Reservoir evaluation of the Eastern Tethyan Middle Eocene Larger Benthic Foraminifera dominated carbonates from Pakistan.

Abstract

The Eocene Large Benthic Foraminiferal limestones of the Tethyan margins are proven hydrocarbon reservoirs. This study examines the controls on porosity and permeability development of the producing Middle Eocene Pirkoh Member of the Eastern Tethys using well core data from Well Zx-01 from the Punjab Platform area, Central Indus Basin, Pakistan. The standard petrographic and core analysis techniques were adopted to determine these controls. Petrographic studies revealed three microfacies comprising bioclastic packstone to floatstone, bioclastic wackestone and mud-wackestone microfacies. The allochems belong to benthic foraminifera including Nummulites sp., Discocyclinids sp.., Coralline algae, Assilina sp., Lockhartia sp., Coralline algae and Gastropods along with the other bioclastic debris. These allochems were found embedded in the micritic matrix with extensive cementation of calcite. The diagenetic events included cementation, compaction, dolomitization, fracture filling and neomorphism. Based on microfacies, the environment of deposition is interpreted to be a high-energy zone of the middle ramp in shallow marine environment. Subsurface core indicates that the porosity of the reservoir is low to high ranging between 6 to 27.47% and permeability is found low to fair ranging between 0.03 to 7.73 mD, found closely related to microfacies and diagenetic modifications. This study is a first attempt to document the reservoir facies of the non-reefal carbonates of the Pirkoh limestone that would help to exploit the reservoir's potential of the carbonate platforms developed in similar settings worldwide.