Control of hydrocarbon seepage intensity on level of biodegradation in sea bottom sediments

Offshore surface geochemical surveys, which target the surface expression of potential migration pathways for sampling such as fault scarps or diapiric features, have become a commonly-applied approach in the petroleum industry. Results of such surveys help to reduce risk on key exploration play elements and are used to evaluate prospects and to predict hydrocarbon phase and expected properties. Based on geochemical surveys conducted by ExxonMobil in many basins worldwide, there is an interrelation of the seep intensity (concentration) and level of biodegradation. Results from offshore west Africa, where many active macroseeps show moderate-to-severe biodegradation, and a frontier basin offshore United Kingdom (Rockall Trough), where active microseeps show no evidence of biodegradation, are compared. The specific biochemical controls on the difference in biodegradation-proneness are not known, although it appears that a certain threshold of oil concentration is needed to sustain an active bacterial community, or to exceed clay-adsorption capacities that may protect microseeps from biodegradation. It is notable that the 25-norhopane series, often considered an indication of severe biodegradation in reservoir oils, has not been recognized in even ultra-severely biodegraded seeps. This suggests that different biodegradation pathways may be followed in marine surface seeps versus those in subsurface hydrocarbon accumulations, a likely scenario in light of the fact that physiologically diverse bacterial communities are prevalent under different physiochemical conditions.