Abstract Core-based studies have had material impacts on the understanding of a number of late-life, mature North Sea Brent Group hydrocarbon reservoirs. These studies have included sedimentological, diagenetic and reservoir quality focused evaluations of core. The primary objective of the studies has been to improve conceptual and qualitative models that can be utilized in reservoir modelling and also for infill drilling and well workover evaluations. Most of these studies have been undertaken on old core samples collected in the 1980s and 1990s. Two case studies are described here that provide examples of the utility of core in mature fields. (1) Heather Field calcite: to quantitatively assess the distribution of calcite cements and their impact on hydrocarbon volumes and reservoir quality distribution in Brent reservoirs. (2) Thistle Field Etive Formation barriers and baffles: to characterize and describe the origin and distribution of low-permeability intervals within the Etive Formation reservoir. These two studies used a wide variety of core-based techniques including core logging and description, optical microscopy and petrographical studies, isotope analyses, X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) (FEI Company analysis tool and software, QEMSCAN)-based mineralogy, portable-X-ray fluorescence (XRF), NDTr and Thermo Scientific Inc. NITON TM operational software (NDT) geochemical analysis, as well as image analysis of grain size and texture. These data were then integrated with other subsurface datasets, such as well log, seismic data and well performance data, in order to address the specific reservoir challenge. These new and focused reappraisals of core demonstrate the dual value of core-based studies, which can: improve the understanding of producing hydrocarbon reservoirs, leading to improved productivity and recovery. Core is a full asset life-cycle resource and provides critical insight at all stages of field maturity as production behaviour changes and alternative development strategies are considered; further our general knowledge and understanding of clastic sedimentology and diagenesis using rich and diverse core-based datasets backed up by substantial well log and seismic datasets.

Abstract This chapter describes Middle Jurassic second-order sequences J20 and J30, and their component third-order sequences, J22–J26 and J32–J36. The J22 sequence contains the major Intra-Aalenian Unconformity (‘Mid-Cimmerian’) across a wide area of the North Sea Basin and an equivalent event onshore UK. The base J24 (Lower Bajocian) is marked by the Rannoch Shale (Brent Group) and by the flooding of the Ollach Sandstone, Hebrides Basin. The base J26 (Upper Bajocian) ties to the Mid Ness Shale (Brent Group) and the base of the Upper Trigonia Grit Member, central England. The base J32 (Upper Bajocian) ties to the base of the Tarbert Formation, the base of the Great Oolite Group in central England and the base of the Great Estuarine Group, Hebrides Basin. The base J33 (Middle Bathonian) falls within the Tarbert Formation and the base of the Taynton Limestone, central England. The base J34 (uppermost Middle Bathonian) commonly falls at the top of the Brent Group. The base J36 (uppermost Bathonian) represents a major increase in marine influence, at the base of the Beatrice Formation, in the Inner Moray Firth and at the base of the Staffin Bay Formation, Hebrides Basin.

Abstract The most important North Sea Jurassic–lowermost Cretaceous lithostratigraphic units, as developed in the UK, Norway and Danish sectors, are summarized in this chapter (55 units from the UK, 25 from Norway and 10 from Denmark). Some significant issues remain with the use and application of lithostratigraphic terminology in the Jurassic of the North Sea Basin. In particular, there are inconsistencies in unit definition and nomenclature changes across country sector boundaries that obscure the recognition of regional stratigraphic patterns that exist across the region. To aid clarity and to overcome some issues of definition, some revisions are made to the existing lithostratigraphic schemes. Several informal lithostratigraphic units are described, a number of unit definitions are revised and various formerly informal units are formalized (Buzzard Sandstone Member, Ettrick Sandstone Member and Galley Sandstone Member). It is recommended that use of the Heno Formation in offshore Denmark is discontinued. In addition, four new lithostratigraphic member terms are introduced (Home Sandstone Member, North Ettrick Sandstone Member, Gyda Sandstone Member and Tambar Sandstone Member). All described units are placed into a sequence stratigraphic context. All significant lithostratigraphic boundaries conform with key sequence stratigraphic surfaces.

Abstract The most prolific reservoir package in the SW Barents Sea is currently the Upper Triassic–Middle Jurassic Realgrunnen Subgroup, comprising the main hydrocarbon accumulations in the Goliat, Snøhvit and Johan Castberg fields and the Wisting discovery. The interval continues to be the main target as hydrocarbon exploration ventures further into this region. However, the package varies considerably in thickness and reservoir quality throughout the basin, and it is therefore very important to understand how this package developed and what has affected it in the time since it was deposited. Here we review controls on the tectonostratigraphic evolution and facies distribution within the Realgrunnen Subgroup, and exemplify the variability in reservoir characteristics within the subgroup by comparing some key wells in relation to their depositional environment and provenance. New provenance data that record a turnover from reworked Triassic- to Caledonian-sourced mature sediment support facies observations which suggest temporal changes in the depositional environment from marine to fluvial. Much of the variability within the subgroup is attributed the tectonostratigraphic development of the basin that controlled accommodation, facies transitions and sediment distribution. This variability is reflected in subtle differences in reservoir quality important both for exploration and production in the remaining underexplored basin.