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45284-G2
Hydrogeochemistry of Shallow Gas Accumulations in Fractured Devonian Black Shales in the Appalachian Basin
Accumulations of microbial methane in shallow coal beds and organic-rich shales account for a significant portion of the U.S. annual natural gas production. Research conducted on Upper Devonian organic-rich shales in the Michigan and Illinois basins has shown that secondary microbial gas generation was enhanced by Pleistocene glaciation. Loading and unloading of kilometer thick ice sheets extended the natural shale fracture network and depressed the salinity of basinal brines by recharge of dilute glacial meltwater. The northern margin of the Appalachian basin has a similar glacial history to the Michigan and Illinois basins, and age-equivalent organic-rich shales. However, the lack of published geochemical data of Appalachian basin fluids and gas has precluded comparative studies.
This study is focused on the following research questions: 1) Is there any geochemical evidence for microbial methane in Upper and Middle Devonian organic-rich shales across the northern margin of the Appalachian Basin, 2) if so, what is the spatial distribution of microbial versus thermogenic methane, and 3) what hydrologic and geologic factors may have inhibited or enhanced methane generation?
To address these questions, 49 gas samples and 28 co-produced formation waters were collected from active Upper and Middle Devonian oil and gas wells from the northern margin of the Appalachian basin (western NY) towards the basin center (western PA), during the summer of 2007. The depth of shale wells sampled ranged from 450 to 7400 feet. Devonian shales in this region have high organic carbon contents (>3%), and variable thermal maturities (<1 to >4 Conodont Alteration Index) that increase laterally from west to east, and with depth towards the basin center. Major, minor, and trace elemental analyses, alkalinity, and stable isotopes (O, H, and C) were performed on formation waters. Gas composition and compound specific isotopes of CH4, CO2, and C2 were also measured. Carbon-14 will be measured on select water samples this fall. Additional fluid and gas samples will be collected in Spring 2008 at shallow depths near Lake Erie to better define areas of microbial methanogenesis.
Methanogens have been shown to be inhibited at Cl concentrations greater than ~2 mole/L, and SO4 values greater than 5 mmole/L. Cl concentrations of shale formation waters in western NY and PA ranged from 1.3 to 4.3 mole/L, and SO4 concentrations ranged from 0.6 to 12.3 mmole/L. Acetate concentrations were all below detection limit. Cl/Br ratios show significant dilution of formation water salinity by freshwater recharge at the northwestern basin margin. Alkalinity concentrations are relatively low (0.1 to 12.0 meq/kg) with variable d13C of DIC values (-26.2 to +27.3 ‰), and CO2 concentrations were less than 0.2% in all of the wells. The carbon and hydrogen isotope values of methane (-31.5 to -54.7 ‰, and -157 to -315 ‰, respectively) show an enrichment trend corresponding to the thermal maturity of the shales. There also does not seem to be a strong correlation between the hydrogen isotope value of the formation waters and co-produced methane, as would be expected for microbial methanogenesis. These initial results may indicate that thermogenic gases are dominant in the Devonian shales across the northern basin margin, although there may be biogenic methane in some shallow areas with low Cl, low SO4 and elevated d13C-DIC values.
The second year (completion) of this research will be focused on integrating the fluid and gas geochemistry with the rock properties (i.e. total organic carbon content, thermal maturity, fracture density, lithology), hydrology (i.e. fluid flow directions, glacial history, hydrostratigraphy), and subsurface redox conditions to constrain the origin and distribution of natural gas in Devonian shales, and environmental factors necessary for establishing and sustaining microbial methanogenesis. As previously mentioned, ~10 more fluid and gas samples will be collected to better define areas of microbial methane, and select waters will be analyzed for Carbon-14 to constrain the timing of freshwater recharge. Stephen Osborn (Ph.D. student), Justin Clark (undergraduate student, SUMR Scholar), and Trevor Calamel (summer research assistant) were directly involved in every aspect of this research.
