Main factors controlling unconventional gas enrichment and high production in the first member of Permian Maokou Formation, southeastern Sichuan Basin, SW China

Abstract

Based on the data of drilling, logging, experiment and gas testing in the Nanchuan area, southeastern Sichuan Basin, the hydrocarbon generation potential, gas genesis, occurrence state, migration, preservation conditions, pore and fracture features and accumulation evolution of the first member of Permian Maokou Formation (Mao 1 Member) are systematically studied, and the main controlling factors of unconventional gas enrichment and high production in marlstone assemblage of Mao 1 Member are discussed. (1) The enrichment and high yield of unconventional natural gas in the Mao 1 Member are controlled by three factors: carbon-rich fabric controlling hydrocarbon generation potential, good preservation controlling enrichment, and natural fracture controlling production. (2) The carbonate rocks of Mao 1 Member with carbon rich fabric have significant gas potential, exhibiting characteristics of self-generation and self-storage, which lays the material foundation for natural gas accumulation. (3) The occurrence state of natural gas is mainly free gas, which is prone to lateral migration, and good storage conditions are the key to natural gas enrichment. Positive structure is more conducive to natural gas accumulation, and a good compartment is created jointly by the self-sealing property of the Mao 1 Member and its top and bottom sealing property in monoclinal area, which is favorable for gas accumulation by retention. (4) Natural fractures are the main reservoir space and flow channel, and the more developed natural fractures are, the more conducive to the formation of high-quality porous-fractured reservoirs and the accumulation of natural gas, which is the core of controlling production. (5) The accumulation model of unconventional natural gas is proposed as “self-generation and self-storage, preservation controlling richness, and fractures controlling production”;. (6) Identifying fracture development areas with good preservation conditions is the key to successful exploration, and implementing horizontal well staged acidizing and fracturing is an important means to increase production and efficiency. The study results are of referential significance for further understanding the natural gas enrichment in the Mao 1 Member and guiding the efficient exploration and development of new types of unconventional natural gas.