Impact of Fractures and Diagenesis on Reservoir Potential of Inner Ramp Paleocene Carbonates Exposed in Western Part of the Lesser Himalayas of Pakistan

The outcrop investigations provide a better comprehension to interrelate facies-diagenesis and fracture networks for the evaluation of reservoir potential of the carbonate rocks. In this paper, we targeted Kahi-Section (Nizampur Basin) and Peeran Tangai-Section (Kalachitta Range) to analyze structural-kinematics, Discrete Fracture Network Modelling, microfacies identification and diagenesis to interpret their impact on reservoir potential of Lockhart Limestone (Paleocene). The structural grain within the study area mostly represents the typical east-west trending tight to overturned folds and north-dipping thrust faults that mimic the north-south Indo-Eurasian collision. However, a second phase of deformation related to east-west compressions also identified which rotated the axes of preexisting structures. Fracture analysis revealed that extensional fractures are oriented at high angle to bedding and are differentiated into three orthogonal sets trending northeast-southwest, northwest-southeast and east-west, whereas, the shear fractures formed two conjugate sets trending northeast-southwest. The Lockhart Limestone was deposited in the inner ramp setting and microfacies types are packstone, wackestone and wacke-packstone with seven sub-microfacies types. It has been identified that the Lockhart Limestone has the heterogeneous distribution of diagenetic and tectonic features throughout its extent. The observed diagenetic sequence is micritization, calcite cementation, dissolution, neomorphism, pyritization and compaction. The results highlight that open and partially filled fractures may provide an interconnected network to promote fluid mobility, leading to higher values of fracture permeability. The porosity values of the pore matrix were greater than fracture, resulting a significant impact on reservoir storage capacity. In contrast, a negative impact on reservoir potential has been shown by stylolites, veins and dissolution seams. However, based on the overall studies, the Lockhart Limestone revealed the prospect of a good reservoir unit in the study area.