oa Determination of Methane Cracking in Pyrolysis Experiment and its Geochemical Significance

The methane will be cracked in a certain thermal maturity stage, but it is uncertain if the methane of shale gas at high-over maturity has been cracked. In the present work, methane pyrolysis experiments were carried out in closed gold-tube system to investigate methane cracking and its carbon isotope fractionation.

From the pyrolysates, we detected the cracking of CH4 and the generation of bigger hydrocarbons like C2H4, C2H6, and C3H8 from the methane pyrolysis experiments within the shale gas maturity scope. CH4 cracking and recombined to C2H6 will cause the residual CH4 enriched whereas the produced C2H6 depleted in 13C, however, the secondary reaction of C2H6 will complicate the isotope distribution features of the pyrolysates in the methane pyrolysis experiments.

Based on our experimental results and the data of the shale gas reservoirs from different regions of the world, we suggested 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.