Relation between Liquid Hydrocarbon Reserves and Geothermal Gradients--Norwegian North Sea

BAIRD, ROBERT A., Virginia Water Control Board, Richmond, VA

Comparison of average geothermal gradients and initial liquid hydrocarbon reserves for 28 Norwegian North Sea fields indicates

that gradients in the largest North Sea oil fields cluster around 2.1 degrees F/100 ft. No reserves are found where gradients are lower than 1.8 degrees F/100 ft or higher than 2.3 degrees F/100 ft. At 6.89 billion barrels, reserves for 14 fields falling between 2.05 and 2.15 degrees F/100 ft total over four times the reserves for all other fields put together. Reserves for seven fields at gradients lower than 2.05 degrees F/100 ft and for seven higher than 2.15 degrees F/100 ft total 594 and 991 million barrels, respectively.

The conclusion is that 2.1 degrees F/100 ft is the optimum gradient for generation of liquid hydrocarbons in the Norwegian North Sea, given the depth, kerogen type, and source rock potential of the Kimmeridge Clay, the primary source rock there. Gradients lower than this have not stimulated maximum generation from the source rock. At higher gradients, increasing gas production from source rocks and thermal cracking of previously generated liquid hydrocarbons to gas are effective in limiting liquid hydrocarbon reserves. The 2.1 degrees F/100 ft gradient should be a useful pathfinder in the search for new oil reserves in the Norwegian North Sea. Determination of the optimum gradient should be a useful pathfinder in other regions as well.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)