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

Abstract

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 km 2 and 254 km 2 , 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 C 2 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 13 C 1 values of natural gases from the Luliang Basin are within the range of -72.1‰--73.3‰, and the δ D CH 4 values, -242‰--234‰, indicating that the bacterial gas generation is dominated by the way of CO 2 reduction. It has been evidenced that under continental-facies fresh water conditions there did occur the CO 2 reduction as a process of bacterial gas generation. The δ 13 C 1 values of natural gases from the Baoshan Basin are within the range of -62.5‰--63.5‰, and the δ D CH 4 values, -252‰--260‰. These isotopic characteristics are fallen into transitional phase of acetate fermentation and CO 2 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 13 C 2 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 δ 13 C 2 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 13 C 2 values are the result of isotope fractionation of ethane of thermal origin. The δ 13 C 2 values of natural gases from the Luliang Basin are all less than -60‰, and

Longitudinal distributions of two formation pathways of biogenic gases in continental deposits: A case study from Sebei 1 gas field in the Qaidam Basin, western China

“…δ 13 C 2 in Luliang gas field of China was lower than -60‰. Xu et al [22] eliminated the possibility of migration fractionation of thermal ethane, and confirmed that trace wet gases should be bacterially produced during the formation of biogenic methane. It was regretful that carbon isotopes of C 2+3 were unavailable in all of our samples.…”

“…(2) Biogenic heavy gaseous components exist in nature. The biogenic ethane with light carbon isotope has been discovered in recent years [21,22] . δ 13 C 2 in Luliang gas field of China was lower than -60‰.…”

Geochemical characteristics and origin of light hydrocarbons in biogenic gas

“…The Pliocene is developed with the maximum thickness of 1700 m. The source rock and reservoir are in the fourth member of the Neogene Yangyi Formation. The source rock is composed of mudstone, with the thickness of 323-520 m and the total organic carbon (TOC) of 0.21%-2.44% (1.06% on average) [10]. The type of organic matter is mainly typeⅡ with R o values of 0.31%-0.45%.…”

“…Most of geochemical studies on low maturity gas and biogenic gas focused on the components of methane, ethane and isotopic compositions [7][8][9][10]. However, the studies on composition and origin of light hydrocarbon in biogenic gas are scarce [11].…”

Geochemical characteristics of light hydrocarbons in natural gases from the Turpan-Hami Basin and identification of low-mature gas

“…In recent years, great strides have been made in the productive practice and theoretical study of low-mature oil/gas, especially in the analysis and test technologies for light hydrocarbons in natural gases, thus greatly promoting the geochemical research on light hydrocarbons in natural gases. It should be mentioned that Xu et al [14] and Xiao et al [15] obtained C 5 -C 8 light hydrocarbon compounds during their studies of biogenic gases in the Baoshan Basin of the Yunnan Province (δ 13 C 1 values are within the range from -63.6‰ to -62.5‰). Their heptane and iso-heptane values are about 16 and 0.8, respectively.…”