ORIGIN AND DISTRIBUTION OF NATURAL GAS IN UPPER DEVONIAN ORGANIC-RICH SHALES, APPALACHIAN BASIN

Upper Devonian organic-rich fractured shales from the Great Lakes and Northeast regions of the United States host large economic accumulations of natural gas. Significant portions of these shale gas resources are microbial in origin and are actively being generated along the shallow margins of the glaciated Michigan and Illinois basins. Methane and CO₂ are produced by microbial methanogenesis in-situ with dilute Pleistocene meltwaters and adsorbed onto the organic matter. In contrast, thermogenic gas resources are generated by thermal degradation of organic matter at depth within sedimentary basins, and are often associated with saline fluids. Determining the origin of natural gas in fractured shales and coals is critical for gas exploration and production strategies and for making reasonable resource estimates. The lack of published formation water and gas chemistry from Upper Devonian shales in the Appalachian basin has prevented an adequate assessment of glacial recharge history, fluid flow and salinity gradients, and sources of natural gas. It has also precluded comparative studies with shale gas plays in adjacent Paleozoic basins.

Twenty-eight formation waters and gas samples were recently 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). The depth of shale wells sampled ranged from 450 to 7400 feet, and formation water salinity ranged from 82 to 236 g/L total dissolved solids. Upper 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, stable isotopes (O, H, and C), and ¹⁴C_DIC, were performed on formation waters. Gas composition and compound specific isotopes of CH₄, CO₂, and C₂ were also measured. The combined geochemical analyses will be used to constrain the paleohydrology at the basin margin, origin and distribution of natural gas, and environmental factors responsible for establishing and sustaining microbial methanogenesis.