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ABSTRACT: High Molecular Weight Hydrocarbon (>C40) in Source Rock Extracts

DEL RIO, JOSE CARLOS, I.R.N.A., C.S.I.C., Sevilla, Spain, and R. PAUL PHILP,* University of Oklahoma, School of Geology and Geophysics, Norman, OK

The past two decades of organic geochemistry have seen some dramatic advances catalyzed to a large extent by developments in analytical chemistry, particularly those associated with gas chromatography-mass spectrometry. In this paper data are presented to demonstrate that another significant advance is currently underway in organic geochemistry into an area that has not been previously exploited. Hydrocarbons used in most geochemical studies are in the carbon number range C1-C40. However, in the past two years the availability of high temperature gas chromatography columns has meant that it is now possible to analyze hydrocarbon fractions up to temperatures of approximately 500 degrees C. This, in turn, means that the carbon number range of compounds that can be determined has been ex ended up to approximately C100.

In this paper results will be presented from a study of source rock extracts and associated crude oils, which will illustrate the presence of the n-alkanes, monocyclic and branched acyclic extended hydrocarbons and tricyclic terpanes in the carbon number range. The origin, significance and utility of these compounds will be discussed.

The discovery of the presence of these highs molecular weight hydrocarbons will lead to a reevaluation of some of the older geochemical concepts. For instance, it has been commonly stated that extracts from marine derived source rocks maximize around C20. However, it is now evident that many marine source rock extracts and oils show a second maxima about C40, previously not observed due to the temperature limitations of the GC columns.

 

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)