ABSTRACT

The study area, offshore Troms, is located from 16 to 25°E long. and 70 to 72°N lat. The geologic border between continental and oceanic crust is defined by a dramatic increase in water depth in the western part of the area. The area with water depth less than 500 m (1,640 ft) covers approximately 6,000 km2 (2,300 mi2). In this area, several highs and basins are defined. The Troms and Hammerfest basins are situated between the Loppa high on the north and the Troms-Finnmark basement high on the south. The southwestern continuation of the Troms basin is a strongly faulted depression, the Harstad basin. Thinned continental crust under the Troms basin is covered with more than a 15 km (9.3 mi) thick sedimentary sequence of Paleozoic and younger rocks. Early Permian evaporites have formed a few large diapiric structures. The main rifting phase in the Troms basin occurred during Late Jurassic-Early Cretaceous. Several thousand meters of Cretaceous claystones were subsequently deposited. During early Tertiary, a second rifting phase resulted in sea floor spreading west of the studied area. During the cooling stage, the southwestern Barents Sea acquired a westward dip. Tertiary sediments onlap the Upper Cretaceous unconformity from the west. Quaternary sediments lie directly on Cretaceous sediments in the eastern part of the area.

The north-south-oriented Ringvassøy-Loppa fault complex separates the Troms from the Hammerfest basin. In the Hammerfest basin, the Permian is developed as a shallow shelf carbonate facies. Tertiary and Mesozoic sedimentation was dominated by clastics. The Harstad basin is dominantly filled with Jurassic and younger sediments, and Paleozoic sediments are thought to underlie. Minor amounts of diapirism are present. Several compressional structures, formed by mid-Cretaceous strike-slip forces, are associated with the Troms-Finnmark fault complex. The Loppa high is a triangular-shaped structure covered by relatively thin Jurassic to Quaternary sediments. The southeastern part is an inverted Triassic basin. Shallow shelf carbonates define a good seismic marker in the upper Paleozoic.

Seismic control of the area offshore Troms, at present, is generally a 2 by 4 and 2 by 2 km grid. More detailed surveys cover the licensed areas, totaling 2,200 km2 (850 mi2). The first exploratory well was spudded in spring 1980, and 15 wells, dominantly wildcats, have now been drilled. Five gas discoveries have been made in the western Hammerfest basin, two of them with recoverable reserves in the range of 50-100 × 109 Sm3 (1.76-3.52 tcf) gas.

All discoveries have been made in Early to Middle Jurassic sandstones. Rocks of Triassic and Permian age will be increasingly important toward the east and north. In the Troms basin, these sub-Cretaceous rocks are deeply buried, so younger reservoir zones have been drilled. Jurassic and younger rocks will be the main targets in the virgin Harstad basin.

The gas has been generated from Lower Jurassic coals and shales. These, and Upper Jurassic shales with oil source potential, will probably also be the source for most hydrocarbons to be discovered in the future. Some deep Paleozoic structures will be dependent on older source rocks.

The hydrocarbon potential of the relatively well-known Troms and Hammerfest basins is considered low and moderate, respectively. More promising today are Paleozoic structures on the Loppa high and structures in the Harstad basin.

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