Advanced Borehole Seismic Calibration in Deep Water Reservoirs, a Case Study in Offshore Angola Block 31

The Plutao-Saturno-Venus-Marte group of fields in offshore Angola Block 31 is a large ultra-deep-water development. First oil was in December 2012. Well placement and performance is the key to successfully delivering resources and sustaining the oil production plateau. Accurate well planning is attained by optimizing the resolution and relative positioning of the image of the sub-surface seen on the seismic data. The original 2004 acquisition was processed in 2005 using an isotropic pre-stack time migration flow (IPSTM) and subsequently in 2009 using an isotropic pre-stack depth migration (IPSDM). Some early pre-development wells indicated that the IPSTM seismic image is mis-positioned in some wells.

Walkaway VSPs were not originally considered for PSVM because the structural regime is essentially extra-salt and the time depth relationships appeared to conform to a “normal” depth of burial trend with no significant or sharp changes in velocity. However, the mis-positioning of the seismic image on the steeply dipping parts of the Plutao field strongly suggests this is not the case throughout the field.

Some of the mis-positioning observed in the Isotropic volumes will result from uncertainties in the depth conversion as anisotropy has not been taken into account. A significant uplift in imaging is observed when anisotropic velocities are accounted for in the velocity model, as observed in other Angolan extra-salt deep water fields located in Block 31.