Abstract: How Accurately Can Seismic Image Complex Structures in Foldbelts? A Case Study in Pakistan

Wayne P. Wilson, William J. Sercombe, Margaret A. Stratton, Martin L. Albertin, Andrew P. Longo

Interpreters must be aware of how acquisition and processing practices have affected the resolution of their data and have limited seismic's ability to image complex structures. A primary goal for exploration teams working foldbelts is to have seismic data which accurately images structures in their area of interest, but this is rarely achieved in complex foldbelts. After Amoco drilled the Tolanj ^sharp1 on what appeared to be a reliable seismic structure, a complete structural re-evaluation of the Potwar basin and the Kohat Plateau was initiated to understand how well seismic was imaging structures in these areas.

Much of the seismic acquired in the Potwar basin was shot with low frequency sweeps and recorded with 330 foot group intervals. Synthetic seismic sections, using finite difference modeling, were generated from different structural models to show how accurately seismic could image complex structures using different group lengths and intervals. These models also show how poor post-stack time migrations put diffraction energy and dipping events into their correct positions. Pre-stack depth migration (MBS) proves to be the most accurate method to enable the exploration team to make more accurate structural interpretations. Amoco's MBS method allows the processor to derive interval velocities from the seismic data to develop a velocity model independent of geologic models which are develop d from preconceived ideas or seismic interpretations.

Our re-evaluation of the Potwar basin shows the major style of deformation is not uni-directional low angle thrusting, but basement involved transpression which has formed a complex wrench fault system. As acquisition and processing tools develop, exploration teams will continue to be able to reveal structures more accurately.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994