Hydrothermal dolomitization of the Bekhme formation (Upper Cretaceous), Zagros Basin, Kurdistan Region of Iraq: Record of oil migration and degradation Howri Mansurbeg a, ⁎, Daniel Morad b , Rushdy Othman a , Sadoon Morad c , Andrea Ceriani c , Ihsan Al-Aasm d , Kamal Kolo a , Pavel Spirov e , Jean Noel Proust f , Alain Preat g , Hemin Koyi h a Soran University, Faculty of Science, Department of Petroleum Geosciences, Kurdistan Region, Iraq b Department of Earth Sciences, University of Oslo, Oslo N-0316, Norway c Department of Petroleum Geosciences, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates d Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario, Canada e Soran University, Faculty of Engineering, Department of Petroleum Engineering, Kurdistan Region, Iraq f Géosciences, Université de Rennes 1, UMR CNRS 6118, Rennes, France g Department of Geology, Free University of Brussels, Brussels, Belgium h Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University, 752 36 Uppsala, Sweden abstract article info Article history: Received 27 February 2016 Received in revised form 21 May 2016 Accepted 25 May 2016 Available online 06 June 2016 Editor: Dr. B. Jones The common presence of oil seepages in dolostones is widespread in Cretaceous carbonate successions of the Kurdistan Region of Iraq. This integrated field, petrographic, chemical, stable C, O and Sr isotopes, and fluid inclu- sion study aims to link dolomitization to the origin and geochemical evolution of fluids and oil migration in the Upper Cretaceous Bekhme carbonates. Flux of hot basinal (hydrothermal) brines, which is suggested to have oc- curred during the Zagros Orogeny, resulted in dolomitization and cementation of vugs and fractures by coarse- crystalline saddle dolomite, equant calcite and anhydrite. The saddle dolomite and host dolostones have similar stable isotopic composition and formed prior to oil migration from hot (81–115 °C) basinal NaCl–MgCl 2 –H 2 O brines with salinities of 18–22 wt.% NaCl eq. The equant calcite cement, which surrounds and hence postdates saddle dolomite, has precipitated during oil migration from cooler (60–110 °C) NaCl–CaCl 2 –H 2 O brines (14– 18 wt.% NaCl eq). The yellowish fluorescence color of oil inclusions in the equant calcite indicates that the oil had API gravity of 15–25° composition, which is lighter than present-day oil in the reservoirs (API of 10–17°). This difference in oil composition is attributed to oil degradation by the flux of meteoric water, which is evidenced by the low δ 13 C values (-8.5‰ to -3.9‰ VPDB) as well as by nil salinity and low temperature in fluid inclusions of late columnar calcite cement. This study demonstrates that linking fluid flux history and related diagenesis to the tectonic evolution of the basin provides important clues to the timing of oil migration, degradation and reservoir evolution. © 2016 Elsevier B.V. All rights reserved. Keywords: Hydrothermal dolomitization Stable isotopes Fluid inclusions Oil migration and degradation Cretaceous reservoirs Kurdistan 1. Introduction The formation of hydrothermal dolostone reservoirs in sedimentary basins around the world (Al-Aasm, 2003; Smith, 2006; Wilson et al., 2007; Davies and Smith, 2006; Saller and Dickson, 2011), including the Kurdistan Region of Iraq (Di Cuia et al., 2003), has gained increasing recognition in recent years. Davies and Smith (2006) defined hydro- thermal dolomite as “dolomitization occurring under burial conditions, commonly shallow depths, by fluids (typically very saline) with temperature and pressure (T and P) higher than the ambient T and P of the host formation” (see also Machel and Lonnee, 2002). The flux of dolomitizing (Mg-rich), hot basinal brines (referred to as hydrothermal in the literature) into cooler strata occurs along deeply- seated faults in sedimentary basins (Duggan et al., 2001; Packard et al., 2001; Lavoie and Chi, 2010; Shah et al., 2010; Sharp et al., 2010; Dewit et al., 2012, 2014; Haeri-Ardakani et al., 2013a, 2013b; Martín-Martín et al., 2013; Corbella et al., 2014). The petrographic, geo- chemical and microthermometric characteristics of the resulting dolostones and associated saddle dolomite cement have been constrained in the literature (Lavoie and Morin, 2004). However, the links between the flux and evolution of these dolomitizing fluids and timing of hydrocarbon migration and degradation are not equally well constrained. The aim of the paper is to use integrated field, petrographic, bulk chemical, C-, O- and Sr isotopes, and fluid inclusion to decipher the Sedimentary Geology 341 (2016) 147–162 ⁎ Corresponding author. E-mail address: howri.mansurbeg@soran.edu.iq (H. Mansurbeg). http://dx.doi.org/10.1016/j.sedgeo.2016.05.015 0037-0738/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo