Seismic Guided EM Inversion in Complex Geology - Application to the Bressay and Bentley heavy Oil Discoveries, North Sea

Abstract

A novel method for integrating Towed Streamer EM and dual-sensor seismic data is introduced. The method is referred to as seismic guided EM inversion, where a sparse-layer depth model defined by seismic is used to suggest resistivity boundaries without a rigid constraint. This makes good sense since a reservoir can be hydrocarbon-charged to an unknown degree corresponding to the spill-point or less, and the final EM model boundaries do not have to be exactly identical to the seismic determined ones. The inversion workflow is described in detail as applied to a complex geological region where the heavy oil fields known as Bressay and Bentley are located in the North Sea. Seismic imaging over these fields is challenging since they are rich in injectites, having steep and irregular features. There are also other resistive features such as the Balder tuff, granite intrusions and the basement that can interfere with a fully unconstrained EM inversion. The method introduced here is applicable for exploring complex geological regions, in particular in a frontier exploration, where CSEM and seismic data co-exist.