Catagenesis of light cycloalkanes in petroleum

Abstract

The article analyzes the distribution and genesis of light, monosubstituted and multi-substituted cycloalkanes in crude oils. Experimental studies of the formation of naphthenic compounds in thermoradical and cationic reactions of heavy n-alkanes, fatty acid, n-alkyl-substituted cycloalkanes and 5-α-cholestane provided the basis for the new formation mechanisms of light naphthenic products in petroleum. Cyclohexanes with one linear alkyl substituent are prominent components of crude oils. The model studies indicate a possibility of a thermoradical formation mechanism of these compounds from n-alkyl chains during petroleum catagenesis. This route starts with thermolysis of heavy n-alkanes and fatty acids into mixtures of lighter n-alkanes and olefins, and involves cyclization of olefinic radicals into n-alkyl-substituted cyclohexanes. The second group of light naphthenic compounds in crude oils includes cyclopentanes and cyclohexanes with several small alkyl substituents (methyl and ethyl). The model studies indicate that these compounds can be formed abiotically, in two types of reactions: • (a) cyclization of olefinic radicals (the same reactions that generate mono- n-alkyl-substituted cycloalkanes); and • (b) cationic isomerization reactions of mono- n-alkyl-substituted cyclohexanes and cyclopentanes with acidic clays. The clays convert monosubstituted cycloalkanes into the equilibrium mixtures of various multi-substituted cyclohexanes and cyclopentanes. The thermolysis studies of 5α-cholestane and several ancient sediments suggest that the cracking of heavy multi-ringed napththenic compounds (triterpanes and steranes) in the ancient sediments cannot be regarded as the principal source of light naphthenic compounds in mature crude oils.