Petrophysical Study of the Reservoir Heterogeneities in the Kingriali Formation (Triassic) Kohat-Potwar Sub Basin, Pakistan

Abstract

The Kingriali Formation is well exposed in various sections of Surghar Range. In Vanjari Section the Formation is well exposed having thickness of 117 meters. The subsurface thickness of Kingriali Formation encountered in the Isakhle-01 is 114 m and present at the depth of 3286- 3400 m. The current study is focused on correlation of surface and subsurface reservoir character of Kingriali Formation, lithofacies identification from well data and investigation of microfacies from surface data and its effect on porosity of Kingriali Formation. Field observations shows that the reservoir character of Kingriali Formation is well developed evidenced by vuggy porosity, brecciation and fractures. Petrographic studies were carried out on 39 representative samples which shows that dolomites are both Planar and Non-planar with crystals vary in size from very fine grain to coarsely crystalline, anhedral to euhedral, displaying tight packing with unimodal and polymodal distributions. Fine grained microfacies are unimodal while coarse grained microfacies are polymodal. Pervasive and Replacive dolomitization has occurred defined by various microfacies. The porosity is polymodal in Kingriali Formation including intercrystalline, intergranular, Intracrystalline, intragranular, microchannel, microfractures, solution enlarged fractures, and microvuggy porosity. Subsurface lithofacies analysis utilizing NPHI and RHOB logs crossplots indicates the presence of four lithofaices in Kingriali Formation namely Dolomite, sandy Limestone, shale/marl and sandy dolomite. The subsurface lithology determined for the Kingriali Formation is mostly Dolomite with thin interbeded shales and sandy limes tone. These lithofacies are also present in the outcrop and correlation is excellent. Well data and outcrop samlpes are correlated by utilizing the GR (gamma ray) log, Density log (RHOB) and Neutron log (NPHI). The shale horizons marked by High GR readings gives good correlation results with outcrop. High Density and low porosity zones identified from wireline logs are correlated with out crop compact horizons giving excellent results. The porosity correlation revealed that cementation highly occluded the porosity in some zones. The petrophysical analysis of Isa Khel-01 reveals eight promising reservoir intervals at various depths in ranging in thickness from 1.125 to 3.875 m with total thickness of 18 meters. Negligible difference in LLS and LLD, high MHI and dispersed BVW values indicates that these reservoir zones are water wet in Isakhel-01 having no commercial hydrocarbons. Based on outcrop studies, petrophysical analysis and their correlation, it is concluded that the Kingriali Formation possess good reservoir properties in the study area.