Subsurface Geometry of Potwar Sub-Basin in Relation to Structuration and Entrapment

Abstract

  The Potwar sub-basin is located at the northern margin of the Indian Plate, in Pakistan. It is filled with thick Infra- Cambrian evaporites, thin Cambrian to Eocene calcareous-siliciclastic sediments of Indian Plate and relatively very thick Miocene-Pliocene molasse deposits of the Indus Foredeep.
The Potwar fold region is a prolific area for hydrocarbon occurrence. The first commercial discovery from the Himalayan Foreland was made in Potwar subbasin at Khaur in 1914.This sub-basin hosts multiple structural leads. Hydrocarbons have been reported from different reservoirs. The Potwar sub-basin has experienced severe deformation during Himalayan orogeny in Pliocene to Middle Pleistocene. Regional unconformities in the stratigraphic profile of the basin reflect the severity of tectonic pulses.
To develop understanding of its structural style, fault types, decollement levels and influence of tectonics, transects in a grid fashion and a regional time structure map at the Base Miocene level have been developed. The mentioned time structure map is also more or less conformable for deeper horizons.
In the Potwar sub-basin occasionally, surface geological features mismatch the subsurface geometry. Interpretation of seismic reflection profiles has confirmed two decollement levels; the lower one in the Infra-Cambrian Salt Range Formation and the upper one at the interface of Eocene and Neogene molasse sediments. The structural architecture of the sub-basin has been strongly influenced by these decollements which led to offset and structural variation above and below these decollements. The lower decollement is a low angle thrust within Salt Range Formation that carries the entire sedimentary section to the south. Foreland verging Salt Range Thrust is the surface manifestation of this decollement. It is believed that the structures have been formed due to southward propagation of faults related to salt movement activated by southward thrusting of sedimentary wedge.
Attempt is made to relate hydrocarbon entrapment in distinctive structural domains. The orientation, styles and geographic distribution of structures and migration of hydrocarbon from the source beds to reservoir via fault planes is discussed. For the carbonate reservoirs it is considered that faults and flexures have enhanced their porosity by developing fractures. In addition, sub thrust plays and hydrocarbon plays on different leads and fairways have been identified. New identified plays include popup, sub thrust and “snake-headed” structures, salt-cored anticlines, and associated imbricate and triangle zones in different parts of this subbasin which are considered to be the primary drilling targets in future. It will open new horizones for hydrocarbon exploration in the Potwar sub-basin, Pakistan