Controlled Source Electro Magnetics (CSEM) is promising exploration technology, in that it has the ability to distinguish resistive hydrocarbon reservoirs from conductive wet sands in the subsurface. CSEM data is particularly powerful when combined with seismic and well data. Using seismic and well log data to aid in constraining an inversion of CSEM data may significantly improve the results of the inversion. This paper describes how well data may be used to provide a link between acoustic properties (p-wave velocity) and electro-magnetic properties (resistivity).
Four wells in the Vøring Basin, Norwegian Sea were used in the study, the goal being determine a transform from p-wave velocity to resistivity. The wells were analyzed for mineralogy, saturation and porosity, and poor quality elastic data (Vp and Rhob) was identified and thrown out. Depth plots of Vp, density and resistivity were made for the shales only (sand data was filtered out), using depth below mudline. The depth trends show a coherent trend for velocity and resistivity, however, one of the wells has an anomalous density trend in the Tertiary, which is subject most likely related to micro-porosity associated with diatomaceous ooze. Faust (1953) developed a depth dependant exponential resistivity to velocity transform. This empirical transform does not adequately fit the Vøring Basin wells. The cross plot of velocity and resistivity shows a very good correlation. This indicates similar clay mineralogy and similar water resistivity in the wells. Hacikoylu et al. (2006) developed a deterministic transform from velocity to resistivity, by combining rock physics theory and Archie's Formation Factor. This algorithm, too, did not adequately fit the Vøring Basin data.
A new velocity to resistivity transform has been developed for the Vøring Basin. This model is a continuation of the Hacikoylu model, but also considers clay resistivity by introducing an additional term related to the cat-ion exchange coefficient. The transform, though developed using well log data, may be applied to velocities derived from seismic data to establish resistivity estimates away from well control, to better constrain CSEM inversions.