An improved understanding of the origin and mechanism of Permian- Triassic natural gas-condensate accumulations in the Gavbendi High, Southwest Iran: An integrated approach Asghar Ahanjan a , Ahmad Reza Rabbani a, * , Mohammad Reza Kamali b a Faculty of Petroleum Engineering, Amirkabir University of Technology, Tehran, Iran b Exploration and Production Department, Research Institute of Petroleum Industry (RIPI), Tehran, Iran article info Article history: Received 10 August 2016 Received in revised form 6 October 2016 Accepted 14 November 2016 Available online 16 November 2016 Keywords: Gas-condensate reservoir Permian-Triassic Kerogen cracking Multiple charging Southwest Iran abstract Numerous gas-condensate discoveries have been made in the Permian-Triassic reservoirs in the south- west of Iran. However, the origin and post-generation alterations of these gas-condensate accumulations remain unresolved. Thus, a multifaceted approach was utilized to investigate their origin and formation mechanism. For this study, molecular and isotopic compositions of 11 gases and their associated condensate samples together with source rock extracts were integrated to better address the source of the gases. The molecular data on associated condensates suggested that the gas-condensates were derived from two different source rocks. The reservoirs in central and northern parts of the study area with their low concentrations of tetracyclic terpanes and high extended tricyclic terpane ratios were generated from the Early Triassic Kangan source rocks, whereas the condensates at eastern and western parts with higher contents of tetracyclic terpanes and lower values of extended tricyclic terpane ratios showed good affinity with the Silurian Sarchahan Formation. The gas-condensates at central parts of the Gavbendi High with high amounts of saturate to aromatic ratios together with high fractionation index were accumulated through migration of gas from the Kangan source kitchen in northern part of the Gavbendi High. However, the gas accumulations in western side of the Gavbendi High were formed by migration of gas from the western Silurian source kitchen. Moreover, the Permian-Triassic gas reserves with their relatively low differences between carbon isotope values of ethane and propane and less negative carbon isotope values of methane together with less negative carbon isotope values of CO 2 and negligible H 2 S contents were subjected to low extent of oil cracking and generated mainly by kerogen cracking through progressive maturation of source rock. In addition, multiple charging of the reservoirs in northern part of the study area plays an important role in the Permian-Triassic sequence which has not been previously identified. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The major Upper Permian-Triassic gas-condensate accumula- tions of Southwestern Iran are located in a regional paleo-high known as Gavbendi High, which is a continuation of the QatareFars Arch into the onshore part of the Persian Gulf (Fig. 1). The Upper Permian (Upper Dalan) and Lower Triassic (Kangan) reservoirs are akin in terms of lithology and form a continuous reservoir unit regarding reservoir pressure and properties (Esrafili- Dizaji and Rahimpour-Bonab, 2013; Motiei, 1993), but they are separated from Middle Permian Lower Dalan Formation via Nar Member in most parts of the Qatar-Fars Arch. There are two regional seals, namely, the Triassic Dashtak Formation anhydrite and shales, sealing the Kangan and upper part of Dalan reservoirs, and the Nar Member anhydrite capping the Lower Dalan reservoirs. The Lower Silurian Sarchahan Formation has been considered as the main source rock for the Permian-Triassic reservoirs (Aali et al., 2006; Alsharhan and Naim, 1997; Bordenave, 2008; Galimov and Rabbani, 2001; Ghavidel-Syooki et al., 2011; Saberi et al., 2016; Saberi and Rabbani, 2015). However, recent investigation by Rabbani (2013) indicated moderate to good source potential for the shaly mudstone intervals of the Early Triassic Kangan Formation in the Gavbendi High. In the northern part of the Gavbendi High * Corresponding author. E-mail address: rabbani@aut.ac.ir (A.R. Rabbani). Contents lists available at ScienceDirect Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse http://dx.doi.org/10.1016/j.jngse.2016.11.026 1875-5100/© 2016 Elsevier B.V. All rights reserved. Journal of Natural Gas Science and Engineering 37 (2017) 217e233