Evidence and significance of methanogenesis in paleo-oil reservoirs in Junggar Basin, NW China

Methanogenesis is considered to be the dominant type of biodegradation in subsurface oil reservoirs; however, this process is difficult to identify under geological environments. In this study, bitumen, microfossils, and secondary minerals were analysed to reveal the characteristics and conditions of methanogenesis in subsurface oil reservoirs in the Junggar Basin, western China. Bitumen with biodegradation characteristics and nanoscale microfossils were observed in the pores and fractures of reservoirs; these confirmed the existence of microbial activity in the palaeo-oil reservoirs. Secondary calcite that formed simultaneously with bitumen had a high positive δ13C (up to +20 ‰), which further suggested that methanogenesis was the main biodegradation type. Based on comprehensive analyses of δ13C, 87Sr/86Sr, trace element, and fluid inclusions, the methanogenesis is proposed to occur in Mn-rich and hydrocarbon-bearing fluids at 45℃–85 ℃ (∼1000 m to ∼2200 m subsurface) approximately 230 Mya, which is consistent with the earliest hydrocarbon accumulation in the reservoirs. The results effectively reveal the process of methanogenesis and how it was masked by the charging of late oil, thus demonstrating the advantages of inorganic geochemical analyses when solving problems of oil biodegradation. The established geological model was used to discover the other subsurface methanogenesis cases in the basin and also provides a reference for analysing microbially mediated subsurface carbon cycling.