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Three-dimensional modelling and interpretation of CSEM data from offshore Angola

Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Science and Technology , S. P. Andersens vei 15a, N-7491 Trondheim, Norwayand Electromagnetic Geoservices , Stiklestadveien 1, N-7041 Trondheim, Norway

(e-mail: ahb{at}emgs.com)

In 2000, a large marine controlled-source electromagnetic (CSEM) calibration survey was acquired by Statoil as a part of a CSEM evaluation project over known hydrocarbon (HC) reservoirs offshore Angola. The electric field magnitudes measured at the seafloor above the HC reservoirs are 1.5 to 3 times higher compared to the synthetic background responses and ‘off-reservoir’ measurements. The relative strength of the electric field measurement above a salt structure is very strong (normalized magnitudes stronger than 3) compared to the background response. Modelled responses obtained from three-dimensional finite difference time-domain forward modelling, based on seismic and petrophysical data, show good agreement to the measured responses over the HC reservoirs. The modelled response after introducing a shallow resistor to the 3D model explains the strong CSEM anomaly measured in the NE part of the study area with no proven HC reservoir. The 3D forward modelling is also used to predict the sensitivity to the resistivity of the undrilled structure in the SE part of the study area, where no CSEM data were acquired. Integration of structural information interpreted from seismic data and resistivity values estimated from well logs significantly improves the subsurface geo-resistivity modelling and complements the offshore Angola CSEM data interpretation.

KEYWORDS: controlled-source electromagnetic, 3D modelling, resistivity, hydrocarbons, seismic, turbidites, salt