Late Generation of Methane from Mature Kerogens

Abstract

The thermal decomposition of sedimentary organic matter, or kerogen, within the metagenesis zone (T > 160 °C) leads to the formation of large amounts of late gas, mainly composed by methane. With the purpose of understanding and quantifying the mechanisms of late methane generation, artificial maturation experiments were performed in closed system on natural samples of type II and type III mature kerogens (Ro > 1.3%, H/C < 0.65). For each experiment, mass and atomic (C, H, and O) balances were obtained by recovering, fractionating and quantifying the entire pyrolysis effluents. These data were interpreted by means of a kinetic schema based on three bulk reactions: one corresponding to the decomposition of short chain alkylated polyaromatics and diarenylalkanes that constitute partly mature kerogens, another one related to demethylation processes, a last one based on the autohydrogenation of the remaining pyrolysis residue. Kinetic parameters derived from these experiments confirmed that late methane is g...