated by late central eruption, belonging to the volcanic rocks of explosive facies. The high-density clastic flow geological body, composed of hot clastics and high temperature gas produced by volcanic eruption, belongs to the accumulation of volcanic pyroclastic flow. Affected by gravity, hot clastics near the crater were accumulated from the crater along slope to shallow depression zone and then to the empty stacked area of volcanic ash. Large clastics in large particle size of about 60 mm can be seen in the core. The accumulation cycle of pyroclastic lava is the most complete (Well YT1) (Fig. 4). Far from the crater, hot clastics become smaller and thinner, with thick-layered, clumpy or tabular graded bed. The accumulation cycle of the pyroclastic flow volcanic rocks is incomplete (Well TF8). In the far empty stacked area, the fine-grained volcanic ashes are in one-way downwind distribution, forming tuff or tuffite (Well ZJ2) (Fig. 2). In the much farther area, fine-grained volcanic ashes are widely distributed, and can be observed in the profiles of Longshengtang in Leshan, Xiannvdong of Yanjin County, Gongquan of Gong County, Yanjing Village of Junlian Country, even to the east of Huayingshan.

According to the characteristics of volcanic edifice, high-density clastic flow containing hot clastics and high-temperature gas floods along the slope and depression zone, and finally cools down and settles into special geological bodies. Based on outcrops and drilling data and previous studies, the accumulative subfacies of the pyroclastic flow can be further divided into four accumulation zones according to the distance from the crater, i.e. crater rim, proximal, intermediate and distal zones (Fig. 5).

The proximal accumulation zone is a complete volcanic eruption cycle. The early cycle is dominated by dolerite, whereas the middle cycle is dominated by pyroclastic rocks and pyroclastic lava. In the late cycle, effusive basalt near the crater can be seen in the upper lithologic unit, and intrusive diabrite is in the lower unit (Well YT1). The lithofacies of the intermediate accumulation zone is similar to that of the proximal one. But the basalt grain is small, and no dolerite is found in the early cycle. There are pyroclastic rocks, pyroclastic lava and thin tuff in the middle cycle (Well TF8). In the distal accumulation zone (Well ZJ2), the early cycle is dominated by effusive facies basalt, and the late cycle consists of tuff, which is thin-layered but widely distributed. Therefore, the volcanic rock in this area is stratified vertically and zoned horizontally.

Pyroclastic flow reservoirs are mainly composed of pyroclastic rocks and pyroclastic lava. It is rare to find reservoir in basalts. According to the data of 445 samples obtained from core plugs, full-diameter cores and field outcrops, the reservoir lithologies are as follows.

(1) Pyroclastic rock. The pyroclastic rock reservoir developed pores and fractures, which belongs to the fracture-pore reservoir. The pores are mainly dispersive tuffaceous micropores, and less dissolution caves and fractures. The porosity is 1.93%-27.26%, with an average of 13.08% (Fig. 6a). The permeability is (0-4.43)×10^-3 μm², with an average of 0.24×10^-3 μm² (Fig. 6b). Influenced by microfractures, the relationship between porosity and permeability shows two obvious features. The reservoir sample with microfractures is featured by low porosity and high permeability which is (0.1-1.0)×10^-3 μm², and there is no correlation between the porosity and the permeability. For the sample with under-developed microfractures, a positive correlation exists between the porosity and the permeability. With the increase of porosity, the pore structure becomes better and the permeability increases. The multiple correlation coefficient of porosity and permeability of the porous reservoir is 0.38 (Fig. 6c).

(2) Pyroclastic lava. The reservoir is a porous reservoir with abundant pores and rare fractures. Dispersive tuffaceous micropores and ultra-micropores are dominant, followed by dissolution caves and intergranular pores. The porosity is 0.29%-24.27%, with an average of 12.15% (Fig. 6d). The permeability is (0-0.90)×10^-3 μm², with an average of 0.17×10^-3 μm² (Fig. 6e). Whether on the porosity and permeability distribution diagrams or porosity-permeability relationship curve, it can be seen that the permeability becomes better with the increase of porosity. The whole-rock multiple correlation coefficient of porosity and permeability is 0.66 (Fig. 6f).

(3) Basalt. Pores are not developed in basalt reservoir, and with rare fractures. The porosity ranges from 0.35% to 2.16%, with the average of 0.85% (Fig. 6g). The permeability is (0-0.40) ×10^-3 μm², with the average of 0.01×10^-3 μm² (Fig. 6h). There is a poor correlation between the porosity and the permeability, with the multiple correlation coefficient of 0.008 (Fig. 6i).

CT scanning-based analysis on the reservoir of pyroclastic flow volcanic rocks without microfractures shows that the reservoir is distributed in the tuffaceous structure (the red area) under the influence of rock fabric. In the red area, the porosity and the permeability are well correlated, the pores are widely connected with large pore space. The blue and green parts represent pores less connected or isolated. The gray part refers to the area where pores are not developed. On the same transection and longitudinal cylinder, the proportions of red and gray areas are different (Fig. 7a), indicating the strong heterogeneity of the pyroclastic rock reservoir, and the well-developed pores and good connectivity in the tuffaceous matrix (red). Mercury injection analysis also shows that the pore structure of the pyroclastic rock reservoir is generally well sorted, characterized by low drainage pressure, small pore size and ultramicro-fine throat. The maximum pore radius of pore throat is 0.4-0.7 μm. The mercury injection curve is thin and skewed, showing good connectivity (Fig. 7b). High porosity and ultra-micro pore structure are the main characteristics of this set of pyroclastic flow volcanic rocks.

The delineation for spatial distribution of the pyroclastic flow volcanic rocks is a precondition for finding favorable reservoir. The characteristics, such as variable lithology, big electrical property contrast, great difference in physical properties from surrounding rock, and obvious seismic reflection features, provide conditions for seismic identification and distribution description of pyroclastic flow volcanic rocks. In this study, based on the 3D seismic data from Chengdu-Jianyang Block, and the seismic reflection attributes and waveform characteristics, the spatial distribution of the pyroclastic flow volcanic rocks was delineated using horizontal time slices and 3D space engraving.

The thick-layered pyroclastic flow volcanic rocks drilled in Western Sichuan Basin is different in properties from surrounding rocks. The logging data (Fig. 4) shows a large electrical property contrast between the volcanic rocks with different lithologies (Table 1). The logging characteristics of effusive facies basalt and intrusive facies diabase porphyrite include middle-high natural gamma ray (GR), low interval transit time (AC), high density and middle-high resistivity. The AC is 5000-5500 m/s, and the density is about 2.9 g/cm³. The logging characteristics of pyroclastic rock and pyroclastic lava include middle-low GR, high AC, low density and low resistivity. The AC is 3500 to 4500 m/s, and the density is about 2.8 g/cm³. The overlying surrounding rock is characterized by high GR, high neutron porosity, low density and low resistivity. The AC is 5500 to 6000 m/s.

The result of well-to-seismic calibration for Well YT1 (Fig. 8) indicates that, with strong heterogeneity, the pyroclastic flow volcanic rock shows large velocity changes in the vertical and horizontal directions. The seismic reflection characteristic of the volcanic rock is significantly different from those of basalt and surrounding rocks. The seismic reflection of the proximal and intermediate accumulations is characterized by hummocky uplift, poor stratification, middle-weak amplitude, middle-low frequency, inner chaotic and worm-like weak reflection, and local overlying onlap. The seismic reflection of the distal accumulation is smooth and thin (one phase). The top reflection is continuous and medium to strong, while the bottom is intermittent and weak. On the whole, the reflection is subparallel and continuous. The geophysical reflection characteristics of all accumulations are shown in Table 2.

Based on the seismic reflection characteristics mentioned above, the spatial envelope surface of the pyroclastic flow volcanic rocks was interpreted by analyzing seismic facies. The distribution map of volcanic edifices was drawn to depict the spatial distribution by using 3D visualization engraving technology, providing a basis for the establishment of volcanic eruption mode and distribution prediction of volcanic lithofacies. According to the 3D seismic section of instantaneous amplitude from Chengdu-Jianyang Block (Fig. 9), different from the basalt and surrounding rocks, the petrophysical properties of pyroclastic flow volcanic rocks show strong anomaly of instantaneous amplitude, and chaotic inner reflection. The clear reflection characteristics of volcanic edifice indicate that there developed isolated or multiple hummocky uplifts. The depiction for thickness of pyroclastic flow volcanic rocks is shown in Fig. 10. The volcanic rocks in Jianyang Block are clumpy or linear in plane distribution. The area centered around wells YT1, TF2 and TF8, where three volcanic edifices are developed, is the most favorable exploration zone. The volcanic edifice near Well YT1 has the largest anomalous reflection area, which is about 110 km², with a thickness of 80-200 m. The anomalous reflection area of the edifice near Well TF8 is linearly distributed in a nearly NE direction, covering about 90 km² and 50-180 m thick. The anomalous reflection area of the edifice near Well TF2 is in isolated clumpy distribution, covering about 40 km² and 20-110 m thick. The favorable area of pyroclastic flow volcanic rocks predicted by 3D seismic data is up to 500 km². Therefore, the area on the southwest of wells TF2-TF8, where pyroclastic flow volcanic rocks are widely distributed, is favorable to make a breakthrough in future exploration.

The reservoir rocks of Permian volcanic rocks in Western Sichuan Basin are pyroclastic flow volcanic rocks of explosive facies, composed of pyroclastic rocks and pyroclastic lava. The effusive facies basalt is poor in reservoir properties. Core and thin section data show that the pyroclastic rock reservoir is characterized by the high and ultra-high porosity, and the reservoir space is composed of primarily dispersed micro- and ultramicro-dissolution pores, and secondly dissolution vugs, intergranular pores and fractures. The reservoir is also characterized by strong heterogeneity and homogeneous distribution of pore throats. A large number of devitrified micropores are the result from rapid condensation of the pyroclastic flow volcanic rocks (Fig. 3v), and they are easily dissolved by thermal fluid, resulting in the increase of pore space. Fractures are well developed in the basalt reservoir. Weathering and leaching promote the development of dissolved pores (Fig. 3w) and fractures (Fig. 3d) ^[19]. This basic-meso-basic pyroclastic rock reservoir, with the thickness of 70-100 m and formation pressure coefficient of 2.26, belong to the overpressure porous volcanic reservoirs.

The exploration for Permian volcanic gas reservoir reveals that super thick microporous reservoirs of pyroclastic flow volcanic rocks are widely developed at the bottom of the Upper Permian in Sichuan Basin. The distribution of gas pools is primarily controlled by volcanic reservoirs. Based on 3D seismic data, a favorable reservoir distribution area of 500 km² was delineated and confirmed by drilling data. The reservoir is very thick and the cycle is relatively complete in the slope from the proximal to the intermediate accumulation zones, so it is the most favorable exploration zone. There is a good source-reservoir-caprock assemblage in the Western Sichuan Basin, which provides excellent forming conditions of volcanic gas reservoir. The Permian reservoir of pyroclastic flow located at the outer edge of the Lower Paleozoic Deyang-Anyue inner platform rift gets sufficient gas supply from source rocks of the underlying Cambrian Qiongzhusi Formation and the overlying Upper Permian Longtan Formation. The Upper Permian Longtan Formation is a marine-continental transitional source rock, which can provide hydrocarbon for hummocky volcanic reservoir through lateral channels. The underlying Cambrian Qiongzhusi shale is a set of high-quality source rock with high organic abundance and great thickness. It directly supplies hydrocarbon through the source faults from the basement toward inside the Feixianguan Formation, effectively connecting the hydrocarbon source and reservoir, forming an efficient and proximal source-reservoir relationship. Meanwhile, there developed the direct caprock of Longtan Formation and regional caprock of Triassic gypsum salt rock, which provide good preservation conditions. As a direct caprock, the Longtan Formation mud shale overlying the “Emeishan volcanic rocks” not only blocks hydrocarbon from migrating upward, but also acts as a secondary hydrocarbon source^[20,21], providing favorable conditions for lateral sealing and hydrocarbon supply. In conclusion, the volcanic rocks have good geological conditions for gas accumulation.

According to the current exploration, industrial gas flows have been obtained from wells YT1 and TF2 which drilled into reservoirs of pyroclastic flow volcanic rocks. The tested production is (4-20)×10⁴ m³. The resource is estimated to be 3000×10⁸ m³. At present, based on the gravity, magnetic and electrical survey, and two- and three-dimensional seismic data, a favorable zone of 6000 km² in Chengdu-Jianyang-Santai area has been preliminarily delineated, showing the great exploration potential of pyroclastic flow volcanic rocks in this area.

“Emeishan volcanic rocks” are widely developed in Western Sichuan Basin. Vertically, there are two lithologic units. The lower unit is the result of fissure type eruption, which is dominated by effusive facies basalt. The upper unit is the result of central eruption, which is the typical accumulation of pyroclastic flow of explosive facies, including pyroclastic rock, pyroclastic lava and tuff. The two lithologic units may be in unconformity contact.

The accumulative subfacies of pyroclastic flow in Western Sichuan Basin can be divided laterally into four accumulation zones, including the crater rim, proximal, intermediate and distal zones. The plane distribution of the accumulative subfacies is controlled by crater location and eruption period, while the vertical distribution is mainly controlled by eruption period. The thickness of pyroclastic rocks and pyroclastic lava developed in the upper unit gradually decreases from the proximal zone to the distal one.

Reservoir rocks from the proximal to the intermediate accumulation zones include pyroclastic rock and pyroclastic lava which are very thick and have abundant pores. The reservoir space is almost composed of micropores and local ultra-micropores, which are well sorted. Characterized by strong heterogeneity and relatively homogeneous distribution of pores and throats, the reservoir in this area is the most favorable exploration target.

The pyroclastic flow volcanic rocks have special geological features, wide distribution range and distribution space, and distinct features and attributes of seismic reflection. The seismic profiles show strong amplitude anomaly and big phase dislocation. 3D seismic attributes and visualized engraving technique are effective for describing the spatial distribution of the pyroclastic flow volcanic rocks. It’s found that pyroclastic flow volcanic rocks are widely developed in Jianyang Block. Three volcanic edifices have been found around wells YT1, TF2 and TF8, covering an area of about 500 km². It is the most favorable zone for volcanic rocks exploration.