The Use of CSEM within an Integrated Exploration Project "Best of EAGE"

Lovatini, Andrea ¹; Medina, Elena ¹; Campbell, Tracy ²; Myers, Keith ³
(1)Integrated EM Center of Excellence, WesternGeco, Milan, Italy. (2) WesternGeco, Baku, Azerbaijan. (3) WesternGeco, Perth, WA, Australia.

We discuss the integration of multiple geophysical methods in a single project basin study in the offshore Potiguar basin, located in northeastern Brazil, where there is production from the nearshore area, but no production and limited exploration from the deepwater portion.

The existing 2D seismic dataset, initially available only as prestack time migrated data, underwent prestack depth migration using new tools for noise attenuation and multiple removal. The results showed significant spectral content improvement at shallow depths and fault and reflector definition and continuity in the deeper section. The dataset was interpreted to provide regional scale horizons and used as input with other geological and geochemical data for a petroleum system model of the whole basin. Satellite images were processed to provide a map of sea-surface oil slicks.

The CSEM method was applied at a different stage than its more common utilization in exploration workflows, which is as a tool to evaluate known prospects. It was applied together with other methods to provide further information on the geology of a poorly explored basin. Oil slicks and seismic interpretation structural and stratigraphic features were used to identify possible areas for CSEM acquisition. 1D modeling was preliminarily used to identify which features were detectable. Approximately twelve targets were prioritized using the 1D modeling results, their seismic characteristics, their level of complexity for CSEM interpretation and also their areal distribution within the entire basin. 2.5D and 3D modeling was used to investigate more complex targets. To cover as broad a range of depths as possible, a spectrum with high energy at multiple frequencies was designed.

To include as many targets as possible in the basin in a cost-efficient way, without losing 3D illumination, a hybrid approach between 2D and 3D layouts was followed in survey design.

Within the basin five separate datasets were acquired. The interpretation workflow included: qualitative interpretation; 1D anisotropic inversions; up-scaling to 2D and 3D resistivity volumes; background model forward modeling assessment; 2.5D and then 3D anisotropic inversions. All the 3D inversions used multiple frequencies to improve both sensitivity and resolution.

The resistivity volumes were then integrated with the seismic data, the interpreted horizons, the oil-slicks and the petroleum system modeling predicted accumulations in a single 3D earth model.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.