Structure and Evolution of the Petroliferous Euphrates Graben …atlas.geo.cornell.edu/syria/AAPG_98_Litak.pdf · 2006. 8. 28. · ABSTRACT The northwest-trending Euphrates graben

ABSTRACT

The northwest-trending Euphrates graben sys-tem is an aborted intracontinental rift of LateCretaceous age that has subsequently been hiddenby Cenozoic burial. Approximately 100 km wide,the system comprises an extensive network ofgrabens and half grabens extending some 160 kmfrom the Anah graben in western Iraq to thePalmyride fold belt in central Syria, where itbecomes more subdued. The youngest prerift rocksare presently at a maximum depth of about 5 km.Based primarily on interpretation of 1500 km ofseismic reflection profiles and data from 35 wells,we mapped a complex network of numerousbranching normal and strike-slip faults, generallystriking northwest and west-northwest. Bothbranched and single-strand linear normal faults ofgenerally steep dip, as well as positive and negativeflower structures, are manifest on seismic sections.No single rift-bounding fault is observed; instead, amajor flexure coupled with minor normal faultingmarks the southwestern edge of the basin, withconsiderable variation along strike. To the north-east, deformation diminishes on the Rawda highnear the Iraqi border.

The Euphrates graben system likely formed in atranstensional regime, with active rifting primari-ly restricted to the Senonian and with an estimat-ed maximum extension of about 6 km. Minor

Cenozoic inversion of some structures also is evi-dent. Approximately 30 oil fields have been dis-covered in the Euphrates graben system since1984. Recoverable reserves discovered to datereportedly exceed 1 billion barrels of oil and less-er amounts of gas. Light oil is primarily found inLower Cretaceous sandstone reservoirs juxtaposedby normal faulting against Upper Cretaceous synriftsources and seals.

INTRODUCTION

Recent detailed studies on a number of continen-tal rifts have shed considerable light on the archi-tecture and evolution of these types of basins (e.g.,Rosendahl, 1987; Morley, 1995). Continental riftshosting major hydrocarbon accumulations includethe North Sea (e.g., Stewart et al., 1992), Gulf ofSuez (e.g., Patton et al., 1994), and many others.The Euphrates graben system in southeastern Syriais an aborted continental rift that also holds signifi-cant petroleum reserves, but a comprehensive anal-ysis of its development has been unavailable beforenow in the public literature. In this study, wedescribe the structure of, and interpret the evolu-tion of, the Euphrates graben system.

Much of the northern Arabian plate consists of amosaic of relatively stable blocks, including theRutbah uplift, the Aleppo plateau, and the Rawda(Khleissia) and Mardin highs, bounded by zones ofpersistent, episodic instability (Figure 1). Deforma-tion in many of these unstable zones, including thePalmyride fold belt, Euphrates/Anah fault system,Azraq/Sirhan graben, and Sinjar and Abd El Azizuplifts, can be traced back at least to the Triassicand possibly to the late Proterozoic (e.g., Best etal., 1993; Litak et al., 1997). Deformation withinthe mobile zones has varied through space andtime, partly dependent upon the orientation of thezones, and apparently has ref lected tectonicevents at nearby plate boundaries (e.g., Chaimovet al., 1992; Barazangi et al., 1993). Recent studiesof the Palmyrides have elucidated much of theirtectonic history, from a possible Proterozoic suture

1173

©Copyright 1998. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received July 22, 1996; revised manuscript received May 29,1997; final acceptance February 3, 1998.

2Institute for the Study of the Continents, Snee Hall, Cornell University,Ithaca, New York 14853. Present address: 218 Alden Avenue, New Haven,Connecticut 06515-2112.

3Institute for the Study of the Continents, Snee Hall, Cornell University,Ithaca, New York 14853.

4Syrian Petroleum Company, Ministry of Petroleum and MineralResources, Damascus, Syria.

Seismic and well data for this study were kindly provided by the SyrianPetroleum Company. This research is supported by Amoco, ARCO, Exxon,Mobil, Unocal, and Conoco. D. Seber and W. Beauchamp reviewed a draft ofthis manuscript. We greatly appreciate the comments of reviewers T. Patton,N. Gorur, P. Yilmaz, and former Elected Editor K. Biddle that helped toimprove the manuscript. Institute for the Study of the Continents contributionnumber 230.

Structure and Evolution of the Petroliferous EuphratesGraben System, Southeast Syria1

Robert K. Litak,2 Muawia Barazangi,3 Graham Brew,3 Tarif Sawaf,4 Anwar Al-Imam,4

and Wasif Al-Youssef4

AAPG Bulletin, V. 82, No. 6 (June 1998), P. 1173–1190.

zone (Best et al., 1990) to a Mesozoic aulacogen(Ponikarov, 1967; McBride et al., 1990; Best et al.,1993), to the Late Cretaceous and Cenozoic inver-sion and uplift (e.g., Al-Saad et al., 1992; Chaimovet al., 1992; Searle, 1994).

Lovelock (1984) briefly described the Euphratesgraben and “Al-Furat fault” in a review of regionaltectonic elements. Sawaf et al. (1993) presented acrustal-scale cross section across eastern Syria. Intwo recent papers, Alsdorf et al. (1995) describedthe junction between the Palmyrides and theEuphrates depression, and Litak et al. (1997) dis-cussed the first-order structures and regional tec-tonic framework along the length of the northwest-trending Euphrates (Al-Furat) fault systemthroughout Syria. Brew et al. (1997) used seismicrefraction and other data to determine depth tometamorphic basement and to examine uppercrustal structure in eastern Syria.

Late Cretaceous rift structures are best devel-oped in southeastern Syria, herein referred to asthe Euphrates graben system. These structures,now buried by up to 2.5 km of Cenozoic sedimen-tary rocks, are largely responsible for the consider-able oil reserves that have been discovered in theregion during the past decade (de Ruiter et al.,

1995). In the context of both continental riftingand regional tectonics, this study focuses on thestructural and geologic evolution of the Euphratesgraben system and its subsequent burial and partialinversion during the Cenozoic.

DATABASE

The principal data for this study include 1500km of seismic reflection profiles and informationfrom 35 exploratory wells (Figure 2) covering anarea of approximately 13,000 km2. These data are asubset of a considerably larger database providedby the Syrian Petroleum Company as part of anongoing cooperative joint project with CornellUniversity. Seismic data were collected by a varietyof contractors throughout the 1980s, and most ofthe lines are migrated. An array of vibrators provid-ed the source for most of the data, although explo-sive sources were used on several lines in theEuphrates River valley. Seismic data were interpret-ed in paper-record form. Stratigraphic tops wereavailable for all wells, and various logs and litholog-ic information were available for several wells,including sonic logs for six wells. The sonic logs

1174 Euphrates Graben System

Figure 1—(A) Map showing simplified tectonic setting of the Arabian plate. (B) Map showing location of studyregion and nearby tectonic features. Location of the area shown in Figure 2 also is shown.

E. Anatolian Flt.

Bitlis Suture

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40 50 E

40

30

20A F R I C A

o

o

o

N

Arabian

O O

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S Y R

I A

TURKEY

IRAN

Caspian Sea

Sea

Red

Gulf of Aden Arabian

Sea

Mediterranean Sea

Arabian Gulf

Dead SeaFault

System

500 km

Zagros Fold Belt

Najd Faults

Fig. 2

(A)

JordanIra

q

Turkey

BitlisSuture

RutbahUplift

East Anatolian Fault

Dea

d S

eaF

ault

Sys

tem

100 km

N

Euphrates

Depression

AleppoPlateau

Fig. 2

Zagros Fold Belt

Bishri

F.

Mardin High

RawdaHigh

Euphrates R.

AnahGrabenPalm

yrides

Sinjar-Abd El Aziz Uplift

Med

iterra

nean

Sea

?

Abu Jir

Normal Fault

Thrust Fault

(B)

were digitized to produce synthetic seismogramsand velocity information for depth-time conversionto facilitate well ties to the seismic data.

Stratigraphy and Seismic Character

The stratigraphy of southeastern Syria records aclastic-dominated Paleozoic section more than 4km thick underlying Triassic–Neogene carbonatesand evaporites interbedded with lesser amounts ofshale and sandstone (Figure 3). The stratigraphy ofeastern Syria is described in some detail by Sawaf etal. (1993), and lithostratigraphic charts have beenpublished by Lababidi and Hamdan (1985); there-fore, we concentrate on the major packages of rockand their seismic signatures. Based on refractionand wide-angle reflection data, Seber et al. (1993)and Brew et al. (1997) estimated the minimumthickness of the entire sedimentary section to beover 8 km on the Rutbah uplift. The Paleozoic sec-tion is generally poorly reflective with a few promi-nent exceptions (Al-Saad et al., 1992). A strong,continuous, multicyclic reflection from the MiddleCambrian Burj limestone is the deepest regionally

correlatable event (McBride et al., 1990; Sawaf etal., 1993), disappearing below 4 s beneath thedeepest part of the Euphrates graben system. Brewet al. (1997) determined basement depth beneaththe Euphrates graben system to be around 9 km.On some seismic lines, the Burj marker is discor-dant with ref lections observed below it, whichmay represent Proterozoic sedimentary rocks [assuggested by Seber et al.(1993)] beneath anEocambrian (Pan-African) unconformity. Goodregional ref lections from the top of the UpperOrdovician Affendi and Lower Ordovician Swabformations can be ascribed to sharp shale/sand-stone and sandstone/shale discontinuities, respec-tively. More than 3400 m of Ordovician section ispenetrated in the Swab well (location shown inFigure 2). The poorly ref lective Silurian TanfFormation, up to 750 m thick, contains a sandstoneunit underlying organic shales thought to representa good potential source rock (Beydoun, 1991). NoDevonian rocks have been recognized in Syria, butthe unconformity (D on Figure 3) has little struc-tural relief, making its seismic identification prob-lematic. Lower Carboniferous rocks of the MarkadaFormation are penetrated in several wells. These

Litak et al. 1175

34°

35°

40° 41°

AS-7

03PS-25

5PS-417

SM 8

0-18

Swab

Dablan

TayyaniQahar

RasinThayyem

SYRIA

IRA

Q

Wells:

Seismiclines

Euphrates R. Kha

bour

R.

Oilfields

A

50 km

NamedUnnamed

NA

Figure 2—Database map of southeastern Syria showing location of oil fields, wells, andseismic profiles. See Figure 1 forarea location. Line AA′ denotesline of cross section in Figure 5.Bold portions of seismic lines areshown in Figures 6, 7, 9, and 13.

rocks are up to 1 km thick and consist mainly ofinterbedded sandstones and shales; these rocks aregenerally less reflective than the overlying section.Locally, however, a dolomite marker forms a promi-nent seismic horizon. The Upper Carboniferous,Permian, and Lower Triassic section is generallyabsent in the study area (Sawaf et al., 1993).

The Triassic–Santonian section in eastern Syriais dominated by carbonates and evaporites, withlesser amounts of shale, chert, and an areallyrestricted Lower Cretaceous sandstone. This sand-stone, the Rutba Formation, was most likelydeposited in a deltaic environment and is animportant hydrocarbon reservoir in the area.Marked thinning of the Upper Triassic section ontothe Rutbah uplift is noted on several seismic linesand is attributed to the emergence of the upliftduring this time (Sawaf et al., 1993). Jurassic car-bonates attain a thickness of over 800 m in the

Bishri and Sinjar areas (and considerably morealong the Levantine margin), but are missing in theEuphrates area, as is the Neocomian section. ThisEarly Cretaceous unconformity (C on Figure 3) isnearly concordant with underlying Triassic rocks,and may represent a regional uplift that erodedJurassic and (partially) Triassic rocks, or may rep-resent simply a prolonged hiatus (Best et al.,1993). Several wells also encounter a thin sectionof basalt in the Lower Cretaceous and lowerUpper Cretaceous. These volcanic rocks are foundin a number of wells throughout Syria and proba-bly are related to regional extension rather thanthe onset of the Euphrates graben sensu stricto.Because of its lithologic heterogeneity, the entireTriassic–Lower Cretaceous section often forms adistinctive zone of high-amplitude reflections thatranges in seismic thickness up to several hundredmilliseconds.


Age Formation MaximumThickness (m)

Notes

Tri

assi

c

Permian

Lower Silurian

Ordovician

Cambrian

Lower

Carboniferous

Upper

Lower

Middle

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Quaternary

Up

per

Cre

tace

ou

sT

erti

ary

Bakhtiary

Upper Fars

Lower Fars

Jeribe

Dibbane

Euphrates

Chilou

Jaddala

Kermav (Aliji)

Shiranish

Soukhne/Massive

Judea

Rutba

Sarjelu

Allan/Muss

Adaya

Butma

Kurrachine Anhydrite

Kurrachine Dolomite

Amanous Shale

Amanous

Markada

Tanf

Affendi

Swab

Sosink

Burj

Zabuk

Khanasser

1000

700

125

250200

500600

600

1600

700

1200500

700

150150

300

400800250

7501200

7501000

1500?

1000?

750+

150+?

150Derro

Main phaseof rifting

Description

conglomerate w/sandstone

sandstone w/ shale

anhydrite, gypsum, sh.

dolomitic lmst. w/ anhyd.

anhydrite over halite

limestone & marly ls.

limestone & marly ls.

marly ls. & limestone

marl & marly shale

marl & argil. limestone

marl over cherty lmst.

red congl. sandstone

dolomitic ls. w/ anhydrite

sst. over some basalt

dolomite w/ shale

dolomite w/ shale

shale, anhyd. & dolomite

dolomite w/ shale

anhydrite

dolomite w/ some ls.

shale

sandstone

sst. w/ some dolomite

shale w/ interbed. sltst

sandstone w/ shale

shale w/ minor sst.

sandstone

quartzitic sandstone

dolomite limestone

quartzitic sandstone (?)

CenomanianTuronian

Coniacian

Santonian

Campanian

Maastricht.

Minorreactivation;transpression

Mapped horizonon Figure 4

A

B

C

D



Figure 3—Generalized stratigraphic column for eastern Syria.

Due to the presence of several unconformities,the completeness of the Cretaceous section differsgreatly from well to well. The late Campanian–Maastrichtian Shiranish Formation is presentthroughout the area, although its thickness variesdramatically. This formation primarily consists ofmarl having poor seismic reflectivity; strong localreflections within the Shiranish interval are likelydue to interbedded limestone or sandstone. TheShiranish Formation overlies either the UpperCretaceous Soukhne or Judea, the Lower CretaceousRutba, the Triassic Serjelu (Mollussa F), or theCarboniferous Markada formations (Figure 3). TheConiacian Derro Formation, unconformably bound-ed at both top and base, is present in only eight of the available wells. The base Coniacian unconfor-mity (B on Figure 3) probably signals the onset of rifting in the Euphrates graben (Lovelock, 1984),and generally exhibits the most pronounced discor-dance of any Cretaceous unconformity in the studyarea. In addition, a seismic discordance of lowerrelief, but considerable extent, is apparent in sever-al areas within the Shiranish itself. This upperunconformity (A on Figure 3) appears to be espe-cially notable in the northwestern part of the studyarea, and is here interpreted as the synrift/postriftboundary.

Paleogene rocks (Chilou, Jaddala, and Kermavformations) grade upward from marl and calcareousshale to limestone, and underlie the lower and mid-dle Miocene evaporites of the Dibbane, Jeribe, andLower Fars formations (Figure 3), deposited in openor sometimes restricted seas (Sawaf et al., 1993).Less than about 400 m of upper Neogene andQuaternary continental clastics (Upper Fars andBakhtiari formations) are exposed at the surfaceover much of the study area. The low-amplitude,discontinuous seismic reflections of these unitscontrast sharply with the strong continuous reflec-tions of the underlying Miocene evaporites.

STRUCTURAL DEVELOPMENT OF THEEUPHRATES GRABEN SYSTEM

Seismic Interpretation

A time-structure map on the base of the UpperCretaceous (the Rutba Formation over most of thestudy area) illuminates the extent and style of defor-mation of the Euphrates graben system (Figure 4).Sonic logs were available for at least portions of sixwells; these logs were digitized to produce synthet-ic seismograms that were tied to the nearest seis-mic lines. The resulting velocity information wasused to tie in stratigraphic tops from the additional29 wells in the study area. Where necessary, wellswere projected up to several kilometers to the

nearest available seismic line by considering seis-mic character and structural trends. Data qualityvaries, with mapping generally most reliable in thesouthwestern part of the study area, and less reli-able in the deeper parts of the graben system.Where strike lines are sparse, fault correlations aremade on the basis of similar seismic expression andconsistency of structural levels.

Deformation is distributed over a wide areaamong many closely spaced faults; only the largeror more continuous faults are shown. Faults gener-ally trend northwest to west-northwest, with onlyminor branches and transfer zones of differing ori-entations. The rift axis generally coincides with theEuphrates River valley, with the deepest part justnorth of the junction with the Khabour River(Figure 4). This map documents two sets of faultdomains. In the north and northwest, faults strikewest-northwest, with several large normal faultstypical of a classical graben. In the south and south-east, however, faults strike more northwest, withmore diffuse deformation and near-vertical flower-type structures suggesting strike-slip motion (e.g.,Harding, 1985). Deformation is transferred in a stepwise fashion toward the southeast, with down-to-the-north fault motion transferred from the DeirAzzor fault to the Thayyem fault, and eventually tothe El Ward fault (Figure 4). The El Ward faultexhibits progressively less throw to the southeast,becoming more of a hinge or flexure. Several faults(notably the El Ward fault) exhibit a convex shapein map view. This pattern is unusual in continentalrifts, with the “ideal” half graben of Rosendahl(1987) being of concave arcuate shape; however,similar faults have been reported in the Suez rift byPatton et al. (1994). We found no evidence formajor transverse structures such as those reportedin the East African rift (e.g., Rosendahl et al., 1986).

The general structure of the Euphrates grabensystem is illustrated by a seismically derived crosssection (Figure 5). Relatively steep (>70°), planarnormal faults are approximately symmetrically dis-tributed around a central graben. Many of thesefaults clearly offset or disrupt the Cambrian BurjFormation between 3 and 4 s, corresponding toapproximately 5–8 km (Figures 5, 6), indicatingthat they penetrate deep into the section, probablyinto Precambrian basement. Basement depth alongthis cross section is between approximately 6 and8.5+ km (Brew et al., 1997). Faults are spaced irreg-ularly about every 5 km, and are mostly depictedhere as single faults with occasional synthetic orantithetic faults; however, only major faults areshown, and observed disruption of seismic datasuggests numerous additional minor faults. Withfew exceptions, faults do not extend upward intothe Tertiary section, constraining the main phase ofactive extension to the Late Cretaceous. From the

Litak et al. 1177

relatively undeformed section of the Rutbah upliftto the southwest, minor down-to-the-northeastfaulting is apparent in front of a f lexural featurewhere Upper Cretaceous and Tertiary rocks beginto thicken fairly abruptly (Figure 5). This “edge” of

the Euphrates graben system is sometimes referredto as the El Madabe step (Sawaf et al., 1993). Thedeepest part of the main graben occurs just east ofthe Euphrates River, where the dominant fault dipchanges from northeast to southwest. This central


34°

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El Ward F.

Deir Azzor F.

Thayyem F.

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> 2.5 s

contour interval 100 ms

20 km

A

A´

N

Figure 4—Time-structure map of the base Upper Cretaceous horizon, in seconds two-way traveltime. Large west-northwest–striking normal faults (Deir Azzor, Thayyem) dominate in the northwestern part of the area, whereasnumerous northwest-striking normal and strike-slip faults abound near the Iraqi border, with no single fault domi-nant. Seismic and well control shown in Figure 2. AA′ denotes line of cross section shown in Figure 5.

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graben area approximately coincides with themain oil-producing trend of the Euphrates play(Figure 2). The northeastern portion of the tran-sect displays a series of southwest-dipping halfgrabens bounded by northeast-dipping planar nor-mal faults (Figure 5). A local structural high atLower Cretaceous levels occurs near the Rasinwell. Based on limited data (see Figure 2), we inter-pret the area northeast of this local high as anotherzone of Late Cretaceous normal faulting between

the Euphrates River and the Rawda high; however,Paleogene rocks continue to thin over this sec-ondary graben, in contrast to the central part of theEuphrates graben system. A southwest-dippingmonocline can be dated to the Neogene (Figure 5).The monocline approximately coincides spatiallywith the underlying half grabens, suggesting minorNeogene reactivation of the earlier structure.

In the vicinity of the El Madabe step the Con-iacian unconformity (B on Figure 3) is most clearly


Neogene

Paleogene

Ku

Kl

TrPzu

Ord

C

Tw

o-W

ay

Tra

vel t

ime

(s)

0

1

2

3

4

SW NE

2 km

PS-255 Figure 6—Seismic exampleof southwestern edge ofEuphrates graben system.Pzu = rocks of Paleozoicage younger than Ordovician. See Figure 2for location. Abbreviationsare explained in Figure 5.

Neogene

Ku

Ord?

1 km

Kl

Pzu

Tr

Paleogene

0

2

1

Tw

o-W

ay

Tra

vel t

ime

(s)

SW NEPS-417 Figure 7—Seismic exampleof Coniacian unconformity(B on Figure 3) truncatingLower Cretaceous rocks attransition from Rutbahuplift to Euphrates grabensystem. Pzu = rocks of Paleozoic age younger thanOrdovician. See Figure 2 forline location. Abbreviationsare explained in Figure 5.

evident. This unconformity progressively cuts outLower Cretaceous and then Triassic rocks to thesouthwest as it merges with base Cretaceous andUpper Triassic unconformities. In wells drilled far-ther to the southwest, the Rutba Formation, theprincipal reservoir in the area, is missing. Thisunconformity can be mapped on seismic reflectiondata in the area (Figure 6, 7). An isopach map of theRutba Formation based on seismic and well dataindicates thickening along the axis of the graben sys-tem (Figure 8). At least part of this thickening likelyis due to erosion. The Cenomanian–Turonian JudeaFormation pinches out closer to the graben axis thanthe underlying Rutba Formation (Figure 8), a sub-crop pattern consistent with postdepositional ero-sion rather than onlap. This updip pinch-out of thereservoir raises the possibility of a combinationstratigraphic/structural play in this area, a conceptthat, to our knowledge, has not been tested.

An excellent example of Euphrates structure isshown on line SM 80-18 (Figure 9). A major normalfault system drops Lower Cretaceous rocks approx-imately 500 ms (∼1 km) down to the northeast.Footwall uplift, perhaps including a component ofstrike-slip faulting, is apparent beneath a clearunconformity. Because a sonic log is not availablefor the Thayyem well, the precise age of thisunconformity is difficult to ascertain; however,

well control indicates that the pre-ConiacianJudea Formation is thicker on top of this structurethan in the downthrown fault block, signifyingthat the Judea has not been cut out by the uncon-formity. Thus, we interpret this unconformity (Aon Figure 3) to occur within the Shiranish (LateCampanian–Maastr ichtian) rather than theConiacian. In fact, the Santonian–early CampanianSoukhne Formation is thinner on the structurethan elsewhere, consistent with a post–earlyCampanian age for the unconformity. The majorthickening, from 200 to 500 ms, occurs in theCampanian–Maastrichtian Shiranish Formation, dat-ing the main phase of extension to that time.Occurring near the top of the Shiranish Formationas seen in Figure 9, the unconformity is likely oflate Maastrichtian age (A on Figure 3), and may rep-resent the boundary between the synrift andpostrift sequences (Litak et al., 1997). Note also theslight rollover of Neogene reflections, with a north-western (counterregional) dip suggesting minorinversion of this structure.

The overall postrift structure is illustrated by ageneralized depth map to the top of Cretaceousrocks (Figure 10). This map, based primarily onwell tops and guided by seismic data, shows abroad depression centered just east of the junctionbetween the Euphrates and Khabour rivers, with amaximum depth exceeding 2600 m BSL. At theOligocene level, the basin is much broader, withNeogene deposits (primarily the upper MioceneLower Fars Formation) extending over much of east-ern Syria (Figure 11). Subtracting these two mapsyields an isopach map of Paleogene rocks, inferredto represent the postrift sequence (Figure 12). Notethat the Paleogene and Neogene depocenters are off-set somewhat. This offset may indicate that,although the existence of the Euphrates weak zonelikely had some inf luence on the location ofNeogene sedimentation, Neogene deposition mayhave been primarily controlled by other factors,which may include the Zagros collision and develop-ment of the Mesopotamian foredeep.

In the easternmost part of the study area, bothpositive and negative flower structures (Harding,1985) are interpreted as indicative of strike-slipfaulting (e.g., Figure 13). These faults also appear tohave been active during the Late Cretaceous.Reactivation of these structures is commonplaceand appears to be diachronous on different struc-tures. Figure 13 depicts subtle deformation ofPaleogene ref lections; other structures evincegrowth during the middle Miocene (e.g., Figure 12)and Pliocene (Quaternary?). These structures bearconsiderable similarity to mild inversion structuresnoted in east Africa by Morley (1995). Cenozoicinversion in the Euphrates graben area is minor andresults in minimal shortening.

Litak et al. 1181

34°

35°

40° 41°E

SYRIA

IRA

Q

100

100

Euphrates R. Kha

bour

R.

50 km

N

0 0

0

100

Figure 8—Isopach maps of the Lower Cretaceous Rutba(thin lines) and Upper Cretaceous Judea (heavy lines)formations (see Figure 3). The subcrop pattern suggestssubsequent erosion along the graben flanks. Contourinterval 50 m.

Magnitude of Extension

The cross section in Figure 5 can be used to pro-vide simple estimates of extension across theEuphrates graben system. Two methods are appliedhere, and both methods yield estimates of not morethan 6 km total extension. First, line-length balancingat the base of the Upper Cretaceous results in an esti-mate of only about 0.9 km; however, line-length bal-ancing commonly gives lower estimates of extensionthan do other methods. In this instance, the low esti-mate of extension primarily is due to the steepness ofthe faults, typically more than 70°, but their true atti-tude is poorly constrained by the seismic data (e.g.,see Figure 10). Because the extension along a givenfault varies as the cosine of the dip angle, slightchanges in fault dip lead to significant differences inbed length measurements. Also, in presenting Figure5 we did not consider many minor faults and splaysthat may accommodate additional extension, so bynecessity these results represent a minimum esti-mate. Marrett and Allmendinger (1991) argued thatsmall faults can account for a significant amount ofthe total strain across a fault system.

An alternative method is to calculate the extensiondirectly from the excess sediment area. This methodassumes that isostatic equilibrium was maintainedthroughout rifting, and that low-density synrift andpostrift sediments are compensated by thinning ofthe crust and lithosphere. The isostatic balance issummarized by Turcotte (1983) in the followingequation:

(1)

where ys = the synrift sedimentary thickness, thefirst term on the right = the combined synrift andpostrift thickness; the second term = the postriftaccumulation. We wish to solve for γC, the crustalthinning factor, and γL, the lithospheric thinningfactor. Absent evidence to the contrary, these termsare assumed to be equal. For many of the variablesused in these terms, we have taken these common-ly used values: ρm = the density of the mantle (3300kg/m3), ρc = the density of the crust (2750 kg/m3),αm = the coefficient of thermal expansivity of typi-cal rocks (3 × 10–5/K), Tm – T0 = the difference intemperature between the surface and the mantle(1250 K), and yL0 = the initial lithospheric thick-ness (assumed to be 180 km).

The initial crustal thickness, yC0, is taken as 37km based on a seismic and gravity transect insouthwestern Iraq that passes close to southeast-ern Syria (Alsinawi and Al-Banna, 1990), and onthe gravity model of Sawaf et al. (1993) and Khairet al. (1997) in eastern Syria. The area of the syn-rift (Late Cretaceous below the postrift unconfor-mity; A on Figure 3) section was measured fromFigure 5, and found to be 50 km2. For a rift zone100 km wide, the measured areas correspond toaverage sediment thickness of 500 m. For thepostrift (primarily Paleogene) thickness we alsosubtracted the average thickness of postrift-agesediments observed outside of the rifted area.This method yields a value of 625 m. The averagedensity of the synrift and postrift sediments (ρs) isderived from the density log of the El MadabeNW-101 (about 15 km northwest of the cross sec-tion). The synrift density is 2.43 kg/m3, andpostrift density is 2.22 kg/m3. An average valuefor the combined synrift and postrift sections,weighted in proportion to their thicknesses, isthus 2.313 kg/m3.

y y

T T

sm c

m sC C

m m m

m sL L

=−( )−( ) −( ) −

−( )−( ) −( )

ρ ρ

ρ ργ

ρ α

ρ ργ γ

0

00

1

0 5 1.


Thayyem 103

Tr

Ord ?

Two

-Way

Tra

velt

ime

(s)

Pzu ?

Ku

5 km

SW NE

Kl

Paleogene

SM 80-18

Neogene

0

1

2

3

4

Figure 9—Seismic exampleshowing Thayyem fieldand fault, an example of a major normal fault into a half graben. Note themarked unconformitywithin the Upper Cretaceous. Pzu = rocks ofPaleozoic age younger thanOrdovician. See Figure 2for location. Abbreviationsare explained in Figure 5.

Using the values in equation 1 yields a thinningfactor of 0.946, corresponding to a stretching fac-tor (β value of McKenzie, 1978) of 1.06. Thus, 6 kmof stretching has taken place, thinning the initial 37km crust to 35 km, and thinning the lithosphereapproximately 10 km, from 180 km to 170 km. Thesecond term of equation 1 also provides an inde-pendent assessment of the postrift thickness basedon thermal subsidence after the end of rifting,which here yields an average postrift thickness of604 m, compared to the observed value of 625 m.These calculations, although necessarily nondefini-tive because they are based on certain assumptions(lithospheric thickness, mantle temperature, etc.),thus suggest that the Euphrates graben systemunderwent stretching of about 6 km, and subse-quent thermal subsidence appears to have extend-ed throughout the Paleogene.

The fact that lower and middle Miocene forma-tions are thin or absent also suggests that postriftsedimentation ef fectively ceased before theNeogene. In addition, the late Miocene depocenteris offset somewhat to the northeast from the locusof rifting (compare Figures 10 and 11). Thus, theupper Miocene section probably is dominated bysubsidence that may be associated with develop-ment of the Mesopotamian foredeep and theZagros continental collision.

The amount of strike-slip along the Euphratesfault system is more difficult to quantify, and reliable

estimates are not possible with the current data set;however, comparing the number, continuity, andapparent seismic disruption of the normal faultsand faults interpreted to involve some strike-slipmotion suggests that the magnitude of strike-slipdisplacement is not greater than the extensionaldisplacement. Mapping of Lower Cretaceous reflec-tions provides little evidence for significant strike-slip movement. In particular, the Euphrates faultsystem probably did not accommodate the nearly60 km of offset necessary for the Abd El Aziz andSinjar zones to represent the northeastward contin-uation of the Palmyrides (see Figure 1B), as suggest-ed by Lovelock (1984).

DISCUSSION

Hydrocarbon Habitat

The trend of major producing oil fields in theEuphrates graben system occurs in a fairwayapproximately 40 km wide (Figure 2). This fairwayis centered northeast of the Euphrates River nearIraq and trends northwest, becoming morewest–northwesterly near the Khabour River.Inspection of the structure map (Figure 4) revealsthat this productive trend follows the fault trendsdiscussed, as well as the rift axis. In addition, thelarger fields generally are located near the deepest

Litak et al. 1183

Euphrates R.

0

0

-1000

34°

35°

40° 41°

SYRIA

IRA

Q

N

-1500

0

-2000

-1500-500

Kha

bour

R.

50 km

34°

35°

40° 41°

SYRIA

IRA

Q

Kha

bour

R.

Euphrates R.

-1000

0

-500

-500

-500

-500

0

50 km

N

Figure 10—Generalized structure map, top of Cretaceous(see Figure 3). Contours in meters below sea level.

Figure 11—Generalized structure map, top of Oligocene(see Figure 3). Contours in meters below sea level.

part of the basin. This correlation suggests that thelocation of productive fields is largely governed bythe thickness and maturity of synrift source rocks.The source rocks are principally the Soukhne andShiranish formations, but others are documented inthe Silurian Tanf formation (Figure 3), and possiblywithin the Carboniferous (de Ruiter et al., 1995).

The Thayyem oil field (Figure 9) is exemplary ofoil-producing structures in the Euphrates grabensystem, albeit most structures are considerably sub-tler. The Thayyem was the first field discovered inthe Euphrates play in 1985 (Beydoun, 1988; deRuiter et al., 1995), and remains one of the mostprolific fields. Primary production is from theRutba Formation, with additional reserves inMiocene carbonates trapped in the invertedrollover structure. The Rutba reservoir is chargedby Upper Cretaceous source rocks, principally theSoukhne and Shiranish formations, downthrown tothe normal fault system. The thick Shiranish sec-tion also provides the seal both above and laterally,with closure achieved against the normal faults.

Most oil fields are similarly located on structuralhighs associated with normal faults and rely onupthrown closure against those faults. The Rutbasandstone is juxtaposed against the marly shales ofthe Shiranish Formation by even relatively smallfaults. Many fields of limited areal extent, but signif-icant productivity, are due to the extremely numer-ous minor faults of the Euphrates system and the

relatively undeformed sealing lithology that existsabove good reservoir rocks. Trap integrity is some-what of a concern only in the areas that have expe-rienced significant reactivation in the northwesternpart of the Euphrates fault system. In addition tothe Shiranish Formation, the shaly Derro clasticshave been proven to be good seals, and severalreservoir/seal pairs have been documented in theTriassic carbonates and evaporites (Beydoun,1991). Although the Rutba sandstone is the mostprolific reservoir in the graben, other reservoir-quality rocks have been documented in the Triassicand Carboniferous, although more drilling is need-ed to fully assess Paleozoic lithology. Sealing of aCarboniferous reservoir might have to rely oninterbedded shales or on being upthrown againstthe Upper Cretaceous carbonates. Proven recover-able reserves in the Euphrates area are estimated atwell over 1 billion barrels of light, sweet oil andmuch lesser amounts of gas. Current productionfrom the Euphrates is around 450,000 barrels perday (OAPEC Bulletin, 1996).

Cenozoic Reactivation

In contrast to the Palmyrides (e.g., Chaimov etal., 1990), Abd El Aziz-Sinjar uplift (Sawaf et al.,1993), and even the northwestern part of theEuphrates fault system (Litak et al., 1997), theEuphrates graben system in southeastern Syria hasexperienced relatively little inversion associatedwith the Cenozoic collision along the northernedge of the Arabian plate. Near the junction withthe Palmyrides, several Late Cretaceous extensionalstructures appear to have been uplifted during theNeogene. In particular, an anticline along theEuphrates River near the main graben axis may beassociated with reactivation of a Cretaceous normalfault (at approximately 34°45′N, 40°10′E on Figure11), and probably accommodates some of the dex-tral transpression along the South Al-Bishri (Figures1B, 4) and related faults of the Palmyrides [seeAlsdorf et al. (1995) for more discussion of thisarea]. In contrast to the South Al-Bishri fault, how-ever, there is no seismicity or topographic evidencefor Quaternary inversion of this structure.Shortening associated with this structure is limitedto a maximum of a few hundred meters, and proba-bly less.

In the northwestern part of the Euphrates faultsystem, Litak et al. (1997) documented significantuplifts and clear instances of true inversion of nor-mal faults. Such features are not apparent in south-eastern Syria. Instead, Late Cretaceous faults seemto have been reactivated as positive flower struc-tures or uplifted blocks (see Figure 13). Thesestructures may be related to their proximity to the


40° 41°

SYRIAIR

AQ

Kha

bour

R.Euphrates R.

34°

35°500

500

1000

1000

1000

1000

50 km

N

Figure 12—Isopach map of Paleogene rocks (see Figure3), contours in meters.

east-trending Anah graben in Iraq (Dunnington,1958; Ibrahim, 1979) and also accommodate mini-mal shortening. Upwarping of Pliocene reflectorscan be noted on several structures. Strike-slipdeformation also may occur along the northernedge of the Euphrates graben system where seismiccontrol is relatively poor. (This end of theEuphrates graben and its relationship to the Abd ElAziz and Sinjar uplifts will be further explored infuture research.) The presence of several linear seg-ments of the Euphrates River and the general coin-cidence of its present course with the mapped faulttrends suggest some Quaternary activity alongthose faults, especially in southeastern Syria(Ponikarov, 1967), but no significant Quaternaryoffsets are observed on the few seismic lines thatcross the Euphrates River valley.

Regional Relationships

Northwest-trending structures are apparent allover the northern Arabian plate, from the Red Seato the Zagros Mountains. Beydoun (1991) suggest-ed that these structures might all be related to thelate Proterozoic Najd fault system exposed in SaudiArabia (e.g., Stoesser and Camp, 1985; Agar, 1987;Husseini, 1989). The Najd deformation may haveimparted a deep-seated regional fabric that con-trolled the locus of later episodes of deformation.Lovelock (1984) conjectured that the Euphratesfault system, after an eastward step at the Anahgraben, continued along the southeast-striking

“Abu Jir” trend. Kader and Marouf (1994) discussedparallel structures farther east along the Tigris Riverthat appear to have a remarkably similar history tothe Euphrates graben system. These workersdescribed flower structures, normal faulting, andgraben formation active during the Campanian–Maastrichtian with minor late Neogene dextraltranspression. Litak et al. (1997) attempted to fur-ther link the Najd fabric to the Sirhan graben andoverlying basalt field in Jordan, and also suggestedthat the Euphrates fault system may represent aProterozoic suture between the Rutbah and Rawdablocks. This conjecture is supported by the gravitymodeling of Brew et al. (1997).

The fault trends mapped in this study generallyare consistent with the exposed Najd faults, lend-ing credence to those interpretations; however, asdiscussed, two fault populations of different char-acteristics and slightly different strikes can be dis-tinguished. The northwest-striking faults in south-easternmost Syria are more nearly parallel to thoseof the Najd trend. Because they also appear toevince more strike-slip movement than the graben-type normal faults farther northwest, one interpreta-tion is that these northwest-striking faults wereoblique to the Late Cretaceous extension direction,whereas the graben-type normal faults may havebeen more nearly orthogonal to extension. This sce-nario implies that the west-northwest–striking faultsinitially may have formed in response to LateCretaceous stresses rather than being reactivatedProterozoic faults. That is, Late Cretaceous intraplatestrain may have been initially accommodated by the

Litak et al. 1185

Neogene

Paleogene

Ku

Kl&Tr

Pzu

Ord

Mid C-

Tw

o-W

ay

Tra

vel t

ime

(s)

0

1

2

3

4

SW NE

2 km

AS-703 Figure 13—Seismic exampleshowing uplifted blockexhibiting apparent strike-slip faulting adjacentto half-graben (to northeast). Note slightupwarping of Paleogenereflections signifyingminor Cenozoic reactivation. This structureis the site of a reported1994 oil and gas discovery.Pzu = rocks of Paleozoicage younger than Ordovician. See Figure 2for location.

Najd faults, but later broke through a new set ofmore favorably oriented structures. This interpreta-tion would suggest that Late Cretaceous motion ofthe Rawda block relative to the Rutbah block wasnorth–northeast rather than north as implied byLovelock (1984) and Litak et al. (1997). Alternately,the change in fault orientation may reflect a changein the preexisting structures at the “corner” of theRutbah block formed by the intersection of theEuphrates and Palmyrides trends.

Comparison With Other Rifts

At about 160 × 90 km, the scale of the Euphratesgraben system in southeast Syria is roughly compa-rable to both the North and South Viking grabens(e.g., Beach et al., 1987), and to the half grabens ofnorthern Lake Tanganyika (Rosendahl et al., 1986).The Euphrates graben system is slightly larger thanthe largest basins in the Rio Grande rift (e.g.,Russell and Snelson, 1994), and about the same


End Early Cretaceous

TriassicPaleozoic

Lower CretaceousSW NE(A)

? ?

Coniacian

TriassicPaleozoic

SW NE

(B)

?

Maastrichtian

TriassicPaleozoic

?

SW NE

(C)

End Paleogene

TriassicPaleozoic

Upper Cretaceous

Lower Cret.

SW NE

(D)

Triassic

PaleozoicLower Cret.

Late Neogene

Paleogene

??

Upper Cretaceous

NESW

Not to scale

(E)

Figure 14—Series ofschematic cross sectionsillustrating conceptualevolution of theEuphrates graben system.Darkest shaded area represents most recentsedimentation in each case.

width, but one-half the length, of the Suez rift (e.g.,Chénet et al., 1987). Thus, the Euphrates systemprobably would be classified as a rift unit in thenomenclature of Rosendahl (1987), although itsaspect ratio is approximately one-half that of typi-cal rifts. In this nomenclature, the combination ofthe Euphrates graben system and the Abu Jir zone,described by Lovelock (1984) as a transtensionalstructure, may represent a rift zone. The offsetalong the Anah graben might then form an accom-modation zone between these two. Litak et al.(1997) demonstrated that Senonian extension,albeit much less pronounced, is also present alongthe Euphrates trend northwest of the Palmyrides,occurring at least as far north as the Turkish border.

Our extension estimate of about 6% (β = 1.06) islower than that reported in the literature for mostother continental rifts. Kusznir and Park (1987)compiled β values ranging from 1.1 to 1.3 in theRhine graben to 1.55 to 1.9 in the North Sea centralgraben, with a representative average of 1.4–1.5.Using a variety of techniques, several workers haveestimated extension in the Gulf of Suez rangingfrom 1.1 in the northern part (Colletta et al., 1988)to 1.65–1.7 in the southern part (Angelier, 1985)[see Patton et al. (1994) for compilation]. Bosworth(1995) recently estimated β for the central basin ofthe southern Gulf of Suez rift at 1.9–2.0, placing itas perhaps the most highly extended of failed conti-nental rifts. The relatively low value for extensionin the Euphrates graben system signifies that it wasarrested at an early stage of development.

In contrast to the “classical” rift structure asseen in the East Africa rift (Rosendahl, 1987), theEuphrates graben system does not exhibit sharpedges characterized by major listric boundingfaults. Instead, the southwestern edge of theEuphrates system is marked by several faults thatbecome progressively smaller onto the edge of theRutbah uplift (Figure 5). The northwestern part ofthe rift (near the Palmyrides) in particular has thegross morphology of a full graben; farther south-east, the southwestern margin is manifested by asteep-sided flexure with minimal throw across theassociated fault (Figures 6, 7). This feature has theappearance of the updip end of a half graben, butdoes not correspond to a major southwest-dip-ping normal fault. Major transverse structures,such as those reported in the East African rift(Rosendahl, 1987), are not observed. Chénet et al.(1987) postulated that transverse faults in theSuez r ift followed the trend of a preexistingcrustal fabric. If Najd-type faults underlie theEuphrates graben system, the preexisting fabric isnearly orthogonal to the extension direction, per-haps explaining the lack of transverse structures.Listric faults are only clearly observed in the deep-est part of the graben.

Although not ubiquitous, high-angle faults anddeep-water sedimentation were argued by Morley(1995) to be common features at early stages of con-tinental extension. Their presence here again sug-gests that the Euphrates rift aborted early on. On thenorthwestern margin of the Española basin in NewMexico, Baldridge et al. (1994) also noted numeroushigh-angle, planar normal faults accommodatingminimal extension and distributed over a broadzone. Baldridge et al. (1994) interpreted that zone asan aborted rift boundary and suggested that thisstyle of deformation may characterize the initial for-mation of rift basins, an inference consistent withour results. The cessation of rifting in the Euphratesgraben may have been due to a change in the state ofstress in the Arabian plate related to the LateCretaceous convergence associated with ophioliteemplacement along the Arabian plate margin.

CONCLUSIONS

The Euphrates graben system in southeast Syria isan aborted continental rift that was primarily activein the Late Cretaceous. Following Lower Cretaceousdeposition of the Rutba Formation (Figure 14A),extension began during the Coniacian with blockfaulting, development of a regional unconformity,and limited deposition of continental clastics (Figure14B); however, the main phase of deformationoccurred during the Campanian–Maastrichtian(Figure 14C), with extensive normal faulting andgraben formation. Faulting essentially ceased by thePaleocene; a Paleogene thermal sag basin (White andMcKenzie, 1988) overlies the graben system (Figure14D). An additional phase of deposition in the upperMiocene overlies part of the Euphrates graben, butprobably is primarily related to the Mesopotamianforedeep associated with the nearby Zagros conti-nental collision. Minor late Neogene transpressionreactivated some of the structures in response to theZagros-Bitlis continental collision (Figure 14E). Thearea appears to be tectonically inactive at present.

The total amount of extension is minimal, proba-bly not more than 6 km, but deformation is extreme-ly widespread and complex considering the amountof extension. This may be partly due to the presenceof a preexisting zone of weakness that served to dis-tribute deformation across numerous preexistingfaults. The amount of strike-slip displacement also isinferred to be minimal. Two distinct fault popula-tions are noted: west-northwest–striking normalfaults with relatively large throws in the northwest-ern part of the study area, and steeply dipping,northwest-striking f lexures and strike-slip faultsnearer to the Iraqi border.

Light oil and minor gas accumulations are foundprimarily in Lower Cretaceous clastic reservoirs

Litak et al. 1187

upthrown to both large and small normal faults.The charge is believed to emanate from lateSenonian synrift source rocks downthrown to thetrapping faults. Larger fields occur in the deepestpart of the basin, probably due to greater thicknessand increased maturity of source rocks.

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Litak et al. 1189

Robert Litak

Bob Litak received his bachelor’sdegree in geology from MichiganState University in 1982. He thenworked for ARCO as a geophysicistuntil 1986 before returning to grad-uate school at Cornell. There, hegained his master’s degree in 1990,and Ph.D. in 1993 before joiningthe Cornell Syria Project as aresearch associate under the direc-tion of M. Barazangi. His interestsinclude regional tectonics of the Middle East and seismicinterpretation.

Muawia Barazangi

Muawia Barazangi is a senior sci-entist and faculty member in theDepartment of Geological Sciencesand Institute for the Study of theContinents (INSTOC), CornellUniversity. He is the associatedirector of INSTOC, and leader andcoordinator of the Syria Project atCornell University. His academicbackground includes a B.S. degreein physics and geology fromDamascus University, Syria, an M.S. degree in geo-physics from the University of Minnesota, and a Ph.D. inseismology from Columbia University’s Lamont-DohertyGeological Observatory. His professional experienceincludes global tectonics, seismotectonics of continentalcollision zones, tectonics of the Middle East, and struc-ture of intracontinental mountain belts.

Graham Brew

Graham Brew graduated fromUniversity College London, UnitedKingdom, with a bachelor’s degreein geophysics in 1995. During hisyears as an undergraduate heworked for a time with RTZ Miningand Exploration in Santiago, Chile.He is currently studying for hisPh.D. in geophysics at CornellUniversity under the supervision ofM. Barazangi. His interests includeseismic interpretation, regional tectonics, and potentialfield methods.

Tarif Sawaf

Tarif Sawaf is a senior geologistand associate head of the regionalgeologic department, Syrian Petro-leum Company. His academicbackground includes a B.S. degreein applied geology from DamascusUniversity, Syria. His professionalexperience includes seven yearsemployment as a petroleum geolo-gist with Sonatrach, Algeria, and hecurrently specializes in the subsur-face and structural geology of Syria.

ABOUT THE AUTHORS

Anwar Al-Imam

Anwar Al-Imam obtained both aB.S. degree in 1965 and a Ph.D. in1974 in applied geophysics fromMoscow State University, Russia.He has worked as a lecturer inDamascus University, as director ofthe General Geological Survey ofSyria, and as head of GeophysicalInterpretation for the SyrianPetroleum Company. He alsoworked as a professor in theInstitute of Earth Sciences in Oran in Algeria,1990–1992. His interests include seismic interpretationand modeling.

Wasif Al-Youssef

Wasif Al-Youssef was the directorof the Exploration Department,Syrian Petroleum Company, 1988–1997. Since 1972, he has held thepositions of director, Service-Contract Department; director,Petroleum Studies Department;and chief of Petroleum ReservoirEngineering. Wasif received hisdiploma-engineering degree inpetroleum geology in 1972, and hisdoctorate in petroleum engineering in 1993 from theInstitute for Geology of Energy and Exploitation ofCombustible Resources, Russia. Wasif has participatedin numerous energy-related conferences as a speaker.


Documents

Structure and Evolution of the Petroliferous Euphrates Graben …atlas.geo.cornell.edu/syria/AAPG_98_Litak.pdf · 2006. 8. 28. · ABSTRACT The northwest-trending Euphrates graben