The Mandal-Ekofisk(!) petroleum system in the Central Graben of the North Sea contains 3292 million m³ of in-place oil, 110 million m³ of in-place condensate, and 1167 billion m³ of in-place gas in Devonian-Tertiary reservoirs. The major reservoir is the Upper Cretaceous to basal Paleogene chalk of the Ekofisk area in the southeastern part of the graben, with 607 million m³ (46%) producible oil and gas as oil equivalent. Here, chalk is the seal except where synsedimentary slumping coupled with an early influx of oil creates reservoir rock. Permeability mainly results from on-structure halokinetic fracturing. The second most important reservoir, with 457 million m³ (35%) producible oil equivalent, occurs in Paleocene s ndstone where fan turbidites are excellent reservoir rocks in the Forties-Montrose and Gannet areas. Surrounding mudstones form the seal. A less significant reservoir rock, with 231 million m³ (18%) producible oil equivalent, occurs at the Upper Jurassic level in the shallow marine Fulmar and Ula sandstones immediately underlying the source rock horizon (seal) on both the eastern and western flanks of the Central Graben. Additional reserves are found in Permian and Devonian reservoir rocks.

The oil, condensate, and gas originated from the Upper Jurassic-Lower Cretaceous organic-rich marine mudstones of the Mandal Formation (equivalent to the Kimmeridge Clay). Geochemical oil-source rock correlation has been established and is backed up by the regional distribution of fields relative to mature source rocks. Source rock thicknesses range from less than 100 m on flanks and highs, to greater than 1200 m in the axis of the Central Graben. Thermal maturity modeling indicates that oil generation was initiated by late Early Cretaceous in the mid-graben. Under steadily increasing sedimentation rates, generation continues to the present day over about 112,750 km² of the Central Graben.

Expulsion efficiency from the Mandal Formation source rock is approximately 60%. Secondary migration is mainly vertical, the oil and gas moving through salt- and fault-related fractures into a variety of reservoir rocks. The generation-accumulation efficiency of 0.56% is obtained using Schmoker's calculation for the petroleum system. The major control on the petroleum system efficiency is the strong vertical component of secondary migration through low permeability strata plus undiscovered reserves.