MORELOS-GARCIA, J. A., and R. M. MITTERER, University of Texas at Dallas, Richardson, TX; and R. SASSEN, P. COMET, and J. BROOKS, Geochemical and Environmental Research Group, College Station, TX

ABSTRACT: Migration-Contamination of Oils Entrapped in Terrigenous Reservoirs: A Potential Problem for Interpretation of Biomarker Parameters

The Southern Tampico-Misantla basin contains oil in Cretaceous reservoirs, composed of reef and forereef deposits, Upper Jurassic reservoirs of oolite grainstones, and Tertiary reservoirs of turbiditic shales and sandstones. Tertiary oils are emplaced in various pay zones that vary from 1500 to 1800 m in depth. Tertiary sediments are thermally immature.

All these oils have a common origin and were generated from a carbonate source rock, containing dominantly algal and bacterial organic matter, deposited in an anoxic environment. Bulk and some molecular parameters indicate that Cretaceous oils are the least mature, followed by Tertiary oils, with the Upper Jurassic oils being the most mature of the study area. However, C<29> 20S/(20S+20R) thermal maturation parameter of Tertiary oils shows a systematic shift toward immaturity. Similarly, source indicators exhibit an increasing Tertiary and/or Upper Cretaceous terrestrial input in the oils. This is, M/Z 191 fragmentograms of Tertiary oils exhibit a gradual increase of oleanane and 28,30-bisnorhopane with increasing upward migration through the shale and sandstone sequence. M/Z 21 fragmentograms also present an increasing proportion of immature R isomers of regular steranes and a higher proportion of C<29> R steranes through the same sequence. Monoaromatic biomarkers have a similar pattern. These patterns are not supported by any other indicators.

Not all biomarkers in shales are integrated into the kerogen structure; some are present in the bitumen fraction. Such compounds can be dissolved and incorporated into migrating oil passing through the porous and permeable zones of the shale in a process called migration-contamination. The appearance of these biomarkers in Tertiary oils of the Tampico-Misantla Basin is due to this process. Biomarkers from oils that have migrated through, and been entrapped in, terrigenous rocks in other regions may also have been subjected to migration-contamination. Conclusions based on these biomarkers may be misleading.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.