Carboniferous-Permian coalbed methane-accumulating stages in the Qinshui basin Shanxi

Abstract：According to the diagenetic sequence of the reservoir, characteristics of oil-gas inclusions and homogenization temperatures of hydrocarbon-bearing saline inclusions and isotope dating of authigenic illite, combined with the tectonothermal evolution history and fission-track path data, an integrated study has been performed on the stages of coalbed methane accumulation in Permo-Carboniferous coalbeds and roof sandstone in the Qinshui basin. The authors think that two stages of hydrocarbon injection occurred in the methane accumulation. The first stage occurred in the terminal Triassic to Early Jurassic, when the main coal seams of the Shanxi and Taiyuan formations were in a mature stage and liquid hydrocarbon and gas-liquid hydrogen inclusions formed; and the second stage took place in the Late Jurassic to Early Cretaceous, corresponding to the phase of thermal cracking of kerogen, when large amounts of coalbed methane were produced, so the stage is the main accumulation stage of Permo-Carboniferous coalbed methane. The age of authigenic illite in roof sandstone, Shanxi Formation, is ~191 Ma, which implies that the Early Jurassic was the earliest time for filling of oil/gas desorbed from coalbeds methane in the sandstone reservoir with basin uplift and erosion.