Overview of the geochemistry of some tar mats from the North Sea and USA: implications for tar-mat origin

Tar mats and oil legs from several marine source-rock derived oil fields (Ula and Oseberg, North Sea and an unidentified North American example) have been geochemically characterized. Tar mats were derived from the same petroleum system as the oil-leg oils and are found in the most porous and permeable portions of the reservoir. It is possible to eliminate some potential tar-mat formation mechanisms using the data from case histories and subsurface asphaltene solubility characteristics. Adsorption of asphaltenes on reservoir clays and light to moderate levels of biodegradation can probably be eliminated as feasible tar-mat formation mechanisms. Thermal degradation of oil in the carrier and reservoir causing asphaltene precipitation may be a major contributing factor to formation of tar mats in the Ula Field, whereas increased gas solution in the oil leg may have contributed to deasphaltation in the Oseberg Field. Based on tentative mass-balance calculations, the extreme pressure dependence of asphaltene solubility in subsurface crude oils and the occurrence of tar mats in zones of elevated permeability, it is suggested that tar-mat formation is initiated during petroleum migration. While tentative, this model may provide a more generic initiation mechanism than others in the literature.