wAbstract
@We consider the effect that commercial gas production has had
on microbiology and water and gas geochemistry in the northern
producing trend of the Antrim Shale, an unconventional gas reservoir
in the Michigan Basin, USA. We analyzed gas, water, and microbial
biomass samples collected from seven wells in 2009 and compared
our findings to the result of analyses performed as early as 1991
on samples collected from the same wells. We also examined production
records associated with six wells. Water production has decreased
sharply over time and is currently at 0.2 to 14.6 % of peak levels.
While this has happened, the chemical and isotopic composition
of gas and water produced from the wells has shifted. The proportion
of CO2 has increased by as much as 15 mole
% while CH4 content has correspondingly decreased.
Isotopically, the Β13C and ΒD values of CH4
decreased for most wells by averages of 1.3ρ and 9ρ, respectively,
while Β13C values of CO2 increased
for most wells by an average of 1.7ρ. Alkalinity in the water
from each well decreased by 10 mM on average and SO42-
content increased from below 50ΚM to over 200ΚM on average in
water from each well with initial values. Microorganisms most
closely related to CO2-reducing methanogens
were the most abundant group in archaeal clone libraries and SO42- reducers were the most abundant
group in bacterial libraries. In contrast, no SO42-
reducers were identified in a nucleic acid-based analysis of a
sample collected in 2002 from one of the wells we sampled. Our
results show that commercial gas production has not only caused
chemical and isotopic changes in water and gas in the Antrim Shale
but also an increase in the abundance of SO42--reducing
microorganisms, a change that can ultimately have a negative impact
on biogenic CH4 formation. Processes that
can explain these changes include ongoing biogeochemical reactions,
groundwater flow, gas desorption, and open-system degassing.
Keywords: Sulfate reduction; Methanogenesis; Antrim Formation;
Michigan Basin; Unconventional natural gas reservoir; Black shalex
1. Introduction
2. Materials and methods
@2.1. Commercial gas wells
@2.2. Sample collection
@2.3. Microbial analysis
@2.4. Chemical and isotopic analysis
@2.5. Field station records
3. Results
@3.1. Microbial community composition
@3.2. Chemical and isotopic composition of water
@3.3. Chemical and isotopic composition of gas
4. Discussion
@4.1. Pathway of CH4 formation
@4.2. Shifts in archaeal community composition
@4.3. Shifts in bacterial community composition
@4.4. Shifts in groundwater geochemistry
@4.5. Shift in gas geochemistry
@4.6. Potential impact of hydraulic fracturing
5. Conclusions
Acknowledgements
Appendix A. Supplementary data
References