It is known that CSEM will detect resistors often associated with hydrocarbon deposits in marine environments; therefore, it represents a significant advance in deepwater oilfield exploration. CSEM has been called the most significant new technology in oilfield exploration since the development of 3D seismic acquisition more than 20 years ago. This technology may be applied to a wide variety of exploration targets; from the near surface to as deep as 4,000 m below the sea floor. The ability to predict reservoir fluid properties ahead of the drill-bit means a considerable risk reduction for exploration programs and also a significant advantage when considering offshore license bidding.
The Method: A low-frequency electromagnetic field is transmitted using a deep-towed electric dipole antenna (source), and the resulting field is sampled at the seafloor. Unlike natural source plane wave electrical methods such as magnetotellurics (MT) where thin resistive layers are effectively transparent, the generated dipole field interacts with such a layer, and the layer's presence, thickness, and lateral extent may be determined. As with any geophysical method, there are limitations on the depth of burial, layer thickness, and resistivity contrasts that affect the viability of a target.
Operations: A high-power low-frequency dipole antenna is towed behind the survey vessel within 50-100 meters of the seafloor. An array of ultrasensitive seafloor electromagnetic receivers is deployed in a pattern appropriate to the target. Transmitter and receivers are tracked and located acoustically. When data acquisition is complete, the receivers are recovered and the data are downloaded and analyzed. The data are then interpreted, first in terms of electrical units, and then as geologic formations taking into account and integrating other geophysical data.
Applications: The primary application of marine CSEM is the identification and characterization of units that are more resistive than the surrounding rocks. Typically, a potential reservoir is identified with seismic data and CSEM used to analyze its resistivity, taking advantage of the resistivity contrast between oil- or gas-saturated rocks and those with a significant water content. The transformation from resistivity to geology, and finally pore fluid content, is an interpretive process that requires careful interpretation and integration of the CSEM data with seismic data and local well control.
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