Brent Sedimentology from Seismic

In the Unst basin 60 miles to the west of the Brent province, Jurassic sands are buried to only 2000 ft. The DTI commissioned RML to see what sedimentological data could be obtained from seismic over this area by examining seismic attributes and inverting the data. Malcolm Pye ( Malcolm.Pye@dti.gsi.gov.uk ) of the DTI reports on the results. The work was undertaken by Arnold Woodruff and Phil Mollicone ( p.mollicone@senergyltd.com ), both of RML.

Over most of the Northern North Sea fields, the Brent sands are too deep for seismic to resolve the sedimentological features required to build 3D geological models. However, 60 miles to the west, Jurassic sands are buried to only 2000 ft in the Unst Basin . Tricentrol drilled two wells here in the 1980s and the area is covered by a dense grid of 2D seismic.

DTI commissioned RML to carry out a study to see what sedimentological data could be obtained from this seismic by examining seismic attributes and inverting the data. The mapping work was carried out on a Landmark workstation. Hampson-Russell software was used for the inversion.

Figure 1: Composite Logs

The two wells in block 1/4 found a Jurassic succession over 400 ft thick composed of sands, shales and coals similar to the Ness Formation of the Brent further east (Figure 1). Seismic resolution at 2000ft is around 50 ft so it was not possible to map individual beds but rather packages of sediments (Figure 2).

Figure 2: Line TOC81-120, Reflection Section Line

The three seismic attributes found to be most useful were Instantaneous Phase, Instantaneous Frequency (Figure 3) and Energy Half Time. These were used to map out the coals and a lower sand.

Figure 3: Line TOC81-120, Instantaneous Frequency Section

The inversion of the 2D seismic data was not straightforward as the software was written for 3D. However, once achieved the inverted data allowed a much more confident mapping of the sedimentary packages (Figure 4) and revealed some internal sedimentary architecture approaching the level of detail required for geocellular modelling (Figure 5).

Figure 4: Line TOC81-120, Impedance Section

Figure 5: Detail Of 1/4-2 Area Impedance, Line TOC81-124

TWT ribbon maps were produced of the upper and lower coals around well 1/4-1 (Figure 6), a "soft" sand around well 1/4-2 (Figure 7) and a speculative channel sand south of 1/4-1.

Figure 6: TWT Map, Upper Coals

Figure 7: TWT Map, Lower Brent "Soft Sand"

The study achieved its objective of mapping sedimentological data from the Jurassic but far more information could be obtained if 3D seismic were available and there were more well penetrations. An area of shallow Jurassic covered by 3D with multiple well penetrations has been identified but unfortunately it is a long way from the Brent area. However, this should make a good MSc project in summer 2004.

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