Mineralogical Analysis of Paleocene Shales from Western Salt Range and Hazara Area, Pakistan

Abstract

  The Paleocene shales of Patala Formation exposed in western Salt Range (Khairabad Section) and its time-equivalent Kuza-Gali Shales from southern Hazara area have been analyzed and compared for the source rock characteristics, based on their mineralogical compositions, diagenetic changes and hydrocarbon generating potential during basin evolution. Qualitative and semi-quantitative analyses were carried out using X-ray diffraction, Coulometric, Scheibler, and scanning electron microscopic techniques for mineral identification and to estimate their concentrations in the sediments. The samples from western Salt Range are mostly calcareous claystones, whereas the sediments from Hazara area can be classified as sandy claystones on the basis of their grain size and composition. The main constituents of the sediments are: calcite, quartz and clay minerals; however, dolomite has also been detected in a fewsamples from Khairabad section. The organic matter in the sediments ranges from 0.5 to 1.2%.
The principal clay minerals identified in the <2 micron fraction are mixed-layer illite/smectite, chlorite/smectite, illite, kaolinite and chlorite, in both the areas. In the Salt Range sediments, randomly stratified mixed-layer illite/smectite is dominant; small amounts of kaolinite and chlorite are also present. The sediments from Hazara area contain mainly corrensite and illite/smectite mixed-layers. However, the samples belonging to northernmost section are mainly composed of illite.
The clay minerals have also revealed the burial diagenetic history of the sediments indicating, probably acid pore water environments in early stage of deposition and increasing alkaline pore water conditions with increase in burial depth. Presence of mixed-layer clay minerals in southern and central part of the study area, represents an intermediate phase of burial diagenesis, Whereas, illite, the principal clay mineral present in northern part, is an indication of dehydrated phase.
The sediments, in both the areas, contain a considerable amount of swelling clays which might have acted not only as a catalyst, generating hydrocarbons, but also might have facilitated in migration of oil and gas in Potwar basin. In Hazara area, apparently, the deeper burial would have produced condensate and eventually gas, which might have been migrated.