Combinative Inversion of Controlled-Source Electromagnetic [CSEM] and 2-D Seismic Data Detects a Commercial Oil Field During Blind Test Offshore Côte D’Ivoire, West Africa

Lars Hübert¹, Lucy McGregor², and Joel D. Walls^1
1Rock Solid Images, Houston, TX
²OHM Surveys, Aberdeen, United Kingdom

A blind test of co-rendering seismic and electromagnetic data across a producing West African offshore oil field provides an invaluable decision forum to match positive resistivity anomalies within controlled source electromagnetic (CSEM) data to geological traps delineated by seismic data. Because both datasets sample fundamentally different subsurface properties (acoustic vs. electromagnetic), a combined inversion specifically offers 1.) rapid quality control during EM processing, 2.) a more efficient matching of the CSEM data to a geological model, resulting in 3.) invariably improved exploration success in frontier areas. The Baobab field, offshore Cote d’Ivoire is a case in point. A CSEM line was recorded over this field and extended southward to additionally cover a prospect of interest. The line was recorded in water depths varying from 200 to 2,500 m, complicating the EM inversion. A staged approach to the inversion was adopted, starting with a simple depth model which systematically added seismically derived constraints increasing the accuracy and complexity of the geological low-frequency inversion model and its predictive prowess. Using standard visualization tools, each intermediate EM inversion result is compared to the 2-D seismic data. Honoring bathymetry data and seismic constraints in the EM inversion allows for a geologically much more plausible approximation of the geologic structure with an overlay of electrical resistivity anomalies illuminating not only subsurface architecture, but, furthermore, its’ alternating fluid conditions.

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