MICROFACIES ANALYSIS AND DIAGENETIC ALTERATION IN JURASSIC CHILTAN FORMATION, ISHA SECTION, NORTH WAZIRISTAN, KHYBER PAKHTUNKHWA, PAKISTAN: IMPLICATION ON RESERVOIR PROPERTIES

Abstract

The research study area is situated in the axial belt North Waziristan, Khyber Pakhtunkhwa, Pakistan. Research is based on petrographic analysis comprising of microfacies and diagenesis study. The Chiltan Formation exposed in the Isha section is having a thickness of 60 meters, with the limestone and dolomite alternating lithology. The field investigation revealed that the formation at the study area is dark grey colored limestone with the some repetition of the dolomite also having color dark grey but at some interval brownish colored limestone is also recorded. The formation contains broad networks of calcite veins, fractures, stylolites, calcite veins with the stock work geometry, dissolution, butcher-chop weathering. The microfacies analysis revealed the presence of five (5) microfacies; Mudstone (CMF1), Peloidal Wackestone (CMF2), Peloidal Packstone (CMF3), Bioclastic Mudstone (CMF4) and Ooidal Grainstone (CMF5). The depositional model is developed on the analyses revealing that; CMF1, CMF2 and CMF3 deposited in the lagoonal environment; CMF4 deposited in peritidal zone and; CMF5 deposited in the shoal environment. Diagenesis revealed the presence of the micritization, fractures, stylolites, pyritization, dissolution, dolomitization and cementation. The paragenetic sequence is developed for Chiltan Formation revealing the history from the marine stage deposition to the uplifting stage. The SEM analysis revealed the presence of porosity at a nano-scale, stylolites and the dolomitization was also present in the formation. The effect of diagenesis on reservoir revealed that the porosity and permeability is not present at a high level to call the formation a good reservoir as the porosity is built due to the presence of the features like fractures, stylolites dissolution and dolomitizaton etc. but the porosity and permeability is killed by the features and processes like cementation and pyritization. Hence, based on the petrographic study linked to the observation of porosity and permeability analysis, the Chiltan Formation is a poor quality reservoir rock in the Isha section.