Natural gas genesis and sources in Jiulongshan Gas Field, northwest Sichuan Basin

Lower Jurassic Zhenzhuchong Formation and the second number of Upper Triassic Xujiahe Formation are two main gas production strata of Jiulongshan Gas Field, however, no conclusion has yet to be reached about gas source for the both strata. Therefore, this contribution makes a detailed analysis about the gas component, carbon isotopic composition, source rocks and reservoir bitumen biomarkers, to clear the gas sources of Zhenzhuchong Formation and the second number of Xujiahe Formation. There are 2 offsets of high quality hydrocarbon sources: the shales of shallow Xujiahe Formation (third number), limestone of deep Maokou Formation and mudstone of Lower Cambrian. In general, gas is dry, and gases from both the Zhenzhuchong Formation and the third number of Xujiahe Formation have similar characteristics, while gases from the second number of Xujiahe Formation have combined characteristics of both shallow and deep gases. δ13C2 value is averaged at -26.73‰ for the gas from Zhenzhuchong Formation, which is similar to the Xujiahe Formation coal source of periphery Wenxingchang and Zhongba Gas Fields. The δ13C2 value is from -29.5‰ to -30.49‰ for the gas from the second number of Xujiahe Formation, which demonstrates characteristics of both shallow coal-derived and deep oil-derived gas. Zhenzhuchong Formation has high tricyclic terpane value and rearrangement sterane/steranes value, which is the same as Xujiahe Formation. C27/C29 value is from 0.93 to 1.00, like shape V. The second number of Xujiahe Formation has similar low tricyclic terpane values and rearrangement sterane/steranes to that in Maokou Formation. Therefore, it is concluded that gases in the Zhenzhuchong Formation is sourced from the third number of Xujiahe Formation, while gases in the second number of Xujiahe Formation are mainly derived from both the third number of Xujiahe Formation and deep Maokou Formation.