Abstract
Shale gas with carbon isotope reversal (δ13Cmethane > δ13Cethane), as a promising indicator for resource “sweet-spot”, tends to occur at high-over maturity shales with high gas dryness index (Cmethane/(Cmethane + Cethane) > 0.97). However, the cause for this reversal is still controversial and remains an open question. In the present work, we propose methane cracking within high-over matured shale rocks, as one of the possible mechanisms, to explain this reversal, which will further enhance the knowledge about carbon isotope reversal of shale gas. In order to validate this hypothesis, methane pyrolysis experiments were carried out in closed gold-tube system to investigate the mechanism of occurrence of carbon isotope reversal in shale gas. From the pyrolysates, the cracking of CH4 and the generation of higher number hydrocarbons like C2H4, C2H6, and C3H8 were detected. The CH4 cracking and the subsequent formation of C2H6 led to the residual CH4 being enriched in 13C whereas the produced C2H6 is depleted in 13C. Based on our experimental results and the data analysis of 583 samples from shale gas reservoirs from different regions of the world, we suggest that CH4 will be cracked and combined into C2H6 in high-over matured shales. When the contribution from methane-derived C2H6 is sufficiently massive, the commonly found order of δ13Cmethane < δ13Cethane will change with increasing maturity (e.g. vitrinite reflectance, Ro > ~2.0%) and a carbon isotope reversal will occur for the shale gas. The Barnett shale from Fort Worth Basin, USA, as the application representative shale gas reservoir, cover the evolution scope of the increase and subsequent decrease of δ13Cethane along with increase of δ13Cmethane and thus the beginning of the carbon isotope reversal.
Published Version
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