Confidence was therefore high when a subsequent EM-SBL survey over the adjacent, fault-bounded Valkyrie prospect revealed somewhat higher EM responses than its Linerle neighbour.
Confidence was therefore high when a subsequent EM-SBL survey over the adjacent, fault-bounded Valkyrie prospect revealed somewhat higher EM responses than its Linerle neighbour.
The Donna Terrace offshore mid-Norway is geologically challenging with potential Triassic to early-Jurassic traps of viscous,heavy, oil and is characterized by large variations in petrophysical rock properties.
Being mindful of this and that EM-SBL anomalies can be generated by certain rock types (e.g. salt and anhydrite), geophysicists cautiously offered two explanations:
- that the prospect indeed contained an approximately 100 metre thick, oil-bearing reservoir at 1 320 metres below the seabed, which accorded well with seismic data
- and/or that there was a 100 to 400 metre thick zone of anhydrites situated 1 500 to 1 700 metres below the seabed, although seismic indications of this were weak.
Of the two fluid- and rock-based hypotheses, the first gained credence. Nevertheless, it was realized that a large electrical anisotropy might also have influenced the data.
Although a succession of Statoil EM-SBL trials and applications has been highly successful, exploration well 6608/11-5 drilled in June 2006 failed to penetrate a commercially viable reservoir.
Admittedly the Jurassic Åre sandstone at Valkyrie was encountered at the prognosed depth, but it only contained an oil column less than 5 metres, which is far too thin to be electromagnetically detected.
Furthermore, only two, relatively thin, low resistivity anhydrite beds were found beneath the reservoir interval, both of which are electromagnetically undetectable. Neither of the two main hypotheses was correct.
Statoil researchers began an intensive evaluation of the Valkyrie results, realizing that it is as important to unravel technical problems as it is to explain technical successes,
Possibilites :
- considerable electrical anisotropy might have been induced by rapid alternations of shale and sandstone beds. Such beds would be much thinner than the EM wavelength (of several hundred metres), causing, for example, macro-scale differences in the electrical response of rock volumes according to the directions the wave fields pass through them (e.g. vertically, horizontally, obliquely). Residual oil can also further increase electrical anisotropy in such situations.
- is that the compaction of older shales may yield micro-scale electrical anisotropy. The older Triassic and Jurassic shales at Valkyrie have been uplifted and the top of the Åre Formation has been eroded by the base Cretaceous unconformity.
Electrical anisotropies such as these could masquerade as false hydrocarbon indicators.
And the well may simply have missed the targeted compartment, of which nine have been identified.
Nevertheless, electrical anisotropy currently appears to be the most convincing explanation.