Geochemical characteristics of natural gas in tight sandstone of the Chengdu large gas field, Western Sichuan Depression, Sichuan Basin, China

Abstract

SINOPEC has made significant progress in natural gas exploration in terrigenous tight sandstone in the Western Sichuan Depression, making it one of the predominant exploration areas in the Sichuan Basin. Although the Chengdu large gas field was located in the main reservoirs of Upper Jurassic tight sandstone, few geochemical proofs were presented to support the explanation of the gas source, and regional differences in natural gas have been poorly investigated. The Jurassic tight gas in the Chengdu large gas field has a dryness coefficient (C1/C1-5) of 0.939–0.982, and the δ13C1, δ13C2, and δD1 values range from −33.7‰ to −30.7‰, −25.4‰ to −22.3‰, and −162‰ to −153‰, respectively, with positive carbon and hydrogen isotopic series of gaseous alkanes. The carbon and hydrogen isotopic compositions of the Jurassic tight gas indicate that it is typically coal-derived gas. The measured vitrinite reflectance (RO) values of source rocks of Member 5 of the Xujiahe Formation, which are demonstrated as the main source of Jurassic tight gas, are consistent with the calculated RO values according to the two-stage fractionation model of coal-derived gas. Effective source rocks are rare in the Lower Jurassic Baitianba Formation of the study area, which is considered to have only a minor contribution to the Jurassic gas reservoirs. The geochemical characteristics of Jurassic natural gas from various gas fields in the Western Sichuan Depression vary depending on the effect of different gas sources and accumulation processes.