Depositional and diagenetic modeling of the Margala Hill Limestone, Hazara area (Pakistan): Implications for reservoir characterization using outcrop analogues

Understanding the depositional and diagenetic processes is critical for the modeling and resource potential of carbonate reservoirs. The Lower Eocene Margala Hill Limestone forms hydrocarbon resources in Pakistan characterized by heterogeneity; however, there is a lack of details about the diagenetic modifiers of this formation in the southeastern Hazara, Pakistan, and an integrated study of outcrop analogues can help in the better exploitation of these important resources. This work focuses on clarifying of depositional environment, diagenesis, and reservoir characterization in five stratigraphic sections of Lower Eocene Margala Hill Limestone in the southeastern Hazara, Pakistan. The integrated sedimentological and stable isotopes analysis were used in this study to better understand reservoir characterization and potential. This study revealed that the Margala Hill Limestone in the studied outcrops is medium-to thickly bedded, nodular in places, and fractured. Based on petrographic studies and allochems to matrix ratio, six microfacies (wacke-to packestone) were identified in the studied formation. The Margala Hill Limestone was deposited in the inner to middle ramp setting. The various types of cement observed in the Margala Hill Limestone are non-Ferroan cements (i.e., granular, blocky, drusy, and microcrystalline) and ferroan cements are also observed. The observed diagenetic processes are cementation, biogenic alteration, neomorphism, compaction, dolomitization, fracturing, dissolution, and nodularity. The diagenetic and stable isotopes (813C and 818O) studies indicate that the Margala Hill Limestone was affected by marine, meteoric, and burial diagenesis. The recorded porosity types include intragranular, intergranular, moldic, vuggy, and fractured porosity, indicating that the Margala Hill Limestone shows a good secondary reservoir for hydrocarbon accumulation. This study could prove a potential carbonate reservoir and help in the future exploration of the under-explored Eocene carbonates in the southeastern Hazara area, Pakistan.

Automated summary

This study focuses on the depositional environment, diagenesis, and reservoir characterization of the Lower Eocene Margala Hill Limestone in southeastern Hazara, Pakistan. Through sedimentological and stable isotopes analysis, it was found that the studied formation exhibits medium-to thickly bedded, nodular, and fractured characteristics. Six microfacies were identified based on petrographic studies. The Margala Hill Limestone was affected by marine, meteoric, and burial diagenesis, and shows various types of porosity, making it a good secondary reservoir for hydrocarbon accumulation.