Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.  All Rights Reserved.
 

Detecting migrated petroleum in shallow cores requires techniques that reliably indicate the presence of thermogenic petroleum in a core extract. Often, the level of petroleum in the extract is very low, and in our experience, many surveys have been prone to some form of contamination. It is important to be able to recognize these contaminants to avoid ascribing contaminants to actual petroleum in the core.

The most common form of contamination in our coring surveys has been distilled vacuum pump oil in the extracts. This arises during the freeze-drying process when the wet cores are subjected to a high vacuum to dry them prior to extraction. In many of our shallow-coring studies, this material was mistakenly interpreted as "condensate" largely because of its fluorescence characteristics. However, GC-MS analysis shows that this material is of low thermal maturity and has a similar composition in coring studies from widely differing parts of the world where very different source characteristics are expected. This constancy in composition is strongly suggestive of contamination. Despite attempts to remove this pump oil with a series of filters in the freeze-drying apparatus, it has proved to be difficult to prevent it from reaching the cores. We now air-dry all our cores to prevent this contamination and have found that our data more clearly show the presence of true petroleum in cores. Several other contaminants have been detected during coring surveys, mainly from contamination in the laboratory. It has proved necessary to carefully monitor each coring survey and the data produced by contractors to achieve reliable interpretations.