T276 Unconventional Energy Resources

Wednesday, 28 September 2016
8:00 AM - 12:00 PM
Mile High Ballroom 2B

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

Paper number: T276-7
Program: Technical Sessions
Time: 9:30 AM – 9:45 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-287735

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