Publications

The use of electromagnetic methods for hydrocarbon exploration began early in the twentieth century. Indeed the first issue of the journal Geophysics in 1936 contained papers on this subject. Marine controlled-source electromagnetics (CSEM) has recently become a significant business tool for upstream applications due to the convergence of many technologies. CSEM provides valuable information on subsurface lithology and fluids independently from seismic data, however, its spatial resolution is much lower. Industry uptake has been dramatic, with more than 300 marine CSEM surveys acquired worldwide since late 2000. Results from deepwater West Africa and Brazil clastic targets demonstrate both the promise and the challenges that lie ahead. These results show that CSEM can detect and map offshore reservoir hydrocarbon resistivity pre-drill at depths exceeding 2000 meters sub-mud. Although the early proof-ofprincipal test surveys over large discoveries in Angola showed robust resistivity responses, many other exploration targets have relatively weak CSEM responses due to any combination of large target depth of burial, small reservoir net thickness, low resistivity contrast with the non-reservoir ",background",, and limited lateral extent. However, careful integration of these data with other geoscience information, plus in particular application of 3D nonlinear inversion and imaging, can produce useful results even in complex geological settings. But remote resistivity determination is hardly a fool-proof method for hydrocarbon identification, since many geologic facies are electrically resistive relative to their surroundings. As marine CSEM matures, it may prove to be the most important geophysical technology for probing below the seafloor since the emergence of 3D reflection seismology 30 years ago. The key determinant of commercial success will be whether the value of CSEM information is worth the money spent, relative to what other data can provide. This presentation is excerpted from the Society of Exploration Geophysicists Spring 2007 Distinguished Lecture.