The petroliferous North Sea basin is classified as an intracontinental failed-rift basin and should as such exhibit "normal to high geothermal gradients," if relating to the world average of 25°C/km.

An updated, regional, present-day geothermal gradient map

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of the whole North Sea basin shows that the mean gradients rarely exceed 40°C/km and are rarely lower than 20°C/km. In broad outline, the gradient pattern seems to reflect the major structural elements. The positive geothermal anomalies are explained in terms of convective heat transport within an actively subsiding basin. Major fault systems serve as conduits for fluid flow. Salt pillows and salt diapirs are centers of hot areas caused by heat conduction. Isothermal maps of key structural horizons and geoisothermal maps are presented to support these interpretations.

The importance of constructing thermograms is emphasized. In the North Sea basin, variations in interval geothermal gradients are explained by a combination of differences in lithology-related conductivity and subsurface fluid flow. Locally, interval gradients twice as high as the mean gradients are observed.

This investigation does not support the idea that geothermal anomalies are uniquely associated with oil or gas accumulations and hence could be used as an exploration tool. Rather, it seems that detailed mapping of geotemperatures can help explain the pattern of subsurface fluid flow of which water (and not hydrocarbons) constitutes the major part.

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