PETROGRAPHIC AND DIAGENETIC INVESTIGATION OF HABIB RAHI LIMESTONE, SULAIMAN BASIN, NORTH WAZIRISTAN, KP, PAKISTAN

Abstract

The early Eocene Habib Rahi Limestone of the North Waziristan, Sulaiman Basin has been studied for microfacies analysis, depositional model, diagenesis, and reservoir potential.This study is mainly focused on investigating and assessing reservoir quality using depositional and diagenetic processes. The Formation is composed of medium to thick-bedded, hard, highly fossiliferous, fractured limestone with interbedded shale and marls. Five microfacies have been identified based on the relative estimated ratio of allochems constituents and micrite. These microfacies are Mudstone, Wackstone, WackPackstone, Packstone, and Peloidal Grainstone and comprises larger benthic forams such as Nummulites, Assilina, Alveolina, Milliolids, Lockhartia, Orbitolites, and green algae. Based on mineral composition, fossil assemblages, and micritic matrix, the depositional environment for the Habib Rahi Limestone is interpreted as proximal inner ramp to distal middle ramp environment. The diagenetic processes observed in this study include micritization, cementation, dissolution, neomorphism, pyritization, mechanical and chemical compaction, stylolitization, sparitization, fracturing, and veins. The depositional, diagenetic, and deformation processes are the controlling factors of the distribution of porosity and permeability. The primary reservoir characteristics of this formation are highly affected by diagenetic processes developing it into a productive reservoir. Fracture and dissolution are significant mechanisms that increased the reservoir potential of the Habib Rahi Limestone at both the microscopic and macroscopic levels. In the studied section, the visually estimated average micro porosities of the Habib Rahi Limestone range from 0.5 to 2.1%. Intergranular porosity, dissolution porosity, moldic porosity, and fracture porosity are among the recognized porosity types.