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

Changjie Liu, Peng Liu, Gregory P. McGovern, Juske Horita

Research output: Contribution to journalArticle

9 Scopus citations

Abstract

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 δ2H values of propane increased by 30‰ with maturity, position-specific hydrogen isotope deviations of propane (Δ2-1) increased by 80‰. Position-specific δ2H values showed that δ2H values of the central H in propane increased by 70‰ 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 ∼310 to ∼6 m.y. at ∼140–185 °C. On the other hand, bulk and position-specific δ13C 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.

Original languageEnglish
Pages (from-to)188-204
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume255
DOIs
StatePublished - Jun 15 2019

Keywords

  • Shale gas geochemistry
  • position-specific isotope
  • water and propane exchange

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