Microfacies and Diagenetic Analysis of Chorgali Carbonates, Chorgali Pass, Khair-E-Murat Range : Implications for Hydrocarbon Reservoir Characterization (T-1410) (MFN 4997)

Abstract

The Chorgali Formation of Eocene age is composed of light to dark grey, thin to medium bedded limestone with shale intercalations, which is exposed in the Chorgali Pass, Khair-e-Murat Range of the Northren Potwar Deformed Zone (NPDZ). Lower contact of Choragli Formation is conformable with Sakesar Limestone, which is dark grey in color, thick to massive bedded and highly fossiliferous and upper contact is conformable with Kuldana Formation. In this study, the exposed stratigraphic section along axis of anticlinal structure is logged and sampled with a total of 6 samples. Three microfacies recognized from petrographic studies are: Bioclastic Wackestone-Packstone Microfacies, Mixed Bioclastuc Wackestone-Packstone Microfacies and Larger Foraminiferal Wackestone-Packstone Microfacies. The depositional texture and faunal association suggested that the microfacies represents deposition in a low energy, distally steepened ramp setting. Due to the deposition in low energy environment the ratio of lime mud is more than bioclasts, due to which the primary pore spaces in the formation are negligible. The analysis further showed that the Chorgali Formation was also subjected to various diagenetic changes, mainly showing the compaction, stylolitization, aragonite to calcite transformation (neomorphism), tectonically induced fracturing and calcite veins passing from marine diagenesis to meteoric diagenesis through burial diagenesis. Fracturing due to later tectonic deformation have had a major influence on porosity generation and in some cases fracturing and stylolites together, have had an important effect on permeability values. The petrophysical analysis shows that the Eocene Chorgali Formation is very good reservoir with average hydrocarbon saturation of 80 %.