Identification of secondary microbial methane and biodegradation: Case study from Luliang oil and gas field, Junggar Basin, China

The “secondary microbial gas” is defined as natural gas generated via the biodegradation of oil reservoirs. Since methane dominates, it is also called “secondary microbial methane”. Taking Luliang oil and gas field in the Junggar Basin as example, the methods for identifying secondary microbial methane and selective biodegradation of alkane gases were fully discussed in this study based on the molecular and stable carbon isotopes of natural gas as well as the geochemical characteristics of low molecular weight hydrocarbons of oil reservoirs. When the thermogenic gas is mixed with secondary microbial methane, its δ13C–CH4 ratio tends to be lighter with the C1/ƩC1–4 increasing. In the meantime, there is a tight relationship between secondary microbial methane, burial depth of gas reservoirs and the existence of biodegraded oil reserves. The methanogenesis is commonly accompanied by the selective biodegradation of C2–4 alkane gases, which results in the increasing of gas dry coefficients and the enrichment of 13C (even the reversal of carbon isotopes). Most currently used empirical diagrams to identify the gas genetic types are only applicable to the original thermogenic gases. All kinds of secondary alterations should be fully taken into consideration when these diagrams are used.