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.
This paper gives a unified treatment of electromagnetic ͑EM͒ field decomposition into upgoing and downgoing components for conductive and nonconductive media, where the electromagnetic data are measured on a plane in which the electric permittivity, magnetic permeability, and electrical conductivity are known constants with respect to space and time. Above and below the plane of measurement, the medium can be arbitrarily inhomogeneous and anisotropic.In particular, the proposed decomposition theory applies to marine EM, low-frequency data acquired for hydrocarbon mapping where the upgoing components of the recorded field guided and refracted from the reservoir, that are of interest for the interpretation. The direct-source field, the refracted airwave induced by the source, the reflected field from the sea surface, and most magnetotelluric noise traveling downward just below the seabed are field components that are considered to be noise in electromagnetic measurements.The viability and validity of the decomposition method is demonstrated using modeled and real marine EM data, also termed seabed logging ͑SBL͒ data. The synthetic data are simulated in a model that is fairly representative of the geologic area where the real SBL were collected. The results from the synthetic data study therefore are used to assist in the interpretation of the real data from an area with 320-m water depth above a known gas province offshore Norway. The effect of the airwave is seen clearly in measured data. After field decomposition just below the seabed, the upgoing component of the recorded electric field has almost linear phase, indicating that most of the effect of the airwave component has been removed.
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