Fault Definition Improvements in the West Abu El Gharadig Concession Using Prestack Depth Migration

By

Amir El Motany¹, Moumir Diab¹, Marwa El Badry², Nabil El Kady²

(1) Agiba Oil Company, N/A, Egypt (2) WesternGeco Data Services Middle East Ltd -- Cairo Data Processing, N/A, Egypt

 The West Abu El Gharadig concession is located in the Qattara depression in the North of the Western Egyptian Desert. The surface geology is complex, Sabkha deposits cover about 50% of the area of the study. Dyanamite was used to collect the data in this areas. The rest of the area is flat rocky desert, where vibroseis was used. We selected four lines, one of which was a strike line, for prestack depth migration. The area presents a geologic structure affected by an intense extensional faulting that deforms the sedimentary sequences. Faulting induces strong lateral velocity variations, creating false pull-up and sags in the seismic data which distort the true geologic structure. An exploratory well was drilled from an interpretation of time-migrated data, but the well could not confirm the interpreted fault locations. To reduce the risk of drilling, a velocity model was built guided by the well information and the seismic data. A 3-D velocity model was built to unify seismic and well information, and to tie the velocity model for the four lines. This velocity was used to run prestack Kirchhoff depth migration on the four selected lines. A second well was drilled from a re_interpretation of the fault location using the depth images. The second well confirmed the location of a major fault plane in the area. At the target depth of about 4000 ft, there is a 2000 ft displacement across the fault. Depths from the seismic data had a satisfactory tie with the well.