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 δ 13C1 values of natural gases from the Luliang Basin are within the range of –72.1‰―–73.3‰, and the δ DCH4 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 δ 13C1 values of natural gases from the Baoshan Basin are within the range of –62.5‰―–63.5‰, and the δ DCH4 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 δ 13C2 values are within the range of –61.2‰―–66.0‰. These carbon iso- topic values have been reported for the first time in China. As compared to the δ 13C2 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 δ 13C2 values are the result of isotope fractionation of ethane of thermal origin. The δ 13C2 values of natural gases from the Luliang Basin are all less than –60‰, andthere is almost no possibility that ethane of thermal origin migrated vertically and horizontally into gas pools within this basins. This demonstrates that in the Luliang Basin the ethane, whose δ 13C2 values are within the range of –61.2‰―–66.0‰, is of purely biogenic, hence providing scientific evidence that ethane can be produced by the bacterial process. This is an important discovery in natural sci- ence, and also provides clues to the comprehensive study of the mechanism of formation of ethane.

The Luliang and Baoshan basins of Yunnan Province are two small-sizedcontinental oil/gas-bearing sedimentary basins, which were developed at the bases of theCarboniferous and Devonian systems during the Late Tertiary, covering an area of 325 km^2 and 254km^2, respectively. Since the 1990s, there have been discovered small-sized natural gas pools inthese two basins. The natural gases are composed mainly of hydrocarbon gases, with nonhydrocarbonsaccounting for less than 2%. Of the hydrocarbon gases, methane accounts for more than 99%, and thecomponents above C_2 account for less than 0.2%. On the basis of previous studies of geologicalbackground, the composition of natural gases and their carbon isotopic composition, it has beendefined that these two gas pools are of bacterial origin. In this work we have comprehensivelymeasured the carbon and hydrogen isotopic composition of natural gases from these two basins andhave gone into the details of the mechanism of gas generation. The δ^(13)C_1 values of naturalgases from the Luliang Basin are within the range of -72.1 per thousand - -73.3 per thousand, andthe δD_(CH_4) values, -242 per thousand - -234 per thousand, indicating that the bacterial gasgeneration is dominated by the way of CO_2 reduction. It has been evidenced that undercontinental-facies fresh water conditions there did occur the CO_2 reduction as a process ofbacterial gas generation. The δ^(13)C_1 values of natural gases from the Baoshan Basin are withinthe range of - 62.5 per thousand - -63.5 per thousand, and the δD_(CH_4) values, -252 per thousand- -260 per thousand. These isotopic characteristics are fallen into transitional phase of acetatefermentation and CO_2 reduction as defined by Whiticar et al. (1986). An important discovery in theLuliang Basin is the carbon isotopic composition of ethane of purely biogenetic origin, i.e., itsδ^(13)C_2 values are within the range of - 61.2 per thousand - - 66.0 per thousand. These carbonisotopic values have been reported for the first time in China. As compared to the δ^(13)C_2 valuesof less than -55 per thousand for the two cases encountered previously in the world, a significantdifference is that in the latter two cases there are obvious signs of contamination caused by ethaneof thermal origin. So it cannot be ruled out that the light δ^(13)C_2 values are the result ofisotope fractionation of ethane of thermal origin. The δ^(13)C_2 values of natural gases from theLuliang Basin are all less than -60 per thousand, and there is almost no possibility that ethane ofthermal origin migrated vertically and horizontally into gas pools within this basins. Thisdemonstrates that in the Luliang Basin the ethane, whose δ^(13)C_2 values are within the range of -61.2 per thousand - -66.0 per thousand, is of purely biogenic, hence providing scientific evidencethat ethane can be produced by the bacterial process. This is an important discovery in naturalscience, and also provides clues to the comprehensive study of the mechanism of formation of ethane.