AAPG Annual Convention and Exhibition

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Noble Gases Help Trace the Behavior of Hydrocarbons in Unconventional Oil and Gas Shales


The occurrence, distribution, and composition of hydrocarbons in the Earth's crust, result from the complex interplay between the tectonic and hydrologic cycles. For example, there is complex association between the tectonics of fold-thrust belts, the deformation of foreland basins, and the generation and migration of hydrocarbons and other geologic fluids in the subsurface. Accurately characterizing the relationship between these factors is critical to predicting the economic success of conventional and unconventional energy plays. One technique that is traditionally used in these studies is the analysis of gas geochemistry, specifically stable isotopic compositions (e.g., δ13C, δ18O, and δ2H) of hydrocarbon gases or CO2. The inert noble gases provide a complementary geochemical technique that can be used in concert with hydrocarbon molecular and stable isotope composition to evaluate the source and migrational history of hydrocarbons in conventional and unconventional plays. Additionally, in some cases, noble gases can be used as an external variable to evaluate the timing of closure for hydrocarbon reservoirs, open vs. closed system behavior and to determine and monitor the residual fluids in place during exploration and production. Herein, we will present noble gas and hydrocarbon molecular and stable isotope data from hydrocarbon plays in the Appalachian Basin (Utica, Trenton-Black River, and Marcellus) and Dallas-Fort Worth (Barnett) basins. Our presentation will focus on insights gained about hydrocarbon stable isotopic roll overs and reversals based on noble gas isotope data. Our preliminary data suggests that producing natural gas wells that exhibit isotopic reversals display distinct noble gas evidence consistent with relatively closed system behavior. Additionally, samples with isotopic reversals retain more than 3x the concentrations of atmospheric (air-saturated water) noble gases suggesting that significantly higher levels of formational waters remain in black shale source rocks that exhibit isotopic reversals.