DIAGENETIC STUDIES OF JURASSIC CARBONATES IN KOHAT REGION, NORTH WEST HIMALAYAS, PAKISTAN: INSIGHTS FROM PETROGRAPHY AND GEOCHEMICAL STUDIES

The middle Jurassic Samana Suk Formation in Kohat region of the North West Himalayas in Pakistan is dominated by limestone and dolomite. Previously, the Formation has been thoroughly studied for stratigraphy, lithology, and paleogeography, however the diagenetic history is still not properly investigated. This study investigates the different diagenetic processes, particularly dolomitization, that have greatly altered the Samana Suk Formation's original mineralogy and reservoir properties. Fieldwork showed that the Samana Suk Formation is dominantly composed of limestone and dolomite. The original limestone is oolitic to bioclastic in nature. Dolomites are classified based on color and sedimentary features. Saddle dolomite cement, calcite cementation, and mechanical and chemical compaction have also been identified. Petrographic investigations show that the Samana Suk Formation has undergone many diagenetic changes. Micritization, bioturbation, and mechanical and chemical compaction cause fractures and stylolites. Calcite cementation includes isopachous, blocky, granular equant, fibrous, and dog tooth cementation, as well as dissolving processes in diverse diagenetic settings. Dolomites ranged in size and shape from finely crystalline to coarsely crystalline, planar euhedral to non-planar anhedral. The stable oxygen isotope values of these dolomites have depleted from their original marine signatures, implying burial-related fault-controlled dolomitization. Petrographic investigations show porosity evolution during depositional and diagenetic stages. The fenestral porosity evolved in peritidal carbonates during deposition, and burial dissolution and diagenetic dolomitization boosted the rock unit's reservoir potential, as confirmed by plug porosity and permeability tests