UNTANGLING MICROBIAL CONTRIBUTIONS TO NATURAL GAS RESERVOIRS

Over the past few decades, the importance of microbial contributions to our natural gas supply has been widely recognized, even leading to efforts to enhance the rate of methanogenesis in reservoirs whether the substrate is oil, coal, or the organic matter in shale. The presence of biogenic gas was first established with gas compositional and isotopic data. More recently, molecular genomic data has provided a glimpse into the constituencies of bacterial and archaeal communities in the subsurface, both in reservoirs where the microbial community was expected by the geochemistry of both water and gas (e.g. Antrim Shale) and in flowback waters from formations where there was no previous indication of anything other than thermogenic gas (e.g. Marcellus Shale). For these microbes, it is not so much a question of “build it and they will come”, but more that the community lies in wait for conditions to improve and allow them to flourish.

Conditions for microbial methanogenesis are constrained by several parameters: temperatures no greater than ~80^oC, low (< 10mM) sulfate concentrations, and chloride concentrations under ~2M. However, the rates at which certain microbes are able convert complex organic matter into methane depend upon environmental conditions that are constantly changing due to the effects of production.. Over the past two decades, active wells in the Antrim Shale have exhibited changes in the geochemistry of formation fluids, most notably a drop in dissolved inorganic carbon of ~10mM. Gas chemistry has also shifted, with increasing concentrations of carbon dioxide that have also become more enriched in ¹³C, while the co-produced methane has either remained the same or become more depleted in ¹³C over the 24 years that these few wells have been monitored. As expected, the microbial community has also shifted with the water’s chemical evolution; for example, the resurgence of sulfate-reducing bacteria concomitant with acid treatments for scale. Most intriguing is the correlation of the deuterium in the water to the methane (indicative of CO₂-reduction) where gas composition seems to be responding to changes in the water source to keep the fractionation between them constant.