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Nature 288, 688 - 690 (18 December 1980); doi:10.1038/288688a0

Generation of light hydrocarbons in sedimentary rocks

J. M. Hunt*, A. Y. Huc & J. K. Whelan*

*Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
Laboratoire de Géologie Appliquée, CNRS–ERA 601, Université d'Orleans, 45046 Orleans Cedex, France

Most petroleum arises from the thermal alteration of organic matter in fine-grained sedimentary rocks. The time–temperature conditions of petroleum formation have been defined by numerous laboratory and field studies1, but little is known of the conditions in which individual hydrocarbons form in nature. The light hydrocarbons are of particular interest because they are present only in traces in surface sediments, yet they constitute about 30% of a typical petroleum. We have found that the distribution of individual C6–C8 hydrocarbons in shale samples from a Gulf of Mexico well indicates that different types of strucures are being generated preferentially at different depths. Hydrocarbons with a tertiary carbon atom apparently formed earlier than those with a quaternary carbon because of the greater stability of the intermediate tertiary carbonium ion or free radical.



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