The Permian extensional tectogenesis, a tectonic movement with widespread impact, is marked in Southwestern China by large-scale basic and ultrabasic magma eruptions. Many (95) wells in Sichuan Basin encountered Upper Permian igneous rock, most of them (55 wells) are located in the southwest of the basin (hereinafter called Southwest Sichuan), some (35 wells) in the east of the basin (hereinafter called East Sichuan) and a few in Jianyang, Moxi and Nanchong in the middle of the basin (hereinafter called Central Sichuan). Early studies suggested that Upper Permian igneous rock in Sichuan Basin was only distributed in Southwest Sichuan (volcanic overflow facies) and East Sichuan (intrusive facies).

Upper Permian igneous rock in Sichuan Basin consists of basic intrusive rock and extrusive rock. The igneous rock in Southwest Sichuan is largely basalt, with a small amount of pyroclastic rocks, resulted from basic magmatic activity in the early phase of Late Permian and in disconformable contact with the underlying Middle Permian Maokou Formation (P₂²) and overlying Upper Permian Shawan Formation (P₃²)^[1,2,3]; the igneous rock in East Sichuan is largely basalt, with a small amount of diabase, between Upper Permian Longtan Formation (P₃²) and Maokou Formation; the igneous rocks in Jianyang area in Central Sichuan are complex, consisting of diabase, basalt and pyroclastic rocks and in disconformable contact with underlying Maokou Formation and overlying Shawan Formation.

Despite oil and gas shows in early exploration, the igneous rocks were largely ignored with low level of research. Well ZG1 drilled in Middle Permian Maokou Formation in 1992 tested gas in Upper Permian basalt. The subsequent Well ZG2 drilled in basalt, however, was unsuccessful. Well YT1 drilled in pyroclastic rock in Jianyang area in 2018 tested high gas flow, suggesting certain oil and gas potential in Permian igneous rocks in Sichuan Basin. Based on recent exploration achievements and analysis data on igneous rocks and taking into account regional geological conditions, the distribution characteristics and gas-bearing property of Upper Permian igneous rocks in Southwest Sichuan and Jianyang area in Central Sichuan are examined in this work (Fig. 1).

2.1.1. Characteristics of the volcanic lava

Volcanic lava is formed by molten magma from volcanoes under condensation conditions on the surface^[4,5]. Upper Permian volcanic lava in Sichuan Basin is dominated by basalt, distributed in the southwest, Jianyang area, Huayingshan area and east of Sichuan. It is most developed and largest in area in Southwest Sichuan.

Analysis of Upper Permian basalt in Southwest Sichuan shows that its mineral compositions include anorthose (35%- 60%), pyroxene (5%-10%), chlorite (12%-30%), ilmenite and magnetite (5%-13%), sphene (2%-10%), calcite (2%- 8%), quartz (2%-7%), siderite (less than 5%) and volcanic hyaline (20%-35%). Its chemical components are dominated by FeAl silicate rock: SiO₂ (47.11%-48.42%), Al₂O₃ (13.69%- 14.23%), Fe₂O₃ (13.57%-14.04%), CaO (6.95%-8.66%), MgO (4.04%-4.56%), Na₂O (1.67%-2.71%), and K₂O (1.10%- 1.86%).

Basalt can be divided by rock structure into cryptocrystalline-microcrystalline basalt, porphyry basalt, vesicular basalt, and almond basalt (Fig. 2).

The cryptocrystalline-microcrystalline basalt is characterized by uniform distribution of minerals and components in the rock, without direction, pore, almond or phenocryst and with intersertal texture, massive structure and generally column-shaped joints.

The porphyry basalt has porphyritic, intersertal and intergranular textures. Phenocryst is formed by multiple anorthose druses in star-scattered, radial, beam and girdle patterns and up to 5 mm×13 mm. The intercrystal pore is filled with granular minerals such as pyroxene, ilmenite, magnetite, sphene and olivine, as well as hyaline or flaky minerals.

The almond basalt refers to the basalt containing almond formed by siliceous matter (quartz, prehnite and chalcedony), chlorite, zeolite, siderite, calcite and asphalt entering pores created after volatile matter in the magma escapes during condensation^[6]. If the pores are not filled or slightly filled, the rock is called vesicular basalt. The pores (almonds) are mostly circle or ellipse shape 1-3 mm in size and sometimes in the shape of tadpole due to lava flow, extrusion, stretching and explosion.

2.1.2. Characteristics of the pyroclastic rock

Pyroclastic rock is often seen in volcanic eruption mechanism, especially central eruption mechanism. Upper Permian pyroclastic rock in Sichuan Basin is mainly distributed in Jianyang area, where Well YT1 encountered 131 m pyroclastic rock. It is undeveloped in East and Southwest Sichuan.

Analysis of 35 cores of Upper Permian pyroclastic rock collected from Well YT1 shows the content of calcite is the highest, up to 95.5% and averaging 42.0%; followed by clay mineral, up to 57.7% and averaging 19.8% and then by anorthose, up to 41.4% and averaging 15.5%. In addition, common minerals also include potassium feldspar (averaging 3.7%), quartz (averaging 9.6%) and analcite (averaging 6.4%); some samples also contain ferrodolomite, dolomite, pyrite and corundum. Main minerals in the pyroclastic rock from Well YT1 are clay mineral and calcite, suggesting strong secondary alteration of the pyroclastic rock. Geochemical analysis shows the content of SiO₂ at 35.44%-52.84% averaging 43.08% and that of Na₂O+K₂O at 2.27%-8.36% averaging 5.91%. It is alkaline basalt, with some points falling into tholeiitic basalt zone in SiO₂-(Na₂O+K₂O) relation chart (Fig. 3). This suggests that the chemical composition of the pyroclastic rock from YT1 is characterized by both alkaline basalts and tholeiitic basalts, and belongs to the subalkaline type of transitional basalt magma eruption, which is different from basalt in Kangdian area. Its content of SiO₂ is lower and that of Na₂O+K₂O higher than the average values of alkaline basalt in continental rift valley and of basalts around the world^[7,8] (Table 1).

Tephra in pyroclastic rock is not regular in shape, and often in knife edge, triangle, harbor, tree branch and lacerate shapes. The grain sizes vary widely from agglomerate to crystal pyroclast. The main composition is basalt resulted from cooled volcanic lava, with breccia, spot and tuffaceous textures.

In addition, transitional rocks between volcanic lava and pyroclastic rock, such as tuffaceous lava and breccia, are frequently observed.

The main intrusive rock is diabase distributed in small areas, as revealed by some wells in East Sichuan, such as 20 m thick diabase revealed by Well TD3, in lower Longtan Formation and 28 m thick diabase revealed by Well YT1 beneath basalt and pyroclastic rock. The main minerals are common pyroxene and basic anorthose, with small amounts of olivine and magnetite. Anorthose has higher idiomorphism degree than pyroxene, the former mostly in semi-idiomorphism to idiomorphism and the latter in anhedron; the pyroxene typically wraps around anorthose and olivine.

Based on analysis of drilling, seismic and outcrop data, Upper Permian igneous rocks in Sichuan Basin are mainly distributed in three regions: large-scale overflow facies basalts in the southwestern part of the basin, from a deep fault zone in Kangdian area^[9]; intrusive rock (diabase), extrusive volcanic lava (basalt) and pyroclastic rock in Jianyang-Santai area in the central and western part of the basin; intrusive and extrusive diabase and basalt in Dazhou-Liangping area in the eastern part of Sichuan (Fig. 4).

Volcanic activity and distribution of igneous rock are controlled by the activity period and intensity of large, deep extension faults. Southern China was in an extensional tectonic environment in Permian^[10]. In this context, large, deep extension faults in the basement activated, enabling magma to go up and intrude into sedimentary rock or erupt from the ground; distribution of igneous rock in the basin is related to the activity of Huayingshan and Longquanshan fault zones.

Upper Permian basalt in the southwest of the basin belongs to fissure type continental overflow facies and is 0-400 m thick, its bottom in unconformable contact with Middle Permian Maokou Formation (P₂²_m) and its top in unconformable contact with Upper Permian Shawan Formation (P₃²). It is dominated by basalt, with pyroclastic rock in local parts, and without diabase. It thins along the Pujiang-Qingshen-Qianwei-Muchuan-Pingshan zone toward the basin, with cumulative thickness ranging from 50 m to 150 m; its pinch-out boundary is located in Qionglai, Yanjinggou, Renshou, Weixi and Yibin, transitioning to Longtan Formation (P₃¹) (Fig. 5).

Distribution of igneous rock in this area has three characteristics: (1) Its thickness decreases from the rim to the interior of the basin, and vary not much overall, without extremely thick or thin zones found. (2) When its thickness decreases to 150 m, it branches into 2-7 or as many as 10 sub-layers, but the basalt section near the bottom keeps stable and extends farther. (3) The varying gradient of basalt thickness in the basin is 3.75%, equivalent to a slope of 13°.

There are obvious rhythmic structures vertically in overflow facies basalt, and the single rhythmic layer consists of: pyroclastic rocks (undeveloped) → gray and dark gray porphyritic basalts (unstable) → dark gray and purple microcrystalline-cryptocrystalline basalts → dark greyish green porous and amygdaloid basalts. The single rhythmic layer is 10.5- 41.1 m thick and can be divided into up to 10 overflow rhythmic layers.

So far 2 wells (YT1 and YS1) in the area have encountered Upper Permian igneous rock, which show that its bottom is in unconformable contact with Maokou Formation and its top in unconformable contact with Shawan Formation (P₃²). The igneous rock consists of intrusive and extrusive diabase, basalt and pyroclastic rock from bottom up, characterized by central volcanic eruption.

The central volcanic eruption shows the rhythm and sequence of vertical volcanic clastic rocks (agglomerate and breccia) → volcanic lava → tuffaceous lava from bottom to top (Fig. 6).

According to the well - seismic calibration, the top of the volcanic rock exhibits interrupted strong reflection and its interior exhibits disorderly bright spot reflection and disorderly wedge-like reflection; its seismic reflection is in hilly patterns (Fig. 7). Prediction of distribution of the pyroclastic rock based on available drilling and seismic data shows 3 pyroclastic rock zones along Longquanshan fault zone, which may represent 3 volcanic eruption systems in blocks horizontally. The 3 systems, from south to north, are 1 650 km² Jian-yang eruption system, 750 km² Zhongjiang eruption system and 2 100 km² Santai eruption system; their center thickness is typically 200-350 m, thinning till pinching out outward.

Boreholes in Huayingshan outcrop and 35 wells in Dazhou and Liangping reveal 4-65 m thick basalt and diabase. Basalt prevails, with diabase observed only in just a few boreholes (Fig. 8), distributed in lower Longtan Formation (P₃¹). That the basalt in Well YA8 is intercalated with thin carbonatite suggests that this area has gone through at least two volcanic eruptions.

The reservoir space of Permian volcanic lava in the southwestern part of Sichuan Basin consists of pores and fissures. Pores include observed eroded void, pore, intragranular (intracrystalline) dissolution pore, gas pores, intergranular (intercrystalline) dissolution pore and intercrystalline pore; fissures include column-like joints and structural fissures (Fig. 9). Different types of volcanic lava all contain some reservoir space, but different somewhat in types. The main reservoir space in almond, porous and porphyry basalts is pore, followed by structural fissure, whereas that in dense basalts is column-like joint followed by structural fissure, with hardly any pores.

The basalt is relatively dense, with poor reservoir property. Samples with a matrix porosity of less than 4.0% account for 93.94%; samples with a matrix porosity of 4.0%-7.0% account for 5.05%; samples with a matrix porosity of more than 7.0% account only for 1.01%; the samples have an average porosity of only 1.39%. The vesicular basalt has the highest porosity, averaging 5.41%; microcrystalline-cryptocrystalline basalt has the lowest porosity, averaging 0.85% (Table 2).

According to logging interpretation, Permian basalt reservoirs in Southwest Sichuan with a porosity of more than 1.0% and moderately developed fissures are 4.3-50.6 m thick in wells, with a reservoir/stratum thickness ratio of 8.9%-57.7%; the reservoirs with a porosity of more than 1.5% is 0-32.1 m thick in the wells, with a reservoir/stratum thickness ratio of 0-29.7%, suggesting poor reservoir property of volcanic lava. Most of the reservoirs are located in the mid-upper or top part of eruptive cycle, and poor in lateral connectivity and comparability^[11].

The reservoir space of Permian pyroclastic rock distributed in the central and western part of the basin consists of pores and fissures. The pores include residual dissolution pores and vugs, residual gas pores, intragranular (intracrystalline) dissolution pores, intergranular (intercrystalline) dissolution pores, intercrystalline pores, condensing shrinkage pores, de-vitrification micropores and inter-gravelly pores; the fissures include column-like joints and structural fissures.

According to analysis of cores from Well YT1, the pyroclastic rock has a porosity of 8.66%-16.48% averaging 13.76% and a permeability of 0.604×10^-3 μm²-4.43×10^-3 μm² averaging 2.446 3×10^-3 μm² (Fig. 10). Logging interpretation shows the reservoir is 100.3 m thick and 12.5% in average porosity.

The Upper Permian basalt at 2 870-2 883 m depth of Well ZG1 in the southwestern part of the basin was tested a gas production of 25.61×10⁴ m³/d; the Upper Permian pyroclastic rock intervals at 5 628-5 644 m and 5 646-5 675 m depths of Well YT1 were tested a gas production of 22.5×10⁴ m³/d.

Natural gas from Wells ZG1 and ZG2 in the southwestern part of the basin is characterized by high content of methane (91.2%-96.59%), high dryness coefficient (99.8%), high isotope composition (δ¹³C₁ -28.1‰, δ¹³C₂ -31.56‰ and δ¹⁸O -135.4‰), low heavy hydrocarbon (without butane or heavier components), low carbon dioxide (0-0.042%) and no hydrogen sulfide (Table 3). According to gas-source correlation, the natural gas comes mainly from Middle Permian and is likely to have some gas from older hydrocarbon source rock formations (Cambrian Qiongzhusi Formation) mixed in. Basalt reservoirs in Southwest Sichuan are characterized overall by ultra-low porosity and permeability, small reservoir thickness and rapid lateral change, and mostly pore-fissure type.

Natural gas from pyroclastic rock in Well YT1 of Jianyang area has similar characteristics as that from Well ZG1, with 98.9% of methane, 0.35% of ethane, 0.03% of propane, no butane or heavier components, no hydrogen sulfide and 0.18% CO₂. Analysis shows the natural gas comes from hydrocarbon source rock in Cambrian Qiongzhusi Formation. The pyroclastic rock reservoirs in Jianyang-Santai area are characterized by medium-high porosity and permeability, large thickness and good quality and are largely fissure-pore type.

The formation of gas reservoir in igneous rock is not much different from that of conventional gas reservoirs and must have generation, storage, sealing, transport, accumulation and preservation conditions in good match. Large high-abundance gas reservoirs can be formed in igneous rock if formation conditions exist and are in good match. Otherwise, reservoir cannot be formed or is small in scale.

The igneous rock itself is unable to produce hydrocarbon and needs external source of hydrocarbon. The hydrocarbon source rock formations beneath volcanic rock in Southwest Sichuan consist mainly of Middle Permian carbonate and Lower Cambrian Qiongzhusi Formation, but because Caledonian movement caused uplift erosion to Sinian-Lower Cam-

brian, the latter has a small residual thickness (of 13.5 m from Well ZG1) and thus limited hydrocarbon generation capacity; so the hydrocarbon comes mainly from Middle Permian^[12], but the Middle Permian carbonate is moderate in hydrocarbon generation capacity. Therefore, the gas source for basalt in the southwest of the basin is slightly insufficient.

The hydrocarbon source rock formations beneath the igneous rock in Jianyang-Santai area consist also of Middle Permian and Cambrian Qiongzhusi Formation, but this area was "Deyang-Anyue Taphrogenic Trough" in Early Palaeozoic Era^[13], where the hydrocarbon source rock in Cambrian Qiongzhusi Formation is thick (200-350 m), with strong hydrocarbon generation capability, and is the main hydrocarbon source layer (Table 4).

Preservation conditions of gas reservoir in igneous rock play an important role in forming gas reservoir. The southwest of the basin is dominated by low, steep, fault-folds with large amplitude and faults. This, on one hand, greatly improves

reservoir permeability conditions and transport system, enabling part of the oil and gas generated in the underlying source rock formation to move to basalt traps to form reservoirs; and on the other, deep fault zones, especially those close to the outcrop area would inevitably damage gas reservoirs. In contrast, the Jianyang-Santai area in Central-West Sichuan transition area has low levels of tectonic deformation, and a small number of faults in small scale; local caprock consists of Upper Permian clay shale and regional cap rock of Triassic Jialing River Formation and Leikoupo Formation gypsum salt rock, providing favorable preservation conditions.

According to analysis of inclusions in basalt outcrop samples from Longmen Cave of Mount Emei, the homogenization temperature of the inclusions is 79.3 °C-133.6 °C and the burial depth of basalt is estimated at 2 200-3 900 m, equivalent to the burial depth of the area in early-medium Jurassic period. This suggests oil and gas migration and accumulation occurred in Yanshan Period.

To sum up, Jianyang-Santai area is the favorable area for oil and gas exploration in igneous rock, with high-quality reservoirs, abundant hydrocarbon source, good preservation conditions and good match of the reservoir forming conditions.

Permian volcanic eruption in Sichuan Basin had multiple cycles in time sequence. Horizontally, the overflow facies is characterized by large area, low gradient and contiguous distribution; the eruptive facies is characterized by lumpy distribution, wide variation in thickness, with multiple eruption systems appearing along the fault like a string of beads.

Permian igneous rocks are mainly distributed in three regions with obvious differences in lithological assemblages and distribution characteristics: the southwestern part of the basin, which is dominated by large-scale overflow facies basalt; the central and western part of the basin, Jianyang-Santai area, where intrusive rocks, volcanic lavas (basalts) and pyroclastic rocks occur; and the eastern part of Sichuan, Dazhou-Liangping area, only diabase and basalt exist.

Volcanic lava reservoirs are characterized by ultra-low porosity and permeability, small reservoir thickness and vertical distribution in the upper and middle parts of the cycle, with unstable lateral distribution and developed fissures, and of pore-fissure type. Pyroclastic rocks are medium to high porous reservoirs, with large thickness and good reservoir quality, and are pore type.

The natural gas in basalt in the southwest of the basin comes mainly from Middle Permian while the natural gas in pyroclastic rock in Jianyang area, the central and western part of the basin, is mainly from Cambrian Qiongzhusi Formation. Comparison shows that Jianyang-Santai area in the central and western part of the basin has better match of reservoir forming elements of pyroclastic rock than those of basalt in the southwest of the basin, and thus is the favorable area for oil and gas exploration in igneous rocks.