45284-G2
Hydrogeochemistry of Shallow Gas Accumulations in Fractured Devonian Black Shales in the Appalachian Basin
Devonian-age organic-rich shales in the Appalachian Basin have recently garnered strong interest due to their potential as a large natural gas play. Previous studies have shown thermogenic gas is present in the shales at depth within the basin. Our research investigated the potential for microbial methane within the fractured shales at the northern basin margin, aqueous environmental conditions that may inhibit or enhance methanogenesis, and recharge history through coupled formation water and gas isotope analyses. Microbial methane accounts for over 20% of natural gas in sedimentary basin world-wide and is the dominant source of natural gas at the glaciated margins of the Michigan and Illinois basins. Pleistocene recharge diluted brines and created an environment conducive to methanogenesis. Given the similar glacial history and large geographic-extent of organic-rich shales, we hypothesized that microbial methane is also an important component of natural gas at the northern Appalachian Basin margin. To test this hypothesis, we collected 50 natural gas samples from active oil and gas wells completed in Devonian shale and interbedded sandstones in western NY and northwestern PA. Twenty-two co-produced formation water samples were collected where available; interestingly, the Devonian shales contained very little water. Dissolved gas samples and groundwaters were also collected from shallow drinking water wells screened in the top of the organic-rich shale bedrock, along lake Erie. Elemental analyses and stable isotopes (O, H, and C) were performed on formation waters. Gas composition and compound specific isotopes of CH4 (C1), CO2, C2-C4 were measured on select samples. Geochemical indicators for microbial activity - Microbial methanogenesis in the Michigan and Illinois basins is inhibited by high salinities (Cl >~2 mole/L) and high SO4 values (>~5 mmole/L). Chloride concentrations in Devonian shale brines in the northern Appalachian Basin generally increase with depth from less than 1.3 to 4.2 mole/L. Sulfate concentrations in brine and shallow groundwater range between <0.08 to 12 mmole/L. There is a wide range of δ13C values of dissolved inorganic carbon (DIC) in shale formation waters; groundwater in shallow bedrock wells have δ13C-DIC values between -25 and +5.9‰, while brines at depth range between -26 and +27‰ VPDB. Low δ13C-DIC values (<~-15‰) may indicate sulfate reduction, while high δ13C-DIC (>~0‰) may indicate methanogenesis. Alkalinity concentrations are relatively low (<12 meq/kg), compared to Devonian Shale waters in the Michigan and Illinois basins (up to 70 meq/kg); this may be the result of limited microbial activity and/or an open hydrologic flow system. The combined carbon isotope values of DIC and alkalinity concentrations suggest that sulfate reduction may be dominant at shallow depth, while microbial methanogenesis is important at depth in select locations across the basin margin. Isotopic evidence for origin of natural gas - Devonian Shale gas samples from the northern Appalachian Basin have low gas wetness values (4-25%), and enriched δ13C (>-50‰ VPDB) and δD values of CH4 (>-314‰), which increase with depth and thermal maturity. There is no correlation between δD values of CH4 and co-produced waters, as would be expected for microbial fractionation. Analyses of select gas samples for carbon isotopes (C1-C4) confirm that thermogenic gas is dominant. This is in stark contrast to the Devonian Shales along the margin of the Illinois and Michigan basins, which dominantly contain microbial gas. Three of 7 dissolved gas samples from drinking water wells contained methane above trace levels (9 to 54%), with relatively negative δ13C values (-74 to -58‰), suggesting microbial methanogenesis. Together with the water analyses, the gas isotope results suggest that microbial methane is present in the Appalachian Basin Devonian shales near the surface, and may be present at depth, however the vast majority of methane was generated in the geologic past via thermogenic processes. Isotopic evidence for recharge source - Unlike in the Michigan Basin, there is no evidence of Pleistocene glacial meltwaters in the Appalachian Basin Devonian shales; oxygen isotope values in brine range between -7.4 and -0.4‰, much more positive than the average estimated δ18O for the Laurentide Ice sheet (<~-15‰). The δ18O and δD values of formation waters plot off of the local meteoric water line along a mixing trend between evaporated Paleozoic seawater and a meteoric water end-member. Oxygen isotope values for the bedrock drinking water wells are between -11.6 and -9.1‰ in the range of modern precipitation. These values plot on the local meteoric water line and more negative than the brine meteoric water end member, suggesting that the Devonian Shale brines had a different recharge source than modern precipitation. The renewed interest in Devonian shale gas in the Appalachian basin may provide a unique opportunity to continue this research with new well completions. We have presented this ACS-PRF funded research at over 7 national and international conferences, and have a manuscript in preparation to submit to Chemical Geology this fall.
