Reservoir Imaging Beneath a High Velocity Layer Using Multi-Azimuth Walkaway VSP in the Mediterranean Sea, Egypt
Authors: Badri M. , Colombo S. , Hashem F. 
Published: December 2007

The presence of an evaporitic complex of varying thickness and geometry above the target of interest in the offshore Nile Delta, Egypt, has masked the detectability of the gas-bearing sand on the 3D surface seismic reflection data acquired in the area. Furthermore, the surface seismic data is characterized by lacking clear reflectivity at the reservoir depth and low frequency content. The evaporitic complex consists of shale beds sandwiched in anhydrite has caused the seismic energy to be trapped within it and little energy penetrated down to the target zone. These factors have imposed a difficulty in reliably mapping the gas-bearing sand. The sediments above the target zone mainly consists of thick shale sequence with P-wave velocity of about 2400 m/sec while the sand P-wave velocity about 3470 m/sec with a thickness of about 13 m. In order to determine the geometrical distribution of the gas-bearing sand, a multioffset-multiazimuth, three-component vertical seismic profiling surveys were acquired using tri-axial downhole seismic array. The main objective of the survey was two folds: first, to obtain a better definition of the reservoir geometry away from the well to assist in positioning future development wells and second to evaluate the usefulness of Walkaway VSP survey in enhancing the structural and stratigraphic interpretation.

The VSP data processing procedure included analysis of the three components, travel time inversion for velocity determination, Hilbert transform for wavefield separation, and GRT migration to transform the VSP data from time-depth domain to VSP-CDP domain. The effect of transverse anisotropy on the migration process was also considered.

The VSP images have provided a detailed image of the structural dip of the gas-bearing sand and its extension along three different azimuths. The derived images have shown that the upper sand level thickens along the northwest azimuth while the lower sand showed a decrease in thickness away from the well along the same azimuth. The results of the Walkaway VSP have been confirmed by well testing analysis and at a later stage by subsequent drilling along the azimuth of the increasing sand thickness using vertical incidence VSP in a deviated well.

Based on the Walkaway VSP images and drilling results of the deviated well, a new structure map of the gas-bearing sand was constructed to provide accurate definition of reservoir extension needed for future development drilling. In conclusion, a multi-azimuth Walkaway VSP survey has provided a clear target definition of the reservoir where 3D surface seismic was limited in imaging it. The Walkaway VSP technique not only bridges the gap between surface seismic and well logs but also complements surface seismic data in imaging a difficult exploration environment.

Introduction

The 3D surface seismic data acquired in the Offshore Nile Delta has revealed new structural plays in the area which have led to successful gas discoveries. However, the exploration task is not an easy process due to the fact that the reservoir is situated below an evaporitic complex of variable thickness which causes absorption of the seismic energy. The recent discoveries have been found at a depth of about 3600 meters where a thick shale sequence forms the cap rock for gas-entrapment. Reservoir thickness varies laterally due to the fact that the depositional environment is related to a turbiditic system.

Recently, a gas discovery has been made in the Offshore Nile Delta area. The location of well is shown in Fig. 1. P. 195^