Carbon isotopic evidence for the catalytic origin of light hydrocarbons

Frank D Mango

doi:10.1186/1467-4866-1-38

Abstract

The molecular proportionality between C6 and C7 isomers reported recently (F. D. Mango, Geochim. Cosmochim. Acta, 2000, 64, 1265; ref. 1) is probably the strongest evidence for catalysis yet published. It implicates two cyclopropane-like precursors, [S6] and [S7] (where S denotes any substrate), of similar structures, each yielding three isomers along similar kinetic pathways: [S6] → n-hexane + 2-methylpentane + 3-methylpentane [S7] → n-hexane + 2-methylhexane + 3-methylhexane This view is supported here by the carbon isotope ratios of these isomers in 36 oils from Western Canada (M. J. Whiticar and L. R. Snowdon, Org. Geochem., 1999, 30, 1127; ref. 2). They exhibit strong correlations in δ13C, consistent with their being formed in triads through isotopically indistinguishable precursors. These results add significantly to the growing body of evidence supporting catalysis.

Highlights

There can be little doubt that light hydrocarbons (C1±C9) can be produced thermally from decomposing hydrocarbons in sedimentary rocks.[3]
They exhibit strong correlations in d 13C, consistent with their being formed in triads through isotopically indistinguishable precursors
Other pathways have always seemed possible, they were rarely given serious consideration until it became clear that (a) ordinary hydrocarbons should remain stable under the time±temperature conditions typically seen in sedimentary rocks,6±9 and (b) thermal cracking in the laboratory does not produce a gas resembling natural gas.8,10±16 Catalysis gained additional recognition in 1987 when an invariance in isoheptanes was disclosed.[17]

Summary

Carbon isotopic evidence for the catalytic origin of light hydrocarbons

1) is probably the strongest evidence for catalysis yet published It implicates two cyclopropane-like precursors, [S6] and [S7] (where S denotes any substrate), of similar structures, each yielding three isomers along similar kinetic pathways:. This view is supported here by the carbon isotope ratios of these isomers in 36 oils from Western Canada They exhibit strong correlations in d 13C, consistent with their being formed in triads through isotopically indistinguishable precursors These results add signi®cantly to the growing body of evidence supporting catalysis

Introduction

Results and discussion

Sylvan Lake