STRUCTURAL REFINEMENT OF THE SOUTHERN KOHAT BASIN AND ADJOINING AREAS: IMPLICATIONS FOR HYDROCARBON POTENTIAL OF THE KOHAT BASIN, PAKISTAN

Abstract

This research work constitutes southern part of the Himalayan Foreland Basin which is one of the largest and dynamic terrestrial basins. The tectono-stratigraphic architecture of the foreland basin has been well understood because of detailed mapping The research area's stratigraphy comprises of Cenozoic platform sedimentary rocks that are unconformably overlain by Plio-Pleistocene Siwalik Group deposits. Structural studies of the major part of Kohat basin have been undertaken by various researchers in an attempt to understand the Himalayan induced deformation and kinematics of the region. However, the southern Kohat Basin remains less understood, due to the non-availability of vintage seismic and well data for research. The surface structural style within the Kohat Basin is disharmonic below the Eocene package characterized by a south directed imbricate thrust system emanating from the regional basal decollement and associated fault propagation/bend folding. The structural geometry nicely translates into the older units and is constrained by the idealized fault propagation folding model. It includes a series of south-verging imbricate thrust faults originating from the regional basal dectachment. Blind back thrusts are commonly associated with the major fore thrusts. The entire tectonic slab of the study area is exposed along the Surghar Fault uplifting rocks as old as the Permian along the range front. Geo-seismic and geological transects coupled with structural modelling have enabled to work out 35% shortening induced by the compressional stresses. These shortening estimates and the variable structural style further led to an understanding that the basal decollement underneath the Kohat Basin may not be located in the Precambrian Salt. The lack of salt may be a reason for the more internal deformation of the Kohat Basin as greater shear resistance was provided due to the involvement of brittle rocks in the horizontal translation of the Kohat tectonic slab above crystalline basement as compared to that of the Karak Trough region. This study has further enabled that the vicinity of Karak Fault Zone is the area where promising structural traps at top Lockhart level are predicted which is a proven reservoir in Kohat Basin.