Using seismic guided EM inversion to explore a complex geological area: An application to the Kraken and Bressay heavy oil discoveries, North Sea

The integrated analysis of controlled source electromagnetic (CSEM) with seismic data can provide valuable information on reservoir characteristics. We introduce in this paper a method for integrating Towed Streamer EM and dual-sensor seismic data and refer to it as seismic guided EM inversion. The inversion workflow is initiated by adopting a sparse-layer depth model defined by the dual-sensor seismic data to suggest resistivity boundaries without a rigid constraint. This makes good sense when considering the uncertainties in the seismic data from the time to depth conversion, and more importantly, the fact that a reservoir can be hydrocarbon-charged to an unknown degree corresponding to the spill-point or less. The anisotropic resistivity variations within the layers are accommodated by the lower and upper boundaries, which can be estimated by the unconstrained 2.5D anisotropic inversions. To demonstrate the workflow, we apply it to a dataset acquired in an area with complex geology resulting in challenging imaging issues of the Kraken and Bressay fields. The two heavy oil reservoirs are rich in injectites, located in close proximity to other high resistivity settings, such as the shallow gas pockets in the overburden, the regional Balder Tuff and a few granite intrusions.