Demethylation as a mechanism for isotopic reversals of shale gas generated at over maturity

Abstract

This paper investigates demethylation as a possible mechanism leading to isotopic reversals of shale gas during the thermal degradation using a model compound representative of methylated monoaromatic hydrocarbons presented in over-mature organic matter. The model compound used is 1,3,5-Trimethylbenzene (TB), which was subjected to isothermal pyrolysis experiments at three temperatures of 400 °C, 425 °C and 450 °C holding from 24 to 480 h at 300 bar. During the process of TB pyrolysis, heavy hydrocarbon gas was generated in addition to methane and hydrogen. Some aromatic compounds-bearing ethyl and propyl were also detected in the liquid products. The series of carbon isotope of hydrocarbon gas shows a reversal feature (i.e. δ13C1>δ13C2) at TB conversion below 80%. The reversal feature is replaced by normal isotopic distribution when the TB conversion is above 80%. According to the yields variation of different products during TB cracking, heavy hydrocarbon gas appears to have generated via two pathways, (1) combination among the CH3 radicals from demethylation and (2) cleavage of long aliphatic chains formed by combination between CH3 radicals and methyl linked on aromatic rings during TB cracking. The reversal of carbon isotope between CH4 and C2H6 was caused by carbon isotopic fractionation during the heavy hydrocarbon gas generation. The TB cracking experiments indicates that demethylation is one of the factors causing the isotopic reversals of shale gas generated at over maturity. The change of carbon isotope from reversal to normal distribution is attributed to the cracking of C2H6 generated at higher thermal dynamics. It is deduced that the phenomenon of isotopic reversal of shale gas would disappear and be replaced by normal isotopic distribution with further decreasing of natural gas wetness.