Geology and petroleum prospects of Upper Triassic sediments, Jordan

Marine and Petroleum Geology 04 "0887# 672Ð790

S9153Ð7061:87 ,08[99 Þ 0887 Elsevier Science Ltd[ All rights reservedPII] S 9 1 5 3 Ð 7 0 6 1 " 8 7 # 9 9 9 3 5 Ð 3

Geology and petroleum prospects of Upper Triassic sediments\Jordan

F[ N[ Sadoonia\�\ A[ Dalqamounib

a Eden Petroleum Services Ltd[\ PO Box 00!448\ Ellerslie\ Auckland 0992\ New Zealandb Department of Earth and Environmental Sciences\ Yarmouk University\ Irbid\ Jordan

Received 20 January 0886^ revised 15 June 0887^ accepted 18 June 0887

Abstract

Subsurface Upper Triassic sediments of northern Jordan represent part of a regressive evaporitic!clastic succession that marks theshrinkage phase of the Late Triassic basin in the northern parts of the Arabian Plate[ Sabkhas developed along the basin margin\whereas\ oolitic shoals formed on the deeper parts of the carbonate platform[ The basin reached a drewdown stage in the Risha\Palmyra and parts of northern Iraq\ where halite was precipitated[ Local shales\ marls and argillaceous limestones are the majorsource rocks[ The total organic content values of the shales and carbonates range between 9[4Ð0[8)[ The main reservoir rocks arethe oolitic limestones with porosities of 7Ð19) and permeabilities that range between 9[90Ð79 md[ Regional swells and troughs thatwere cut by normal and strike!slip faulting are the main structural styles in the area[ Favourable conditions for hydrocarbongeneration and accumulation may be found under the Hauran Basalts in NE Jordan[ Þ 0887 Elsevier Science Ltd[ All rights reserved[

Keywords] Triassic evaporites^ Oolitic limestone^ Sabkhas^ Jordan

0[ Introduction

The exposed Triassic rocks in Jordan comprise around0999 m thick of clastic rocks\ carbonates and evaporites[Wetzel and Morton "0848# introduced the _rst litho!stratigraphic scheme for these outcrops\ which was laterupdated by Bandel and Khouri "0870#[ The successionwas divided among nine formations^ these are from theoldest\ Um Irna\ Ma|in^ Dardun^ Ain Musa^ Hisban^Mukheiris^ Iraq Al Amir^ Um Tina and Abu RuweisFormations[ Other nomenclature schemes for the Triassicsuccession in Jordan are shown in Fig[ 0[

The Jordanian Natural Resources Authority "Andrews\Makhlouf\ Taani\ Al!Bashish + Al!Hiayri\ 0881# sug!gested a new classi_cation for the subsurface sediments[A new unit\ the Ramtha Group\ was re!introduced toinclude all the subsurface Triassic sediments in northernJordan[ The term is a collective one\ originally suggestedby TOTAL geologists to accommodate the Triassic suc!cession encountered in the Ramtha!ER!0!A well\ north!ern Jordan[ Beydoun\ Futyan and Jawzi "0883# reviewedthe geological setting and hydrocarbon potential of Jor!dan including the Triassic sediments[ More recently\ Shi!

� Corresponding author[ Tel[] 9953 8 415 0429^ Fax] 9953 8 460 2103^E!mail] fsadooniÝihug[co[nz

naq "0885# divided the Triassic sediments into _veunconformity!bounded sequences[ Figure 0 comparesthese nomenclatures[

The present study deals with the geology of the LateTriassic Abu Ruweis Formation[ The sequence wasdescribed for the _rst time by Bender "0863# to includethe Upper Triassic sediments "Gypsum Formation#exposed in the Huni Valley "see Fig[ 2 for locations#[The exposed parts of this formation consist mainly oflaminated dolomitic marl and limestone\ clays and thickunits of laminated gypsum[ The thickness of the exposedsection is around 049 m in Huni Valley[ Andrews etal[\ "0881# have suggested a complementary subsurfacesection in the well Risha!0[ The detailed lithologies of theformation in that well are shown in Fig[ 1[

The study aims to investigate the depositional systemand the hydrocarbon prospects of the subsurface sectionof Abu Ruweis Formation using some selected wells innorthern Jordan[

1[ Methods of study

Borehole cuttings and core samples from the AbuRuweis Formation were collected from three wells innorthern Jordan[ These are Ajlun!0^ Northern Hi`hlands!1^ and Risha!1 wells\ "Fig[ 2#[ Gamma Ray\ Resistivity\

F[N[ Sadooni\ A[ Dalqamouni:Marine and Petroleum Geolo`y 04 "0887# 672Ð790673

Fig[ 0[ Comparison between the di}erent stratigraphic nomenclatures used for the Triassic sediments in Jordan[

Neutron:Density and Sonic Logs from these three wellswere qualitatively used to subdivide and correlate theformation into distinctive facies units across the studiedarea and quantitatively\ to calculate the volume of shaleand sonic and neutron porosity[ Thin sections were pre!pared from the collected samples and were examinedunder transmitted and polarised light microscopes[Around 09 selected samples were also examined underthe Scanning Electron Microscope to investigate porosityevolution and the replacement of carbonates by evapo!rites[

2[ Regional setting

The deposition of Abu Ruweis sediments marks theevaporitic stage of the Late Triassic\ which occurrsregionally over the northern part of the Arabian Plate[The base of the formation is determined by the _rstappearance of a thick anhydrite bed "Bandel + Khoury\0870^ Andrews et al[\ 0881# that represents a shift fromthe underlying normal carbonate!shale succession of theUm Tina Formation[

The Abu Ruweis Formation is equivalent to the KurraChina Formation in Iraq and there is a consistent litho!logical correlation between the two formations "Sadooni\0884a#[

In Syria\ the Triassic is penetrated by wells in theJazirah area "NE Syria#\ north!central areas and in thePalmyra range\ 149Ð299 km NNE of northern Jordan

"Cherrife 1\ 2 and Ash!Shaer 0 wells#[ These sedimentsinclude a succession of dolomites\ clays and pyritisedmarls with anhydrite beds[ Where the Triassic is di}er!entiated from the Jurassic\ its thickness is estimated to be799Ð0999 m\ along the axis of the Triassic basin\ "Bach!Imam\ Khoja + Sigal\ 0879^ Babeshev et al[\ 0880#[ TheCarnian sediments of this succession are assigned to theKurra Chine Anhydrite Formation\ "Beydoun\ 0870\Beydoun + Habib\ 0884#[

In Israel\ the Mohilla Formation is equivalent to AbuRuweis Formation[ The Mohilla Formation consistsmainly of gypsum or anhydrite\ dolomite and shale\ "Zak\0852^ Druckman\ 0858#\ "Fig[ 3#[

The Triassic stratigraphy of the region is in need ofgeneral revision and hence no proper correlation of theTriassic successions can be carried out across the indi!vidual countries with the present level of data[

3[ Tectonic Setting

The presence of evaporites "anhydrites and halites ofthe Kurra Chine in Iraq and Syria\ Mohilla in Israel andAbu Ruweis in Jordan# as well as in the occurrence ofplant!bearing sandstones "carbonaceous materials andpyritised wood fragments# as reported by Bandel andKhoury "0870# from the Triassic outcrops in Jordan andthe Minjur Formation in the Gulf States "Alsharhan\0882# indicate the occurrence of repeated events ofrestricted circulation in the Triassic basin[

F[N[ Sadooni\ A[ Dalqamouni:Marine and Petroleum Geolo`y 04 "0887# 672Ð790 674

Fig[ 1[ Lithologic column of Abu Ruweis Formation in its type locality at the well Risha!0\ "modi_ed after Andrews et al[\ 0881#[


Fig[ 2[ The main locations mentioned in the text[

Rifting during the Late Triassic was regionally wide!spread and associated with intrusions such as the Sah!aronim basalt which was described from MakhteshRamon in Israel by Baer\ Heimann\ Eshet\ Weinberger\

Fig[ 3] Stratigraphic nomenclatures of the Triassic sediments in the northern parts of the Arabian Plate[

Musset and Graham "0884#[ These intrusions occur inthe gypsum beds of the Mohilla Formation[ Radiometricdating of these basaltic bodies has suggested a LateTriassic to Early Jurassic age and paleomagneticmeasurements indicate that southern Israel was near theequator at that time[ Other magmatic bodies of approxi!mately the same age are found beneath Mt Carmel\ Israel"Dvorkin\ Amos + Kohn\ 0878#[ A dolerite body morethan 009 m thick was encountered in Risha!1 well[Although no analysis has been carried out on these rocks\they may correlate with the Saharonim basalts of Israel[Other volcanic materials of di}erent ages\ were describedalso from Triassic outcrops by Bandel and Khoury"0870#[

Upper Triassic sediments are restricted to the northernand central parts of Jordan[ To the east and south\ clasticsediments become dominant[ Further to the south\ inSaudi Arabia and the Gulf States\ evaporites are foundwithin the Jilh Formation\ "Sharief\ 0871\ 0872^ Alsh!arhan\ 0882#[

4[ Age and fauna

In the studied wells\ the basal part of the Abu RuweisFormation\ which consists of evaporites\ limestones\shales\ and dolomites has been dated as Early Carnian"SSI\ 0878#\ while the remainder of the succession has


been assigned to the Mid!Late Carnian[ In the well Ram!tha "ER!0A#\ the sequence was given a Carnian and LateTriassic "undi}erentiated# age by Keegan\ Shaheen andMajed "0876a#[ In Risha!0 well the formation is con!sidered of Carnian age\ "Keegan\ Shaheen + Majed\0876b#[

There is little evidence concerning the age of AbuRuweis Formation from outcrop[ On the basis ofMyophoria sp[ "Blake\ 0824#\ Bandel and Khouri "0870#suggested a Carnian age for that section[ This assumptionis also supported by a tentative correlation with Israel\where the equivalent Mohilla Formation is assigned tothat age "Druckman\ Hirsch and Weissbrod\ 0871#[ Theage of the lower member of the Mohilla Formation wasdeduced as Carnian since it overlies the ammonites!bear!ing sediments "Clionites sp[# of Lower Carnian age"Parnes\ 0851#[

More recent studies of the Abu Ruweis outcrops haverecorded the presence of the conodont species\ Mosh!erella newpassensis "Mosher# which suggests a LowerCarnian age\ "Abu Hammad\ 0883#[

5[ Vertical succession

The type section of Abu Ruweis Formation in Risha!0 well consists from bottom of alternations of dolomiteand anhydrite that topped by halite\ followed by a suc!cession of argillaceous limestone\ shale\ dolomite andanhydrite[ The upper part of the formation consists pre!dominantly of argillaceous limestone\ "Fig[ 1#[ A com!bination of gamma ray and sonic logs was used tosubdivide the formation into four distinctive vertical units"Sadooni\ 0884b#[ These are from the top to bottom\ "Fig[4#[

5[0[ Unit AR0

The unit consists of dolomites and anhydrites in Ajlun!0 well but changes to shaly limestones and mudstones inboth Risha!0 and 1 wells[ The relative abundance of cla!stics in Risha area indicates the increasing in~uence ofthe neighbouring continental area to the south and theeast[ Gamma!Ray readings are increasing sharply fromAjlun toward Risha\ which re~ect this situation[ Thethickness of the unit is around 14 m in Ajlun!0 andincreases to 24 m in Northern Hi`hlands!1 but thins againin Risha area\ "Fig[ 4#[

The base of this unit is placed at the top of a thickanhydrite bed[ It represents the _nal stage of Abu Ruweisdeposition that ended with a regional unconformity[

5[1[ Unit AR1

This unit comprises the evaporitic components of thesuccession[ It consists of alternation of thick anhydrite

beds and thin dolomites in both the Ajlun and NorthernHi`hlands wells[ In the Risha area\ the unit is truncatedby a thick dolerite body 009 m thick and contains around29 m of halite in the Risha!0 and of dolomite mainly inthe Risha!1 well[ It is characterised by a spiky Gamma!ray log "except in Risha!0# due to the presence of thinshale or mudstone units between the thick anhydrite beds[The average thickness of the unit is around 79 m but itthins considerably in Risha!0 well[

5[2[ Unit AR2

This is a relatively thin unit that consists mainly of thinshale units alternating with thin dolomitic beds[ It ischaracterised by erratic and spurious response of theSonic log due to the friable nature of the sediments andhigh shale contents as re~ected by the high Gamma!rayreadings "up to 79 API units#[ The thickness of this unitis around 14 m in Ajlun!0 and increases in thickness inNorthern Hi`hlands!1 but it thins again toward Risha!0well[

5[3[ Unit AR3

This is the lowest and the thickest unit of the formation[It consists mainly of alternations of oolitic!peloidal lime!stones with dolomites and shales with thin anhydrite bedsthat grade to halites in the Risha area[ The unit representsthe newly established shallow water carbonate!evaporiteshelf of the Late Triassic in the area[ The unit is charac!terised by a spiky gamma!ray response due to the pres!ence of shale beds but also contains good porosity zonesas re~ected by the Sonic log readings[ The average thick!ness of this unit is around 199 m but may reach the doubleof this in Risha!0 well[

6[ Lithologies

6[0[ Carbonates

Detailed petrographic analysis was made of 019 thinsections prepared on selected samples of cores and cut!tings from the limestone horizons of the Abu RuweisFormation[ These were corrected for lag time with petro!physical logs[

The studied carbonates of the Abu Ruweis Formationhave been distributed among six major facies[ Each ofthese represents a distinct environment or a sub!environ!ment of deposition[

6[0[0[ Bioclastic packstone faciesThis facies is dominated by echinoderm debris and

pelecypod fragments\ embedded in anhydritic\ often darklime mudstone\ "Fig[ 5A#[ The matrix contains abundant


Fig[ 4[ Vertical subdivisions and lateral correlation of Abu Ruweis Formation in the studied wells[ For section orientation see Fig[ 1[


Fig[ 5[ The main carbonate microfacies of Abu Ruweis Formation] "A# Bioclastic packstone of large pelecypod shell fragments\ echinoderm debris[Some of the pores are _lled either with glauconite or anhydrite cement[ NH!1 well\ core 8\ 1979 m\ polarised light[ "B#] Oolitic packstone in sparrycalcite cement[ The ooliths are of normal "a# type[ RH!1 well\ cuttings\ 0309 m\ polarised light[ "C#] Oolitic grainstone of Bahamite type[ Somepatches of glauconite and anhydrite in the groundmass[ AJ!0 well\ cuttings\ 0419 m\ polarised light[ "D#] Spherulitic packstone with rim calcitecement[ Some patches of anhydrite cement[ AJ!0 well\ cuttings\ 0479 m\ polarised light[ "E#] Lime mudstone] homogenous\ _nely fractured with silt!size quartz and patchy anhydritic cement[ NH!1 well\ core 09\ 1204[3 m\ polarised light[

unidenti_ed silici_ed shell fragments\ but the silica maybe diagenetic[ The thickness of this facies reaches 09 mat some intervals[ Glauconite is also present inside someof the shells[

The facies may represent the subtidal\ normal marinesedimentation of the basin or a shelf lagoon[ Normallysuch facies contain benthonic foraminifera also\ but theseare believed to be totally obliterated by severe diagenesis[


The glauconite may be developed in local micro!anoxicenvironments[

6[0[1[ Oolitic grainstone!packstonesDi}erent types of oolites were recorded from the Abu

Ruweis Formation[ Concentric ooliths "Normal ooliths#were found at unit AR!3 of the Abu Ruweis section inthe Ajlun!0 well[ These ooliths are of spherical shape withconcentric cortical laminations[ Their size ranges between099Ð079 mm\ with size reduction and sorting deteriorationdownward[ No skeletal components were found in associ!ation with this type of ooliths[ The matrix is mainlydolomicrite[ Occasionally\ anhydrite replaces either thewhole oolith or its nucleus only "Fig[ 5B#[

Micritic ooliths are found in the upper section of theAjlun!0 well\ and the lower part of the Risha!1 well\ witha total thickness that does not exceed 04 m[ These arecharacterised by micritic cortex without obvious struc!ture with extensive pyrite pigmentation around the exter!nal envelopes of the ooliths[ They ~oat in an anhydriticgroundmass with patches of glauconite "Fig[ 5C#[ Thegrains are large and moderately developed\ but in thelower part of the interval\ the facies changes into a mix!ture of ooids and peloids[ No skeletal components wherefound with this type[

Spherulites are characterised by the presence of radialstructure perpendicular to the concentric layers and orig!inate from a nucleus toward the outer boundaries or arerestricted at the outer part as an external crust on theoriginal oolites[ This type is elongated to hemisphericalin shape "Fig[ 5D#[

Oolitic shoal facies have been described from theTriassic sediments of the Arabian Plate from di}erentlocations "Sadooni "0884a# from Iraq^ Shinaq "0885# fromJordan and Alsharhan "0882# from the Gulf States#[

The oolitic grainstone and packstones are probablyformed in high!energy shoals as a part of a regressivecycle similar to the Recent ooids described by Loreauand Purser "0862# from the Gulf[ It is believed that ooliticgrainstones and packstones are formed in a highstandsetting along basin margins in the form of oolitic shoals\beaches and tidal bars[

Flu�gel "0871# suggested that spherulites are usuallyfound in water with higher salinity as compared to nor!mal marine water and in relatively quiet water as well[This may be the case in some of the Triassic basins in theArabian Plate\ where salinity was higher than normal[These are usually mixed with peloids and other types ofoolites[

6[0[2[ Peloidal grainstones!packstonesMany intervals of the Abu Ruweis Formation are

made up of peloidal limestones[ This facies is prevalentmostly in the upper and upper middle parts of the section[

Peloids in this facies are characterised by di}erent sizes\shapes and sorting[ In the Northern Hi`hlands!1 well\

peloids are compacted and oriented[ This led to the defor!mation of their original shapes resulting in elliptical pelo!ids "Fig[ 6A#[ Peloids range in size from 039Ð059 Someof the studied peloids are associated with extensive biot!urbation or with stromatolites "Fig[ 6B#[ Burrowing andlater _lling of these burrows with mud produces peloidsof di}erent shapes and sizes and most of these are _lledwith pyrite pigments[ In some cases\ peloids are exten!sively dolomitised or replaced by anhydrite thatdestroyed their shape[

In both Recent and ancient carbonate sediments\ pelo!ids are either fecal pellets formed in a low!energy\ shel!tered environment or micritised skeletal particles suchas small foraminiferas or even ooliths as suggested byBathurst "0855#[

Some of the peloids of this facies belong to the secondtype[ They may be originally ooids that have been trans!ported from oolite shoals and accumulated in localdepressions or lagoons where they underwent diageneticprocesses such as micritisation and boring[ The presenceof these grains in quantities su.cient to form grainstonetexture\ their depletion of clayey materials\ and theirgradual\ vertical and horizontal change into oolitic faciessupport the second suggestion[ Others may be formedeither by the action of bioturbation or even by the mech!anical erosion of semi!lithi_ed sediments along smallchannels "Fig[ 6C#[

6[0[3[ Stromatolitic boundstoneThis facies consists of closely spaced thin\ algal lami!

nations with dark\ anhydritic\ and pyritic mudstonematrix[

Stromatolites are encountered at many levels in thestudied wells[ Single laminations are of di}erent thicknessup to 0 m in some places[ The most developed sectionsare found in Northern Hi`hland!1 well\ where they areassociated with extensive bioturbation and peloidaltexture[ In some cases\ alternations of nodular anhydriteand stromatolitic boundstone are found[

Stromatolites are formed by cyanobacterial ormicrobial mats[ The dark colour of algal mats could beattributed to the development of disseminated pyrite dueto local anoxic conditions[ Beneath the mat\ grains areblackened due to reducing conditions and may be alteredby boring algae\ to peloids of lime mud[

Stromatolites ~ourish normally in the upper intertidalzone[ The extensive bioturbation and the peloidal texturemay support this assumption[ Abu Ruweis laminatedcyanobacterial mats may indicate a similar environ!mental setting[ The anhydrite association suggests a typi!cal sabkha or lagoonal sequence[

6[0[4[ MudstoneThis facies is formed of dark brown\ homogeneous\

lime mudstone\ which is fractured and partially replacedby evaporite crystals "Fig[ 5E#[


Fig[ 6[ Peloidal limestones of Abu Ruweis Formation] "A# Compacted\ oriented peloidal grainstone[ Peloides are elongated as a result of compaction[NH!1 well\ core 8\ depth 1965 m\ polarised light[ "B# Extensively burrowed peloidal limestone[ Peloids are darker because of pyrite pigmentation andhigh organic content[ NH!1\ core 8\ depth 1964\ scale 0 ] 2[ "C# Core sample of laminated\ bioturbated limestone[ Local erosion produced smallchannels and peloidal texture[ Fractures are _lled with anhydrite cement[ NH!1 well\ core 09\ depth 1219 m\ scale 0 ] 3[

In the Ajlun!0 well this facies consists of lime mudstone\neomorphosed and fractured\ changing sometimes toargillaceous or shaley mudstone with silt!sized silicagrains[ In some cases the mudstone is extensively replacedby laths of anhydrite or pyrite[

In the Northern Hi`hlands!1 "NH!1# and Risha!1 "RH!1# wells\ the facies is composed of neomorphosed\ argil!laceous\ dense mudstones\ with stringers of black andbrown shales[ Micrite is replaced by patches of anhydritelaths[ Silica cements in_ll some fractures[ It contains alsosome micro!stylolites[ The facies thickness rangesbetween a few centimetres to more than 39 m in someintervals[

This facies is deposited in an evaporative tidal ~at[ Itindicates extremely shallow water conditions[ The pres!ence of evaporites indicates hypersaline conditions andit may represent an integral part of the sabkhasetting[

6[1[ Dolomites

Two types of dolomite were recognised from AbuRuweis Formation[

6[1[0[ Sabkha!type\ early diagenetic dolomiteThis type consists predominantly of _ne rhombs of

pyritised dolomite\ with radial or _ne granular anhydritecement\ which becomes more abundant at the top "Fig[7A#[ It contains locally also organic!rich micrites withrare silici_ed shell fragments that appear as patches andare frequently interbedded with dolomitised\ stro!matolitic mudstone[ The thickness of this type may varyand can reach 04 m in some intervals[

This type is probably formed by the dolomitisation ofthe original argillaceous aragonitic mudstone[ Dol!omitisation may be due to the action of hypersaline brinesin a sabkha setting[ This assumption is supported by


the association of this dolomite with evaporites\ its _ne!grained texture and the predominance of fossil!free\ stro!matolitic lime mud in this facies[ All these suggest ahypersaline lagoon or sabkha setting[

6[1[1[ Late diagenetic dolomiteLate diagenetic dolomite is found as medium! to large!

sized\ partially porous or interlocked rhombs of dolomiteobliterating the original texture of the host sediments[This type is considered as late diagenetic because of itsirregular distribution\ zonation and because it is notnecessarily associated with evaporites[ However in somecases\ voids between rhombs are _lled by anhydritecement\ "Fig[ 7B and 7C#[

The Mg for this late dolomitisation may be releasedduring shale compaction "e[g[ montomorillonite trans!formation to illite#\ or from high magnesian calcite grains"local source dolomitisation#\ or as a result of meteoric!sea water mixture[ Evidence in support of the mixingregime is that\ this dolomite is very irregular in distri!bution\ associated with solution features and highporosities probably due to dissolution by meteoric water\and is rarely associated with evaporites[

6[2[ Sandstones

Sandstone beds occur in the lower part of the AbuRuweis section in RH!1 well "at depths of 0209 and 0289m# with a thickness that does not exceed 4 m[ The termquartz arenite is used here for these clastics[ The faciesconsists of subrounded to subangular grains of poorlysorted quartz grains "Fig[ 8#[ Carbonate lime mud is themain cementing material with traces of anhydrite andmicro!crystalline silica[ Hematite is present at the bound!aries of some quartz grains[

This facies represents the clastic in~ux from the nearbyland in the south and east[ It could represent the distalfront of a complex of ~uvial!lacustrine setting at thesouthern or eastern extensions of the basin[

6[3[ Evaporites

6[3[0[ Gypsum and anhydriteThe uppermost 049 m of the Abu Ruweis Formation

exposed in Zerqa Valley "Fig[ 2 for location# include thickunits of laminated anhydrite "Bandel + Khoury\ 0870#[Ahmad and Nabulsi "0876# described massive gypsumlenses up to 29 m thick from the middle part of thesequence that thin out rapidly and are laterally replacedby shale[

In the subsurface\ anhydrite also occurs in the form ofthick beds\ and as discrete nodules or bands of coalescednodules "Fig[ 09A#\ some of which may take the form ofenterolithic layers[ In some cases\ laths of anhydrite may_ll pores and vugs within carbonates as displacive cement\"Fig[ 09B and 09C#[

Fig[ 7[ Dolomites of Abu Ruweis Formation] "A#] Euhedral!subhedralmedium!grained dolomite[ AJ!0 well\ cuttings\ 0349 m\ polarised light["B#] Interlocked\ coarse!grained\ xenotopic dolomite with some hydro!carbon staining on crystal boundaries[ NH!1 well\ cuttings\ 1989 m\polarised light[ "C#] The same dolomite above as it appears under SEM[

6[3[1[ HaliteHalite is present mainly in the Risha area where up to

four beds were found[ The term Halite Member has beenused by Andrews et al[ "0881# for the main halite layer"11Ð46 m thick# that consists mainly of transparent totranslucent halite interbedded with shale\ anhydrite andanhydritic dolomite[


Fig[ 8[ Carbonate!cemented\ poorly sorted quartz arenite[ NH!1 well\cuttings\ 1099 m\ polarised light[

Similar halite deposits were reported from the Palmyrain Syria "Beydoun + Habib\ 0884# and from northernIraq "Sadooni\ 0884a#[ In most cases\ halite representsthe drawdown stage in the basin development[

7[ Depositional sequences

Abu Ruweis sediments represent the shrinking stageof the Triassic basin[ The sequence starts with a thickanhydrite bed that signalled the formation of marginalsabkhas around the basin[ According to Tucker "0880#\when sea level falls close or below the barrier\ quick waterevaporation will cause the lowering of the sea level belowthe shelf break and evaporites start to precipitate mainlyas gypsum wedges in sabkha and shallow hypersalinewaters[ Andrews et al[ "0881#\ mentioned that the highratio of circumpolles to bisaccate pollens in Abu Ruweissediments in general and toward the top in particular\suggests supratidal conditions[

These sabkhas continued to receive clastic materials atthe outer approaches of the basin from high areas inthe south and east[ The presence of Tasmanites sp[ andCymatiosphaera sp[ were considered by Andrews et al["0881# as indicators of fresh water in~uence[

The isolation of the basin was incomplete and resultedin the alternation of marginal gypsums with platformcarbonates that formed metre!scale cycles or para!sequences[ Andrews et al[ "0881# note that bed thicknessesin the Abu Ruweis Formation rarely exceed 4 m[ Readand Horbury "0882# have reported eustatically and tec!tonically!controlled parasequences from the Late Triassicof the Middle East[

However\ the presence of a carbonate section that con!sists of oolitic!peloidal or bioclastic wackestone!pack!stone beds at the top of the lower part of the formationall over the studied area suggests a return to carbonateplatform deposition[ This situation was later reversed and

evaporites become dominant again[ The basin reached atotal drawdown stage in the Risha area where halite wasprecipitated[ These lithologies represent the _rst depo!sitional cycle of the formation that was followed by asecond sequence of carbonates and evaporites "anhy!drites mainly this time#[ These cyclic alternations areprobably of global nature as re~ected by the eustatic sealevel curves of Haq et al[ "0876# and were characteristicfeatures of the Late Triassic[ The deposition ended withan unconformity and the sequence is overlain by theBathonian!Lower Callovian Azab Group "Huni For!mation# in the Risha and the Lower Jurassic sedimentsin the Northern Highlands area[

8[ Hydrocarbon potential

Oil\ gas and condensate discoveries in the Triassic car!bonates beneath an anhydrite seal have been reportedfrom Iraq "Sadooni\ 0884a#\ Syria "Beydoun and Habib\0884# and Israel[ Although 15 boreholes have penetratedthe Triassic sequence in both Jordan and the West Bank\no commercial oil and gas accumulations have been enco!untered so far in these sediments[ Nevertheless\ the possi!bility of the presence of live oil and gas is indicated by anumber of oil shows in the Late Triassic sediments of theAbu Ruweis Formation[ Fair to good oil shows wereencountered in several wells such as the Ramtha "S!8#well at 0039\ 0209 and 0799 m depths "Ahmad + Nabulsi\0876#

Many large gas:condensate and oil _elds have beenfound in the Upper Triassic sediments of the PalmyraRift and NE Syria "Beydoun + Habib\ 0884#[ Similarly\commercial oil and gas accumulations have been dis!covered in their equivalents in NW Iraq "Sadooni\ 0884a#[

8[0[ Source rocks

In their study on the chemical characteristics of thecrude oil from Jebissa oil_eld in Syria\ Metwalli\ Philipand Moussly "0861# observed that Triassic oils havedi}erent characteristics from Cretaceous or Tertiary oils[May "0880# suggested that the Triassic oil in both Syriaand Iraq can be attributed to Middle and Lower Triassicand Permian shales and limestones or possibly to distantbituminous Jurassic beds[ Beydoun "0877\ 0880#\however\ was in favour of an indigenous oil source asrepresented by the thin\ interbedded shales within theTriassic carbonates[ This is more probable because it isdi.cult to suggest vertical oil migration in a successioncomprised predominantly of thick anhydrite beds[Beydoun et al[ "0883# reported that the Triassic sourcerocks consist of argillaceous sediments that containmixed oil!prone Type II and gas!prone Type III kerogensbut with low!to!moderate organic contents[ The maturit!


ies were found to increase with depth in a NW direction"Ro�9[54Ð9[8)#[

Bilal "0877# investigated the paleothermal history ofthe Triassic sediments in Syria using ~uid inclusions[ Hedetermined a thermal gradient that ranges between 4 and6) and increases from the north!northeast toward thewest!southwest\ i[e[ in the direction of the tectonic andvolcanic activities associated with the Great Syrian Riftwhich is the northern continuation of the Great AfricanRift[ He also concluded that although the gradientincreases with depth in each well but it is mainly related tothe location of the well in respect to the above!mentionedcommon trend[

Shales and argillaceous carbonate rocks within theAbu Ruweis Formation may represent fair to goodindigenous source rocks for oil and gas generation[ Thevolume of shale in each of the studied wells was calculatedfrom the Gamma!ray log[ The results were then\ plottedagainst depth[ The volume of shale is higher in the lowerpart of the formation in general\ and the shale beds forma considerable part of the total rock volume[ Friedman"0871# suggested that evaporites rich with algal materialssimilar to those found in the Abu Ruweis Formationcould be excellent source rocks[

Geochemical analysis of some samples from the AbuRuweis section in the well Risha!0 between 749Ð0279 mdepths has been carried out by Ahmad and Nabulsi"0876#[ The analysed sediments consist of medium!darkclaystone\ slightly carbonaceous to slightly calcareousand where kerogen content consists chie~y of amorphousorganic matter[ Small quantities of miospores\ structuredwood and quantities of tissues are also recorded[ Thesesamples contain good!quality kerogen\ which may gen!erate minor quantities of oil at higher maturity levels[The Maturation "S[T[I[# ranges from 0[7Ð1[9 "0[7NORM#[ Ditch cuttings from 0039 m and core samplesfrom 0198[4 m depth\ showed immature source rock\alkanes and mixtures of immature source!rock alkaneswith oil stains\ respectively "Shinaq\ 0885#[ These sedi!ments may become better developed and more mature ina more basinal\ o} structure setting\ "Collins\ 0873\ 0874#[

In the Northern Hi`hlands!0 well\ the content oforganic carbon "TOC# values of the shale and carbonateintervals "AR!3 unit# are relatively high "9[4Ð0[8)# andthey represent oil!prone source rocks "Dungworth\ Aber!nethy + Smout\ 0876#[ Mature and oil!prone source rocksare expected to occur at depths below 0899 m in the Rishaarea but at shallower depths in the Northern Highlands[This may be related to the nearness of this area to thezone of strong tectonic activity reported by Bilal "0877#in Syria[

8[1[ Reservoir rocks

Considerable parts of the Late Triassic section consistof potential reservoir rocks such as dolomites\ dolomitic

and oolitic limestones\ but are so far unproductive\ prob!ably due to the lack of access to adequate hydrocarboncharge or suitable traps[ The carbonates and dolomitesof the Abu Ruweis Formation yield fair!to good porosity"7Ð19)# and permeabilities within the range of 9[90Ð79md[

8[1[0[ Porosity and permeability[ Porosity values havebeen calculated from sonic and neutron logs and thencompared with other data derived from laboratorymeasurements or visual petrographic examination[

The Sonic log is used to estimate primary porosity\ andsonic porosity values are then plotted as a log for thethree studied wells "Figs 00Ð02# which show the variationsof sonic porosity with depth[ Neutron and sonic porosityvalues seem similar\ which indicate the limited e}ect ofdiagenesis on porosity evolution in the studied sediments[Read and Horbury "0882# noticed that Triassic peritidalsediments of the Middle East were only slightly alteredby dolomitisation because evaporites plug the major partsof the primary porosity[ Values of neutron porosity wereplotted against the bulk density measurement from theAjlun!0 well[ The main points are clustered around anhy!drite!dolomite lithology with average neutron porosityof 19) and bulk density of 1[6 g:cc\ "Fig[ 03#[

High variations in porosity values are recorded withdepth[ These are found to be associated with lithology[The main unit that has high porosity is the reservoirunit "AR!3#\ which consists mainly of oolitic!peloidallimestone interbedded sometime with porous coarse!grai!ned dolomite and anhydritic dolomite[ Other highporosity readings were recorded from the upper unit"AR!0#\ which consists of porous dolomite and fracturedlimestones[

Measured porosity values were plotted against per!meability values from core samples in the North Hi`h!lands!1 well[ The plot shows a strong positive correlationbetween the two measurements\ "Fig[ 04#[

Permeability ranges from 9[90Ð79 md[ The highestreadings are recorded from unit AR!3 unit that consistsof dolomite and dolomitic limestones that are frequentlyfractured or contain intercrystalline porosity that mayenhance the permeability[

8[2[ Seals

Anhydrites\ shales and argillaceous limestones of theAbu Ruweis Formation are the main cap rocks of thepotential reservoirs[

8[3[ Traps

The study area is located within the stable part ofthe Arabian Shelf[ The most common tectonic activitiesduring the Phanerozoic were rifting and epeirogenic


Fig[ 09[ Evaporites of Abu Ruweis Formation] "A#] Patchy anhydritic nodules in algal laminated limestone[ NH!1 well\ core 09\ 1205 m\ scale 0 ] 2["B#] Laths of anhydrite cement _lling a solution vug in aphanitic dolomite[ RH!1 well\ cuttings\ 849 m\ polarised light[ "C#] The contact betweendolomite and anhydrite cement as can be seen under SEM[ Details are as above[ "D# Well!aligned Laths of anhydrite as appear under SEM[ Detailsas above[

movements that lead to the formation of undulated sur!face of regional swells and troughs that were cut by nor!mal and strike!slip faulting\ "Fig[ 05#[ Powell andMohamed "0882# described small horst and graben struc!tures _lled with Permo!Triassic and Triassic sedimentson the eastern shores of the Dead Sea[ Such structuresmay also be present in the northern parts of the basin[

Some of these structural culminations were drilled butno major prospects were found as yet[ The most impor!tant among these is the Ajlun Dome which is around 14km in length and situated on the east side of the JordanValley "Beydoun et al[\ 0883#[ This is only partially evalu!ated and the scarcity of geophysical data makes it di.cultto proceed with any further drilling[

Pinch!outs\ unconformities\ and facies changes in theAbu Ruweis Formation may represent good traps forhydrocarbons if they are located in an up dip position\

but again the identi_cation of such features requires goodgeophysical data[

A favourable combination of source\ reservoir rocksand the necessary entrapment may be found northwardsunder the Hauran Basalts where the sediment thickness\the geothermal gradient and the presence of structuraldeformation allow for the presence of a possible play[More e}ort should be directed to explore these areas inthe future[

8[4[ Destructive factors

There are two major factors that may reduce the poten!tial of the Abu Ruweis Formation as a reservoir]

"0# Intrusive rocks and dykes occur within the penetratedsections of Palaeozoic\ Triassic and Jurassic of north!


Fig[ 00[ Lithological and petrophysical details of Abu Ruweis Formation in Ajlun!0 well[

ern Jordan[ Although the destructive e}ects of suchintrusions are speculative\ they should be taken intoaccount in assessing any future oil prospects[

"1# Erosion associated with the regional unconformityduring the Early Cretaceous may have eroded olderformations toward the southeast\ thus breaching anypossible hydrocarbons in Triassic reservoirs in theseareas[

09[ Conclusions

"0# Subsurface Upper Triassic sediments in northern Jor!dan consist of around 299 m of thin beds of sand!stone\ limestone\ dolomite\ anhydrite and halite withdolerite intrusions of Carnian age[ These sedimentsrepresent the shrinkage phase of the Late Triassicbasin[ They consist of a regressive carbonate!eva!

poritic sequence in which sabkhas formed along thebasin margin while oolitic shoals and subtidal car!bonates were deposited in the deeper parts of thebasin[ The basin also received sporadic clastic in~uxesfrom the southern and southeastern areas[

"1# By using a combination of Gamma!ray and Soniclogs the studied sediments are divided into four dis!tinctive units "AR0ÐAR3#[

"2# Local shales\ marls and argillaceous limestones mayrepresent potential source rocks[ Geochemical analy!sis indicates that the kerogen consists of amorphousorganic matter with miospores\ structured wood andquantities of tissue[ The total organic content valuesof the shale and carbonate intervals range between"9[4Ð0[8)#[

"3# Porosities were measured from some cores and alsocalculated from Sonic and Neutron logs[ They rangebetween 7Ð19) and permeabilities range between


Fig[ 01[ Lithological and petrophysical details of Abu Ruweis Formation in Northern Hi`hland!1 well[

9[90Ð79 md[ Most of the porosity is within the ooliticpackstone facies[ These data indicate that unit AR!3has the best potential as a reservoir unit[

"4# The studied area consists of an undulated surface ofregional swells and troughs that were cut by normaland strike!slip faulting[ Some of these structural cul!minations were drilled without important hydro!carbon _ndings\ others such as Ajlun Dome are stillwaiting further exploration which is hindered at themoment by the lack of detailed geophysical surveys[Pinch!outs\ unconformities\ and facies changes in theAbu Ruweis Formation may represent good traps forhydrocarbons if they are located in an up dip position

A combination of good sediment column\ favourable

geothermal gradient and the presence of structural defor!mation in the Hauran Basalts make this area a potentialhydrocarbon play in Northeast Jordan[

Acknowledgements

The authors are indebted to Hisham Al!Rabi andAbdallah Shehadeh of the Jordanian Natural ResourcesAuthority "NRA# for providing well samples and reportsand to the Department of Earth and Environmental Sci!ences\ Yarmouk University for logistics and equipment[Hakam Mustafa and Ra_|e Shinaq commented on theoriginal manuscript[ Ziad Beydoun and two anonymousreviewers made very useful amendments to the article[


Fig[ 02[ Lithological and petrophysical details of Abu Ruweis Formation in Risha!1 well[


Fig[ 03[ Lithology determination from cross plotting of neutron porosity vs bulk density values\ Ajlun!0 well\ "Schlumberger\ 0861#[

Fig[ 04[ Cross plot of porosity versus permeability values from the Northern Hi`hland!1 well[


Fig[ 05[ Geoseismic sections showing the main structural styles of the studied area\ "A# NE Jordan "Risha Panhandle#[ "B# NW Jordan to AjlunDome area[ "Ter] Tertiary^ K] Cretaceous^ Apt!Cen] Aptian!Cenomanian^ Tur] Turonian^ J] Jurassic^ TR] Triassic^ P] Permian^ Ca] Carboniferous^D] Devonian^ S] Silurian^ O ] Ordovician^ C] Cambrian#\ "Beydoun et al[\ 0883#[

Note from editor

The unpublished reports referred to in this article areavailable to the public through the Library of NationalResources Authority in Amman\ Jordan[

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Geology and petroleum prospects of Upper Triassic sediments, Jordan

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Transcript of Geology and petroleum prospects of Upper Triassic sediments, Jordan