--> --> High Stress Anisotropy Associated With the Highly Deformed Kohat Basin, NW Himalayas, Pakistan: Its Importance and Implication in the Hydrocarbon Exploration and Developmental Strategies

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High Stress Anisotropy Associated With the Highly Deformed Kohat Basin, NW Himalayas, Pakistan: Its Importance and Implication in the Hydrocarbon Exploration and Developmental Strategies

Abstract

This study describes the results of borehole vs in-situ surface stress field anisotropy and its dynamics for the optimisation of hydrocarbon exploration and exploitation in the highly deformed Kohat basin, located within the foothills of the Pakistani Himalayan orogenic belt having post Miocene over thrust deformation styles. Kohat basin has gone through several phases of compressional deformation related to Himalayan orogeny, leading to the development of numerous folds and complicated fault system where structures generally follows an east west trend with prominent swing towards west and southwest of the basin. The structural style of the Kohat basin is thin skinned, overprinted by transpressional deformation and thick succession of Eocene evaporites and shales produce disharmony between surface and subsurface structures. Till date more than 45 wells have been drilled in the area which resulted in 10 major discoveries with overall 65% success ratio though the seismic data is poor due to extremely low signal to noise ratio. Log data within the reservoir and non-reservoir section of the different wells drilled in the northern, central, western and north eastern part of the Kohat basin have been analyzed, which shows drastic shift in the orientation of the principle stress vectors creating stress anisotropy within the different detachment compartments and depths of the same field as well as with the natural fracture system. Moreover, all the stress fields data, induced fractures, break outs of the different wells suggest severe shift and disharmony in their orientations with in the same field of the entire Kohat basin influenced by regional compressional to transpressional episodes linked with the Himalayan orogeny and therefore is divided into three different tectonic stress vector regimes. The kinematic and dynamic analysis of the stress fields using the Geomechanics softwares within the borehole vs surface played a vital role in the hydrocarbon exploration and exploitation in the deformed Kohat basin. Working in highly deformed foreland basins, integration of the stress fields within the subsurface vs surface which entails different deformational events, enables to avoid damaging the natural fractures during drilling, predict optimum well trajectories and prudent fracture system associated with the trapping for the effective hydrocarbon production as well as optimise the drilling costs, wellbore stability and reduce the risk for the future drilling wells.