Geosynthesis of organic compounds: III. Formation of alkyltoluenes and alkylxylenes in sediments

Abstract

Heating ortho-undecyltoluene with an acidic clay catalyst and hexamethylbenzene as a methylating agent resulted in the formation of hydrocarbon mixtures containing undecyltoluenes and dimethylundecylbenzenes. At short heating times, methylation at ring positions 4 and 5 forming 2,4- and 2,5-dimethylundecylbenzenes occurred predominantly, while at longer heating times, methylation and isomerization/transalkylation reactions resulted in higher concentrations of more stable alkylxylene isomers. The distributions of undecyltoluenes and dimethylundecylbenzenes in two mature crude oils were also compared to those formed in the laboratory heating experiments. One crude oil derived from carbonate sediments was shown to have a predominance of ortho-undecyltoluene and 2,4- and 2,5-dimethylundecylbenzenes, similar to the reaction products formed from short-term laboratory heating experiments. The other crude oil, derived from clastic sediments containing abundant clays, was shown to contain a mixture of the undecyltoluenes and dimethylundecylbenzenes similar to the reaction products generated from longer-term heating experiments. These observations suggest that alkylxylenes in crude oils result from sedimentary methylation of ortho-alkyltoluenes, as the ortho-alkyltoluenes isomers are often the most abundant isomers in low maturity sediments. The initial products formed under kinetic control then undergo clay-catalyzed isomerization/transalkylation reactions to yield mixtures with higher abundances of the more thermodynamically stable isomers.