Delineating Salt Bodies With 3-D CSEM Technology — Case Study: Perdido Fold Belt, US GoM

Abstract

Abstract

Salt mapping is challenging in hydrocarbon exploration. Improper delineation of salt bodies with complex geometries often affects the way geophysical data is processed and interpreted. Controlled Source ElectroMagnetic (CSEM) have demonstrated that large differences in electrical properties occur between salt bodies and surrounding sediments. The technology has the potential to delineate the salt bodies and identify possible sedimentary inclusions within them.

In this paper, we discuss a new approach of deriving initial resistivity models for 3D CSEM inversion for the Phase V dataset of the Daybreak survey in Alaminos Canyon, Gulf of Mexico. The survey area is entirely covered by salt, and the standard approach for deriving an EM start model produced an inversion result that could not find a solution that closely matched the measured data where only limited changes could be made to the initial salt (start) model. From this outcome a new workflow was defined where the resistivity edges (top and base) delineating the salt body had a smooth transition between the salt and sedimentary section. This approach allows the inversion to accurately change the boundaries and recover a salt volume that matches the response of the measured data.

Once the inversion process is completed, the output resistivity volumes are loaded into a workstation and co-rendered with the seismic data (Phase 45 from TGS) for QC and investigation of interpretation updates. The output resistivity data revealed, in part, significant updates to the seismic interpretation of top and base salt. Results show that the resistivity within the salt body is not continuous and in some regions is as low as 1 Ωm. The low resistivity regions are believed to be related to sedimentary inclusions that are entrained between salt sutures. Early seismic interpretations did not capture these features. The impact of the new information suggests that PSDM salt velocity models may have significant short comings that distort ray-path propagation and ultimately the image of the subsalt structure.

The combined workflow to invert resistivity data with seismic interpretation and processing, may provide opportunities to build more accurate salt structures, refine base salt, and the subsalt image through integration of greatly improved geo-bodies in the salt velocity model. The add-on value is better seismic processing workflows that reduce cost, improve structural analysis and ultimately better drilling decisions.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016