Methane generation from methylated aromatics: Kinetic study and carbon isotope modeling

The aim of this work was to elaborate a mathematical model that accounts fo r the carbon isotopic composition of methane generated during the thermal c racking of two model compounds: 9-methylphenanthrene (9-MPh) and 1-methylpy rene (1-MPyr). Pyrolysis experiments were carried out; in an anhydrous clos ed system (gold vessels) during times ranging from 1 to 120 h under isother mal conditions (400-475 degreesC) at a constant pressure of 150 bar. Global rate constants were determined for methane generation from l-methylpyrene decomposition, similar to those determined by Behar et al. (see ref 36 in t he text) for 9-MPh thermal cracking. Two main processes of methane formatio n were recognized: one related to the loss of the methyl group and the seco nd corresponding to the opening of the aromatic rings, the second of which is hydrogen pressure dependent. The derived apparent first-order kinetic pa rameters were determined only for the first process: E = 55.6 kcal/mol and A = 4.2 x 10(12) s(-1). These parameters are in the same range as those fou nd for methane generated from 9-MPh (ref 36). When these results are extrap olated to geological conditions, methane generation occurs at temperatures lower than 200 degreesC and, thus, constitutes a significant source for nat ural gas accumulations. This source of natural gas can compete with late me thane generation from kerogen. Based on the global kinetic scheme proposed for methane generation, a model of carbon isotopic fractionation was elabor ated for predicting the isotopic composition of methane. Results show that very high isotopic fractionation can take place when the methylated aromati cs are thermally degraded: the demethylation reaction leads to an isotopic fractionation between the generated methane and its source which is signifi cantly dependent upon the isotopic heterogeneity of the aromatic compound. This study shows that specific isotopic signatures in natural gas might fin gerprint the secondary cracking of aromatics in deep reservoirs.