Geostatistical Depth Conversion

The depth conversion workflow is an integral part of our reservoir characterization workflow. It is especially relevant for lithology/fluid prediction. The primary objective is to produce a velocity model with a stable and geologically plausible background trend which will enable accurate depth conversion over the study area.

The background trend is a stable, long wavelength solution in the form of a polynomial function of realistic geological quantities. These may include velocity information, seismic horizons and well markers. Other available data deemed likely to influence the observed variations in velocity may also be included. A final krigging is then applied to the background model to obtain the required match at well locations.

The most geologically plausible update is obtained by using the most realistic inputs as constraints. Blind well testing is used to select the most appropriate set of trend terms and to help quantify the uncertainty away from well control. Numerous combinations of parameters are tested to deliver the best prediction result.

The geostatistical depth conversion process involves:

• Tying the well data to the seismic data to obtain calibrated time-depth mappings
• The calculation of the errors in the existing velocity field with respect to the calibrated time-depth mappings
• Building a 3D model of these errors using geostatistics
• The application of this velocity error volume to the existing model, to create the final scaled velocity model