Discovery of the giant Ekofisk field in block 2/4 in the Norwegian part of the North Sea in 1969 was a major turning point in the exploration for petroleum in Western Europe. Since that time, the North Sea has proven to be one of the best areas for exploration anywhere in the world. Current production is 1.5 million barrels of oil per day, and North Sea proven reserves total 18 billion barrels, with estimates of ultimate reserves as high as 40 billion barrels.

Ekofisk is located in the Central Graben in the southern part of the Norwegian sector of the North Sea. Although several periods of tectonism have affected this area, it has remained an intercratonic basin since Devonian time. The main elements of the tectonic fabric were established during the Caledonian and Hercynian orogenies, and later remained as controlling features for facies and sediment distribution.

A stratigraphic history reveals petrography of the main chalk group, environment of deposition, and diagenetic history of the area. A section outlines the preservation of porosity which has allowed the Ekofisk and Tor Formations to retain an average porosity of 30 to 40 percent which would not have been possible under normal circumstances. It is appropriate to state that the anomalously high porosity in Ekofisk field is probably due to a combination of: (1) overpressuring of the reservoir, (2) magnesium rich pore fluids, and (3) early introduction of hydrocarbons.

The six Greater Ekofisk fields now being developed were all located during the 1960s by reconnaisance seismic work. The history of the geophysical exploration is outlined and a study has been made on source rock analysis and geopressures. Also outlined is the three-phase, Greater Ekofisk development program.