Structure and Evolution of the Petroliferous Euphrates Graben System, Southeast Syria1 Robert K. Litak,2 Muawia Barazangi,3 Graham Brew,3 Tarif Sawaf,4 Anwar Al-Imam,4 and Wasif Al-Youssef4 ABSTRACT Cenozoic inversion of some structures also is evi- dent. Approximately 30 oil fields have been dis- The northwest-trending Euphrates graben sys- covered in the Euphrates graben system since tem is an aborted intracontinental rift of Late 1984. Recoverable reserves discovered to date Cretaceous age that has subsequently been hidden reportedly exceed 1 billion barrels of oil and less- by Cenozoic burial. Approximately 100 km wide, er amounts of gas. Light oil is primarily found in the system comprises an extensive network of Lower Cretaceous sandstone reservoirs juxtaposed grabens and half grabens extending some 160 km by normal faulting against Upper Cretaceous synrift from the Anah graben in western Iraq to the sources and seals. Palmyride fold belt in central Syria, where it becomes more subdued. The youngest prerift rocks are presently at a maximum depth of about 5 km. INTRODUCTION Based primarily on interpretation of 1500 km of seismic reflection profiles and data from 35 wells, Recent detailed studies on a number of continen- we mapped a complex network of numerous tal rifts have shed considerable light on the archi- branching normal and strike-slip faults, generally tecture and evolution of these types of basins (e.g., striking northwest and west-northwest. Both Rosendahl, 1987; Morley, 1995). Continental rifts branched and single-strand linear normal faults of hosting major hydrocarbon accumulations include generally steep dip, as well as positive and negative the North Sea (e.g., Stewart et al., 1992), Gulf of flower structures, are manifest on seismic sections. Suez (e.g., Patton et al., 1994), and many others. No single rift-bounding fault is observed; instead, a The Euphrates graben system in southeastern Syria major flexure coupled with minor normal faulting is an aborted continental rift that also holds signifi- marks the southwestern edge of the basin, with cant petroleum reserves, but a comprehensive anal- considerable variation along strike. To the north- ysis of its development has been unavailable before east, deformation diminishes on the Rawda high now in the public literature. In this study, we near the Iraqi border. describe the structure of, and interpret the evolu- The Euphrates graben system likely formed in a tion of, the Euphrates graben system. transtensional regime, with active rifting primari- Much of the northern Arabian plate consists of a ly restricted to the Senonian and with an estimat- mosaic of relatively stable blocks, including the ed maximum extension of about 6 km. Minor Rutbah uplift, the Aleppo plateau, and the Rawda (Khleissia) and Mardin highs, bounded by zones of persistent, episodic instability (Figure 1). Deforma- ©Copyright 1998. The American Association of Petroleum Geologists. All rights reserved. tion in many of these unstable zones, including the 1Manuscript received July 22, 1996; revised manuscript received May 29, Palmyride fold belt, Euphrates/Anah fault system, 1997; final acceptance February 3, 1998. Azraq/Sirhan graben, and Sinjar and Abd El Aziz 2Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York 14853. Present address: 218 Alden Avenue, New Haven, uplifts, can be traced back at least to the Triassic Connecticut 06515-2112. and possibly to the late Proterozoic (e.g., Best et 3Institute for the Study of the Continents, Snee Hall, Cornell University, al., 1993; Litak et al., 1997). Deformation within Ithaca, New York 14853. 4Syrian Petroleum Company, Ministry of Petroleum and Mineral the mobile zones has varied through space and Resources, Damascus, Syria. time, partly dependent upon the orientation of the Seismic and well data for this study were kindly provided by the Syrian Petroleum Company. This research is supported by Amoco, ARCO, Exxon, zones, and apparently has reflected tectonic Mobil, Unocal, and Conoco. D. Seber and W. Beauchamp reviewed a draft of events at nearby plate boundaries (e.g., Chaimov this manuscript. We greatly appreciate the comments of reviewers T. Patton, et al., 1992; Barazangi et al., 1993). Recent studies N. Gorur, P. Yilmaz, and former Elected Editor K. Biddle that helped to improve the manuscript. Institute for the Study of the Continents contribution of the Palmyrides have elucidated much of their number 230. tectonic history, from a possible Proterozoic suture AAPG Bulletin, V. 82, No. 6 (June 1998), P. 1173–1190. 1173 1174 Euphrates Graben System (A) (B) Black Sea Caspian Sea East Anatolian Fault Zagros Fold Belt o TURKEY 40 N Suture Bitli N Bitlis s S Mardin High E. Anatolian Flt. u yyyyyyyyyyyyyyyyyyytu re Turkey yyyyyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyyyyyZagros Thrust Zone Mediterranean Sea yyyyyyyyyyyyyyyyyyy Sinjar-Abd El Aziz Uplift S Y R I A Euphrates R. yyyyyyyyyyyyyyyyyyyZagros Fold Belt IRAN Dead Sea Aleppo Rawda yyyyyyyyyyyyyyyyyyy Fault Fig. 2 Plateau ? High System yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy Euphrates Depression yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy o yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy 30 A yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy ra Bishri F. Anah yyyyyyyyyyyyyyyyyyyyyyyyyyyyyybia Graben n Sea Palmyrides Abu Jir yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyNajd Faults G Red ulf Mediterranean yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy Rutbah yyyyyyyyyyyArabian Uplift yyyyyyyyyyy Fig. 2 Iraq yyyyyyyyyyyShield yyyyyyyyyyy o A F R I C A Jordan yyyyyyyyyyy 20 Normal Fault yyyyyyyyyyy Sea yyyyyyyyyyy Thrust Fault yyyyyyyyyyy yyyyyyyyyyy Dead Sea Fault System 100 km 500 km Gulf of Aden Arabian Sea O O 40 50 E Figure 1—(A) Map showing simplified tectonic setting of the Arabian plate. (B) Map showing location of study region and nearby tectonic features. Location of the area shown in Figure 2 also is shown. zone (Best et al., 1990) to a Mesozoic aulacogen 1995). In the context of both continental rifting (Ponikarov, 1967; McBride et al., 1990; Best et al., and regional tectonics, this study focuses on the 1993), to the Late Cretaceous and Cenozoic inver- structural and geologic evolution of the Euphrates sion and uplift (e.g., Al-Saad et al., 1992; Chaimov graben system and its subsequent burial and partial et al., 1992; Searle, 1994). inversion during the Cenozoic. Lovelock (1984) briefly described the Euphrates graben and “Al-Furat fault” in a review of regional tectonic elements. Sawaf et al. (1993) presented a DATABASE crustal-scale cross section across eastern Syria. In two recent papers, Alsdorf et al. (1995) described The principal data for this study include 1500 the junction between the Palmyrides and the km of seismic reflection profiles and information Euphrates depression, and Litak et al. (1997) dis- from 35 exploratory wells (Figure 2) covering an cussed the first-order structures and regional tec- area of approximately 13,000 km2. These data are a tonic framework along the length of the northwest- subset of a considerably larger database provided trending Euphrates (Al-Furat) fault system by the Syrian Petroleum Company as part of an throughout Syria. Brew et al. (1997) used seismic ongoing cooperative joint project with Cornell refraction and other data to determine depth to University. Seismic data were collected by a variety metamorphic basement and to examine upper of contractors throughout the 1980s, and most of crustal structure in eastern Syria. the lines are migrated. An array of vibrators provid- Late Cretaceous rift structures are best devel- ed the source for most of the data, although explo- oped in southeastern Syria, herein referred to as sive sources were used on several lines in the the Euphrates graben system. These structures, Euphrates River valley. Seismic data were interpret- now buried by up to 2.5 km of Cenozoic sedimen- ed in paper-record form. Stratigraphic tops were tary rocks, are largely responsible for the consider- available for all wells, and various logs and litholog- able oil reserves that have been discovered in the ic information were available for several wells, region during the past decade (de Ruiter et al., including sonic logs for six wells. The sonic logs Litak et al. 1175 40° 41° Figure 2—Database map of southeastern Syria showing location of oil fields, wells, and seismic profiles. See Figure 1 for R. area location. Line AA′ denotes Euphrates line of cross section in Figure 5. Bold portions of seismic lines are ° shown in Figures 6, 7, 9, and 13. 35 R. Khabour A Thayyem Rasin IRAQ Qahar Tayyani SM 80-18 Dablan AS-703 PS-417 Oil 34° fields PS-255 SYRIA Swab Wells: Named Unnamed A N 50 km Seismic lines were digitized to produce synthetic seismograms correlatable event (McBride et al., 1990; Sawaf et and velocity information for depth-time conversion al., 1993), disappearing below 4 s beneath the to facilitate well ties to the seismic data. deepest part of the Euphrates graben system. Brew et al. (1997) determined basement depth beneath the Euphrates graben system to be around 9 km. Stratigraphy and Seismic Character On some seismic lines, the Burj marker is discor- dant with reflections observed below it, which The stratigraphy of southeastern Syria records a may represent Proterozoic sedimentary rocks [as clastic-dominated Paleozoic section more than 4 suggested by Seber et al.(1993)] beneath an km thick underlying Triassic–Neogene carbonates Eocambrian (Pan-African) unconformity. Good and evaporites interbedded with lesser amounts of regional reflections from the top of the Upper shale and sandstone (Figure 3). The stratigraphy of Ordovician Affendi and Lower Ordovician Swab eastern Syria is described in some detail by Sawaf et formations can be ascribed to sharp shale/sand- al. (1993), and lithostratigraphic charts have been stone and sandstone/shale discontinuities, respec- published by Lababidi and Hamdan (1985); there- tively. More than 3400 m of Ordovician section is fore, we concentrate on the major packages of rock penetrated in the Swab well (location shown in and their seismic signatures. Based on refraction Figure 2). The poorly reflective Silurian Tanf and wide-angle reflection data, Seber et al.