Development types and natural gas accumulation patterns of volcaniclastic reservoirs in the first member of Upper Permian Wujiaping Formation in northeastern Sichuan Basin

The Sichuan Basin and its periphery are rich in natural gas resources, and unconventional oil and gas reservoirs in igneous/volcaniclastic rocks show great exploration potential. Influenced by the Emei Taphrogenesis, a set of volcaniclastic rocks developed during the deposition of the first member of the Wujiaping Formation (Wu1 member) in the northeastern Sichuan Basin. However, the reservoir types and natural gas accumulation patterns of these volcaniclastic rocks remain unclear. Focusing on well LY1 and well YB701, combined with logging data from multiple exploration wells, core descriptions, geochemical data, and reservoir information, this study clarified the development types and accumulation patterns of natural gas reservoirs in volcaniclastic rocks of the Wu1 member in the northeastern Sichuan Basin. The results showed that: ① The vertical lithological assemblages in the Wu1 member varied greatly across different regions, exhibiting strong heterogeneity, and were generally characterized by interbeds of bioclastic limestone and volcaniclastic rocks. ② The Wu1 member developed two exploration types of reservoirs: "self-generating and self-storing" type and "near-source" type. The "self-generating and self-storing" reservoir type was characterized by high total organic carbon (TOC), high gas content, and high porosity, whereas the "near-source reservoir" type was characterized by low TOC, low gas content, and high porosity. ③ The spatial distribution of the two reservoir types was controlled by the distance from volcanic vents and depositional environment. The reservoir storage space was dominated by clay mineral shrinkage pores, dissolution pores, and microfractures, while organic pores were underdeveloped. The pore structure was dominated by mesopores, followed by micropores, and the preservation conditions were excellent. ④ Two types of natural gas accumulation patterns were developed in the Wu1 member, and there were differences between the two patterns in terms of reservoir spatial distribution and hydrocarbon supply. The Puguang area was characterized by a "sandwich" structure with multiple sources of hydrocarbon supply, whereas the Yuanba area was characterized by a "two-story" structure with a single source of hydrocarbon supply. This study provides guidance and reference significance for the exploration and evaluation of natural gas from volcaniclastic rocks in the Wu1 member.