Brent Group

Ziegler K. and Coleman M. L. (2001) Palaeohydrodynamics of fluids in the Brent Group (Oseberg Field, Norwegian North Sea) from chemical and isotopic compositions of formation waters. Appl. Geochem. 16, 609-632. 

Walderhaug O. (2000) Modeling quartz cementation and porosity in Middle Jurassic Brent Group sandstones of the Kvitebjorn Field, northern North Sea. Am. Assoc. Petrol. Geologists Bull. 84, 1325-1339. 

Fig. 1. Data localities for the present study (KvalvSgen, Spitsbergen Tilje Formation, Haltenbanken Brent Group, Tampen Spur Brora, Scotland Gulf of Corinth, Greece).

Several cored intervals from the Brent Group which contain faults have been examined. The present depths range between ca. 2500 and 2800 m, and most faults display similar characteristics to those described from Haltenbanken. Mesoscopic faults with small throws (0.5-2.0 cm) are easily identified, commonly as dark seams. Larger faults, which exhibit displacements above the seismic resolution (>ca. 20 m), are characterized by enrichment of phyllosili-cates, sand injection and shale smear. 

Juxtaposition seal of reservoir against non-reservoir can be assessed by fault-plane diagrams. Additional seal may be developed (at reservoir juxtapositions) if fault-plane processes increase the capillary entry pressure. In Oseberg Syd, clay smearing is considered to be dominant because of the relatively shaly nature of the Brent Group and the shallow burial depths during faulting (<500 m). 

The main block-bounding normal faults of the Oseberg Syd region have throws in the range of 200-500 m in the reservoir section (the Brent Group). The following structural elements are defined by these faults (Fig. 2) ... 

Fig. 4. Seismic and geoseismic cross-section through the 30/9-3A and -4S wells, illustrating the sedimentary growth across faults in the Oseberg Syd area. Note the dramatic thickness increase of the Brent Group of ca. 100% from the Gamma to the Omega North structure, and an -i-additional 30 ms increase to the westernmost fault block. The growth observed in the Dunlin Group and StatQord Fm indicate that most of the faults were active through most of the Jurassic period.

The above studies suggest that sealing by clay smear may be predicted deterministically frorn a consideration of the thickness and offset of individual shale beds. However, such an approach is difficult to apply directly in the Brent Group because of the heterogeneity of the sequence. It is not feasible to map... 

Fig. 11. Graph illustrating the pore-pressure profile through the Brent Group on each side of Fault 1. FW, footwall HW, hangingwall. Note the upwards increase in pressure difference through the hydrocarbon columns. The two aquifer gradients are believed to be coincident (within the uncertainty of the tool measurements) since the reservoir is continuous around the southern end of the fault (see Fig. 3).

Fig. 15. The SGR for Fault 9 reveals that the lowest values are found in the lower part of the Brent Group. The weakest point with respect to leak would consequently be expected to be found in the lower one third of the Brent Gp, whereas the upper two thirds would be expected to seal well.

The calculated SGR is generally above 20%, except for the R2A unit, where it is less than 15% in areas of small throw. If the SGR thresholds that we have obtained on the G structures are representative for the C structure, the upper two-thirds of the Brent Group juxtaposition are expected to seal well, and the lower part would be open to flow (Fig. 16). Note that this is in contrast with the faults in the western area, where seal is poorest on the upper parts of the faults (Tarbert Fm). 

Morad, S. De Ros, L.F. (1994) Geochemistry and diagenesis of stratabound calcite cement layers within the Rannoch Formation of the Brent Group, Murchison Field, North Viking Graben (northern North Seal-comment. Sediment. Geol., 93, 135-141. 

McAulay, G.E., Burley, S.D., Fallick, A.E. Kusznir, N.J. (1994) Palaeohydrodynamic fluid flow regimes during diagenesis of the Brent Group in the Hutton-NW Hutton reservoirs constraints from oxygen isotope studies of authigenic kaolin and reverse flexural modelling. Clay Miner., 29, 609-626. 

Diagenetic carbonate cement in reservoir sandstones of the Oseberg Formation (Brent Group) in the Oseberg field, Norwegian North Sea, occurs as disseminated siderite and ankerite, and as massively calcite-cemented intervals. Other diagenetic features include extensive feldspar dissolution and K-feldspar, quartz, kaolinite and dickite cements. Conditions of carbonate cementation are constrained on the basis of textural, geochemical and fluid inclusion evidence. 

Fig. 3. Geological cross-section through the Alpha and Gamma structures (see location on Fig. 2), showing the eastward tilt of Brent Group sediments and the Late Jurassic-Early Cretaceous Cimmerian erosional unconformity (between Shetland and Viking Groups). Note how the top part of the Brent Group can be affected by the Cimmerian erosion in crestal areas.

Eocene a significant oil column and gas cap were established, at least in the upper units of the Brent Group. The main pulse of oil charging occurred in the Oligocene and Miocene, with maximum oil filling (to spill point) in the latest Miocene. Later (Pliocene-present) the oil-water contact moved up (10-15 m) owing to gas leakage and pressure increase. Pre-production oil-water contact was at 2710 m relative to mean sea level (MSL) (2600 m relative to sea floor) over most of the field. 

Fig. 4. General stratigraphy of the northern North Sea sediments in the Norwegian sector and depositional environments of successive formations in the Brent Group.

Fig. 5. Reconstructed time-temperature curves for the Brent Group in the Oseberg field, and timing of oil emplacement. After Dahl Yiikler (1991) and Walderhaug (1994).

Bjorlykke, K., Nedkvitne, T., Ramm, M. Girish, C. (1992) Diagenetic processes in the Brent Group (Middle Jurassic) reservoirs of the North Sea an overview. In Geology of the Brent Group (Eds Morton, A.C., Haszel-dine, R. S., Giles, M.R. Brown, S.). Spec. Publ. Geol. Soc. Ixjndon, 61, 263-287. 

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