Microfacies Analysis and Diagenetic Overprints of Pirkoh Formation in Zindapir Anticline, Sulaiman Province, Dera Ghazi Khan, Pakistan: Reservoir and Source Rock Implications

Abstract

The present study is revealing the depositional facies, diagenetic overprints, reservoir properties and source rock potential of middle to late Eocene Pirkoh Formation in Zindapir Anticline of eastern Sulaiman Province, Dera Ghazi Khan, Pakistan. The microfacies analyses of Pirkoh Formation is representing total fifteen microfacies (MF1-MF15) categorized in three major facies assemblages i.e., inner, middle, and outer shelf based on their depositional setting on carbonate platform. The facies assemblages are typical representative of Tethyan carbonate shelf. The inner shelf facies are deposited on tidal setting (MF1), open lagoons (MF2), backreef shelf (MF3), reef patch (MF4), forereef shelf (MF5-MF7). The middle shelf facies are represented by (MF8-9) characterized by lime mudstone texture and low P/B ratio less than 5%, while the outer shelf facies are (MF10-15) consist of wackestone and packstone texture display high P/B ratio greater than 90% with co-occurrence of extensive echinoid fragments and large flat and well-preserved nummulites and orthophragminids in pelagic matrix. The diagenesis overprints of the Pirkoh Formation are studied in chronological sequence from early to late diagenetic stage. The early diagenetic stage is characterized by micritization, and dolomitization in shallow marine setting, which is followed by the first generation isopachous rim cementation during eogenesis. The mesogenetic phase of diagenesis display fabric selected dissolution in vadose zone of meteoric regime, which is subsequently followed by precipitation of blocky calcite cementation. The physical and mechanical compaction is found to be significant in burial stage of diagenesis resulting to suture seems and selective dolomitization along the fluid pattern. In the latest stage of diagenesis fracturing and telogenetic calcitization take place during tectonic uplift of late Eocene. The depositional facies of the Pirkoh Formation display reservoir potentiality index (RPI) as tight reservoir displaying air porosity (2.67 to 5.64 %) and air permeability (0.06 to 0.29 mD). The source rock results are indicating fear to good quality, but immature source rock with good TOC (1.28 to 1.58 wt.%), indigenous hydrocarbons, mix type II & III kerogen of more anoxic environment in pre-oil window. The Pirkoh formation is secondary reservoir based on the high-density fractures at the outcrop scale and can be a good source rock at certain depth of favorable thermal maturation.