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The distribution of oil and gas fields in the northern North Sea closely reflects the structural patterns of Mesozoic-graben development. Late Mesozoic-Tertiary basin subsidence in the Central graben has resulted in a very favorable burial history for source rock maturation.
Time-stratigraphic information and present-day average temperature gradients were used from several wells to calculate depths of oil and gas windows in the area. By intersecting this depth-to-generation trend with the Late Jurassic-Cretaceous unconformity surface, the resulting map view reflects thermal maturation at this structural level plus the underlying Kimmeridge Clay. Average depths to the onset of moderate hydrocarbon generation range from 8,000 to 10,000 ft (2.438 to 3,048 m).
A combination of rapid sedimentation and sufficient subsurface temperatures in the Central graben promoted early source rock maturity as compared with the northern North Sea as a whole. The absolute timing of oil generation could in part be dependent on the magnitude of paleotemperature changes possibly associated with thermal subsidence of the basin. Early oil generation and migration may have promoted preservation of high chalk porosities as discussed in published works on the Ekofisk area.
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