Abstract: Focusing on the C5~C7 light hydrocarbons in natural gas, combined with natural gas components and carbon isotopes, the paper studies the organic precursor type and maturity, hydrocarbon generation temperature and secondary hydrocarbon reconstruction in Yingdong, Wunan and Zhahaquan oilfields in southwestern Qaidam basin. The results show that the C7 compounds in the natural gas of the 3 oilfields exhibit high n-heptane contents and the C7 compounds in Wunan oilfield have relatively high methylcyclohexane content. Based on the carbon isotope analysis, it is considered that the natural gas from Yingdong, Wunan and Zhahaquan oilfields mainly belong to oil-type gas, and coal-type gas and mixed gas are found in Wunan and Zhahaquan oilfields. The heptane and isoheptane values indicate that the natural gas are mostly in the mature-highly mature stage, based on the δ13C1-Ro% relationships, the calculated natural gas maturity is 0.8%~1.5% with the average value of 1.2%, indicating these gases were mainly generated in the mature stage. The calculated natural gas generation temperatures can roughly reflect petroleum forming temperatures of the main part of the modern reservoir. The petroleum generation temperatures in Yingdong, Wunan and Zhahaquan oilfields (124~129 ℃) are higher than those in Qigequan and Shizigou oilfields (108~109 ℃), showing that the natural gas maturity in Yingdong, Wunan and Zhahaquan oilfields are even higher. Through light hydrocarbon analysis, several oil and gas reservoirs suffered from biodegradation, evaporative fractionation and water washing have been identified