Geochemistry of shale gases from around the world: Composition, origins, isotope reversals and rollovers, and implications for the exploration of shale plays

Abstract We studied a large dataset of ∼2600 shale gas samples from 76 geological formations in 38 sedimentary basins located in eleven countries. Shale gases contain mostly hydrocarbons dominated by methane. Shale gases can have primary microbial, secondary microbial and thermogenic origin. However, gases produced from most commercially successful shale plays (e.g., the Marcellus, Haynesville, Eagle Ford and Barnett in the USA, the Vaca Muerta in Argentina and the Wufeng-Longmaxi in China) are thermogenic. It appears that formations with greater gas endowment such as the Marcellus and the Haynesville contain late-mature thermogenic gas. Shale plays with early-mature thermogenic and secondary microbial gas such as the Antrim (USA) and the New Albany (USA) formations have relatively low endowment of recoverable gas. Shale plays with primary microbial gas are not significant from commercial exploration perspective. Isotope reversals (δ13C of methane > δ13C of ethane) are observed in shale plays with mature organic matter (vitrinite reflectance >2%) that experienced significant uplift (>2 km). It appears that isotope fractionation during desorption from depressurized late-mature shales leads to isotope reversal in the residual gas produced from shale formations (e.g., the Wufeng-Longmaxi). Significant contribution of adsorbed gas (enriched in 13C-rich C2+ hydrocarbons relative to the co-occurring free gas) in the production from some plays (e.g., the Fayetteville, USA) may result in isotope rollovers.