Relationships of the iC 4C 4 and iC 5C 5 ratios with maturity of coal-derived gases of Triassic Xujiahe Formation in central Sichuan Basin, SW China

摘要

The original gas reservoirs in different areas and different layers of the Triassic Xujiahe Formation in the central Sichuan Basin are studied to reveal the relationships of iC4C4and iC5C5ratios in coal-derived gas components with maturity using conventional natural gas geochemical research methods. The testing results of 73 gas samples from 8 gas fields show that the iC4C4and iC5C5ratios in coal-derived gas have a good positive correlation, and the correlation coefficient is above 0.8. Both the iC4C4and iC5C5ratios become higher with the increase of natural gas dryness coefficient (C1/C1+) and the methane carbon isotope becoming less negative. These parameters are highly correlated. This study not only reveals characteristics of heavy hydrocarbon isomers generated by coal formation, but also puts forward new identification indicators reflecting the maturity of coal-derived gas, the regression between iC4C4, iC5C5and Ro, which can provide an important reference for maturity, migration and accumulation of coal-derived gas, and late stage reformation of coal-derived gas reservoirs.