Marine Controlled Source Electromagnetics (mCSEM) for De-Risking Key Prospects in a Complex Turbiditic Environment

Frederik Pivot, Antony D. Price, and Jean-Noel Ferry
E&P Angola, TOTAL, Luanda, Angola

mCSEM techniques have reached an industrial level of maturity, and many examples are available to demonstrate that this technique can be used as a complementary geophysical method for improved charactorisation of fluid content for a given prospect. However, as mCSEM deals with diffusion of electromagnetic energy, and the entire subsurface as well as possible resistive bodies contribute to the recorded electromagnetic response. The inversions of the CSEM data are of much lower spatial resolution than seismic, and interpretation can become very difficult if many resistive bodies are spread in the area of interest. In the case of a prospect surrounded by other resistive bodies, one way to avoid a false interpretation is to simulate the electromagnetic responses of a family of realistic 3D models covering many geological scenarios. We propose here using advanced seismic interpretation techniques and geomodelling tools for rapid construction of 3D CSEM models, testing the impact of upscaling and averaging effects; subsequently, the full 3D CSEM response of each model is computed and careful comparison between them allows evaluation of the target versus other pheneomena which can mask the response of interest. Such a workflow is illustrated here on a realistic case of a turbiditic prospect , deep offshore Angola. For this prospect, that the desired goal was to evaluate the ability of the mCSEM technique to test the observed seismic anomalies for the presence of oil bearing sands or low gas saturation sands, despite a highly complex background environment.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery