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<p>The Late Early-Middle Eocene Kirthar Formation, exposed at the Shinki Post section of North Waziristan, Pakistan, records deposition on a carbonate ramp along the eastern Tethys margin. This study integrates field logging, petrography, Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM–EDS), and X-ray diffraction (XRD) to evaluate its depositional architecture, diagenetic evolution, and reservoir quality. Six microfacies were recognized, ranging from restricted inner-ramp mudstones (MFT-1) and Alveolina-rich wacke-packstones (MFT-4B) to open-marine, bioclastic wackestones (MFT-2), larger benthic foraminiferal wackestones (MFT-3), Assilina wacke-packstones (MFT-4A), and Nummulitic/Assilinal packstones (MFT-5A, MFT-5B), culminating in peloidal grainstones (MFT-6) of high-energy inner ramp. These microfacies represent an environmental gradient from tidal-flat to middle-ramp settings →  ~ 10–130&#xa0;m, with fossil assemblages dominated by Nummulites, Assilina, Alveolina, Miliolids, and Orbitolites. XRD analyses reveal a calcite-dominated mineralogy with minor quartz, feldspar, ankerite, and clays, reflecting limited siliciclastic input and localized burial alteration. Diagenetic processes including micritization, cementation, neomorphism, stylolitization, dissolution, and multiphase fracturing profoundly modified reservoir fabrics. Integrated petrographic and SEM analyses confirm a diverse pore system, with the development of secondary porosity types including vuggy, moldic, fracture, and intercrystalline pores being a key control on reservoir quality. A petrographic physical property analysis, based on visual data, reveals that these effective pores are commonly observed in the larger benthic foraminifera (LBF)-rich wacke-packstones. SEM–EDS analyses further reveal Fe-enriched calcite cements and clay-associated pore-throat reduction, highlighting distinct geochemical controls on pore evolution. Reservoir potential is fundamentally microfacies-dependent. Relatively better reservoir characteristics are developed in Assilina- and Nummulitic-dominated wacke-packstones and packstones (MFT-4A, MFT-5A, MFT-5B), where dissolution and late fracturing locally enhance pore connectivity. In contrast, peloidal grainstones (MFT-6), despite their high-energy depositional origin, are pervasively cemented and retain poor reservoir properties, while micritic mudstones (MFT-1, MFT-2) exhibit only microporosity with negligible permeability. Overall, the Kirthar Formation contains moderate to good reservoir intervals, with its heterogeneity governed primarily by the interaction between depositional microfacies and diagenetic overprints. These findings contribute to a broader understanding of Eocene carbonate ramp reservoirs across the Indus Basin and their correlation with other Tethyan systems.</p>

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Microfacies and diagenetic synthesis of the late early-middle eocene Kirthar Formation, Waziristan, Pakistan: implications for reservoir characterization

  • 期刊论文

  • 发表日期：

    2026年2月23日

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  • Fawad Ahmed,
  • Abdullah Khan,
  • Nowrad Ali,
  • Muhammad Zafar,
  • Ihtisham-ul-Haq,
  • Mujahid Ahmed Khan

摘要

The Late Early-Middle Eocene Kirthar Formation, exposed at the Shinki Post section of North Waziristan, Pakistan, records deposition on a carbonate ramp along the eastern Tethys margin. This study integrates field logging, petrography, Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM–EDS), and X-ray diffraction (XRD) to evaluate its depositional architecture, diagenetic evolution, and reservoir quality. Six microfacies were recognized, ranging from restricted inner-ramp mudstones (MFT-1) and Alveolina-rich wacke-packstones (MFT-4B) to open-marine, bioclastic wackestones (MFT-2), larger benthic foraminiferal wackestones (MFT-3), Assilina wacke-packstones (MFT-4A), and Nummulitic/Assilinal packstones (MFT-5A, MFT-5B), culminating in peloidal grainstones (MFT-6) of high-energy inner ramp. These microfacies represent an environmental gradient from tidal-flat to middle-ramp settings →  ~ 10–130 m, with fossil assemblages dominated by Nummulites, Assilina, Alveolina, Miliolids, and Orbitolites. XRD analyses reveal a calcite-dominated mineralogy with minor quartz, feldspar, ankerite, and clays, reflecting limited siliciclastic input and localized burial alteration. Diagenetic processes including micritization, cementation, neomorphism, stylolitization, dissolution, and multiphase fracturing profoundly modified reservoir fabrics. Integrated petrographic and SEM analyses confirm a diverse pore system, with the development of secondary porosity types including vuggy, moldic, fracture, and intercrystalline pores being a key control on reservoir quality. A petrographic physical property analysis, based on visual data, reveals that these effective pores are commonly observed in the larger benthic foraminifera (LBF)-rich wacke-packstones. SEM–EDS analyses further reveal Fe-enriched calcite cements and clay-associated pore-throat reduction, highlighting distinct geochemical controls on pore evolution. Reservoir potential is fundamentally microfacies-dependent. Relatively better reservoir characteristics are developed in Assilina- and Nummulitic-dominated wacke-packstones and packstones (MFT-4A, MFT-5A, MFT-5B), where dissolution and late fracturing locally enhance pore connectivity. In contrast, peloidal grainstones (MFT-6), despite their high-energy depositional origin, are pervasively cemented and retain poor reservoir properties, while micritic mudstones (MFT-1, MFT-2) exhibit only microporosity with negligible permeability. Overall, the Kirthar Formation contains moderate to good reservoir intervals, with its heterogeneity governed primarily by the interaction between depositional microfacies and diagenetic overprints. These findings contribute to a broader understanding of Eocene carbonate ramp reservoirs across the Indus Basin and their correlation with other Tethyan systems.