Downhole Seismic Array Applications for Reservoir Delineation in Pakistan

ABSTRACT The key oil producing formations in northern Pakistan are significantly affected by extensive and complex faulting. The detection of these faults has been an objective of offset Vertical Seismic Profiles (VSPs) for the past several years. Recently, two land walkaway VSP surveys were undertaken in a cased development well 2750m deep. The objective was to delineate the fault pattern in the Cambrian sandstone and Eocene limestone reservoir formations. A single setting of the eight-receiver Downhole Seismic Array (DSA*) tool enabled the data to be acquired in a single run per line, using two vibrators operating at a regular shot point spacing of 75m, with a maximum offset of 3km from the well. The shot points were located on two orthogonal lines centered at the well. An important factor of the acquisition, affecting the interpretation, was the correct determination of static corrections which were obtained by running refraction spreads. These were then applied to the VSP data prior to velocity filtering. The final interpretation involved the integration of surface seismic, VSP, walkaway VSP and well logs, including the Formation MicroScanner (FMS*) log. The walkaway VSP results appeared to be superior in resolution when compared to the surface seismic section. In addition, they delineated the fault pattern around the well, thus giving a better understanding of the complex tectonic framework of the area. The major reverse fault crossing the wellbore and causing an abnormal thickness of the Cambrian sandstone was clearly identified on the zero-offset VSP, and its lateral extension away from the well in all the directions was imaged by the walkaway VSPs.