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 Wireline and seismic acoustic impedance imaging show that the marine part of the clastic Brent Group reservoir in the Heather Field, northern North Sea, contains much calcite cement in the flank parts of the structure. The non-marine Ness Formation and crest parts of the structure contain negligible calcite cement. This localized calcite cement has led to relatively poor reservoir performance since first oil in 1978, although a new suite of wells has boosted production with plans to keep the field active until 2030. Understanding the origin and distribution of calcite cement would help the development of more realistic reservoir models and boost production rates through optimum well location. We have thus used a suite of techniques, including standard point counting, SEM-EDS mineralogy, BSE microscopy, fluid inclusion thermometry and stable isotope analysis, to develop new and improved models of calcite distribution. Calcite seems to have attributes of both early and late diagenetic cement. A 30–40% intergranular volume in calcite cemented beds seems to support pre-compactional growth but high-temperature fluid inclusions and the presence of primary oil inclusions suggest late growth. Much calcite may have developed early but it seems to have recrystallized, and possibly undergone redistribution, at close to maximum burial or had a late growth event. Calcite cement probably originated as marine-derived micrite, bioclasts or early marine cement but adopted the isotopic characteristics of high-temperature growth as it recrystallized. Quartz grains have corroded outlines in calcite-cemented areas with one sample, with 79% calcite cement, displaying signs of nearly total replacement of quartz grains by calcite. The flank localization of calcite cement remains to be explained, although it could be due to primary depositional factors, early diagenetic loss of calcite from crestal regions or late diagenetic loss of calcite from crestal regions. Controversially, the growth of calcite seems to be associated with quartz dissolution, although the geochemical and petrophysical cause of this remains obscure. Diagenetic loss of quartz from sandstones cannot easily be explained by conventional modelling approaches and yet seems to be an important phenomenon in Heather sandstones.

Abstract The need for a new Schiehallion full field reservoir simulation model was driven by the requirement to re-evaluate the reserves in the field: the existing model indicated that the modelled volumes were potentially too conservative. This, coupled with a 50% increase in the wells database through ongoing development drilling, was the main reason for building the new model. An integrated multidisciplinary team consisting of BP and Shell staff was set up to build a new full-field reservoir simulation model for reserves re-evaluation. The paper outlines the workflow employed in building the new model, FFM2003, and describes elements of this workflow in more detail, concentrating on lessons learned during the process.

Abstract The Trois Evêchés outcrop of the Grès d’Annot (Annot Sandstones: Eocene-Oligocene, SE France) represents one of the world’s best exposed examples of a confined sandy turbidite system and has exceptional exposures of ‘channelised lobe’ or ‘amalgamated sheet’ sand-bodies. The sand bodies have simple tabular external geometries (lobe/ sheet-like) but show complex internal organisation characterised by a combination of scouring, bypass and aggradational features (channel-like). We have targeted the best exposed, most laterally continuous sand body (termed the FB unit) for very detailed studies at a scale appropriate to reservoir modeling. The result is a west northwest-east southeast panel 35 m thick, 1700 m long parallel to palaeocurrent direction (‘downdip’), within which all zones are characterised in terms of key properties (grain-size, sorting, cementation, and primary and secondary structures) and all key surfaces are absolutely correlated ( i.e. , directly physically traced). This unique database allows quantification of lateral facies relationships and deterministic definition of both architectural components and stratal hierarchy. The FB body is a thick, tabular, high-net:gross unit having a simple tabular external geometry but a complex internal structure. Key observations are that: (1) the system shows considerable lateral variability; (2) the well-defined base of the FB sand body is a different genetic surface in different places; (3) a few beds and surfaces are laterally persistent but, because of erosion, many are not traceable for more than 100-200 m; (4) packages within the sand body are erosive based and vary laterally in thickness, having an element of compensation between successive packages; (5) the erosive-based packages show a back-stepping arrangement of successive points of deepest erosion ( i.e ., migrating towards the east-southeast, uppalaeocurrent). The FB sand body appears relatively uniform at a distance but in detail contains numerous heterogeneities at variable scales which would have a considerable impact on fluid flow through an apparently homogeneous sandstone.