Petrophysical and Microfacies Analysis of Sakesar Limestone, Balkassar and Fimkassar Oil Fields, Potwar Sub Basin, Pakistan

Abstract

The Potwar Basin, is the prolific hydrocarbon prospect zone due to a number of discoveries recorded in the Eocene carbonate succession which also include the Sakesar Limestone. The present study evaluated the Sakesar Limestone in terms of petrophysical properties with emphasis on microfacies analysis. An attempt was made to interpret Sakesar Formation using well logs data set and samples from exposed section of Sakesar Formation at Padhrar, Central Salt Range. The limestone of Sakesar Formation is planar to cherty and nodular in nature. Based on field and petrographic study, five microfacies have been identified 1) Nummulitic wackestone (PSL-1), (2) Benthic foraminiferal wackestone (PSL-2), (3) Lockhartia rich mud wackestone (PSL-3), (4) Bioclastic wacke- packstone (PSL-4), and (5) Miliolidal wackestone microfacies (PSL-5). Based on the allochems, orthochems and fossil assemblages, the identified microfacies of the Sakesar Limestone are interpreted to be deposited in the distal middle ramp to restricted inner ramp settings of homoclinal carbonate ramp environment. These microfacies include species of Nummulites, Lockhartia, Assilina, Alveolina and miliolids along with skeletal fauna of shallower fauna like echinoderms, Rotalia, algae and their bioclasts. Among the diagenetic fabrics, the dissolution and microfracturing enhance the reservoir efficacy and cause the formation of secondary (inter and intragranular) porosities like moldic, channel and fenestral porosities. Petrophysical analysis of Sakesar Formation encountered at Fimkassar-02 and Balkassar OXY-01A wells shows acceptable ranges of permeability and effective porosity. In Fimkassar-02, a 26m thick zone of interest at the depth (-3008m to 3034m) represents hydrocarbon saturation of 61%, the average effective porosity of 0.049% and permeability of 1.353 mD indicating good reservoir potential. In Balkassar OXY-01A, a zone of interest of -9m at the depth (-2496m to 2505m) indicates fair reservoir properties having 58% of hydrocarbon saturation with average permeability 0.06% and 0.02% effective porosity. The dissolution and microfracturing mainly reflect positive correlation with surface Fimkassar-02 and result in porosity generation in the Sakesar Limestone. In Balkassar Oxy-01A, the physical and chemical compactness negatively affect the reservoir aspect of the formation that could result by cementation and overburden pressure.