Elemental and isotope geochemistry of Appalachian fluids: Constraints on basin-scale brine migration, water-rock reactions, microbial processes, and natural gas generation

This study utilizes new geochemical analyses of fluids (formation water and gas) collected predominately from Devonian organic-rich shales and reservoir sandstones from the northern Appalachian Basin margin to investigate basin scale hydrologic processes, water-rock reactions, microbial activity, and natural gas generation. Elemental and isotopic composition of co-produced formation waters and natural gas show that the majority of methane in Devonian organic-rich shales and reservoir sandstones is thermogenic in origin with localized accumulations of microbial gas. Microbial methanogenesis appears to be primarily limited by redox buffered conditions favoring microbial sulfate reduction. Thermal maturity (bioavailability) of shale organic matter and the paucity of formation waters may also explain the lack of extensive microbial methane accumulations.;Iodine and strontium isotopes, coupled to elemental chemistry demonstrate basin scale fluid flow and clay mineral diagenesis. Evidence for this is based on anomalously high 129I/I values sourced from uranium deposits (fissiogenic production of 129I) at the structural front of the Appalachian Basin. Radiogenic 87Sr/86Sr (up to 0.7220), and depleted boron and potassium concentrations support smectite clay diagenesis at temperatures greater than 120°C. The development of fissiogenic 129I as a tracer of basin scale fluid flow is a novel application of iodine isotopes provided that the sources of cosmogenic and anthropogenic 129I are reasonably well constrained.;The anomalously high 129I/I in Appalachian Basin brines may be alternatively explained by microbial fractionation based on a correlation with decreasing delta13C-DIC values and decreasing sulfate concentrations in the range of sulfate reduction. These results demonstrate that the microbial fractionation of iodine isotopes may be possible and an important consideration when interpreting 129I/I, regardless of the source of 129I.;Results from this study have important implications for understanding the controls on and origins of natural gas production in sedimentary basins; tectonically and topographically driven basin scale fluid flow, including diagenetically induced water-rock reactions and mineral ore deposition related to orogenesis; and an improvement of the use of iodine isotopes for understanding large scale fluid flow, and possibly its use as a tracer of organic matter diagenesis and the distribution of radionuclides in the environment.