Hydrogen isotopes of hydrocarbon gases from different organic facies of the Zhongba gas field, Sichuan Basin, China

Abstract

In order to investigate the hydrogen isotopic variation of gases sourced from different organic facies, experimental pyrolytic results of a limestone sample (kerogen) and a coal sample from the Zhongba gas field, Sichuan Basin, and a coal sample from the Songliao Basin, China were compared. The results demonstrate that hydrogen isotopes of methane, ethane and propane all increase with increasing thermal maturity, i.e., thermogenic gases become more enriched in 2H with increasing thermal maturity. There is remarkable variation of hydrogen isotopes of gases sourced from different organic matters. Gases derived from the limestone have the heaviest δ2H, followed by that sourced from the coal in Sichuan Basin and then the coal from Songliao Basin. This trend is just opposite to that of the carbon isotopic variation in different organic facies. Such trend is due to the differences in the salinity of water medium of the depositional environment, rather than the types of the organic matter. Unlike the carbon isotopes, hydrogen isotope is more susceptible to influence from the depositional environments, which might mask the influence from different fractions of the organic matter. Field natural gas samples also proves the strong influence from depositional environment on the hydrogen isotope. This work demonstrates that hydrogen isotope itself is not a reliable index to determine the type of organic matter.