Application of Geostatistical and Velocity-Volume Fractal Models to Determine Interval Velocity and Formation Pressures in an Oilfield of SW Iran

In seismic methods, the estimation of formation pressures is acquired by transforming the seismic velocity to the pore pressure and contrasting it with the velocity log and effective pressure obtained during the well-test program. This study is regarding the velocity studies in one of the oilfields in SW Iran, which is generally carbonated. Except for the Kazhdumi Formation, they do not have shale interbeds. This study is based on information from 23 wells and seismic interpretation. Compressional (Vp) and shear velocity (Vs) models are determined from combined geostatistical models and compared with the value-volume fractal method, especially the velocity-volume model. Based on vertical Seismic Profiling (VSP) data, the maximum Interval velocity is 2760-2900 m/s in the northeast of the field related to the Gotnia Formation. In order to determine the formation fracture pressure, the shear velocity cube is modeled using exploratory well-cores and dipole sonic imager (DSI) shear velocity logs. The final cube with a coefficient of 0.95 has been determined for the shear velocity data obtained from the porosity, lithology, and primary DSI shear velocity data. The final amounts of inverted acoustic impedance (AI) in the deeper formation of the field are mainly in the range of 8000-15000 [(m/s)*(gr/cm3)], which it could be referred to as calcareous formations. Based on the calculation of the logratio matrix obtained from the Velocity-Volume (Vp-V) fractal model, the maximum overall accuracy (OA) in the dominant limestone intervals is 0.74. It indicates a high correlation of the compressional velocity cube model obtained from a combination of sequential Gaussian simulation (SGS) and co-kriging models with acoustic impedance inversion (AI). In the final Vp cube’s vertical Variogram, the sill is 0.34, and in major and minor is 0.96. Anisotropy range for vertical variogram range is 96 meters and for major and minor directions is 11850 meters.