Reservoir Quality Assessment of the Upper Permian Chhidru Formation, Salt and Surghar Ranges, Pakistan

Abstract

The Late Permian succession of the Upper Indus Basin in northeastern Pakistan is represented by the carbonate‐dominated Zaluch Group, which consists of the Amb, Wargal and Chhidru formations, which accumulated on the southwestern shelf of the Paleo‐Tethys Ocean, north of the hydrocarbon‐producing Permian strata of the Arabian Peninsula. The reservoir properties of the mixed clastic‐carbonate Chhidru Formation (CFm) are evaluated based on petrography, using scanning electron microscopy (SEM), energy dispersive x‐ray spectroscopy (EDX) and x‐ray diffraction (XRD) techniques. The diagenetic features are recognized, 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). Major porosity types include fracture and moldic, while inter‐ and intra‐particle porosities also exist. Observed visual porosity ranges from 1.5%–7.14% with an average of 5.15%. The sandstone facies (CMF‐4) has the highest average porosity of 10.7%, whereas the siliciclastic grainstone microfacies (CMF‐3) shows an average porosity of 5.3%. The siliciclastic mudstone microfacies (CMF‐1) and siliciclastic wacke‐packestone microfacies (CMF‐2) show the lowest porosities 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 CFm in the Nammal Gorge, Salt Range shows promise and at Gula Khel Gorge, Trans‐Indus, the lowest porosity.