Troll field, a giant gas field underlain by an oil rim of variable thickness, is located approximately 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 eastward-tilted fault blocks capped by clays of Upper Jurassic to Paleocene age. 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 divided into three communicating hydrocarbon provinces that coincide with the major fault blocks. In Troll East, a gas cap of up to 250 m overlies a thin oil rim (0 to 4 m thick). In Troll West Gas, a maximum gas cap of 210 m covers an oil rim some 12 m thick. In Troll West Oil, a modest gas column of 43 m caps up to 28 m of oil. The reservoir rocks exhibit excellent properties, with porosities up to 35%, permeabilities in the darcy range, and no shale interbeddings.

Since the discovery of Troll in 1979, 25 exploration and appraisal wells have delineated initial in place hydrocarbon volumes of 1670 billion standard m³) of free gas and 615 million m³) of oil, as reported for planning purposes by the joint operators.

Phase I of the field development plan will constitute exploitation of the main gas accumulation in Troll East from an offshore concrete gravity structure together with onshore processing facilities. First gas is planned for 1996. Successive development of Troll West Oil could require horizontal wells feeding a floating or fixed

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production platform. The independent Troll-Oseberg Gas Injection (TOGI) project will export first gas in 1991 from a six-slot subsea template located in the south of Troll East for injection in the Oseberg field.