Sequence Stratigraphic Framework of the Jurassic Samana Suk Carbonate Formation, North Pakistan: Implications for Reservoir Potential

Abstract

The paper aims to establish a possible link between the sequence stratigraphy and reservoir potential of the Middle Jurassic carbonates of Samana Suk Formation, Kala Chitta Range, North Pakistan. The Samana Suk Formation is comprised of a monotonous sequence of thin- to thick-bedded bioturbated, cross-bedded, ripple marked, sandy, bioclastic carbonates with shale interbeds. The rock unit is the best analog of shallow to marginal marine carbonates due to its pronounced thickness, enhanced lithological variations, and diverse diagenetic features. To establish a possible link between the sequence stratigraphy and reservoir potential of the rock unit, the outcrop and petrographic features are used to record eight microfacies, deposited in a wide range of shallow to marginal marine environments including mudflats, lagoon, back shoals, and sand shoals. Based on the vertical stacking pattern of microfacies in a given time of deposition (170–160 Ma), one second-order local cycle (SLC-1), three regressive systems tracts (RSTs) and two transgressive systems tracts (TSTs) are delineated within the succession. The primary and secondary porosity of selected microfacies from each systems tracts are investigated using petrography, scanning electron microscopy, and energy-dispersive spectroscopy. The direct measurements of porosity and permeability of these microfacies types are carried out using high overburden pressure plug porosity and permeability analyses. Based on these investigations, higher reservoir potential is recorded for mudstones/wackestone microfacies of RST as compared to grainstone microfacies of TST. The higher porosity within RST and higher interconnectivity of available pores both in RST and in TST suggest that the Samana Suk Formation carries good reservoir potential.