Stratigraphic, structural and depositional history of the Jurassic in the Fisher Bank Basin, UK North Sea

Abstract

The Fisher Bank Basin forms the northernmost segment of the eastern Central Graben. It is filled with Jurassic sediments and contains proven hydrocarbon reservoirs of Late Jurassic age. These sediments were deposited in response to rifting, subsidence and fault block tilting, superimposed on a background of eustatic sea-level rise. Sedimentation commenced in the Middle Jurassic (?Bathonian) with the establishment of coastal swamps, depositing shales, coals and sandstones which comprise the Pentland Formation. This was followed by the development of a coastal barrier/lagoon complex in the Callovian depositing sandstones and shales which are laterally equivalent to the Hugin Formation. Both the Pentland and Hugin formations are relatively uniform in thickness throughout the study area and, although the formations are separated by an unconformity, there is no evidence of contemporaneous fault movement. A phase of intense rifting commenced in the Early Oxfordian and continued until the latest Oxfordian. This resulted on a broad scale in the development of the Fisher Bank Basin as a half-graben, with a wedge of sediment deposited which thickens eastward towards the Jæren High. On a more local scale, the distribution of the Middle to Upper Jurassic appears to have been strongly influenced by salt withdrawal forming local depocentres. In the Early and Middle Oxfordian, a shoreface environment prevailed on the western side of the basin, with shallow-marine, more offshore conditions existing to the east. This resulted in the deposition of cross-bedded and bioturbated sandstones in the west, with a lateral transition into bioturbated shales in the east. In the Late Oxfordian, the sandstones were progressively onlapped by the shales, with aerobic, slightly deeper water conditions becoming established throughout the basin. Previous workers have attributed these sediments to the Fulmar and Heather formations, respectively. The sandstones are, however, demonstrated to be both older and geologically separate from the Fulmar Formation, and the shales younger and lithologically different to the Heather Formation. To emphasize these differences, new names are proposed for these units: the sandstones are referred to the informally termed ‘Frigate Formation’ and the shales the ‘Lower Kimmeridge Clay Formation’. A regional maximum flooding event of latest Oxfordian/earliest Kimmeridgian age marks the transition from the ‘Lower Kimmeridge Clay’ to the ‘Upper Kimmeridge Clay’. The latter ranges in age from Kimmeridgian to Volgian (and locally Ryazanian) and consists of olive-black to greyish-black, laminated shales that are envisaged to have been deposited in a largely anaerobic deeper-marine environment. Rifting appears to have become much less prominent in the latest Oxfordian/Early Kimmeridgian, with the ‘Upper Kimmeridge Clay Formation’ forming a fairly uniform blanket of sediment across the basin and adjacent highs. Erosion of Upper Oxfordian and Lower Kimmeridgian shales took place in the later Kimmeridgian, particularly over the Forties-Montrose High and Jæren High. This was accompanied by localized deposition of sands in the Kimmeridgian and Volgian, with the Kimmeridgian sandstones being of possible shallow-marine origin and those in the Volgian forming in deeper water as turbidites.