- Copyright © 2002 Society of Exploration Geophysicists
Detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry. Seismic methods have traditionally been used in this context, but the results can be ambiguous. Another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids. Modeling presented by Eidesmo et al. (2002) demonstrates that by using seabed logging (SBL), a special application of frequency domain controlled source electromagnetic (CSEM) sounding, the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified. Such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying.
In November 2000, a full-scale trial survey was carried out from the research ship RRS Charles Darwin offshore Angola, in an area with proven hydrocarbon reserves. The project was a collaboration among Statoil, Scripps Institution of Oceanography, and the Southampton Oceanography Centre. The object was to demonstrate that SBL, developed by Statoil (Eidesmo et al., 2000; Ellingsrud et al., 2001), could direct detect hydrocarbon-filled layers in the subseafloor.
The petroleum prospects offshore Angola are in a deep Tertiary basin consisting of a thick (10-20 km) sequence of prograding sands and shales. The area is characterized by allochthonous salt of Aptian age, and deepwater channel sands with petroleum potential. Well logs show sediment resistivities typically around 0.7 Ωm that rise to around 100 Ωm in petroleum reservoirs. The survey site was on the continental slope in water depths of about 1200 m, with a known petroleum reservoir about 1100 m below seafloor. Shallow salt occurs in the northeast corner of the area.
The marine CSEM method employed in this survey uses a horizontal electric dipole (HED) source to transmit …