Print this pageABSTRACT: Troll: Norway's Giant Offshore Gas Field
L. Bolle, H. Henningfeld
Troll, a giant gas field underlain by an oil rim of variable thickness, is located around 80 km offshore Norway on the northwestern edge of the Horda Platform and adjacent to the eastern margin of the Viking Graben in waters between 300 and 355 m deep. The field extends over four Norwegian Blocks (31/2, 31/3, 31/5, and 31/6) and covers an area of 710 km2.
Hydrocarbons are trapped in three easterly tilted fault blocks capped by Upper Jurassic to Paleocene clays. The reservoir comprises sediments of the Fensfjord, middle Heather, Sognefjord, and upper Heather formations of the Jurassic Viking Group. Deposition took place as a cyclic shallow-marine sandstone sequence with alternations of transgressive sands and silts and progradational shoreface facies induced by regional sea level fluctuations.
The field is split into three communicating hydrocarbon provinces, which coincide with the major fault blocks. In Troll East, a gas cap of up to 250 m overlies a 0-4-m-thin oil rim. In Troll West gas, a maximum gas cap of 210 m covers an oil rim of some 12 m thickness. In Troll West oil, a modest gas column of 43 m caps up to 28 m of oil.
The reservoir rock exhibits excellent properties with porosities up to 35%, permeabilities up in the darcy range, and no shale interbeddings.
Since the discovery in 1979, 25 exploration and appraisal wells delineated initial in place volumes of 1670 × 109 Sm3 free gas and 615 × 106 m3 oil.
The field development envisages exploitation of the main gas accumulation in Troll East from a concrete gravity structure as phase one with first gas planned for 1996. Successive development of Troll West oil could require subsea completions on horizontal wells feeding a floating production platform. The independent Troll-Oseberg gas injection scheme will export first gas in 1991 from a subsea six-slot template located in the south of Troll East for injection in the Oseberg field.
AAPG Search and Discovery Article #91000©1990 AAPG Conference-Giant Oil and Gas Fields of the Decade 1978-1988 Conference, Stavanger, Norway, September 9-12, 1990