Methane, ethane and propane chemical and isotopic composition changes (inversion) in natural gases from Barnett and Fayetteville shale plays

Inversion of the isotopic composition of carbon in methane, ethane and propane is common in the highest productivity shale formations in the United States. Roll-over effect is the reversion of normal isotopic trend from delta C-13-C-1 <delta C-13-C-2 <delta C-13-C-3 to delta C-13-C-1 > delta C-13-C-2 > delta C-13-C-3. The reasons for this inversion have not been clarified, but the most common explanation is the presence of secondary cracking of crude oil in shale source rock. Zumberge and colleagues determined the molecular and isotopic composition of more than 200 samples of gas from the Barnett formation (Fort Worth basin) and Fayetteville formation (Arkoma basin). Thermal maturity of these basins was determined, respectively, from about 1 to 2%, and from about 2 to 3% vitrinite reflectance equivalent [9, 10]. The results of these analyzes were used to determine the differences in gas interpretation with formulas and diagrams developed for the conventional gases when used for unconventional gases.