First discovery of biodegraded petroleum in the South Qilian Basin, Northwest China: Implications for the origin of gas hydrates

[Display omitted] Biodegradation can greatly affect the physical properties of petroleum and cause difficulty in assessing its origin and maturity. Gas hydrates and petroleum have been discovered in the South Qilian basin, Northwest China, but biodegradation of petroleum has not been recognized. In this contribution, we present new geochemical evidence for biodegradation in light hydrocarbons (C5-C8), long-chain hydrocarbons from siltstones, oil shales and mudstones collected from boreholes that penetrated the Lower Jurassic strata in the South Qilian basin. The results show that with increasing depth the contents of n-alkanes in light hydrocarbons and long-chain hydrocarbons decrease, while the relative concentration of cyclo-alkanes, iso-alkanes and aromatic hydrocarbons from light hydrocarbon increase. Furthermore, natural gases, light hydrocarbons and long-chain hydrocarbons have undergone slight to moderate biodegradation near the surface (with depth of 0–150 m) of the Muli Depression. Therefore, for the first time, this study documented biodegraded petroleum in the Jurassic strata in the South Qilian basin, which provides important information for further determining the origin of petroleum and gas hydrates in the region.