Controls on Upper Jurassic sediment distribution in the Durward–Dauntless area, UK Blocks 21/11, 21/16

Abstract

The Upper Jurassic shallow marine Fulmar sands are widespread in the West Central Graben but have an increasingly irregular distribution when traced westwards across the West Central Shelf (Western Platform). This is strikingly illustrated in the Durward and Dauntless fields (Blocks 21/11 and 21/16), located 20 km west of Kittiwake, where several exploration, appraisal and development wells failed to find the Fulmar sands objective. The main aim of this paper is to offer an explanation for this irregular sand distribution in the Durward/Dauntless area. This study represents an integrated analysis of the thin (50–250 ft) Upper Jurassic succession (equivalent to the Fulmar and Kimmeridge Clay formations) based on around 90 wells and a sub-regional 3D seismic dataset, which covers most of the West Central Shelf. A combination of sedimentological, biostratigraphical and well log criteria has been used to define up to nine genetic depositional sequences bounded by regionally correlatable maximum flooding surfaces. This stratigraphic framework provides a linkage between the previously unpublished Durward Dauntless area and the Kittiwake Field. Typical ‘Fulmar sands’ facies are restricted to the Kimmeridgian in this area and occur in up to five depositional sequences, which extend from the Base Jurassic unconformity (earliest Kimmeridgian) up to the eudoxus and autissiodorensis maximum flooding surfaces (mfs) (latest Kimmeridgian). These sequences display marked lateral facies changes from sand to mud, while stratal patterns and thickness variations indicate onlap against earlier deposits. This is in striking contrast to the younger mud-dominated depositional sequences (Volgian to Ryazanian), which display complex local subsidence patterns bearing little relationship to Fulmar sand isopach patterns within the Kimmeridgian. The 3D seismic data have enabled mapping of the Base Cretaceous, Top Zechstein Salt and Top Basement (pre-Zechstein), but is unable to directly resolve the thin Upper Jurassic interval. However, these data have enabled a regional structural/stratigraphic model to be developed, particularly in relation to the effects of the nature and rate of accommodation space creation. Conclusions derived from the 3D seismic interpretations have been integrated with those from the well-based sequence stratigraphic studies. The resulting geological model for the Upper Jurassic concludes that Fulmar sand distribution was largely controlled by pre-existing topography, which had evolved during a significant period of (late Triassic–pre Upper Jurassic) subaerial exposure. This topography was critical in controlling highstand sand distribution and its ultimate preservation potential. The importance of pre-Upper Jurassic topography in determining accommodation space became secondary to dissolution of the Zechstein during the suspension-dominated sedimentation of the Volgian and Lower Cretaceous. These local controls vary on a kilometre scale and are superimposed upon the longer wavelength control of regional eastwards dip, which evolved from the cymodoce mfs until the Albian and caused a general westwards onlap of Upper Jurassic and early Cretaceous successions across the West Central Shelf. The model suggests that 3D topographic reconstruction of the pre-Jurassic substrate is the single most critical factor in predicting Fulmar reservoir distribution on the transgressed surface of the West Central Shelf.