Petroleum and Coal Pet Coal (2017); 59(2): 135-145 ISSN 1337-7027 an open access journal Article Open Access PETROPHYSICAL EVALUATION AND RESERVOIR QUALITY OF ILAM FORMATION (LATE CRETACEOUS), AHVAZ OIL FIELD, DEZFUL EMBAYMENT, SW IRAN Mohammad Sadegh Ravanshad 1 , Bahman Soleimani 2 , Ehsan Larkee 3 , Masoud Soleimani 1 1 Geology Department, Shahid Chamran University of Ahvaz, Iran 2 Petroleum Geology, Earth Sciences Faculty, Shahid Chamran University of Ahvaz, Iran. 3 Geophysics-Seismic Exploration, Well logging NIDC, Ahvaz Received January 5, 2017; Accepted April 5, 2017 Abstract Petrophysical characteristics such as porosity, permeability, saturation, shale volume, and lithology are important in evaluating the petroleum potential of a reservoir. In this research, Ilam Formation (late Cretaceous age) in Ahvaz oil field, (SW Iran) has been evaluated using petrophysical parameters. Therefore, the available integrated digital petrophysical logs, and core analysis data consisted of input data of Geolog software (7.1). Petrographic thin sections studies were also made. The results indicated that limestone, dolomite and shale are main components. All documents revealed that the lithology is composed of clean porous limestone with a less quantity of shale (sporadically) and dolomite as well. Average effective porosity and water saturation of Ilam Formation are 15% and 27%, respectively. The shale volume, calculated using gamma ray logs as a shale indicator, is less than 10% indicating a clean formation. The estimated permeability is 8 mD, which is accommodated with the petrophysical logs and core analysis data. Pores are commonly including of intercrystal, cavity and fracture types. Reservoir fluids are distributed in a differentiated pattern (light oil in upper part and heavy oil in lower part) that water saturation degree and gravity derive mechanism (second migration) can be considered as the responsible factors. Keywords: Ahvaz oil field; Ilam Formation; Shale volume; Reservoir characteristics; Gravity derivation. 1. Introduction Physical rock properties and fluids flow in the reservoir are the main purposes of petro- physical studies. Factors such as geometry, temperature, pressure and lithology of the for- mation can play an important role in assessment, completion and exploitation of the hydro-carbon reservoir. The main reason of applying well logs is the low cost operations than other methods such as coring process. In fact, well logs are continuous recording of rocks properties of the well [1] . The variation distribution of petrophysical parameters in reservoir intervals can lead to revise zonation and change the pay zone thickness. Because of the reservoir passed behind the main production phase, primary assessments need to review and re-evaluate, so it may be beneficial for the field development. Porosity and permeability of reservoir rocks are the most important petrophysical characteristics related to storage and transport of fluids in the reservoir. The proper understanding of these two features along with fluid properties is necessary to predict future performance of the oil field [2] . Neutron, density and acoustic logs are the most important petrophysical logs of porosity. Since determining the permeability associates with low accuracy by petrophysical logs, the core data should be used. To identify porosity types and porosity relation to permeability, petrographic thin sections can be used [3] . 135