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2011 SEG Honorary Lecturer Europe. Well logs provide a high-resolution measurement of the properties of a reservoir and the surrounding strata; however, properties can only be determined in a small area local to the well. Often measurements of reservoir properties across the extent of a field are desirable for reservoir management or production optimization. Remote geophysical measurements are therefore required. Seismic data are most commonly used for this purpose; however, in recent years CSEM methods, which measure the resistivity structure of the seafloor, have also been widely applied.

CSEM use a high-powered source to transmit low-frequency signals through the Earth to an array of receivers. By interpreting the received signals using forward modeling and inversion approaches, the resistivity structure of the seafloor can be determined. Resistivity well logs often show that commercial hydrocarbon deposits may be many times more resistive than surrounding lithologies. In principle, such variations should be readily detected using CSEM tools. In contrast, seismic data are sensitive to boundaries between lithologic units but are often less sensitive to fluid changes within these units. Given high-quality seismic and well data and sophisticated seismic inversion and rock physics tools, we can sometimes relate seismic changes to saturation effects. Nevertheless, the change in resistivity caused by variations in saturation should be much easier to detect.

However, despite the sensitivity of resistivity data for determination of saturation, there are two inherent challenges to interpreting CSEM data. First, the structural resolution of CSEM data is poor. Second, the cause of resistivity anomalies (particularly high-resistivity features) cannot be uniquely linked to the presence of hydrocarbons in the subsurface when taken in isolation. In many situations, these are equally likely to be caused by other high-resistivity material (for example, tight carbonates, salt, or volcanics). These limitations can be mitigated using an integrated approach to geophysical interpretation. Seismic information can outline the reservoir structure (but potentially not its content or extent), and we have independent constraints on the surrounding strata within which it is embedded. This is therefore a constrained interpretation problem and one that the CSEM data are in a much better position to answer.