Application of 3-D Versus 2-D Velocity Model: Implications of Depth Conversion Hayat-Yaser-Kenz Fields (Khalda Concession,Western Desert, Egypt)

By

 Ali Mohamed Bakr¹, Fred Wehr¹, Sheldon Plahn¹

(1) Apache Egypt, Cairo, Egypt

 Multiplying a simple 2D velocity grid by a time grid to generate a depth grid can result in inaccurate predictions of geological tops, faults, fault patterns, and reservoir volume. This paper addresses the problem and demonstrates the value of 3D depth conversion in the Khalda area.

Apache has acquired approximately 2500 km2 of 3D seismic data in the Khalda Concession in the Western Desert of Egypt. Hayat, Yasser and Kenz are major oil fields in the Khalda concession, producing from reservoirs in the Bahariya and AEB Formations (Cretaceous). Three main 3D velocity models were constructed over Hayat-Yasser- Kenz and compared to the 2D estimating method. The three 3D velocity models are: (1) stacking/migration velocity (using only seismic stacking velocities), (2) time-depth velocity (using only time-depth pairs from wells data), and (3) migration velocity calibrated with T-D pairs (using both stacking velocities and well control).

Calibrated models provide more reliable results, including a more accurate tie to wells as well as a more structurally admissible fault pattern. The entire seismic data set, including traces, horizons (time), and faults are converted to the depth domain using the proper velocity model. This enables much tighter integration of seismic interpretation and well data.

The reservoir depth map out of this volume was used for validity check and it shows ± (10 feet) prediction error. Results confirm more reliable well top prediction and possible additional propectivity using the 3-D calibrated velocity models versus the 2-D average velocity methodology.