Divergent development of two neighbouring basins following the jurassic north sea doming event: the danish central graben and the Norwegian-Danish Basin

Abstract

The two neighbouring basins, the Danish Central Graben and the Norwegian-Danish Basin were both affected by the regional uplift of the North Sea and adjacent areas in the early Middle Jurassic that caused the formation of a regionally extensive unconformity. The uplifted area was not a simple dome structure but of a more irregular shape with an east-west oriented branch that included the Ringkøbing-Fyn High and much of the Norwegian-Danish Basin and the Fennoscandian Border Zone. Late Aalenian-Bajocian deposition was confined to fault-controlled depocentres in both the Danish Central Graben and the Norwegian-Danish Basin. An initial southward slope in the Danish Central Graben changed to a north- to eastward slope before the end of the Middle Jurassic, and the change possibly coincides with the formation of a conspicuous sequence boundary in the Bathonian. The depositional area began to expand in the late Middle Jurassic as a result of a regional sea-level rise. In the Danish Central Graben, accelerating half-graben subsidence during the Callovian-Early Kimmeridgian enhanced the sea-level rise. Several periods of rapid subsidence during the Callovian-Volgian (mainly in the Oxfordian-Early Kimmeridgian and latest Kimmeridgian-Middle Volgian) gave accommodation space to more than four kilometres of marine mud. A break in subsidence in the late Kimmeridgian, probably related to a change of fault directions, resulted in deposition of shallow marine sandstones on platforms and hanging-wall slopes. The Norwegian-Danish Basin was characterised by a small rate of subsidence and continuous expansion of the depositional area throughout the Late Jurassic. The slow subsidence and a large supply of sediment from the Fennoscandian Border Zone caused repeated progradational events from the northeast. Hydrocarbon discoveries are known only from the Danish Central Graben where Middle Jurassic and Upper Jurassic reservoirs have been charged from Upper and to a smaller degree Middle Jurassic source rocks. Within the Norwegian-Danish Basin, reservoir rocks are abundant in the Upper Triassic-lowermost Jurassic, the Middle Jurassic and Upper Jurassic successions. The presence of mature source rocks, however, is the main risk factor as they most likely only occur within Lower Jurassic mudstones deeply buried in rim-synclines and in local grabens.