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 com
XU Yongchang1, LIU Wenhui2, SHEN Ping1, WANG Wanchun1, WANG Xiaofeng1, Tenger1, YAN Yaomin3 & LIU Ruobin3 1. Lanzhou Center of Oil & Gas Resources, Institute of Geology and Geophsics, Chinese Academy of Sciences, Lanzhou 730000, China
