Troll field represents the largest petroleum discovery within the entire North Sea area in oil equivalents, with 74% of the accumulated petroleum present as dry gas and 26% as a heavy biodegraded oil leg. The field is divided into several provinces based on the distribution of gas and oil, and the gas and oil have been suggested to be cogenetic. The migration and filling model presented in this paper suggests that the oil and gas represent two different migration phases and that gas migration and filling predate oil emplacement. Two different oil populations have been characterized and mapped in Troll field applying conventional 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 two oil populations have been found in neighboring oil and gas discoveries, 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 migrate into the reservoir and mixed with the residue of the biodegraded oil. The field was tilted downward to the west in the Neogene, and oil and gas remigrated within the field with a possible spillage of gas. Tilting resulted in a dominantly upward movement of the oil phase whereas gas migrated laterally. Residual oils in the water zone 

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

¹Manuscript received July 18, 1995; revised manuscript received March 14, 1996; final acceptance August 16, 1996.

²Saga Petroleum a.s., Kjørboveien 16, P.O. Box 490, N-1301 Sandvika, Norway.

³Newcastle Research Group, Drummond Building, University of Newcastle upon Tyne, Newcastle-upon-Tyne NE1 7RU, United Kingdom.

We would like to thank the partners in the Troll license who allowed us to publish these results. Per Erling Johansen and Tone Molvik are acknowledged for the analytical work, and Snorre Olaussen, Nigel Mills, and Terje Hellem are thanked for helpful discussions and for assistance with the back tilting of the IRAP reservoir field model. Jill Sonrier is acknowledged for typing. We acknowledge K. M. Wolgemuth, John F. Brint, and two anonymous reviewers for helpful comments that improved the paper significantly.