A regional study of Brent Group diagenesis and reservoir properties has been undertaken in order to determine the main controls on porosity and permeability in the sandstone reservoirs. Data from 44 wells from block 211/7 in the north to block 3/8 in the south and spanning current depths from 6700 to 13 400 ft include 9000 porosity, permeability and grain density determinations, quantitative petrographic information from 850 thin sections, and stable isotopic and K/Ar analyses of authigenic phases. The diagenesis of the sediments is similar across the study area and most of the diagenetic phases occur in all formations. The sequence of precipitates and dissolution events reflects early porewater evolution in shallow burial environments and later reactions which were essentially isochemical and controlled largely by increasing burial depth and therefore temperature. Early diagenetic products include siderite, calcite, chlorite and vermicular kaolinite. Only where the calcite cements form concretions or cemented horizons have they a significant effect on reservoir properties. Local dissolution of feldspars and carbonate cements took place on the crest of some fault blocks inferred to have been emergent during the Jurassic and this has caused local enhancement of porosity in some crestal wells. In general, the porosity of each of the reservoir facies decreases systematically with depth but permeability only starts to decrease significantly at depths greater than 10 200 ft (3109.0 m). The general decrease in porosity can be attributed to compaction, together with burial cementation by quartz and iron-rich carbonates. Secondary porosity resulting from feldspar dissolution is increasingly common at depth but there is no net increase in porosity as much of the dissolution was evidently accompanied by the precipitation of authigenic quartz initially with kaolinite but at greater depths with illite. The systematic changes in porosity and the decrease in permeability are compatible with thermally driven dissolution of feldspar and the local reprecipitation of the authigenic silicates; the decrease in permeability corresponds to the presence of increased quantities of illite at depth.

A biostratigraphical zonation of the Brent Group, applicable within local oil field areas, has been established. It comprises 11 palynozones and subzones, eight of which are confined to the Ness Formation. Palynomorph associations and palynofacies types are shown to broadly correspond to specific palaeoenvironments. A regional ‘event’ scheme is outlined which establishes chronostratigraphical correlations and suggests temporal relationships between deltaic and delta front sediments. Integrated palynofacies data have allowed the development of deltaic sub-palaeoenvironments and lithofacies to be examined with respect to time and, in this way, to chart the possible progress of the delta as it prograded. Similarly, this approach enables possible areas of concentrated fluvial channel activity to be identified and the likely migration of these fluvial fairways with time. The implications of this depositional model for reservoir development and performance is discussed, showing the varying orientation of the delta front and barrier sands as it changed from a predominantly N-S trend during its early stages to WNW-ESE in its final stages.

The deltaic Brent Group sequence consists of three formations. The Rannoch is a shoreface section; the Etive is a barrier facies. The Ness contains three members: the Enrick, representing a marginal marine back barrier environment; the Oich, an organicrich open lagoonal shale and the Foyers, a largely fluvially-dominated 'delta-top or coastal plain deposit. During the Bajocian, facies belts rapidly prograded northwards until the northern part of the area was reached. Here progradation stalled and thick delta front sediments were deposited. Shortly before the delta began to retreat, a temporary relative sea-level rise pushed the delta front southwards resulting in the deposition of an intra-Ness barrier. The progradational sequence is underlain by the Aalenian to lowermost Bajocian Broom Formation fan delta and overlain by the transgressive Tarbert sequence of largely Bathonian age. Downloaded from• a e o J q 3 u u o s ) o n o a 9 U ! l U n ( I "£I "~!,tl 000,< m ,ooo~-,oo~ ,oo~-,oo~ m ,oo~-,oo~ ~ ,oo~-,oo~v--1 ,oo ~ ~ / t

The oil and gas industry has been active in the West of Shetlands area for the last 40 years, but less than 200 wells have been drilled and only four fields have been developed. In the last ten years activity has picked up significantly with very active licensing rounds and increased drilling; however, the challenges of the complex geology and deep-water location, environmental constraints and commercial considerations have also increased at the same time. To fully develop the region successfully an attitude of compromise and collaboration between all the stakeholders must be nurtured, but with the scale of investment required this will remain a challenge for the future.

Fulmar Formation sandstones have been assessed for reservoir quality in terms of their primary textural and sedimentological characteristics with respect to conventional core analysis data. Some 6000 ft of core were characterized in total using a comprehensive lithofacies scheme and the results integrated with porosity and permeability data to create a statistically valid data set. From this dataset the primary influences affecting reservoir quality have been evaluated for a discrete area of the Central North Sea. Simple cross-plotting techniques indicate general trends in reservoir quality related to lithofacies type and hence depositional energy levels in shoreline/shallow marine shelf environments. Dominant grain size trends are identified that also influence reservoir quality. Broad prograding, aggrading and retrograding sequences are recognized which can be used to sub-divide the reservoir and locally predict reservoir quality.