Print this pageMicrobial Oxidation of Gaseous Hydrocarbons in Reservoirs: Antrim Shale/Michigan Basin Provides Time Constraints
SCHOELL, MARTIN, ANNA M. MARTINI and 
LYNN
M. WALTER
The Antrim shale in the Michigan Basin is a fractured reservoir where formation and alteration of gaseous hydrocarbons can be related to the influx of young fresh waters that have been dated to less than or equal to 20,000 ybp. We will show here that alteration of gaseous hydrocarbons in reservoirs is a rapid process that proceeds in the order of thousands of years and is, therefore, much faster than most reservoir equilibration processes.
Natural gases produced from the Antrim Shale at the northern rim of the Michigan basin vary in composition from methane-rich "dry" gases (C[2+] approx. 0.01 percent) to "wet" gases with appreciable contents of ethane, propane and butane (C[2+] approx. 2-15 percent). Carbon-13 contents of ethane and propane are low in the wet gases (delta{13}C approx. -45 and approx. -37 o/oo, respectively) but increase significantly in the methane-rich gases (delta{13}O approx. -37 and approx. -16 o/oo, respectively). Carbon isotope values of CH[4] and associated CO[2] tend to increase with decreasing C[2+] from approx. -54 to approx. -49 o/oo and from approx. +17 to approx. +23 o/oo, respectively.
Gas composition and isotopic properties of the Antrim shale gases are clearly related to water chemistry. Dry gases tend to occur in association with low salinity waters that result from mixing of young recharge of fresh waters with basinal brine, whereas wet gases occur more basinward in association with more saline waters.
The systematic {13}C-enrichment in ethane and propane is diagnostic of bacterially mediated oxidation of gaseous hydrocarbons. The association of {13}C-rich gases with low salinity waters suggests that this process is triggered by the influx of fresh waters into the Antrim shale reservoir. Because the influx of these fresh waters is dated to less than or equal to 20,000 ybp, we can constrain the time necessary for bacterial alteration to less than or equal to 20,000 years. Reservoir equilibration processes such as diffusion or density driven turnover require much longer time for equilibration of reservoirs (10{5} to x{*}10{6} years). This implies that even pressure equilibrated biodegraded reservoirs are compositionally heterogeneous. Examples from other reservoirs will be shown to demonstrate implications for oil and gas production.