Abstract The Central Graben area was filled with a thick pile of sediments during the Middle Miocene - Quaternary, corresponding to a period of 15 Ma. As hydrocarbon expulsion from the most prolific source rock, the Upper Jurassic Bo Member, was initiated only 20 Ma BP and still occurs today, the Middle Miocene - Quaternary evolution is important. In the Middle Miocene, the Central Graben area was covered by a sea with water depth of 500–700 m. During the Late Miocene (Tortonian), the basin was successively filled by prograding slope and deltaic sediments from the northeast. The progradational infill resulted in local tilting of the substratum due to the loading effect of the deposits. In the latest Late Miocene (Messinian), the main input of sediments occurred from the south, as illustrated by a thick onlapping succession of upper Messinian sediments. Pliocene sedimentation was characterized by regular infill from the east within a shelf to shallow marine depositional environment. Following the Miocene and Pliocene, the North Sea Basin tilted due to strong uplift of the Fennoscandian shield and increased subsidence and sedimentation rates within the Central Graben area. This further complicated the maturation of the source rock, migration pathways and accumulation of hydrocarbons. The consequence of this complex burial history is exemplified by the Kraka and Halfdan fields. The Kraka Field has a large down-flank oil accumulation, which is the result of a porosity anomaly resulting from an early invasion of oil in this position before the late tilting of the North Sea Basin. The history of the non-structural accumulation of the Halfdan Field can be readily modelled; it constituted a simple four-way dip closure during the Late Miocene when peak oil migration occurred. The Quaternary tilting of the North Sea Basin due to uplift of the Fennoscandian Shield and strong subsidence of the Central Graben area resulted in a distinct gradient favouring long-distance migration of hydrocarbons. The occurrence of viable migration routes, especially within Paleocene sand layers, has resulted in long-distance migration of oil into the Siri submarine valley system. The most northern indication of hydrocarbons has been recognized as far as 75 km from the source area. Long-distance migration of hydrocarbons is also indicated by direct hydrocarbon indications (DHIs) throughout the Cenozoic succession in the Danish North Sea. DHIs are particularly prominent above known hydrocarbon accumulations in the Central Graben. This indicates pronounced vertical migration, for instance along active faults, above these structures.

Abstract The Ribe Formation is a regionally extensive Miocene sand aquifer that is present in western Denmark at depths ranging from 100 to 300 m below ground surface. Groundwater chemistry and isotope data collected from more than 40 wells show that the Ribe Formation mainly contains high quality Ca-bicarbonate type groundwater of Holocene age (100–10 000 a bp ). Pleistocene age groundwaters, identified on the basis of stable isotopes, noble gases and corrected 14 C values, are present below the island of Rømø, in discharge areas near the coast, and in hydraulically isolated inland areas. The groundwater age distribution in the Ribe Formation was successfully simulated with a numerical groundwater flow model and particle tracking only when the 14 C content in groundwater was corrected for both geochemical reactions and diffusion. The results indicate that geochemical and physical processes significantly influence the 14 C content of groundwater and that the correction factors required for the two processes are of the same magnitude. Flow modelling results indicate that Pleistocene groundwaters were emplaced at depth within the Ribe Fromation under low base-level conditions that prevailed throughout the late Pleistocene – near the coast these waters are essentially isolated from the present flow system, and Pleistocene freshwater may be present offshore. Seismic surveys show that conditions offshore are favourable for the presence of Pleistocene freshwater within the Ribe Formation and other aquifers.