PETROLEUM MIGRATION, ALTERATION, AND REMIGRATION WITHIN TROLL FIELD, NORWEGIAN NORTH-SEA

Troll field represents the largest petroleum discovery within the enti re North Sea area in oil equivalents, with 74% of the accumulated petr oleum present as dry gas and 26% as a heavy biodegraded oil leg. The f ield is divided into several provinces based on the distribution of ga s and oil, and the gas and oil have been suggested to be cogenetic. Th e migration and filling model presented in this paper suggests that th e oil and gas represent two different migration phases and that gas mi gration and filling predate oil emplacement. Two different oil populat ions have been characterized and mapped in Troll field applying conven tional geochemical techniques. We suggest that the two oil populations migrated into the structure through two different migration systems. Oil and gas were subsequently biodegraded within the reservoir. The tw o oil populations have been found in neighboring oil and gas discoveri es, and an oil-oil correlation with these discoveries has been used to determine the location of field filling points and regional migration routes. When oil biodegradation terminated, fresh oil continued to mi grate into the reservoir and mixed with the residue of the biodegraded oil. The field was tilted downward to the west in the Neogene, and oi l and gas remigrated within the field with a possible spillage of gas. Tilting resulted in a dominantly upward movement of the oil phase whe reas gas migrated laterally. Residual oils in the water zone have been used to reconstruct the paleoconfiguration of the field that controll ed the current distribution of oil populations within Troll field.