Light Hydrocarbon (C5–C7) Geochemistry in the Northern Margin of the Qaidam Basin, Northeastern Tibetan Plateau: Gas Mixing and Hydrocarbon Charge History

Twenty natural gases from the northern margin of the Qaidam Basin were collected and examined for the composition and stable carbon isotopic characteristics of gas and light hydrocarbons (C5–C7). The results reveal that the carbon isotopes of iso-alkanes and cyclo-alkanes in light hydrocarbons are mainly controlled by the bioprecursors, whereas the carbon isotopes of n-alkanes and aromatics in light hydrocarbons are primarily influenced by the bioprecursors and maturity. Based on the genetic types obtained from C1–C3 and C5–C7 fractions, three types of gases are identified: coal-type gas, oil-type gas, and mixed gas. Coal-type gas dominates the northern margin of the Qaidam Basin, oil-type gas is mostly distributed in the Lenghu no. 3 field, and mixed gas is mainly developed in the Dongping and Mabei fields. According to the maturity obtained from δ13C1 and heptane and isoheptane ratios, the petroleum charge period is studied in combination with burial history and hydrocarbon generation history, and the result is roughly well matched with the research of homogeneous temperatures of petroleum inclusions. Furthermore, the generation temperature of the major reservoired hydrocarbons calculated from C7 light hydrocarbon compositions ranges from 125.0 to 135.9 °C, suggesting that a major petroleum charge event may occur primarily during the Shangganchaigou period (N1) in most fields. The deep paleo uplifts adjacent to the hydrocarbon-generating depressions at the margin of the basin and uplift zone in the inner basin are estimated as favorable areas for further exploration in the northern margin of the Qaidam Basin.