ABSTRACT

Secondary microbial gas: characteristics, worldwide distribution and significance 
Alexei V. Milkov Colorado School of Mines, Golden, Colorado, USA

Distribution and scale of methanogenic biodegradation in the world’s petroleum accumulations and the significance of its terminal product, secondary microbial methane, in the global gas endowment and carbon cycle are largely unknown. I will discuss geological and geochemical criteria to recognize secondary microbial methane in petroleum reservoirs. These include the presence of biodegraded oil (as pools, legs or shows) in the reservoir or down-dip, the relatively dry (i.e., methane-dominated) gas containing methane with ?13C values between -55‰ and -35‰ and, most importantly, CO2 with ?13C > +2‰. Based on these criteria, the presence of secondary microbial methane is identified in numerous sedimentary basins around the world. Secondary microbial methane may account for ~5–11% of the global conventional recoverable gas endowment and appears more abundant than primary microbial gas (~3–4% of the global gas endowment). Most of the generated secondary microbial methane probably is aerobically and anaerobically oxidized to CO2 in the overburden above petroleum reservoirs. However, some secondary microbial methane may escape from shallow reservoirs into the atmosphere and affect present and past global climate.