The Brent Group


Uppermost Lower Jurassic to Middle Jurassic
(Upper Toarcian–Bajocian)


The Brent Gp has its type area in the East Shetland Basin and contains five formations; the Broom, Rannoch, Etive, Ness and Tarbert Fm. On the Horda Platform, the Oseberg Fm is defined as part of the Brent Gp. Type and reference well for the Brent Gp is well 211/29-3 (UK) and well 33/9-1. For the Oseberg Fm the type well is 30/6-7. The lower boundary is the marine silts and mudstones of the Dunlin Gp. The upper boundary is the Heather/Draupne Fm marine mudstones of the Viking Group, forming a regional seal.

The Brent Gp is found in the East Shetland Basin and is recognizable over most of the East Shetland Platform and the northern part of the Horda Platform. South of the Frigg area, broadly equivalent sequences to the Brent Gp are defined as the Vestland Group. To the North, the deltaic rocks of the Brent Gp shales out into marine mudstones between 61o30´N and 62o N. The thickness of the group varies considerably due to differential subsidence and post Middle Jurassic faulting and erosion. Variable amounts of the group may be missing, particularly over the crests of rotated fault blocks.

The deposition of the Brent Gp records the outbuilding of a major deltaic sequence from the south and the subsequent back-stepping or retreat. The Oseberg sandstones form a number of fan-shaped sand-bodies with a source area to the east. The sandstones in the lower part are deposited in a shallow marine environment, overlain by more alluvial sands and capped by sand which has been reworked by waves.

Due to the Upper Jurassic faulting, uplift/erosion and differential subsidence, the Brent Group is located at a wide range of depths, varying from 1800 m on the Gullfaks Field to more than 3500 m on the Huldra Field. As a result there is a complex distribution of porosity and permeability.

The Broom Fm (Upper Toarcian to Bajocian) is thin and locally developed. It consists of shallow marine, coarse-grained and poorly sorted conglomeratic sandstones and is a precursor for the regressive sequence of the overlying Rannoch Fm.

The Rannoch Fm (Upper Toarcian to Bajocian) in the type area is well-sorted very micaceous sandstones, showing a coarsening upwards motif, deposited as delta front or shoreface sands. The upper boundary is defined by cleaner sandstones of the overlying Etive Fm. The thickness of the Rannoch Fm in the type area varies between 35 and 63 m.

The Etive Fm (Bajocian) contains less micaceous sandstones than the underlying Rannoch Fm. The upper boundary is the first significant shale or coal of the overlying Ness Fm. The depositional environment for the Etive Fm is interpreted as upper shoreface, barrier bar, mouth bar and channel deposits. The thickness of the formation varies considerably from 11 m to more than 50 m.

The Ness Fm (Bajocian to Bathonian) consists of an association of coals, mudstones, siltstones and fine to medium sandstones. Characteristic features are numerous rootlet horizons and a high carbonaceous content. The upper boundary is the change to the more massive and cleaner sandstones of the overlying Tarbert Fm. The formation is interpreted to represent delta plain or coastal plain deposition. The amount of silt and mudstones in the formation may act as a local seal. The Ness Fm shows large thickness variations ranging from 26 m up to about 140 m.

The Tarbert Fm (Bajocian to Bathonian) consists of grey to brown sandstones. The base of the formation is taken at the top of the last fining upward unit of the Ness Fm, either a coal-bearing shale or a coal bed. It is deposited in a marginal marine environment. Thickness in the type area varies between 14 and 45 m.

The Oseberg Fm (Upper Toarcian to Lower Bajocian) consists of relatively homogenous coarsegrained sandstones defined from the Oseberg Field (block 30/6) between the Viking Graben and the Horda Platform. The base of the formation is shales of the Dunlin Gp and the upper boundary is the micaceous sandstones of the Rannoch Fm. The formation has been correlated with various formations of the Brent Group, but whereas the Brent Group forms a deltaic unit building out from the south, the Oseberg Fm has its source area to the east. The thickness in the type area is between 20-60 m. The sandstones in the lower part are deposited in a shallow marine environment, overlain by alluvial sands and capped by sand reworked by waves.

Burial depth of the Oseberg Fm varies between 2100 and 2800 m and porosities and permeabilities in the order of 23-26% and 250- 2000 mD, respectively, are reported.

A time equivalent to the Brent Gp is the Vestland Gp which is defined in the southern part of the Norwegian North Sea.


Fig-4-026 - Download pdf



Fig-4-027 - Download pdf



Fig-4-028 - Download pdf



Fig-4-029 - Download pdf

Core photo well 30/6-7, 2679-2683 m (Ness Fm)



Contents - CO2 storage atlas

Main page >

Preface >

1. Introduction >

2. Petroleum activity on the NCS >

3. Methodology >

4. The Norwegian North Sea >

4.1 Geology of the North Sea >

4.2 Storage options in the North Sea >

4.2.1 Saline aquifers >

4.2.2 Abandoned hydrocarbon fields >

4.2.3 Summary >

5. The Norwegian Sea >

6. The Barents Sea >

7. Summary. Storage capacities of the NCS >

8. Storage options with EOR >

9. Monitoring >

References >

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