Geochemical Characteristics of Natural Gas and Hydrocarbon Charge History in the Western Qaidam Basin, Northwest China

31 natural gases in the western Qaidam Basin of China were collected and analyzed for gas composition including light hydrocarbons (C5-C7) and carbon isotopic characteristics. Based on genetic type obtained from C1-C3 and C7 fractions, four types of gases are identified: oil-type gas, coal-type gas, biodegraded gas, and mixed gas. The oil-type gas is the predominant-type gas in the western Qaidam Basin; coal-type gas is mainly distributed in the Zhahaquan and Nanyishan fields; mixed gas is mainly in the Zhahaquan, Wunan, and Nanyishan fields; and biodegraded gas is mainly distributed in the Huatugou and Yuejinerhao fields. According to the empirical relationship between δ13C1 and the equivalent vitrinite reflectance (Ro, %) of source rock, the Ro values of gas range from 0.6% to 1.5%, with an average value of 0.9%. The generation temperatures of major reservoired hydrocarbons (GTMRH) calculated from the C7 components range from 115.6°C to 141.7°C, with an average value of 126.5°C. These two maturity indicators have relatively positive correlation and reveal that the maturity of gas increases from west to east in the southwestern Qaidam Basin. Moreover, combining GTMRH with the homogenous temperature of petroleum inclusions, it is inferred that major petroleum charge in the western Qaidam Basin mainly occurred during the late period of the Himalayan movement. Deep hydrocarbon fluid sources were found in the Shizigou, Yingdong, Zhahaquan, and Nanyishan fields; thus, the deep reservoirs of paleouplifts adjacent to the hydrocarbon-generating depressions are estimated as a favorable area for further exploration in the western Qaidam Basin.