MICROFACIES ANALYSIS AND RESERVOIR EVALUATION OF LOCKHART FORMATION, KOHAT SUB BASIN, PAKISTAN

Abstract

The present research study is dealt with the Late Paleocene-Early Eocene Lockhart Formation for detailed sedimentological investigations and reservoir characteristics in Kotal Pass section of Kohat sub-basin. A total of 25 samples were collected out of which 20 samples were thin sectioned and were studied for the petrographic investigations and diagenetic imprints. Based on the allochems to matrix ratio, a total of four microfacies are identified; Bioclastic foraminiferal wacke-packstone (KLL-1), Miscellanea wackepackstone (KLL-2), Foraminiferal wackestone (KLL-3) and Lockhartia rich wacksetone (KLL-4). Based on the faunal assemblages and depositional fabrics, these microfacies of the Lockhart Formation are interpreted to be deposited the in an open ramp/inner lagoon of Inner ramp to proximal open marine middle ramp settings of the homoclinal carbonate ramp environment. The faunal assemblage of the Lockhart Formation includes the larger benthic foraminifera (LBF) species of Miscellanea (Miscellanea miscella, Miscellanea juliettae other species of Miscellanea), Lockhartia (Lockhartia haimei, Lockhartia conditi, Lochkartia tipperi), Rotalia. Milliolid and very rare Ranikothalia sp. (Ranikothalia sindensis) along with the invertebrate fauna like echinoderm and dasycladales algae. Based on these index fossil of species of Lockhartia, Miscellanea and Ranikothalia sindensis, the Thanatian age of Late Paleocene is assigned to the Lockhart Formation. Among the diagenetic fabrics, the secondary porosities generated by dissolution (in the form of molds, vuggs) and microfractures (channel porosity) have played the pivotal role which enhance the reservoir characteristics of the formation envisaging the carbonates of the Lockhart Formation could be an acceptable reservoir. Based on the SEM analysis, among the identified microporosities, vuggy, dissolution and intergranular are the dominant porosities that can play effective roles to act the Lockhart Formation as a good reservoir. Based on the petorphysical analysis, a zone of interest between 2687m to 2710 m having thickness of 23m is marked in Maramzai-02 well. This zone is typified by low values of Gamma ray log and shale volume with average porosity and effective porosity values ranging from 7-9% and 4-6% respectively. This zone has low Sw which is 26.40% and relatively fair to good effective porosities of 4 to 6% with permeability in range of is 1-3md envisaging that the Lockhart Formation in Maramzai - 02 bears fair to acceptable reservoir properties.