Geological characteristics and enrichment model of Permian Mao 1 Member shale gas reservoirs in Nanchuan area

Abstract

To understand the shale gas reservoirs in the first member of the Permian Maokou Formation (Mao 1 Member) in Nanchuan area of the Sichuan Basin, we used outcrops, cores, mineralogical and geochemical datas, thin-section analysis, and argon-ion-polishing scanning electron microscopy results with logging and seismic attribute prediction techniques to study the sedimentary facies, source rocks, reservoir properties and types. The geological characteristics and enrichment model of shale gas reservoirs of the Mao 1 Member were revealed and are summarized as follows: (1) the Maokou Formation can be divided into four members from bottom to top. The Mao 1 Member is located in the relatively deep-water gentle slope of a carbonate platform, comprising well-developed dark-gray/grayish-black carbonaceous mudstone, calcareous mudstone, argillaceous limestone, and bioclastic limestone. Carbonaceous and calcareous mudstones, with a relatively high total organic carbon content (TOC0.5–5.1%) and moderate thermal maturity (Ro: 2.0–2.3%), are potentially good hydrocarbon source rocks with possibly new type of atypical shale gas reservoir; (2) organic pores, dissolution pores, grain margin fractures, and shrinkage fractures are developed in the Mao 1 Member and may form a reservoir of a complex “pore–fracture–network,” which is beneficial to gas storage and migration; (3) the shale gas in the Mao 1 Member has the accumulation and enrichment model of “integrated source and reservoir, continuous distribution, and structure-fracture dual reservoir control” with a continuous distribution area exceeding 500 km2 and a geological resource volume exceeding 70 billion cubic meters; therefore, it has great exploration potential for shale gas. The research results are crucial in improving the understanding of new type of shale gas geology and guide shale gas exploration in southeastern Sichuan.