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<p>Evaporite-related sediments are vital in petroleum geology. They play multiple roles in petroleum systems by acting as seals and traps, preserving thermal maturity, and providing a favorable environment for organic matter accumulation. No previous studies addressed the biomarkers characteristics of organic matter in the Kohat basin Eocene sediments. This study evaluates the shale intervals of Eocene Bahadur Khel Salt and Jatta Gypsum in Kohat Basin, Pakistan, to assess their source potential, source and origin of organic matter input, thermal maturity, and depositional environment using geochemical proxies like TOC/Rock–Eval screening, gas chromatography-mass spectroscopy, and organic petrology. TOC results indicate that the Bhagan and Dharangi stratigraphic sections exhibit very good, whereas the Malgin section has excellent source rock potential. Extracted organic matter yields &gt; 1000&#xa0;ppm for the analyzed samples, showing good to excellent source quality. Rock–eval pyrolysis indicates the organic matter of Bhagan and Dharangi sections is mixed with&#xa0;Type II/III kerogen, while the Malgin section has Type III kerogen. Similarly, pyrolysis–gas chromatography data reveal mixed of type II/III and type III kerogen types. Thermal maturity of the analyzed rocks lies within the oil window with vitrinite reflectance (%R<sub>o</sub>) values range&#xa0;between 0.52 and 0.72% and T<sub>max</sub> between 430 and 438°C, except for the Malgin section showing anomalously high pyrolysis Tmax. Biomarker fingerprinting shows anoxic depositional settings with marine organic matter input, facilitating organic matter preservation. Palynofacies composition in the study area is dominated by structureless&#xa0;amorphous organic matter, suggesting oil-prone shaly intervals as potential hydrocarbon sources. Globally these shales correlate with the age equivalent Eocene Pabdeh Formation in Iran.</p>

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Organic geochemistry and petrology of organic-rich shales within Eocene evaporite sequences in the Kohat Basin, Pakistan

  • 期刊论文

  • 发表日期：

    2025年8月6日

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  • Naqib-Ullah,
  • Khairul Azlan Mustapha,
  • Nimat Ullah Khattak,
  • Mutasim Sami Osman,
  • Nasar Khan

摘要

Evaporite-related sediments are vital in petroleum geology. They play multiple roles in petroleum systems by acting as seals and traps, preserving thermal maturity, and providing a favorable environment for organic matter accumulation. No previous studies addressed the biomarkers characteristics of organic matter in the Kohat basin Eocene sediments. This study evaluates the shale intervals of Eocene Bahadur Khel Salt and Jatta Gypsum in Kohat Basin, Pakistan, to assess their source potential, source and origin of organic matter input, thermal maturity, and depositional environment using geochemical proxies like TOC/Rock–Eval screening, gas chromatography-mass spectroscopy, and organic petrology. TOC results indicate that the Bhagan and Dharangi stratigraphic sections exhibit very good, whereas the Malgin section has excellent source rock potential. Extracted organic matter yields > 1000 ppm for the analyzed samples, showing good to excellent source quality. Rock–eval pyrolysis indicates the organic matter of Bhagan and Dharangi sections is mixed with Type II/III kerogen, while the Malgin section has Type III kerogen. Similarly, pyrolysis–gas chromatography data reveal mixed of type II/III and type III kerogen types. Thermal maturity of the analyzed rocks lies within the oil window with vitrinite reflectance (%Ro) values range between 0.52 and 0.72% and Tmax between 430 and 438°C, except for the Malgin section showing anomalously high pyrolysis Tmax. Biomarker fingerprinting shows anoxic depositional settings with marine organic matter input, facilitating organic matter preservation. Palynofacies composition in the study area is dominated by structureless amorphous organic matter, suggesting oil-prone shaly intervals as potential hydrocarbon sources. Globally these shales correlate with the age equivalent Eocene Pabdeh Formation in Iran.