Geochemical characteristics of natural gases in the Sichuan basin

The Sichuan basin is one of the largest gas-oil-bearing basins in China. Ool and. gas pools occur in Mesozoic, Paleozoic and Proterozoic strata in this basin, with natural gases being dominant. A good wealth of data from 2000 drill wells on the distribution of natural gases (hydrocarbons: CH₄ C₂H ₆ ⁺ ; non-hydrocarbons: H₂S, CO₂, N₂; noble gases: He, Ar) show that natural gases in the basin are predominated by oil-thermocracked and coal-series gases. Geological-factor analysis of the geochemical characteristics of natural gases provide evidence suggesting that the occurrence of natural gases, especially dry gases, is attributed to the high maturity of organic matter, and the multi-productive formation has a great bearing on the multi-source rocks; the anomalies of some components (e. g. H₂S) are related not only to the type of primary organic matter, but also to the lithological characters of reservoir beds. Also discussed in this paper are some geochemical characteristics of coalseries and noncoal-series gases at the same degree of maturat ion, demonstrating that the former is characterized by high proportions of CH₄ and gaseous Hg, high C₁/C₂ ratio, high δ¹³C, low C ₂ ⁺ , and high iC₄/nC₄.