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 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.
Figures & Tables
Giant Oil and Gas Fields of the Decade 1978-1988
The success of Memoir 14 and the worldwide interest shown for data on giant fields prompted AAPG to schedule a symposium on giant fields at the end of each subsequent decade. The 1968-78 symposium was held in Houston, Texas, April 1-4, 1979, and the papers were published in AAPG Memoir 30, December 1980.
The Stavanger Conference "Giant Oil and Gas Fields of the Decade: 1978-1988" was held in Stavanger, Norway, September 9-12, 1990, and is a continuation of the Giants of the Decade series.
Scientific studies and projections of future world energy demand indicate that although alternative-energy fuel sources must be actively pursued and developed, there also must be adequate petroleum supplies to bridge the gap. For the international petroleum industry, the years covered by this conference, 1978-88, were complex. They were years of boom and bust. The world's energy consciousness was boosted sharply by the effects of the 1979 Iranian revolution and the resulting embargo, which sent world oil prices to record heights. Global petroleum exploration soon surged, leading to the industry's all-time drilling high in 1981. Then came the oil price collapse in 1985, and the following years were characterized by falling oil prices and drastic budget cuts for exploration and development.
Although exploration dropped sharply during the latter part of the decade, there was a steady flow of giant oil and gas field discoveries. Using the giant field designation criteria of 500 million bbl of oil recoverable for fields in Asiatic Russia, North Africa, and the Middle East,