Brent Group: sequence stratigraphy and regional implications

Abstract

Abstract Previous published models for the stratigraphic evolution of the Middle Jurassic (Aalenian—Bathonian) Brent Group and its equivalents rely largely upon lithostratigraphy, with few reliable time lines for correlation across the Northern North Sea basin. To rectify this a new, largely well-based, multi-disciplinary regional study was undertaken, combining detailed biostratigraphic and sedimentological data from more than 450 wells across the whole of the Northern North Sea basin. A basin-wide sequence stratigraphy for the Middle Jurassic has been developed, comprising two tectono-stratigraphic units (J20, Aalenian—Late Bajocian and J30, latest Bajocian—Middle Callovian), subdivided into five sequences: J22 (Aalenian), J24 (Early Bajocian), J26 (Early—Late Bajocian), J32 (latest Bajocian—Late Bathonian) and J34 (latest Bathonian—Middle Callovian). The tectono-stratigraphic units developed in response to large scale (second order) tectonic processes: pre-rift thermal uplift (J20) and the onset of rifting (J30), whilst sequence development was controlled by a combination of changes in basin subsidence, sediment supply and eustatic sea level. A series of palaeogeographic and isopach maps outline the basin evolution and sedimentary response for each sequence. They describe a progressive basinward shift of the J22 and J24 depocentres in response to the thermal uplift; by the end of J26, sediment supply could not keep pace with increased basin subsidence caused by the crustal extension. J32 and J34 represent the first periods where active rifting can be clearly identified in the North Sea basin, with the development of tilted fault block geometries in the East Shetland Basin and Bruce—Beryl Embayment. Continued extension and rifting in these areas led to the development of an overall retrogradational system which extends through to the latest Jurassic. The study provides a model for North Sea basin evolution and a predictive sequence stratigraphic framework within which the local lithostratigraphy can be resolved and basin potential evaluated.