Carbon and Hydrogen Isotopic Compositions: generation pathway of bacterial gas in China

Key words: 
• bacterial gas

Abstract: The carbon and hydrogen isotopic composition of biogenic gas could indicate its forming mechanism and poolforming characteristics. In this paper, the forming ways and poolforming characteristics of the biogenic gas are researched according to the carbon and hydrogen isotopic composition of thirtyone samples from ten biogenic gas reservoirs. The studies show that the hydrogen isotopic composition of these biogenic gas reservoirs can be divided into three zones, i.e. δDCH4>200‰, 200‰>δDCH4<250‰, and δDCH4<250‰. The research results of forerunner showed that the main way of biogenic gas forming under the condition of continental fresh water facies is acetate fermentation. Our research results show that the typical way of biogenic gas forming under marine and saline lake facies is CO2 reduction, and the biogenic gas have heavy hydrogen isotopic composition, such as the values of δDCH4 are greater than 200‰; however, the main way of biogenic gas forming under the condition of land facies is the same way too, but they have light hydrogen isotopic composition which may relate to salty degree of ancient lakes as water medium, such as the values of δDCH4 are between 200‰ and 250‰. As compared with the relational data of Qaidam Basin, it is showed that the hydrogen isotope composition of the biogenic methane has the same trend as salty degree of water medium increases. The biogenic gas reservoir formed in continental transition region, in which CO2 reduction generated CH4 in company with fermentation together, may relate to constant salinity of ancient water medium and relatively high geothermal gradient，such as Baoshan basin. It has light hydrogen isotope, such as the value of δDCH4 less than 250‰, and heavy carbon isotope, which result in a negative correlation between its carbon and hydrogen isotope. The carbon and hydrogen isotope composition of biogenic gas controlled by the formation mechanism and ways can be used to identify whether it has industrial utilization value. In general, the biogenic gas from acetate fermentation is not favorable for the formation of gas reservoir.