Genetic identification of natural gases from shallow reservoirs in some oil- and gas-bearing basins of China.

Natural gases of shallow reservoirs with the carbon isotopic compositions of methane ranging from -50‰ to -60‰ (PDB) were considered as mixed gases of biogenic and thermogenic origins previously and some of them were considered as low-mature (or low temperature thermogenic) gases lately. In this paper natural gases with the carbon isotopic compositions of methane in the above range were identified using the molecular and stable carbon isotopic compositions of methane, ethane and propane. The mixed gases of biogenic and mature thermogenic origins display the characteristics of δ 13C1 ranging from -50‰ to -60‰, δ 13C2 > -35‰, Δ values (δ 13C3 - δ 13C3) < 5‰ and C1/ΣC2+ ratios < 40. Immature to low-mature gases display the characteristics of δ 13C1 ranging from - 50‰ to - 60‰, δ 13C1 < -40‰, Δ values (δ 13C3 - δ 13C3) > 7‰, and C1/ΣC2+ ratios >60. [ABSTRACT FROM AUTHOR]