We propose a method of surface and marine electrical prospecting using controlled-source excitation. The method is designed to detect hydrocarbon deposits at depths of a few kilometers and to map their boundaries. The technique is based on imaging the induced-polarization ͑IP͒ parameters of the geologic formation. We use the fact that, because of the imaginary part of the electric conductivity, polarized media support wave propagation processes whose nature is similar to displacement currents induced by the dielectric permittivity. However, unlike displacement currents, these processes reveal themselves at much lower frequencies and, therefore, at greater depths. It is established that the ratio of the second and the first differences of the electric potential does not decay after the current turn-off in polarized media, whereas it decays quickly if the IP effect is absent. Thus, the IP response can be observed directly and separated from the electromagnetic ͑EM͒ response. We use a vertical focusing of the electric current to decrease the effect of laterally adjacent formations to apply a 1D layered model in a 3D environment. This method obtained promising results in several regions of Russia.
A novel focused-source electromagnetic (FSEM) method focuses the EM field in the vertical direction to provide deep-reading resistivity data. FSEM offers better spatial resolution and greater depth of investigation than the conventional controlled-source electromagnetic (CSEM) method for land and marine EM surveys. We have proven the high efficiency of FSEM by analyzing 3D models of various complex geologic formations in the presence of seafloor bathymetry, shallow resistive gas-hydrate overburdens, and secondary gas reservoirs formed above deeper oil reservoirs. Combining the power of our focusing technique with the power of our 3D numerical modeling method, we have developed exceptionally challenging test cases to conclude that FSEM automatically cancels unwanted shallow effects and allows simple visual interpretation of deep reservoir responses. In addition, FSEM is insensitive to imperfections in the setup geometry. We achieve these advantages using a proper combination of measurements acquired in the receiver excited by transmitters situated at different space points. The method is promising in anisotropic formations as well.
We evaluated the results of a large-scale commercial project that illustrated the capabilities of advanced time-domain electromagnetic (TDEM) technologies powered with integrated interpretation of geologic and geophysical data. To study the hydrocarbon prospectivity of a field in Eastern Siberia, we developed a survey design, and then acquired, processed, and interpreted the TDEM data from 30 profiles (total length 772 km) covering an area of approximately [Formula: see text]. The data were acquired using the conventional TDEM and a novel high-resolution version of TDEM, the focused-source electromagnetic method. We described the geologic framework, data acquisition methodologies, and key results obtained using integrated TDEM, seismic, and well-logging data. The interpretation was used to select well locations for additional exploratory drilling. Postsurvey drilling supported our interpretation. The presented case study demonstrates the value of TDEM in the exploration workflow.
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