Gas geochemical evidences for biodegradation of shale gases in the Upper Triassic Yanchang Formation, Ordos Basin, China

Fractionation of chemical components and hydrogen/carbon isotopic compositions associated with biodegradation of shale gases were determined from typical shale profiles of the Upper Triassic Yanchang Formation, Ordos Basin, China. The results show that the light hydrocarbons (C1-C5) of the shale gases were partially degraded by microorganisms and the degradation degree decreased with depth in the profile of Well YY22. However, the degradation degree of Wells FY2 is basically unchanging at different depths and there is no obvious evidence for biodegradation in Well YY18. Biodegradation of C1 and C2 can occur and the degradation of hydrocarbon gas does not occur in any obvious sequence, but instead there are only small differences between the relative degradation rates. The relative degradation rates in descending sequence were C3 = n-C4 = n-C5 > i-C5 = i-C4 > C2 > C1. With intensification of degradation, δ13C1 of shale gas showed no obvious change, whereas a marked alteration took place in δ2HC1. The δ13C values of C3 and n-C4 did not alter significantly during early biodegradation; thus, use of exceptional heavy isotopic compositions of those two components to recognize moderate levels of biodegradation is limited. In contrast, the δ13C and δ2H values of C2 became more positive even at a low degree of biodegradation; hence, these are effective parameters for identifying biodegraded gases with a low degree of degradation. No obvious change of isotopic composition was observed for the n-C5 and i-C5 during degradation. Due to the low relative degradation rates but high content of C1 and C2, biodegradation of C1 and C2 will severely destroy and decrease the gas content of shale gas. There is a positive relationship between gas content and total organic carbon (TOC) of shale samples without biodegradation. In contrast, samples that have experienced biodegradation show an obvious decrease in gas content with TOC.