Relationships of the iC 4 / nC 4 and iC 5 / nC 5 ratios with maturity of coal-derived gases of Xujiahe Formation in central Sichuan Basin , NW China

The original gas reservoirs in different parts and different layers of the Xujiahe Formation in the central Sichuan Basin are studied to reveal the variation of iC4/nC4 and iC5/nC5 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 iC4/nC4 and iC5/nC5 ratios in coal-derived gas have a good positive correlation, and the correlation coefficient is above 0.9. Both the iC4/nC4 and iC5/nC5 ratios become higher with the increase of natural gas dryness coefficient (C1/C1+) and the methane carbon isotope becoming less negative. The trends of these related 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 iC4/nC4, iC5/nC5 and 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.