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Depositional Setting and Reservoir Quality of the Lockhart Limestone (Lower Paleocene) in the Hazara-Kashmir Basin (NW Lesser Himalayas, Pakistan)

Abstract

The study area forms part of the Hazara-Kashmir basin, where Collision of Kohistan Island Arc and Indian Plate developed a crustal bulge at 67±2ma that subsided in Danian to deposit a mega sequence of Paleocene-Eocene succession. The Lockhart limestone (lower Paleocene) that represents thick carbonate ramp is a retrogradational/progradational (trangressive/highstand system tract) deposit, comprising mainly of nodular limestone with some shales. Based on biostratigraphic studies, paleoecology and geodynamic settings, 3D depositional model for Lockhart Limestone has been proposed. In addition, available well data from adjoining Kohat-Potwar sub-basin has been utilized to understand variation in surface to subsurface facies characteristics and petrophysical properties. In order to achieve above mentioned objectives, 661 samples from 7 selected outcrops were studied for conceptual depositional modeling and quantitative porosity studies. In addition, sonic, neutron and density logs were utilized to understand porosity distribution in the selected seven wells and their relationship to the outcrop studies. Based on petrographic studies, five microfacies (MF) are identified. These include; (i) Rotaliidae MF, (ii) Miscellaneidae MF, (iii) Discocyclinidae MF, (iv) Nummulitidae MF and, (v) Planktonic foraminifera/Algal/bioclastic MF in decreasing order of abundance. Inner to middle ramp depositional settings are inferred based on the paleoecology of the observed fauna. Late stage meteoric diagenesis is mostly evident in the studied samples, which is also confirmed from geochemical analysis (major and trace elements and stable isotopes). Petrophysical studies revealed that quantitative analysis of the studied samples show variable porosity values (i.e., 0.5 to 3.0%). However, petrophysical well data indicate tight to 4% porosity. In addition, plug porosity using Helium injection indicate 4.5% porosity. All of the above mentioned results indicate that there is little variation in porosity values from surface to subsurface dataset. Furthermore, such porosity is related to fracture development during Himalayan compression and Lockhart Limestone can be a potential reservoir in the Hazara-Kashmir basin.