Geologically Constrained Pre-stack Depth Imaging over the Greater Burgan Oilfield
Zahran, Wael A.*1; Mukherjee, Pradip K.1; Al-Ghareeb, Aisha Y.1; Cunnell, Chris 2; Saleh, Tamer 2; Abdel-Wahab, Hossam M.2
(1) Exploration, Kuwait Oil Company, Ahmadi, Kuwait. (2) WesternGeco, Cairo, Egypt.
We present a case study for pre-stack depth imaging over the Greater Burgan oilfield, with an emphasis on the use of geological constraints to provide more accurate and reliable velocity models and seismic images.
The Greater Burgan oilfield is located in Kuwait and is defined by a low-relief anti-clinal dome draped over a basement horst structure. The primary producing reservoirs are the Wara and Burgan. The shallowest mapped horizon is the RUS, characterized by high velocity anhydrites.
The objective was to provide improved imaging compared to recent pre-stack time migration, focusing on the section between the primary reservoirs, the intra-reservoir clastic sequences and minor faults. A reflection tomography workflow was used to refine the velocity model. This is a data driven approach, however additional geological constraints were also imposed during the model building. The careful use of these constraints produced a more geologically plausible velocity model in several ways.
The primary use of geological constraints was in the definition of the near surface velocity model. The long wavelength component of the refraction statics solution was incorporated directly into the velocity model, and the remainder applied as high frequency statics. However inspection of the initial depth images indicated a residual mis-match between the Top RUS markers and the seismic volume. A targeted model update was performed to minimize this mis-match. Checkshot velocity profiles were also used to guide and constrain the strong velocity inversion directly below the RUS formation.
Additional geological constraints were introduced by using interpreted horizons to guide the tomography picking, checkshots to derive anisotropic parameters, and formation marker depths to enable mis-tie analysis.
The result was a geologically reasonable model that provides a close match to known borehole information, coupled with improved structural imaging, fault definition and amplitude handling.
The success of the Greater Burgan pilot demonstrates the uplift in image quality and interpretation confidence that can be achieved via the careful application of a fit-for-purpose PSDM workflow, despite the limitations of a conventional narrow azimuth acquisition dataset. However it also points the way to further enhancements in the imaging process through improved acquisition geometries as well as more advanced imaging techniques such as Full Waveform Inversion and alternative migration algorithms.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain