Biodegradation of light hydrocarbon(C5-C8) in shale gases from the Triassic Yanchang Formation, Ordos basin, China

Evidence based on the molecular, carbon and hydrogen isotopic composition of shale gas has shown that the biodegradation level of the Yanchang Formation shale gas decreases markedly with increasing depth in some parts of the Ordos basin. The compositions of a suite of 30 shale desorbed gases from the Upper Triassic Yanchang Formation, Ordos basin, China, were analyzed in order to assess the effect of variable levels of biodegradation on the distribution of light hydrocarbons in the shale gas and to investigate the effect of biodegradation on common light hydrocarbon parameters. By comparison and analysis of the gas chromatograms of shale desorbed gas from different depths, a series of light hydrocarbon parameters reflecting biodegradation were established. The results indicate that the order of preferential biodegradation of light hydrocarbon fractions (C5-C8) is C5>C6>C7≈C8 and that the C6 fraction composition is n-Hexane (nC6)>2-methypentane (2-MP) ≈3-methypentane (3-MP)> Methylcyclopentane (MCP)>2,3-Dimethybutane (2,3-DMB)>2,2-Dimethybutane (2,2-DMB)>Cyclohexane (CH). There are three main controls on the susceptibility to biodegradation of light hydrocarbons: carbon skeleton, degree of alkylation, and position of alkylation. If the carbon skeleton is the same, a higher degree of alkylation represents a stronger ability to resist biodegradation. If the degree of alkylation is the same, those alkanes with larger carbon skeletons are more resistant to biodegradation. Finally, light hydrocarbon fractions with two methyl groups in the same position are more resistant to biodegradation than hydrocarbons with two methyl groups in different positions. Most of the light hydrocarbon parameters were influenced by biodegradation, except the Mango parameters K1and K2.