Publications

Seismic imaging of the geological structure of the Bekapai field is challenging. The complex shallow geology of the overburden, including fault networks in unconsolidated sediments, overshadowed by gas clouds, causes several imaging challenges. Large lateral velocity variations, absorption, shadowing zone and push down effects, are hurdles difficult to tackle with conventional depth imaging workflow. Total in-house Full Waveform Inversion technology is addressing some of these challenges. The latest high-fold long-offset wide-azimuth Ocean Bottom Cable acquisition over the Bekapai field, acquired in 2012, provides a favorable seismic dataset for the successful application of this technology. Full Waveform Inversion is a data-matching wave-equation-based tomographic method. The method constructs a high-resolution velocity model capable of explaining and characterizing the complex near-surface structures and properties of the overburden. Full Waveform Inversion provides new evidence which reinforces and improves our understanding of the geology of the Bekapai field. Some of the geological features, such as relatively thin carbonates patches, were already known and interpreted on seismic images, but are observed for the first time in a velocity model. The main findings are the extension and thickness of the shallow gas anomalies within the gas cloud area, and low velocity anomalies linked to gas trapped beneath shallow normal faults. These latter anomalies were never detected before. These low velocities anomalies characterization enables Full Waveform Inversion to resolve some complex image push-downs and concomitantly to improve deeper targets reservoirs imagery. The previous model building methodology which relied on reflection tomography, could not correct for these push-downs. Therefore, Full Waveform Inversion also constrains the deeper velocity model which will be updated by reflection tomography. The integration of Full Waveform Inversion into the model building workflow is the key to its success and encourages this approach on similar exploration and development projects.