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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 44 (1960)

Issue: 7. (July)

First Page: 1256

Last Page: 1257

Title: Carbon Isotopic Evidence on Mechanisms of Petroleum Maturation: ABSTRACT

Author(s): Sol R. Silverman

Article Type: Meeting abstract


The C13/C12 ratio of gas (largely methane) separated from crude petroleum is about 1 per cent lower than that of the whole crude. Analyses of liquid distillate fractions of petroleum indicate that the C13/C12 ratio is highest in the lowest boiling fraction (gasoline) and decreases gradually in consecutive higher temperature fractions; the rate of C13/C12 ratio decrease is about 0.015 per cent per 100°F. increase in boiling range.

To account for the observed C13/C12 ratio increase in the transition from complex petroleum constituents to the simpler hydrocarbons of the gasoline fraction, we postulate that the lowest molecular weight hydrocarbons are formed by decomposition of high molecular weight components. This decomposition is accompanied by carbon isotope fractionation that enriches the lowest molecular weight products (methane and ethane) in C12. The residue left behind after methane formation, therefore, has undergone a reduction in molecular weight and an increase in C13 content. As decomposition

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continues, the residual molecules become correspondingly smaller (and lower boiling) and isotopically heavier (higher C13 content).

If a material balance is attempted by redistributing the C13/C12 abundances of the gas fraction among the various liquid distillate fractions, we arrive at an isotopic composition of the resulting gas-free petroleum that is close to the isotopic composition of the highest boiling fraction. Available data on the isotope effect accompanying the thermal decomposition of simple organic molecules indicate that C12--C12 bonds are ruptured about 8 per cent more frequently than are C12--C13 bonds. This fractionation is in the same direction and of the same order of magnitude as the fractionations apparently involved in the decomposition of complex petroleum constituents.

These observations, which can readily be explained by assuming that the lighter petroleum fractions are derived from high molecular weight compounds, must be considered in evaluating existing hypotheses of petroleum origin and evolution.

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Copyright 1997 American Association of Petroleum Geologists