The surface oil showings of West Pakistan occur mainly in Eocene beds, more particularly in porcellaneous limestones laid down during a transition from open-water marine to brackish-water inland sea conditions; the changing environment may have played some part in the origin of the oil. The limestones and shales of the Eocene are overlain unconformably by great thicknesses of sandstones, clays, and conglomerates of Miocene to Pleistocene age known collectively as the Murree-Siwalik or Nimadric formation; these beds are of fresh-water origin and contain abundant vertebrate fossils.
Oil is being produced in the far northern part of the province from four open anticlinal folds in the outer foothills of the Himalayas. At Khaur, the first field to be found, most of the oil has been obtained from sandstones of the Nimadric system. The oil occurs in fissures in the sandstones at all depths from the surface to beds overlying the Eocene at about 5,400 feet. Some oil has been produced also from the Eocene limestones. The Nimadric deposits have no resemblance to any known source rocks and the oil in them must have migrated upward from the Eocene. It has been suggested that this migration occurred along fault planes, but the distribution of oil in the sandstones has no direct relationship with the known faults; it seems more probable that the migration took place through cracks and fissures in the crest of the acute anticline.
At another field, Dhulian, where the folding is more open, small quantities of heavy oil occur irregularly in the Nimadric rocks a few hundred feet above the Eocene; the inspissated character of this oil suggests that it occurred as surface seepages in Miocene time.
Almost the whole of the present production of West Pakistan is from the Eocene limestones; total production from the fresh-water Nimadric rocks has been about 4 million barrels.