Microfacies, diagenesis and hydrocarbon potential of Eocene carbonate strata in Pakistan
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ORIGINAL ARTICLE
Microfacies, diagenesis and hydrocarbon potential of Eocene carbonate strata in Pakistan Umer Khitab1 · Muhammad Umar2 · Muhammad Jamil1,3 Accepted: 26 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The role of microfacies and diagenesis was focused to assess the hydrocarbon potential of Eocene carbonate strata exposed in Sulaiman–Hazara mountain ranges. Thin- to thick-bedded Eocene limestone exhibits wackestone, packstone, mudstone, and grainstone texture and comprises larger benthic forams such as Nummulite, Assilina, Lockhartia, Discocyclina, Orbitolites and Operculina. The accumulation of the strata occurred in lagoon, inner to deeper shelf. The microfacies and textural variations revealed stable shallow shelf conditions in early Eocene. The marine sedimentation ceased in Hazara and Salt ranges, but Kohat and Sulaiman regions remained under water during middle Eocene. The deposition of Habib Rahi Formation in deeper shelf attests to local scale normal faulting due to the northward movement of the Indian Plate. The Eocene reservoir zones were sourced by Patala and Ghazij Formations and capped by thick cover of fluvial shale, establishing favourable conditions for stratigraphic traps. Thick limestone packages of foraminiferal wackestone/packstone and dolomitized/dolomitic wackestone microfacies are regarded as estimable targets for hydrocarbons exploration in the region. Several diagenetic processes were observed in the studied limestone which includes: micritization, dolomitization, neomorphism, cementation, mechanical and chemical compaction, fracturing and dissolution which were caused in marine, meteoric and burial diagenetic environments. Compaction was the main factor for the destruction of primary porosity along with cementation effect. The porosity in the limestone was significantly enhanced by late stage dissolution, fractures enclosed by calcite cement and dolomitization. Keywords Eocene limestone · Microfacies · Shelf morphology · Diagenesis · Hydrocarbon exploration · Pakistan
Introduction Carbonates produce nearly 50% hydrocarbon (oil and gas) resources in the world on account of the tendency to impart multiple porosity system (Fan et al. 2007; Jiang et al. 2008). Microfacies analyses (e.g. Flügel 2004) along with diagenetic studies of carbonate rocks (e.g. Scholle and Scholle 2003) are useful to predict carbonate depositional environments as well as petroleum reservoir characteristics. The understanding of type, nature and geometry of the carbonate shelf is significant to establish reservoir units for oil and * Muhammad Umar [email protected] 1
Department of Earth Sciences, COMSATS University Islamabad, Abbottabad Campus, Islamabad, Pakistan
2
Department of Geology, FATA University Dara Adam Khel, Akhorwal, KPK, Pakistan
3
Department of Geosciences, Universiti Teknologi PETRONAS, 32610 Perak, Malaysia
gas exploration (Kenter et al. 2005; Santantonio et al. 2013; Ni et al. 2016). Diagenesis influences the primary texture, miner
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