Sequence Stratigraphy, Facies Analysis, Depositional Model And Reservoir Potential Of Sakesar Limestone, Eastern Salt Range, Upper Indus Basin, Pakistan

Abstract

The sedimentological study for Early Eocene Sakesar Limestone in part of the upper Indus Basin is carried out. The stratigraphic sections include Mustafa Abad, Badshah Pur, and Tatral section located along the Kallar Kahar – Choa Sedan Shah road section of the Eastern Salt Range. Twenty-eight thin sections were prepared for microfacies analysis. Detailed sedimentological investigations based on outcrop observations and thin section evaluation show that lithologically light to medium grey nodular, massive, fractured, hard, and fossiliferous limestone of Sakesar limestone is mainly comprised of the wackestone and packstone microfacies and fine-grained matrix with abundant larger and smaller benthic foraminifera. Sakesar Limestone contains abundant larger benthic foraminifera as compared to planktons and nanofossils belonging to genera Nummulites, Alvolina, Lockhartia, and Assilina. This suggests deposition in a shallow shelf environment. Microfacies of Sakesar Limestone (18 m) in the Mustafa Abad section include bioclastic wackestone (MsSMF-1), benthic foraminiferal packstone (MsSMF-2) and Nummulitic packstone (MsSMF-3). These microfacies were deposited in the middle to outer, inner, and middle portions of the shelf respectively. Microfacies of Sakesar Limestone (18.7 m) in the Badshah Pur section include Numulitic wackestone (BsSMF-1) and Millioloidal wackestone (BsSMF-2). These microfacies were deposited in the middle portion of the shelf. Microfacies of Sakesar Limestone (6.8 m) in the Tatral section include bioclastic wackestone (TsSMF-1). These microfacies were deposited in the middle portion of the shelf. Deposition models are prepared for these three stratigraphic sections. Diagenetic features observed in Sakesar Limestone include phenomena of neomorphism, dissolution, cementation and compaction, and calcite-filled fractures. Reservoir potential for Sakesar Limestone is evaluated through visual porosity estimation technique using twenty-eight stained thin sections and wireline log analysis of Amir Pur-1 well. No visual porosity is observed in thin sections except two thin sections of bioclastic wackestone (TsSMF-1) in the Tatral section. Visual porosity in TsSMF-1 is estimated to be 14 %. Wireline log interpretation also depicted negligible porosity (2.2 % neutron porosity, 6.47 % average porosity, and 3.47 % effective porosity) in the zone of Sakesar Limestone in the Amir Pur-1 well. The diagenetic features including compaction, neomorphic and calcite-filled fractures might have choked the pore throats hence leaving no space for fluid accumulation. Sakesar Limestone may be produced where fracture porosity is developed considerably in parts of Upper Indus Basin. Sequence stratigraphy of Sakesar Limestone is evaluated based on microfacies analysis including outcrop observations and using Gamma Ray log trends. Sakesar limestone represents a pro gradational facies depositional style which is indicative of deposition in High stand system tract (HSST)