Abstract: Biomarkers from Asphaltene Pyrolysis: An Additional Tool for Oil Correlation

Harry Dembicki, Jr., M.D. Mathiesen

Compounds typically used for oil correlation are the biological marker hydrocarbons (biomarkers) including isoprenoids, terpanes, and steranes. Sometimes the biomarkers in crude oils and source-rock bitumens can be altered or obscured by biodegradation or contamination. In these cases, the ability to make correlations can be significantly impaired. One part of the crude oil and bitumen that is not affected by either biodegradation or most forms of contamination is the asphaltene fraction. Asphaltenes are a high molecular weight, polycyclic, NSO (nitrogen, sulfur, and oxygen) bearing fraction of crude oils and bitumen. They are generally considered to be formed in the source rock by simple cleavage of a portion of the much larger kerogen polymer. These smaller pieces of the kerogen are held in a colloidal solution in the sediment bitumen and can be carried along when the bitumen migrates to form a crude oil. Because asphaltenes are small pieces of the kerogen from the source rock, they should carry the same chemical information as the kerogen. By pyrolyzing asphaltenes it might be possible to release biomarkers similar to the way that biomarkers are released from the kerogen during the generation process. These biomarkers might be useful in correlation studies.

Previous attempts at obtaining biomarkers from asphaltene pyrolysis using temperatures of 300°C or greater have yielded terpane and sterane distributions dissimilar to naturally occurring bitumens and oils. However a lower temperature pyrolysis-gas chromatography-mass spectrometry (P-GC-MS) method has been developed for obtaining biomarker data from asphaltenes that are comparable to biomarker data from the saturate fractions of bitumens and crude oils. The method includes pyrolyzing the asphaltenes at 250°C for 20 min, collecting the pyrolysis products directly on the gas chromatographic column, and then analyzing the biomarkers with the mass spectrometer in selective ion monitoring mode. These distributions can be useful for oil-to-oil and oil-to-source rock correlations w ere saturate fraction biomarkers may have been obscured by biodegradation or contamination.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994