Natural gas geochemistry of low evolutionary stage of medium- and small-sized basins in Yunnan Province, southwestern China

The biogenic gas and premature-low mature associated gases in some medium and small-sized basins of Yannan Province, such as Luliang, Yanglin, Baoshan and Jinggu basins, have beed researched. The results show that the biogenic-gas consists mainly of methane which is more than 99% in gasous hydrocarbons and of lighter carbon isotopic composition with δ¹³C₁ values from −60.0‰ to −75.4‰ The methane carbon isotopic compositions in the Baoshan Basin is relatively heavy (δ¹³C₁, −60‰ – −65‰), but those in Luliang and Yanglin basins are lighter (δ¹³C₁, less than −70‰), which implies that the gas field of the Baoshan Basin formed earlier than the others. In the Jinggu Basin, where crude oil is premature-lower mature, the natural gas of associated oil relatively wet and the relative content of methane about 58%–95% in gasous hydrocarbons. Constituently the gas composition is much similar to associated one, but the methane carbon isotopic compositions from −53.8‰ to −57.8‰ are obviously richer in¹²C than those of general oil fields and similarly characterize the biothermocatalytic transitional zone gas. Their ethane carbon isotopic compositions from −34.6‰ to −29.0% show that they may be derived from type I or II source rock. But for the associated gas from lower evolutionary stage, the heavier ethane carbon isotopic composition as well as the reversed order among the carbon isotopic composition of ethane, propylane and butane also implies that some gases from the type III organic matter are mixed. Thed¹³ C_CO2 \delta ^{13} C_{CO_2 } of the samples essentially less than −10‰ may be generated from organic matter.