Molecular and intramolecular isotope geochemistry of natural gases from the Woodford Shale, Arkoma Basin, Oklahoma

A combined bulk and position-specific isotope study was conducted on natural gases collected from an unconventional gas field in the Late-Devonian to Early Mississippian Woodford Shale in the southwestern Arkoma Basin in Oklahoma. The set of natural gases records a hitherto underreported case, where gases become wetter with decreasing isotope compositions in an early stage of maturation. A first dataset of position-specific isotope compositions of propane with demonstrated accuracy and precision show that while the bulk delta H-2 values of propane increased by 30 parts per thousand with maturity, position-specific hydrogen isotope deviations of propane (Delta(2-1)) increased by 80 parts per thousand. Position-specific delta H-2 values showed that delta H-2 values of the central H in propane increased by 70 parts per thousand with maturity, but those of terminal H in propane remained constant. This contrasting behavior could have resulted from the hydrogen isotope exchange between shale water and propane with a half-time of the center H, varying strongly from similar to 310 to similar to 6 m.y. at similar to 140-185 degrees C. On the other hand, bulk and position-specific delta C-13 values of propane remain nearly constant during the maturation. Position-specific isotope compositions of propane at the highest maturity yield an equilibrium temperature that is consistent with its maturity, thus possibly serving as single-compound isotope geothermometry. (C) 2019 Published by Elsevier Ltd.