oa Limits to Hydrocarbon Stability in Deep Basins: Evidence from Stable Isotope Reversals and Noble Gas Geochemistry

We report reversals of both carbon and hydrogen isotopic compositions in natural gases in a deep sedimentary basin (Appalachian basin, eastern USA). The isotopic and molecular compositions together with unique geochemical properties of the source organic matter allow us to seperate mixing and source contributions to the gases so that we can identify trends due to Raleigh fractionation during hydrocarbon destruction by redox reactions. Data from noble gas geochemistry and the isotope geochemistry of carbon dioxide, nitrogen, and hydrogen sulfide in the gases support our arguements. Rapid development and exploitation of shale gas and other unconventional reservoirs has begun to encounter gases with partial to complete carbon isotope reversals. Our interpretation of the origin of reversals due to destruction of higher hydrocarbons at high thermal maturities in sedimentary basins may place constraints on the potential volume of resource from these types of gas accumulations.