Microfacies analysis and reservoir characters of eocene carbonates of Khair-i-Murat Range, Northern Potwar Deformed Zone (NPDZ), Sub-Himalayas, Pakistan
Abstract
Khair-i-Murat Range is a fundamental part of Northern Potwar Deformed Zone (NPDZ), Sub-Himalayas. The geological investigations were carried out to understand the stratigraphy, sedimentology and tectonics of the area. The surface geology of the area is comprised of early Eocene to recent rock units. Eocene rocks namely; Margalla Hill Limestone, Chorgali Formation and Kuldana Formation are exposed along the Khair-i-Murat reverse fault. The petrographic study of Eocene carbonates of Chorgali Formation and Margalla Hill Limestone shows microfacies of various types. The Margalla Hill Limestone comprised of dolomitic, fossiliferous, nodular and fractured packstone to wackestone facies. The Chorgali Formation is dominated by sparsely fossiliferous, dolomitized and micritic limestone. The important microfacies of these carbonates are biomicrite, containing a variety of benthonic forams including different species of Assilina, Nummulities, Alveolina, Milliolids, Soritids and Textularia. The other common microfacies is dolomite. Cementation and supplementary alterations within the carbonate of these formations indicate that these are of early diagenetic origin. The major factor responsible for the development of secondary porosity and permeability in carbonate rocks was the fractures caused by intense structural deformation and development of thrust faults in Khair-i-Murat area. The presence of Eocene benthonic foraminiferal assemblages and dolomitization in the carbonate sequence of Margalla Hill Limestone and Chorgali Formation show the shallow marine, open shelf and intertidal to supratidal environments of deposition. The Early Eocene Margalla Hill Limestone, Chorgali Formation and Middle Eocene Kuldana Formation also indicate different transgression and regression periods of the Tethyan Ocean.
Introduction
The Khair-i-Murat area is the part of Potwar foreland sub-basin, which is formed by tectonic compression at the northern margin of Indian Plate (Gansser et al, 1964: Fig. 1). In Middle to Late Eocene, the uplifting of Himalayan Ranges began due to continentcontinent collision (Molnar and Tapponnier, 1975) and collision between the Indian-Eurasian plates cause structural deformation. Structurally, Potwar sub-basin is bounded by Margalla Hills in the north, left lateral Jhelum fault to the east, the Salt Range Thrust to the south and right lateral Kalabagh fault in the west (McDougall and Khan, 1990; Kazmi and Rana, 1982). The rocks of Potwar sub-basin represent thin-skinned compressional deformation. The cover sequence is deformed on the basal decollement of Salt Range Formation (Lillie et al., 1987). The Khair-i - Murat area lies in Northern Potwar Deformed Zone (Jaswal, 1990), and formations exposed in the area are entirely sedimentary in nature. These include Margalla Hill Limestone, Chorgali Formation, Kuldana Formation, Kamlial Formation and Murree Formation. In these formations Margalla Hill Limestone and Chorgali Formation are fossiliferous and contain wide variety of foraminifers. In Khair-iMurat Range, the carbonates of Chorgali Formation and Margalla Hill Limestone are exposed in the central part of the Khair-i-Murat anticline at Chorgali Pass (Fig. 2). On the basis of accessibility and continuity of exposure, a complete sequence of rocks along the northern limb of Khair-i-Murat anticline was selected for detailed petrographic studies. Chorgali pass is the type locality of Chorgali Formation. According to petrographic studies, the carbonate rocks are mainly composed of micrite, allochems and sparry calcite. Rock types are classified and these rock types are considered as microfacies. Initially Brown (1943) suggested the term microfacies and has been defined as “the total of all the paleontological and sediment logical criteria which can be classified in thin sections, peels and polished slabs”. This study is carried out to determine the microfacies and reservoir characteristics of Eocene carbonates (Margalla Hill Limestone, Chorgali Formation) of Khair-i-Murat Range, Northern Potwar Deformed Zone (NPDZ), Sub-Himalayas, Pakistan.
Source : Microfacies analysis and reservoir characters of eocene carbonates of Khair-i-Murat Range, Northern Potwar Deformed Zone (NPDZ), Sub-Himalayas, Pakistan