Isotopic reversal (‘rollover’) in shale gases produced from the Mississippian Barnett and Fayetteville formations

Ethane, propane, and carbon dioxide show reversed carbon isotopic maturity trends in natural gas produced from the Barnett and Fayetteville Shales at thermal maturities greater than ∼1.5% VRE. At this high level of thermal maturity, the iso-butane to n-butane ratio also reverses, suggesting wet gas cracking has occurred, generating more gas molecules in the same volume resulting in overpressure, and increased stabilized production rates in the Barnett. Hydrothermal fluids from the nearby Ouachita Thrust front apparently enhanced the maturity of the Fayetteville and Barnett shales. Water-hydrocarbon reactions at these high maturities may account for the isotopic reversals.

► C2+ shale gases show reversed carbon isotopic trends at high maturities.
► i/n-butane ratios also reverse, suggesting wet gas cracking with resulting overpressure.
► Hydrothermal fluid-methane reactions may cause carbon isotopic reversals.