Sedimentology and diagenesis control on reservoir quality of Sheikhan Limestone (Eocene) at the Panoba and Sheikhan Nala Section, Kohat Basin, Pakistan

The Sheikhan Limestone of Eocene age has been studied in detail covering its microfacies analysis, depositional setting, being a reservoir potential, and diagenetic sequence. For this purpose, two best exposed geological sections at Sheikhan Nala and Panoba section, Kohat Basin (Pakistan) have been selected. The techniques used for this research work are petrographic analysis, X-ray diffraction (XRD), cathodoluminescence (CL), pore plug porosity, and Scanning Electron Microscope (SEM) analysis. Typically, the Sheikhan Limestone is compacted, fossiliferous, and yellowish-grey in color, medium-thin bedded limestone with interbedded shale partings. The thickness of the studied limestone is 32 m to 44 m in the Sheikhan Nala and Panoba sections, respectively. A petrographic study reveals that calcite occurs in the form of micrite with a minor quantity of spare. Carbonate-filled fractures are common, as micro-veins. Detailed study reveals the diagenetic processes, and these include the imprints of micritization, neomorphism, cementation, compaction, dissolution, and fracturing. The dissolution and fractures enhance the reservoir characteristic, whereas clay minerals, micritization, compaction, and cementation reduces the reservoir potential of Sheikhan Limestone. The outcrop and petrographic studies have been used as a powerful tool to identify different microfacies. Based on all petrographic studies and microfacies analysis, it is suggested that the Eocene Sheikhan Limestone has been deposited under a shallow neritic inner shelf environment. Dissolution and fractures enhance the reservoir characteristics, whereas micritization, cementation, and compaction reduce the reservoir quality.

Automated summary

The Sheikhan Limestone of Eocene age was studied in detail, including its microfacies analysis, depositional setting, reservoir potential, and diagenetic sequence. Petrographic analysis, X-ray diffraction, cathodoluminescence, pore plug porosity, and Scanning Electron Microscope analysis were used in this research. The Sheikhan Limestone is compacted, fossiliferous, and yellowish-grey in color, with interbedded shale partings. Petrographic study revealed the occurrence of calcite in the form of micrite and carbonate-filled fractures. Diagenetic processes such as micritization, neomorphism, cementation, compaction, dissolution, and fracturing were identified. The dissolution and fractures enhance reservoir characteristics, while clay minerals, micritization, compaction, and cementation reduce reservoir potential. Based on all petrographic studies and microfacies analysis, it is suggested that the Eocene Sheikhan Limestone was deposited in a shallow neritic inner shelf environment. This study provides important insights into the geology and petroleum resources of the region.