Offset VSP's ? examples of their application to exploration in the Timor Sea

Abstract

BHP Petroleum has acquired seven Offset Vertical Seismic Profiles (OVSP's) since commencing exploration in the Timor Sea and all were acquired with the aim of defining more accurately the position of a bounding fault close to the well. OVSP's were acquired at Montara-1 and Bilyara-1 to assist in planning proposed sidetracks. In both of these cases there were significant differences between the fault location determined from the OVSP and the fault position interpreted on the seismic data. The discrepancies were in excess of 100 m in both cases and show that migrated 2D data do not have the accuracy that we sometimes assume. A comparison of two OVSP images using different receiver spacings (10 m and 20 m) showed that there was a significant improvement in resolution and clarity of the image when using the 10 m spacing. The OVSP's acquired at Montara-1 and Bilyara-1 provided a more accurate image of the subsurface close to the well bore and therefore were useful in planning options such as sidetracking the wells.