The Central Graben is a major hydrocarbon province. Oil, condensate, and gas are found in a variety of horizons ranging in age from the Devonian to Eocene. The principal economic zones are the Upper lurassic Fulmar Sandstone, the Upper Cretaceous Chalk Group, and the Paleocene Forties Formation.
The Upper lurassic sandstones in the Central Graben vary from being normally pressured, 0.01 MPa/m (0.45 psi/ft) near the graben margins, to pressure gradients in excess of 0.02 MPa/m (0.867 psi/ft) in the center of the graben. The Paleocene sandstones consist of sheet sandstones forming a normally pressured regional aquifer, and the Chalk Group, where overlain by these sandstones, is similarly normally pressured. In the southern part of the graben, the Paleocene consists of clay stones that act as a seal to both overpressure and hydrocarbons in the Ekofisk Formation of the Chalk Group.
The origin of the overpressure was formerly considered to result from compaction disequilibrium. In recent years it has been recognized that hydrocarbon generation can play a crucial role in generating the extreme overpressures that are present in the Jurassic sandstones. The Kimmeridge Clay stone Formation is an excellent source rock with a TOC locally greater than 10%. It varies from sub-mature at the graben margins, to late-stage gas generation within the depocenter of the graben.
In highly overpressured rocks, pore pressures may approach the fracture gradient and the dynamic interplay between pore pressure and fracture gradient is indicative of a dynamic overpressure system. In this type of system, the overpressure is produced by the volumetric expansion associated with the generation of oil and gas. When the pore pressure exceeds the minimum confining stress, episodic fluid expulsion occurs through fractures in the seal.
Recognition that the overpressure is generated from hydrocarbon generation in a dynamic system is important both in modeling the distribution and magnitude of the overpressure and to understanding the relationship between migration and distribution of hydrocarbons in the Central Graben of the North Sea.
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Abnormal Pressures in Hydrocarbon Environments
Abnormal pressures, pressures above or below hydrostatic pressures, occur on all continents in a wide range of geological conditions. According to a survey of published literature on abnormal pressures, compaction disequilibrium and hydrocarbon generation are the two most commonly cited causes of abnormally high pressure in petroleum provinces. In young (Tertiary) deltaic sequences, compaction disequilibrium is the dominant cause of abnormal pressure. In older (pre-Tertiary) lithified rocks, hydrocarbon generation, aquathermal expansion, and tectonics are most often cited as the causes of abnormal pressure.
The association of abnormal pressures with hydrocarbon accumulations is statistically significant. Within abnormally pressured reservoirs, empirical evidence indicates that the bulk of economically recoverable oil and gas occurs in reservoirs with pressure gradients less than 0.75 psi/ft (17.4 kPa/m) and there is very little production potential from reservoirs that exceed 0.85 psi/ft (19.6 kPa/m). Abnormally pressured rocks are also commonly associated with unconventional gas accumulations where the pressuring phase is gas of either a thermal or microbial origin. In underpressured, thermally mature rocks, the affected reservoirs have most often experienced a significant cooling history and probably evolved from an originally overpressured system.