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The continental shelf offshore Mid-Norway was opened for exploration drilling in 1980. The area extends southward to the North Sea sedimentary basins and northward to the Barents Sea continental shelf. The central part is a proven petroleum province. By use of mainly seismic and well data, the tectonic evolution, sedimentary facies, reservoir potential, and hydrocarbon generation and distribution have been studied.
After peneplanation of the Caldeonides, a large epicontinental basin connected the region between Greenland and Norway during the late Paleozoic and Mesozoic. Pre-Triassic rocks have not yet been recorded in the wells and nothing is known about their exact composition. The main targets for the exploration drilling have been the Triassic-Jurassic succession. The Triassic consists mainly of continental red shales with sandstone and salt intervals of poor to no source or reservoir potential. During the Triassic, regional extension initiated the formation of half grabens.
A change in climate to more humid conditions toward the end of the Triassic led to coastal plain deposition that persisted into the Early Jurassic. These carbonaceous sediments are important source beds for gas and condensate. The reservoir potential is low because of expected lack of continuity of the channel sands and extensive kaolinite cementation.
A major transgression took place during the Early Jurassic, leading to deposition of a sequence of shallow marine sands, tidal-flat sands, and offshore muds of medium to low reservoir quality. A Middle Jurassic regression resulted in deposition of shallow marine sandstones presently representing the main reservoir in the area. Diagenesis has not been detrimental to the reservoir properties, and a similar porosity-depth trend as seen in the North Sea is present.
Normal and growth faulting during the Triassic to Early Jurassic culminated with the main Kimmerian (pre-Callovian) tectonic phase that resulted in extensive horst and graben development, with subsequent erosion of structural highs. The Upper Jurassic consists of marine shales, of which the upper part is an oil-prone shale of excellent source rock characteristics.
The base of the Cretaceous is developed as a regional unconformity (late Kimmerian) onlapped by Cretaceous marine shales, marls, and minor limestone with no reservoir potential. Differential subsidence created the main platforms and basins. The resulting Cretaceous thickness ranges between 0 and 3 km (9,800 ft), with the main depocenter at the outer part of the shelf.
The Tertiary represents a period of epeirogenic subsidence leading to rapid deposition of marine clastic sediments. The northward progress of the North Atlantic rift is seen in the sedimentary record as a series of tuffaceous layers within the upper Paleocene-lower Eocene. Along the margin, a volcanic high was formed. As the continental slope and shelf subsided, damming of glacial deposits landward of the outer high created the Voring Plateau. Reactivation of older fault zones took place, and there is evidence for strike-slip movements and folding, especially in the northern area.
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