Subsalt depth imaging using 3-D VSP technique in the Ras El Ush field, Gulf of Suez, Egypt

Abstract

In 1995 oil was discovered in the pre-Miocene Matulla and Nubia Sandstones in the Ras El Ush field, Gulf of Suez, Egypt. The discovery was based on an aeromagnetic anomaly from a basement high. After drilling several delineation wells, based on a geological model, it became evident that the field is very complex as it is broken into tilted and rotated compartmental blocks by two perpendicular fault systems. Also the 2-D seismic data were of poor quality beneath the thick Miocene South Gharib Evaporite. Since part of the field lies below shallow-water, 3-D seismic was considered to be too costly. When a delineation well did not encounter the reservoir, due to an unanticipated fault, a 2-D walkaway Vertical Seismic Profile (VSP) was acquired. It clearly revealed the presence of a cross fault. The success of the 2-D VSP in imaging the fault led to the acquisition of the first Middle East 3-D VSP survey in the following well. A downhole, tria-axial, five geophone array tool was used to acquire the 3-D VSP. The 3-D volume of the final migrated VSP data provided the means for the reliable mapping of horizons beneath the South Gharib Evaporite. These maps improved the definition of the field and helped detect previously unrecognized prospective blocks. Four further successful delineation wells confirmed the 3-D VSP interpretation.