ORIGIN OF TAR MATS IN PETROLEUM RESERVOIRS .2. FORMATION MECHANISMS FOR TAR MATS

Tar mats and oil legs from several marine source rock derived oil fields, including the Ula and Oseberg fields in the North Sea and an unidentified North American example, have been analysed geochemically. In all cases the tar mats were derived from the same petroleum system as the oil leg oils and were found in the most porous and permeable portions of the reservoir. Comparing case history data with published data on the subsurface solubility characteristics of asphaltenes in petroleums, it is possible to eliminate some potential tar mat formation mechanisms and suggest the most likely causative mechanisms. Thus the adsorption of asphaltenes on reservoir clays, and light to moderate levels of biodegradation, are considered unlikely to contribute to tar mat formation. In the Ula field thermal degradation of the oil in the carrier and reservoir causing asphaltene precipitation may be a major contributing factor to the tar mats there. In the Oseberg field increased gas solution in the oil leg may have contributed to deasphaltation. Tentative mass balance calculations, the extreme pressure dependence of asphaltene solubility in subsurface crude oils, the occurrence of tar mats in zones of elevated permeability, plus a simple transport model, suggest that during secondary migration an asphaltene-rich petroleum phase may be produced in dipping carrier beds and may be carried into the reservoir, contributing in some instances to tar mat formation.