Microbial Production and Modification of Gases in Sedimentary Basins: Constraints from Isotopic Compositions

Abstract

Unconventional natural gas reservoirs, such as organic-rich shales and coalbeds, are gaining in economic significance worldwide. Many of these fields contain some portion of microbial methane, either enhancing or replacing thermogenic gas. Importantly, these microbes produce isotopic and compositional effects on the various gas components that often obfuscate their origin. An expanding data set for gases produced from Devonian black shales in the Michigan and Illinois basins, USA, has allowed for a detailed examination of the related chemical and isotopic compositional changes in gas-water systems that trace these processes. The Antrim Shale, within the northern margin of the Michigan Basin, has recently been shown to produce economic quantities of methane that is dominantly bacterial in origin, based on integrated chemical and isotopic analyses of formation waters and co-produced gas (Martini et al., 1996). The most compelling evidence for microbial generation is the correlation between deuterium in methane and coproduced water (Fig. 1). Exploration for economic microbial gas deposits has been linked to low thermal maturity of the host rock organic matter, development of an open, permeable fracture network, and hydrologic conditions that permit freshwater recharge. Further expansion in the Michigan and Illinois basins has allowed us to develop this hypothesis in areas with differing geologic histories and hydrologic settings. Expansion along the western and southern margins of the Michigan Basin at shallow depths within the Antrim Shale has yielded results that are very similar to those previously reported for the northern margin. Waters show evidence of meteoric influx (salinities increasing basinward from subcrop) and microbial activity. Gas chemistries suggest the occurrence of bacterial methanogenesis. In contrast, deeper, midbasin Antrim Shale development has proven unsuccessful. Here, waters show no signs of meteoric mixing, having salinities and stable isotope compositions (O and H) typical of basinal brines. The gases produced include significant volumes of ethane and propane. Importantly, the correlation between the deuterium in methane and that in co-produced waters is absent. These results suggest the gas produced is thermogenic. The isotopic effects of microbial activity are also seen in the 6a3C values of ethane. The 513C values for methane and ethane are plotted against the 51So values of co-produced waters in Fig. 2. The 6~80 values represent the influx of dilute waters, with the most negative values being from shallow wells near the subcrop of the shale, and the most positive values being from deeper wells within the basin. The methane values are invariant while those for ethane become increasingly higher as the ~lSo values of