Journal of Petroleum Science and Technology Source Rock Evaluation of the Cenomanian Middle Sarvak (Ahmadi) Formation in the Iranian Sector of the Persian Gulf Maryam Mirshahani*, Mohammad Kassaie, and Arsalan Zeinalzadeh Geochemistry Group, Faculty of Upstream Petroleum Industry, Research Institute of Petroleum Industry (RIPI), Tehran, Iran ABSTRACT The middle Sarvak formation (Cenomanian) is one of the stratigraphic units of the Bangestan group in the south of Iran. This formation is stratigraphically equivalent to the Ahmadi member of Kuwait and Iraq. There is geochemical evidence that indicates this unit has a high level of organic richness and can be a possible source rock in various locations. This study focuses on the organic geochemistry of the middle Sarvak formation in the Persian Gulf region. Rock Eval pyrolysis, organic petrography, and kerogen elemental analyses were used in order to evaluate the thermal maturity and determine the kerogen type of the middle Sarvak formation. The results of this study show that middle Sarvak formation has entered the oil window in eastern and western parts of the Persian Gulf, but it is immature in the central parts. This regional pattern of organic maturation is probably a consequence of the regional uplift in the Qatar-Fars arch area. The higher maturities in the eastern and western parts are, on the other hand, attributed to its greater depth of burial. The results of the screening analyses by the Rock-Eval pyrolysis in parallel with the results of maceral and elemental analyses show that the kerogen type in the middle Sarvak formation is mainly a mixture of Types II and III. Keywords: Middle Sarvak Formation, Rock Eval Pyrolysis, Organic Petrography, the Persian Gulf INTRODUCTION is present throughout the Persian Gulf area. A number The Persian Gulf region is known for its prolific of geological and geochemical studies on the Sarvak petroleum systems owing to the large number of formation in the south of Iran, particularly in the active source rocks. The middle Sarvak formation is Zagros area, have been carried out [2-5].The goal of considered as a candidate source rock in the studied this study is to evaluate the source rock potential of area based on geochemical observations. This the middle Sarvak formation in the Persian Gulf area. formation is a part of the Bangestan group and is stratigraphically equivalent to the Mauddud, Ahmadi, Geological Setting The Persian Gulf basin encompasses a thick Rumaila, and Mishrif formations of the southern sedimentary succession with alternating clastic, Persian Gulf region [1]. The middle Sarvak formation *Corresponding author Article history Maryam Mirshahani Received: October 31, 2015 Email: [email protected] Received in revised form: May 10, 2016 Tel: +98 21 4825 3027 Accepted: June 19, 2016 Fax: +98 21 4473 9723 Available online: July 22, 2017 Journal of Petroleum Science and Technology 2017, 7(3), 100-116 http://jpst.ripi.ir © 2017 Research Institute of Petroleum Industry (RIPI) 100 Journal of Petroleum Source Rock Evaluation of the Cenomanian Middle Sarvak (Ahmadi)... Science and Technology carbonate, and evaporite sediments, which makes the NW-SE trending folds) [16]. Apart from these events, area particularly prolific for hosting large hydrocarbon most of the structures in the Persian Gulf basin are deposits. The oldest sediments in the area are affected by episodic salt movements to varying extents believed to be the evaporites, shales, and dolomites [17]. of the Late-Precambrian Hormuz series [6]. Generally, The morphology of the Persian Gulf is highly affected there is little data about the sedimentary history of by the Qatar-Fars Arc [18]. The Qatar-Fars Arc is a first- the Lower Paleozoic in the Persian Gulf region, and the order structure that was created in the central Persian sedimentary record comprises mostly of shales and Gulf following the tectonic movements during the Late sandstone with rare carbonates in the Devonian and Precambrian to Early Cambrian in the region (Figure 2). Early Carboniferous [7]. During the Permian, carbonate It is a very large (over 100 km wide and 300 km long) shelf deposits of the Dalan formation were deposited regional gentle anticline [19]. According to offshore under warm and shallow-water conditions. More seismic data in the study area, this structure has a arid conditions during Mid-Late Triassic formed the northeast–southwest direction in the Iranian sector of evaporite deposits of the Dashtak formation, which the Persian Gulf and continues southwards to the Qatar mark the end of the carbonate cycles. The middle peninsula [20]. As Figure 3 demonstrates, the thicknesses Jurassic sediments mainly consist of normal marine of the Pabdeh, Gurpi, and Kazhdumi formations and organic-rich carbonates (the Surmeh formation) [8] Ahmadi member significantly decrease toward the which are capped by extensive evaporite deposits central parts of the study area with a noticeable thinning (Hith formation) of Tithonian age [9, 10]. During which can be due to the effect of the Qatar-Fars Arc Cretaceous, three main stratigraphic sequences [11] Paleohigh during depositional time [14]. were recorded in the Persian Gulf area: the Lower Cretaceous deposits of the Fahliyan, Gadvan, and Dariyan formations; the middle Cretaceous sediments comprising of the Kazhdumi and Sarvak formations; and the Upper Cretaceous deposits of the Ilam, Laffan, and Gurpi formations [12, 13]. A regional unconformity marks the end of the Cretaceous and the boundary between the Late Cretaceous and Early Tertiary sediments [14]. Orogenic folding of the adjacent Zagros during the Late Tertiary resulted in rapid uplift, extensive erosion, and the formation of a thick clastic wedge (Figure 1). The Paleozoic structural evolution of the area generally took place along regional basement highs [15]. Recent deformation episodes, however, included a Late Cretaceous event (producing NNE-SSW trending faults) and a Late Cenozoic Zagros Orogeny event Figure 1: A generalized stratigraphic column of the study (reactivating previous folds and causing a new set of area [24]. Journal of Petroleum Science and Technology 2017, 7(3),100-116 http://jpst.ripi.ir © 2017 Research Institute of Petroleum Industry (RIPI) 101 Journal of Petroleum Science and Technology M. Mirshahani, M. Kassaie, and A. Zeinalzadeh Figure 2: Location of the Qatar-Fars Arc and distribution of Hormoz Salt in the study area [21]. Figure 3: Lithostratigraphic cross section in the Iranian sector of the Persian Gulf through Jurassic to Quaternary (modified after [23]). Journal of Petroleum Science and Technology 2017, 7(3), 100-116 http://jpst.ripi.ir © 2017 Research Institute of Petroleum Industry (RIPI) 102 Journal of Petroleum Source Rock Evaluation of the Cenomanian Middle Sarvak (Ahmadi)... Science and Technology Persian Gulf Figure 4: Location map and selected fields of the study area. The sediments of the middle Sarvak formation The Ahmadi member is a continuation of the middle were deposited on a passive margin of the Neo- Sarvak in the northwestern Persian Gulf (towards Tethys Ocean. This formation is divided into four Kuwait). Lithologically, it consists of 30 to 60 meters depositional members: the Mauddud, bituminous of highly eroded shale with intercalation of thin shaley limestone, Khatiyah in the central and western limestone beds. Most of the present Persian Gulf parts of the Persian Gulf, Ahmadi with shaley facies area was beyond the deltaic influence, and until in the northern Persian Gulf, and the Mishrif reefal the deposition of the Ahmadi shales, continued to limestone [1]. receive sub-littoral to shallow marine carbonates [25]. The middle Sarvak is present in all wells drilled in the The Ahmadi member also consists of fossiliferous Persian Gulf. The thickness of the formation varies limestones. The shale and limestone of the Ahmadi from a minimum 41 meters up to a maximum of 223 member were deposited during Early to middle meters in the northwestern Persian Gulf [1]. Cenomanian in open-marine conditions on the outer Journal of Petroleum Science and Technology 2017, 7(3),100-116 http://jpst.ripi.ir © 2017 Research Institute of Petroleum Industry (RIPI) 103 Journal of Petroleum Science and Technology M. Mirshahani, M. Kassaie, and A. Zeinalzadeh shelf at depths between 100 to 200 meters [1]. was attained, followed by the flotation of the isolated For the purpose of this study, in order to evaluate source organic matter (kerogen) within zinc-bromide solution rock potentiality of middle Sarvak formation, cutting (specific gravity=2) to remove residual inorganic samples from a total of 18 wells through the Persian matter. Gulf area have been analyzed with organic geochemical According to the results obtained from the screening methods. Figure 4 shows the location map of these wells analyses, a total of 36 samples were selected for in the Persian Gulf. Four study blocks were delineated vitrinite reflectance measurements. Vitrinite reflectance to present the northwest (Block A), west-central (Block measurements were carried out on both polished whole BC), east central (Block D), and east parts of the Iranian rock block and kerogen concentrate samples by using sector of the Persian Gulf. a Leitz-MPV-SP photometer microscope at RIPI. The measurements were carried out in a random mode EXPERIMENTAL PROCEDURES according to the ASTM standard method [26]. Materials and Methods Nine polished blocks and equal number of thin In order to evaluate middle Sarvak formation for sections were prepared from the extracts for the hydrocarbon source rock potentiality, Rock-Eval identification of kerogen type and the determination pyrolysis results were used in combination with of thermal alteration index (TAI). organic-petrography methods. The latter included The kerogen extract samples were used for the vitrinite reflectance measurements, visual kerogen elemental analyses to determine the elemental inspection, thermal alteration determination, and composition of C, H, N, O, and S. The elemental kerogen elemental analysis following generally composition of the isolated kerogens (C, H, N, and S) published procedures.