Geochemical characteristics of biogenic gases in China

A number of contributions have demonstrated that the stable carbon and hydrogen isotopic compositions together with molecular compositions have a great significance on determination for gas origin and gas generation pathways of biogenic gases. Together with some data from published work, a total of 143 gas samples were compiled from different biogenic gas producing locations in China, to investigate the gas origin and gas generation pathways by the molecular composition, stable carbon and hydrogen isotopes. The results demonstrate that biogenic gases in China are dominated by methane with low level of C2+3 (generally less than 0.5%) and low delta C-13(CH4) values (generally <= 55 parts per thousand). The relatively high level of C2+3 (up to 11.79%) in the biogenic gases from the Baise and Songliao basins indicates the possible mixture of low-mature thermogenic gases. Biogenic gases from the Jiangzhe coastal area (including Shanghai, Zhejiang, and Jiangsu provinces) and Yingqiong basin have delta D-CH4 values greater than -190 parts per thousand, indicating marine salt water depositional environments. However, all other gases have delta D-CH4 values less than - 190 parts per thousand, possibly indicating a terrestrial freshwater or freshwater to slightly saline water depositional environment. The methanogenic pathway of biogenic gases in China is dominated by biogenic carbonate reduction, though a number of these biogenic gases were generated in a terrestrial freshwater or freshwater to slightly saline water depositional environments. Compared to the bacterial carbonate reduction methanognic pathway in the Luliang basin, gas formation mechanism in the Baoshan basin was more toward bacterial methyl-type fermentation, likely due to the relatively higher geothermal gradient in the Baoshan basin. (C) 2012 Elsevier B.V. All rights reserved.