Structural Interpretation of Potwar Sub-basin, Western Lesser Himalayas, Pakistan

Shahina Tariq, Zia Ul Hasan Shah, Mohammad Zafar, Maas Siddiqui, Sidra Shahid, Salwa Waheed Pal, Ghayoor Ahmed, and Waqar Ahmed

The present study includes seismic and structural interpretation of Chak Naurang and Joya Mair area, which lies in the Potwar sub-basin in the of western lesser Himalayas in Pakistan. The main objectives include to delineate the subsurface structure and to locate the prospective zones in the study area and to assess the remaining recoverable reserves of Joya Mair Oilfield. The total estimated reserves of Joya Mair Oilfield are 23-25 million US barrels (mmbbl), whereas, at present the recoverable reserves are only 10.45 mmbbl. To achieve these objectives, reinterpretation of four seismic lines O/881-CW-06 (Strike line) and O/881-CW-08, O/881-CW-09, and O/881-CW-10 (Dip lines) was carried out. Four prominent reflectors namely Kamlial Formation (Miocene), the target horizon of Chorgali Formation (Top Eocene), a very thin layer of Permian age and the basement consisted of Pre-Cambrian Salt were marked. Two thrust faults were marked which showed fault-propagating folds in the form of anticlines of Chak Naurang and Joya Mair. A time contour map for the Upper Eocene Chorgali Formation was also developed which shows a long east-west oriented fault that is continued along the Pre-Cambrian basement

Upper Eocene Chorgali formation has been used for contouring and average velocities to find the depths of the formations for the geoseismic section. Well data of Joya Mair was used to mark the thicknesses of the individual formations on the geoseismic section from Cambrian to Recent. The geoseismic section showed that the area was structurally deformed due to salt decollement and compressional tectonic movements.

Restoration of the depth sections using Fixed Pole method for dip lines help to know the mechanism of the tectonic movements in the area. The major causes have been the thrust faults in the area that invoked the basement salt movement and eventually form the anticlines (as they truncated against the basement fault). Salt probably moved into these anticlines due to compressional movements indicating thin-skinned tectonics in the study area that doesnot involve the basement. Hydrocarbon traps were probably developed in these anticlines and thus the prospective zones for hydrocarbons in Chak Naurang and Joya Mair area. Furthermore, on the basis of this seismic and structural studies and correlating the results with availble gravity and magnetic data: it is concluded that Joya Mair thrust and Mengan thrust deformed the structural trap in Joya Mair area. It also indicates that Joya Mair anticline is not a single structure, rather, it is faulted into blocks. That is why the expected reserves potential of Joya Mair Oilfield are not recovered so far.