| ABSTRACT
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. |
