Hydrocarbon Prospects of Karak anticline, NW Pakistan: Myths and Facts

Abstract

 The Karak anticline (KA) is a prominent structural high of the southern Kohat Basin in northwest Pakistan. In recent past the Kohat Basin has emerged as a prolific hydrocarbon bearing province of the country. In 1977- 78,Texas Gulf drilled an exploratory well Karak-1 to test Eocene carbonate reservoir. The well was abandoned within Siwaliks at the depth of 4459 meters. Since then the exploration chapter remained closed on the assumption that Siwaliks are enormously thick and the reservoir is too deep. This paper discusses issues related to hydrocarbon prospectivity of the KA incorporating redefinition and reinterpretation of the data. Current field investigations reveal that the KA is about eighteen kilometers long, running ENE-WSW, is a well-defined, slightly asymmetrical fold with a relatively broad crest. The closures to the NW, west and along the southern flank are well marked. The fold is cut on the north eastern boundary by the Mitha Khel Fault, which is steeply north dipping. The attitude data on the limbs of KA suggest the presence of a subcrop, south facing thrust fault. The seismic definition of the fold is in harmony with the surface data and depicts two prospective highs at the level of Eocene related to subcrop fault: one in the hanging wall with Eocene top at 3300m depth from ground level and the second one in footwall with Eocene top at 4700m depth from ground level. Geo-seismic transect constructed across the KA depicts that Karak-1 well was placed around 800msouth of the hanging wall prospect and penetrated the steeply dipping forelimb, the subcrop fault and repeated sequence of Siwaliks and was abandoned approximately 200m above the footwall high. Reported oil and gas seepages, kerogen shales (oil shales) associated with gypsum, faint film of oil (about 1 mile NW of Karak), and discoveries in the adjoining areas indicate that KA lies within the hydrocarbon province and, since the anticline is bounded by well-marked structural lows, it would be expected that any petroleum generated in the structural lows would have migrated updip, into the likely reservoirs in the anticline. The Bannu Plain is the hydrocarbon kitchen for KA. The reservoir rocks beneath the KA include Eocene Sakesar Limestone and Paleocene Lockhart/Hangu formations which are the main producing horizons of the oil fields in adjoining areas.