ACETATE AS A POSSIBLE METHANOGENIC SUBSTRATE IN THE ANTRIM SHALE? - RELATIONSHIP TO SALINITY AND THE MICROBIAL COMMUNITY

The Antrim Shale in Michigan Basin hosts a mixed biogenic-thermogenic gas derived from degradation of shale organics. The produced gas has become somewhat more biogenic-appearing with time, implied by slight declines in δ¹³C-CH₄ and increases in δ¹³C-CO₂ in ~20 yr of production (Kirk et al. 2012, Chem. Geol., v332, p15). There is interest in identifying active biogeochemical pathways that yield biogenic gas. Pathway-specific intermediates (e.g. acetate) may identify active substrate production and consumption more specifically than examining accumulated CH₄ and CO₂. This study illustrates a pathway-specific approach applied to a possible methane precursor, acetate, examined across a significant variation in environmental conditions (pH/salinity).

In 2013 samples, δ¹³C-CH₄ (-53.3 to -50.5‰) and δ¹³C-CO₂ (21.7-25.5‰) showed little variation across a basinward transition from dry, biogenic-dominated gases to wet gases with (C₁/(C₂+C₃) range 10⁴ to 10¹. Along this transition, formation waters ranged from fresh to Na-Cl^- brines (total salinity 4-211 g/L), and pH declined from 7.0 to 5.3. Acetate concentrations were <20-630 µM, and compound-specific δ¹³C-acetate was -26.7 to -12.3‰. δ¹³C-acetate was highest at high acetate concentrations, high salinity, and low pH.

Acetate concentrations and δ¹³C-acetate record a balance between acetate production and consumption. The high acetate concentrations in the most saline wells imply that acetate consumption is sluggish. The ¹³C-enriched acetate in these wells may imply that acetoclastic methanogenesis is active. While intriguing, this is seemingly inconsistent with (1) the published salinity limits of acetate-utilizing methanogenesis, which these wells exceed (Oren 2011, Environ. Microbiol. v13 p1908); and (2) previous microbial characterizations that do not indicate abundant acetate-using methanogens in the most saline Antrim Shale wells. Another possibility is pH-dependent isotope effects (e.g. acetate carboxyl-DIC exchange), which may have systematic effects across this large pH range. Finally, the patterns in δ¹³C-acetate may relate to acetate production (biotic or abiotic) rather than consumption.