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The Sichuan basin is one of the largest gas-oil-bearing basins in China. Ool and gas poolsoccur in Mesozoic, Paleozoic and Proterozoic strata in this basin, with natural gases being do-minant. A good wealth of data from 2000 drill wells on the distribution of natural gases (hydro-carbons: Ca_4 C_2H_6~+; non-hydrocarbons: H_2S, CO_2, N_2; noble gases: He, Ar)show that naturalgases in the basin are predominated by oil-thermocracked and coal-series gases. Geological-factor analysis of the geochemical characteristics of natural gases provide evi-dence suggesting that the occurrence of natural gases, especially dry gases, is attributed tothe high maturity of organic matter, and the multi-productive formation has a great bearing onnthe multi-source rocks; the anomalies of some components(e.g.H_2S)are related not only tothe 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 non-coal-series gases at the same degree of maturation, demonstrating that the former is characte-rized by high proportions of CH_4 and gaseous Hg, high C_1/C_2 ratio, high δ~(13)C, low C_2~+, andhigh iC_4/nC_4.
The Sichuan basin is one of the largest gas-oil-bearing basins in China. Ool and gas poolsoccur in Mesozoic, Paleozoic and Proterozoic strata in this basin, with natural gases being do-minant. A good wealth of data from 2000 drill wells on The distribution of natural gases (hydro-carbons: Ca_4C_2H_6 ~ +; non-hydrocarbons: H_2S, CO_2, N_2; noble gases: He, Ar) show that naturalgases in the basin are predominated by oil-thermocracked and coal-series gases. Geological-factor analysis of the geochemical characteristics of natural gases provide evi-dence suggestive of the occurrence of natural gases, especially dry gases, is attributed tothe high maturity of organic matter, and the multi-productive formation has a great bearing onnthe multi-source rocks; the anomalies of some components (egH_2S) are related not only tothe 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 non-coal-series gases at the same degree of maturation, demonstrating that the former is characte-rized by high proportions of CH_4 and gaseous Hg, high C_1 / C_2 ratio, high δ ~ (13) C, low C_2 ~ +, andhigh iC_4 / nC_4.