1 Running title: Geochemical Assessment and Lateral Reservoir Continuity in Iran Geochemical Assessment and Lateral Reservoir Continuity of Asmari Reservoir Oils in Cheshmeh-Khush Oilfield, SW Iran Farid Tezheh 1 , Bijan Beiranvand 1,* , Hormuz Ghalavand 2 , Mohammad Reza Kamali 1 , Rahim Bagheri Tirtashi 3 1 Research Institute of Petroleum Industry (RIPI), Tehran, Iran 2 Deputy of Development and Engineering, National Iranian Oil Company (NIOC), Tehran, Iran 3 Geochemistry Department, Exploration Directorate, National Iranian Oil Company (NIOC), Tehran, Iran Abstract: In the present study, a total of 6 oil samples were systematically selected from six Asmari Reservoir wells in Cheshmeh-Khush Oilfield for geochemical evaluation of hydrocarbon system based on reservoir geochemistry and oil fingerprints. An investigation on distribution pattern of normal alkanes and tricyclic and tetracyclic trepans along with characteristic biomarkers of the depositional environment and sedimentary facies indicated that, source rock of the studied hydrocarbons was deposited in a reducing aquatic environment with low input of terrigenous material and dominantly carbonate lithology derived from organic algal matter. The studied oil samples exhibited moderate maturity, as was further confirmed by the parameters extracted from light hydrocarbons. Statistical clustering based on different biomarker parameters indicated the presence of two oil families. Additionally, the application of branched and cyclic compounds in light hydrocarbons (C 5 – C 11 ) to evaluate lateral continuity of the Asmari reservoir further confirmed the presence of the two oil families. Different evidences proved the presence of a NE-SW trending fault separating the wells X5 and X6 from other wells in the field. In other wells across the field, good lateral reservoir continuity was observed despite the presence of faults on the northern and southern plunges of the structure. Key words: oil fingerprint, lateral continuity, Asmari Reservoir, petroleum families, Cheshmeh-Khush Oilfield E-mail: biranvandb@ripi.ir 1 Introduction Reservoir geochemistry can serve as a tool for determining paths for or barriers against fluid flow in oil and gas fields (Hwang et al., 1994). All around the world, several studies have been performed on reservoir continuity and statistical calculation of the connection between different reservoirs across oil and gas fields. It should be noted that, if the oils occurred across the same field had undergone similar geological history (in terms of, for example, source and maturity), the oils are expected to exhibit no significant difference in physical properties, such as density, specific gravity, sulfur content, and even biomarker compounds. Therefore, the best instrument for determining molecular composition of an oil sample is Gas Chromatography technique (Kaufman et al., 1990). The chemical composition identified via GC (that is also known as fingerprint) can serve as an indicator of reservoir characteristics. For example, the fingerprint technology can be used in addressing exploitation engineering and reservoir development problems, so as to identify connected reservoir horizons across different wells. This technology can be further used in determining the extent (reservoir continuity) and volume of reservoir across the oilfield based on which one can prepare reservoir extension maps and propose the required number and location of wells to be drilled (Kaufman et al., 1990; England et al., 1995; Smalley and Hale, 1996). Cheshmeh-Khush Oilfield produces oil from Asmari Formation as reservoir. This study was focused on identifying and evaluating the trends of changes in geochemical parameters across the reservoir. This is the first study in the region in which hydrocarbon samples were systematically taken from oil-producing wells across the field and subjected to comprehensive geochemical studies. Accordingly, the samples were geochemically evaluated by accurate determination of biomarker variations (fingerprints) and then, performing high-resolution gas chromatography (HRGC) and finally lateral reservoir continuity was examined across the field. 2 Geological Setting Cheshmeh-Khush Oilfield as a producing field was explored in 1962. It is located in the west of Dezful Embayment as a part of the Zagros fold-thrust belt (Figure 1). The Zagros basin is generally divided into three individual parts of thrusted zone, imbricated zone, and simply folded zone (Sepehr and Cosgrove, 2004). These This article is protected by copyright. All rights reserved. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/1755-6724.13884.