Depositional, diagenetic, and sequence stratigraphic constrains on reservoir characterization: a case study of middle Jurassic Samana Suk Formation, western Salt Range, Pakistan

The present study integrates microfacies and diagenesis to construct a sequence stratigraphic framework and evaluate petroleum reservoir prospects of the middle Jurassic Samana Suk Formation, exposed in the western Salt Range, Pakistan. The rock unit is mainly composed of limestone, and marls with minor sandstone lithofacies. Based on the detailed petrographic, scanning electron microscopic, and energy-dispersive X-ray spectroscopic data, a total of four microfacies are identified, suggesting deposition of Samana Suk Formation in three different depositional environments including open marine, lagoon, and beach settings. Based on interpreted sea-level fluctuations from the microfacies data, a sequence stratigraphic framework is constructed. The Samana Suk Formation has witnessed one 2nd-order local cycle and 3rd-order sequences. The 3rd-order sequence holds three Highstand Systems Tracts (HSTs), two Transgressive Systems Tracts (TSTs), and a Lowstand Systems Tract (LST). The LST sandstone deposits show relatively higher microscopic porosity than the carbonates of HST and TST. The diagenetic modifications including compaction, shallow micritization, and heavy cementation have diminished the porosity of TST and HST limestone deposits. However, the effect of such diagenetic alterations is relatively less pronounced in LST deposits. The sea-level curve constructed from the local depositional pattern mismatch with the global sea-level curve probably because of the effect of local tectonics. The integrated porosity measurements based on outcrop, petrography, and scanning electron microscopy data revealed poor to moderate reservoir potential for the limestone and sandstone lithofacies of the studied rock unit, respectively.