ABSTRACT

Three-dimensional surveying is the method of choice in marine controlled-source electromagnetic (CSEM) exploration for hydrocarbons in frontier regions, but robust interpretation of the typically large-size field data faces significant challenges, including how to determine the correct resistivity, depth, and lateral limits of hydrocarbon-saturated reservoirs in the presence of heterogeneous host rocks or anisotropy and how to relate CSEM information to the key elements of geologic prospect evaluation (the presence of source rocks, migration and charge, reservoir rock, trap, and seal). We have developed a simple geologic risk-tailored approach for multiattribute analysis and first-pass interpretation of CSEM data in frontier exploration in which little prior information is available. First, geometric normalization of electric field amplitudes at each receiver location yields “phase-consistent” sounding curves that directly represent subsurface electrical structure (and can indicate reservoir rock presence). It enables accurate determination of seafloor resistivity (whose areal variation and direct correlation with seepage-induced geochemical and seismic shallow-gas anomalies can indicate the presence of a working petroleum system). Edge-detection attributes are then used to determine the geographical position and boundary shape of anomalous 3D resistive bodies (the trap presence and structural closure). Keeping these known parameters fixed, the most likely burial depth and resistivities of the sought 3D bodies are found using a simple line search technique involving rigorous 3D modeling and the results are validated and optimized post facto using seismic depth constraints to locally improve the prediction of the size and resistivities of hydrocarbon-charged or water-bearing sections crucial for prospect derisking, reserve estimation, and well placement.

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