Carbon and hydrogen isotopic characteristics of natural gases from the Luliang and Baoshan basins in Yunnan Province, China

The Luliang and Baoshan basins of Yunnan Province are two small-sized continental oil/gas-bearing sedimentary basins, which were developed at the bases of the Carboniferous and Devonian systems during the Late Tertiary, covering an area of 325 km2 and 254 km2, respectively.Since the 1990s, there have been discovered small-sized natural gas pools in these two basins. The natural gases are composed mainly of hydrocarbon gases, with nonhydrocarbons accounting for less than 2%. Of the hydrocarbon gases, methane accounts for more than 99%, and the components above C2 account for less than 0.2%. On the basis of previous studies of geological background, the composition of natural gases and their carbon isotopic composition, it has been defined that these two gas pools are of bacterial origin. In this work we have comprehensively measured the carbon and hydrogen isotopic composition of natural gases from these two basins and have gone into the details of the mechanism of gas generation. The δ13C1 values of natural gases from the Luliang Basin are within the range of -72.1‰--73.3‰, and the δDcH4 values, -242‰--234‰, indicating that the bacterial gas generation is dominated by the way of CO2 reduction. It has been evidenced that under continental-facies fresh water conditions there did occur the CO2 reduction as a process of bacterial gas generation. The δ13C1 values of natural gases from the Baoshan Basin are within the range of -62.5‰--63.5‰, and the δDCH4 values, -252‰--260‰. These isotopic characteristics are fallen into transitional phase of acetate fermentation and CO2 reduction as defined by Whiticar et al. (1986).An important discovery in the Luliang Basin is the carbon isotopic composition of ethane of purely biogenetic origin, i.e., its δ13C2 values are within the range of -61.2‰--66.0‰. These carbon isotopic values have been reported for the first time in China. As compared to the δ13C2 values of less than -55‰ for the two cases encountered previously in the world, a significant difference is that in the latter two cases there are obvious signs of contamination caused by ethane of thermal origin. So it cannot be ruled out that the light δ13C2 values are the result of isotope fractionation of ethane of thermal origin. The δ13C2 values of natural gases from the Luliang Basin are all less than -60‰, and there is almost no possibility that ethane of thermal origin migrated vertically and horizontally into gas pools within this basins. This demonstrates that