The Cambrian sedimentology and reservoir properties of the hybrid Abbottabad Formation in the Kotli area, Sub-Himalaya, Pakistan: insights into the Proto-Tethys paleoenvironment

The study area (Kotli region) is situated within the Kashmir Basin, which is structurally influenced by the Hazara Kashmir syntaxis (HKS). It is imperative to assess the potential of the exposed Abbottabad Formation as a hydrocarbon reservoir, particularly in anticlinal structure (Tatta Pani), within the Kashmir Basin. The Abbottabad Formation, which is bounded by unconformities, consists of dolomite-chert interbeds (LA-1), stromatolitic dolomite (LA-2), and ankerite (LA-3) lithofacies. Petrographic analysis reveals three microfacies: fine crystalline dolomite (MF-I), algal mat-stromatolitic dolomite (MF-II), and ankerite (MF-III). Different features contribute to the reservoir characteristics of the formation, including fenestral and moldic structures, tidal channels, chopboard fractures, intergranular, and vuggy/karst porosities found within dolomite and dolomitic limestone. Additionally, intense structural deformation further enhances the reservoir qualities of the formation. X-ray diffraction, energy dispersive X-ray diffraction, and scanning electron microscopy have revealed the existence of distinct minerals in different facies. MF-I contains chalcedony, chlorite, pyrite, hematite, and siderite. Chalcedony denotes silica substitution, chlorite shows hydrothermal alteration, and pyrite implies anoxic conditions and rapid precipitation. MF-II suggests a connection between high biological production and development of algal mats. MF-III provides insight into the alteration processes and interactions between ankerite- and manganese-rich hydrothermal solutions. The Abbottabad Formation, originating from the Proto-Tethys Ocean at the start of the Cambrian, displays coarsening upward sediment layers and common dissolution and cementation. The formation was dolomitized through refluxal processes, revealing saddle dolomite, replacive minerals, coarser texture, selective dolomitization of rock fabrics, and unusual crystallographic orientations, indicating reflux mechanisms. The deepest subtidal facies were deposited in the lower part, while regressive intertidal to supratidal facies were deposited toward the top, eventually capped by a subaerial unconformity (SU). In Kashmir and adjoining basins, deposition of the Proto-Tethys Ocean ceased, and the area experienced uplift that persisted until the Danian age.