Origin of natural gas in Jurassic Da'anzhai Member in the western part of central Sichuan Basin, China

The distribution pattern of oil and gas in the western part of central Sichuan Basin is complicated, there are many different views on the sources of natural gas, which restrict further exploration and deployment of oilfields. Therefore, by analyzing geochemical characteristics and quantitative calculation of natural gas in study area, combined with the data of structure and gas-oil ratio, the origin and sources of gas are discussed and analyzed. The results show that the dryness coefficient of the natural gas ranges between 0.71 ∼ 0.93, δ13C values range between −45.3 ∼ −38.0‰ in methane, −32.0 ∼ −25.6‰ in ethane and −30.0 ∼ −25.3‰ in propane. Crossplots of isotope data and quantitative calculation indicate that natural gas in Da'anzhai Member is characterized by the mixture of sapropelic and humic natural gas, the humic gas largely occupies the proportion of 20-40%, up to 70%. It is the result of upward migration of natural gas in Triassic Xujiahe Formation. The abnormally high gas-oil ratio is an important manifestation of this upward intrusion. Based on the study of high gas-oil ratio wells, the distribution of humic natural gas that intruded from Xujiahe Formation and enriched in Da'anzhai Member is unevenly distributed. It inherits characteristics of “Plaque accumulation” of natural gas in Xujiahe Formation. The inhomogeneity of this plane distribution is controlled by fault location, fault scale and the size of underlying Xujiahe Formation gas reservoirs. The humic gas from Xujiahe Formation occupies a large proportion in blocks of Da'anzhai Member which are favorable for intrusion, accumulation and preservation of Xujiahe Formation natural gas, whereas for blocks without these conditions, natural gas is often associated gas.