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.
