Reservoir Quality Assessment of The Late Permian Chhidru Formation, Salt and Surghar Ranges, Pakistan Muhammad ZAHIR 1 , Muhammad AWAIS 2 , Mukhtiar GHANI* 3 , Mohibullah MOHIBULLAH 4 ,Osmans . HERSI 5 , Irfan U. JAN 6 , Sajjad AHMAD 7 ,and Hamid IQBAL 8 1 Centre of Excellence in Mineralogy, University of Balochistan 2 Department of Geology, University of Swabi 3 Geological Survey of Pakistan, HQ, Quetta 4 Department of Geology, University of Balochistan 5 University of Regina, Geology 3737 Wascana Parkway Regina, SK, CAN S4S 0A2 6 National Centre of Excellence in Geology, University of Peshawar 7 Department of Geology, University of Peshawar 8 Oil and Gas Development Company, Islamabad *Office No: 14, Block-7, Geological Survey of Pakistan, Sariab Road, Quetta. mukhtiarghani@uop.edu.pk Abstract: The late-Permian succession of the Upper-Indus Basin is represented by carbonate dominated Zaluch Group, which consists of Amb, Wargal and Chhidru Formations. These formations accumulated on the southwestern shelf of the Paleo-Tethys Ocean, north of hydrocarbon-producing Permian strata of the Arabian Peninsula. The reservoir properties of mixed clastic- carbonate Chhidru Formation are evaluated based on Petrography, Scanning Electron Microscopy (SEM), Energy Dispersive X- rays-Spectroscopy (EDX) and X-ray Diffraction (XRD) techniques. The diagenetic features, ranging from marine (isopachous fibrous calcite, micrite), through meteoric (blocky calcite-I, neomorphism and dissolution) to burial (poikilotopic cement, blocky calcite-II-III, fractures, fracture-filling and stylolites) are recognized. Major porosity types included fracture and moldic, while inter-and intra-particle porosities also existed. The visual porosity from 1.5-7.14% with an average of 5.15% was recognized. The sandstone facies (CMF-4) had the highest average porosity of 10.7%, while siliciclastic grainstone microfacies (CMF-3) showed average porosity of 5.3%. The siliciclastic mudstone microfacies (CMF-1) and siliciclastic wacke-packestone microfacies (CMF- 2) showed the lowest porosity of 4.8% and 5.0%, respectively. Diagenetic processes like cementation, neomorphism, stylolitization and compaction have reduced the primary porosities. However, processes of dissolution and fracturing have produced secondary porosity. On average, the Chhidru Formation in Nammal Gorge (Salt Range) showed promising and at Gula Khel Gorge (Trans-Indus) lowest porosity. Key words: Upper Permian, Chhidru Formation, Diagenesis, Reservoir, Salt Range, Surghar Range, Pakistan 1. Introduction The Permian sedimentary strata form a part of the thick Pre-Cambrian to Recent sedimentary succession of the Upper Indus Basin, Pakistan (Kummel, 1970). The Permian succession is represented by the Nilawahan and Zaluch groups exposed in the Salt, and Trans-Indus Surghar and Khisor ranges (Kazmi and Abbasi, 2008; Shah, 2009; Fig. 1). The Permian Tethyan carbonate-siliciclastic successions, exposed in Salt Range and Trans-Indus ranges of Pakistan bear sedimentological and paleontological similarities with time-equivalent successions from Arabia, Turkey and Iran (Stephenson et al., 2013; Ahmad et al., 2016; Ghani, 2017; Jafarian et al., 2018). The Permian succession represents prolific hydrocarbon bearing reservoirs in the Middle East, Arabian Peninsula, and Persian Gulf. These reservoirs are represented by Khuff, Akbarah, Unayzah, Dalan and Gharif, formations (Stephenson et al., 2013). The Permian successions of these region have been extensively studied for hydrocarbon prospects (e.g. Egeran and Tasman, 1951; Martin et al., 2008; Edgell, 1977; Stephenson et al., 2003; Stephenson and Al-Mashaikie, 2010; Stephenson et al., 2013). In Pakistan, the Permian succession has been studied for biostratigraphy, paleogeography and sedimentology (Jan et al., 2009; Shah, 2009; Jan and Stephenson, 2011; Ahmad et al., 2015) however the diagenetic and reservoir studies on these successions have not been conducted. The current research focusses to unveil the reservoir potential of the Chhidru Formation. This article is protected by copyright. All rights reserved. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/1755-6724. . 14764