Electromagnetic Seabed Logging (SBL) is a resistivity-based method for direct hydrocarbon detection prior to drilling.
Statoil invented SBL in the late 1990s and established ElectroMagnetic GeoServices (EMGS) in February 2002 to commercialize the technique. SBL is an application of the marine Controlled Source Electro-Magnetic (CSEM) method for hydrocarbon exploration.
The marine CSEM method normally employs a horizontal electric dipole source with user specified frequency content. The source is towed along a towline usually close to the seafloor. In a conventional 2D survey, the receivers are positioned on the seafloor along the towline. In a 3D survey, the receivers are positioned in a grid on the seafloor (see figure).
The state-of-the-art data analysis includes modelling and inversion techniques to obtain one or more resistivity models of the subsurface that explain the acquired data. These resistivity models can then be correlated to seismic and geological data.
In 2009, Statoil evaluated the CSEM performance by an internal review study including more than 60 CSEM surveys acquired between 2001 and 2008. Of these, 21 CSEM surveys were acquired prior to drilling and 11 were performed for calibration on drilled structures. The study shows that newer data have a clear risk impact whereas older surveys are in many cases inconclusive.
CSEM has a short commercial history of only 7-8 years and there have been tremendous developments and improvements in data quality, analysis and interpretation techniques. The positive trend in CSEM exploration is related to improvements in all of these elements and we are continuously improving our ability to apply CSEM in appropriate geological settings.