Microfacies analysis of Wadi Waqb member (Miocene) in Wadi Aynunah, northwest of Saudi Arabia

MICROFACIES ANALYSIS OF WADI WAQB MEMBER (MIOCENE) IN WADIAYNUNAH, NORTHWEST OF SAUDI ARABIA

MaarufHussain and Khalid Al-RamadanDepartment ofEarth Sciences, King Fahd University ofPetroleum and Minerals, Dhahran 3126,

P.o. Box 1400 , Saudi Arabia

ABSTRACT: Easier access to most of spectacular sedimentological features ofMidyan region along the northern coastof Red Sea has enabled a more detailed examination of the Miocene succession that could not have been possible exceptfrom deep subsurface drilling. The Wadi Waqb member ofJabal Kibrit Formation hosted the Wadi Waqb reservoir in thisregion. In this study the microfacies ofthe exposed discontinuous fringing rhodolith and coral reefcomplex ofWadi Waqbmember that seated unconformably on steep cliffs ofgranitic basement are investigated. Four microfacies were recognizedas follows: MF-l) Quartz-peloid packstone and Red algal bioclastic boundstone, MF-2) Bioturbated rhodophyte-bioclasticgrainstone, MF-3) Calc-allochemic sandstone, and MF-4) Intraclastic wackestone.

Microfacies 1 and 2 are considered to be deposited at the upper and mid reef front. The depositional environment of theMF-3 is considered to have occurred in periodical flash floods and microfacies 4 is considered to be a lower reef frontdeposit. Careful microfacies analysis of mixed reefal carbonate and siliciclastic deposits provides a better understanding

ofhow and why carbonate microfacies develop.

INTRODUCTION

The excellent exposures of Miocene Carbonate unit(Wadi Waqb member) of Jabal Kibrit Formation in

the Midyan region at the east flank of Wadi Aynunahnear Al Khuraybah within longitudes 35° 11' 36.0"Eand latitudes 28° 04' 15.2"N along the northern coastalarea ofthe Red Sea (Figs. 1,2)provides an ideal site for

Figure 1. Location map showing the study area in the Midyan Peninsula that lies within the angle formed by the GulfofAqaba and the Red Sea (after Mougenot and AI-Shakhis 1999).

Carbonates and Evaporites, v. 24, no. 2, 2009, p. 139-149.

MICROFACIES ANALYSIS OF WADI WAQB MEMBER IN WADI AYNUNAH, SAUDI ARABIA

Figure 2. Satellite image of the Midyan region showing localities and structural aspects of the studied region. In thisimage, outcrops ofanhydrite show up turquoise (Hughes et al. 1999).

Several authors, including Bogue (1953), Richter­Bemburg and Schott (1954), Skipwith (1973),Shearman (1979), Remond and Teixido (1980), Mottiet al. (1980), Le Nindre (1981), Dullo et al. (1983),Clark (1986), Jado et al. (1990), Taj (1991) amongmany others have contributed to the stratigraphy ofMidyan region. The Midyan regional geology andtectonic development have been studied, mappedand described by Clark (1986), Haq et al. (1988),Gardner et aI. (1996), and in recent time by Hughes

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detailed sedimentologic and microfacies investigationof the unit. The studied outcrops provided a greataccess to rock unit that is usually obtainable fromthe subsurface. This exposed Wadi Waqb member,located in the east flank of Wadi Aynunah near AlKhuraybah, consists of abundant shallow marineallochthonous bioclasts and terrigenous sediments.These shallow-marine bioclasts are probably sourcedfrom the in-situ rhodolith-coral reefs. The terrigenoussediments are highly immature that reflect the effectoferosion ofthe underlying granitic basement.

HUSSAIN AND AL-RAMADAN

et al. (1999), Haq et al. (1988) in Hughes and Johnson(2005) and Sulaiman (2008).

This paper relates the microfacies of Wadi Waqbmember with the depositional environment anddiagenesis. Also, it demonstrates the reservoirpotentiality. In this study, Flugel (2004) StandardMicrofacies Type was used as a model and a briefassessment of the background geology of the studiedarea is explained with reference to work by Hughes etal. (1999).

BACKGROUND GEOLOGY

The geology of the Midyan region is, and has beena subject of great interest for many years, and morerecently owing to the hydrocarbon exploration alongthe northern coast of Red Sea in Saudi Arabia. TheMidyan region which covers an extensive areabetween the Gulf ofAqaba and the Red Sea has beenextensively mapped and described by Hughes etal. 1999 (Figs. 1 and 2) and by Haq et al. (1988) inHughes and Johnson 2005 (Figs. 3 and 4).

The Midyan boundaries are the Proterozoic rocks inthe north and the northeast, while Gulf of Aqaba liesin the west and Red Sea in south. In the eastern partis Wadi Ifal plain which covers more than half of theregion eastern part. The Proterozoic rocks consistsof ultramafic, metavolcanic, and metasedimentaryrocks and granitic plutons, which have been intrudedby basalt, rhyolite, and dolerite dikes. In Gardner etal. 1996 the basement has been dated at about 600 to700 million years old and is considered to have beenformed along an accreting Proterozoic volcanic arc.

The Red Sea stratigraphy of Saudi Arabia, whichhas spectrum of various lithologies includingevaporites, carbonates, and deep- and shallow-marinesiliciclastics, vary in age from the Late Cretaceous(Campanian) to Late Pliocene. The fluviatile UpperCretaceous Addafa formation, which represents theoldest sedimentary rocks, lies unconformably on theProterozoic basement complex. The deepmarine lowerMiocene Burqan Formation rests unconformably onthe Tayran group, and on top of this lie the marinemudstones, carbonates, and evaporites of the middleMiocene Maqna group (Fig. 4). The poorly exposedmiddle Miocene Mansiyah and middle to upperMiocene Ghawwas formations consist of marineevaporites and shallow to marginal marine sediments,respectively. The youngest rocks are alluvial sands

and gravels ofthe Pliocene Lisan Formation.

The Gulf of Aqaba like its counterpart in Egypt,the Gulf of Suez, is a pull-apart basin that containsrift-related sequences. These sequences are usuallyof high hydrocarbon potentials. The hydrocarbonexploration along the Saudi Arabia Red Sea sprang upthe interest in the Midyan region. The region outcropsoffer an excellent means to examine the lithofaciesand biofacies that are usually obtainable from thecore data. While also, seismic and drilling activitiesby Saudi Aramco in the 1990s proved the existenceof hydrocarbon-bearing carbonate reservoirs in theMidyan region (Kamal and Hughes 1995).

The Wadi Waqb member ofthe Jabal Kibrit Formationbelongs to Magna group that hosts an importanthydrocarbon reservoir in the Midyan region. Kamaland Hughes 1995 reported the lithology ofWadi Waqbmember as carbonate rocks which was deposited in ashallow marine environment. The carbonates of theWadi Waqb member represented by a discontinuousfringing rhodolith and coral reefcomplex that is seatedon steep cliffs ofgranitic basement. Exposures ofthemember in Wadi Waqb, located in east of the mouthofWadi Aynunah near Al Khuraybah (Fig. 5), consistof abundant shallow marine allochthonous bioclasts.G.W. Hughes (2008) considered these shallow-marinebioclasts to have been derived from the rhodolith-coralreefs exposed to the east.

The succession is 205 ft thick, and consists ofpredominantly rhodophytes- corals reef and otherbioclasts. Bedding attitudes vary, but generally strikeat about NI OooE, with a dip of450 to 500S. The studiedsection of Wadi Waqb starts at the lower part withpackstone and bioclastic boundstone that is overlainby bioclastic grainstone (Fig. 6). The grainstone isdominated by angular detrital quartz grains withsome ostracod. The upper most part of the sectionhas intraclastic wackestone that is characterized byallomicrite matrix (Fig. 6).

MATERIALS AND METHODS

Twenty Carbonate and sandstone samples werecollected from a steeply dipping fore-reef at amaximum interval of3m. Thin sections were preparedfor all samples subsequent to vacuum impregnationwith blue epoxy. Modal analyses were performed bycounting 300 points in each thin section. Scanningelectron microscope (SEM) was used to study crystal

141

MICROFACIES ANALYSIS OF WADI WAQB MEMBER IN WADI AYNUNAH, SAUDI ARABIA

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143

MICROFACIES ANALYSIS OF WADI WAQB MEMBER IN WADI AYNUNAH, SAUDI ARABIA

Figure 5. Fieldphoto showing steeply-dippingfore-reefrhodolithic and coralline shallow-marine carbonates ofthe WadiWaqb member that is unconformble on the Proterozoic basement in the Wadi Aynunah, near AlKhuraybah (28°04'15.2 "N;35°11 '36.0 "E).

habits and paragenetic relationships among diageneticminerals. The interpretation of subsurface withsuch outcrop studies has been validated and clearlydemonstrated in the literature (e.g. Van Wagoneret al. 1990; Eschard et al. 1993; Kerans et al. 1994;Grammer et al. 1995; van Buchem et al. 1995a).

Microfacies Description

Thin section studies of all samples revealed fourdifferent microfacies (Fig. 7) that characterize thestudied outcrop as follows:

Microfacies 1 (Fig. 7A).- Quartz-peloid packstoneand Red algal bioclastic boundstone. This microfaciesconsists of poorly sorted detrital quartz grains andmicrobial peloids with in situ encrusting and bindingred algae (Lithophylum Spp) associated with intensemicroboring and cavity -filling sparite cement. Theencrusting red algae are characterized by encrustingstrips that provided much of the binding of microbialpeloids and quartz-peloid forming a wave resistantframework. This wave-resistant framework indicatesa high energy environment,

Microfacies 2 (Fig. 7B).- Bioturbated rhodophyte­bioclastic grainstone characterized by grain-supportedencrusting rhodophyte with rare ostracod and scatteredangular detrital quartz fragments. The angular quartzfragments suggest proximity to the source. Thedifferent orientation ofthe bioclasts and quartz grainsindicate high energy environment.

Microfacies 3 (Fig. 7C).- Calc-allochemic sandstone.This facies composition is greatly variable fromcarbonate to clastics. The most common variety isarenaceous biocalcarenite with abundant angulargrains of detrital quartz, feldspar, and mica withassociated few bioclasts ofmicrobialite fragments thathad been cemented by calcite. The abundance ofbothunstable detrital grains (e.g. feldspar and mica) andangular grains indicate a compositionally immaturesandstones and short transportation (i.e. closenessto the granitic basement), respectively. Bioclastswere probably reworked and redeposited within thesiliciclastic sediments.

Microfacies 4 (Fig. 7D).- Intraclastic wackestone.Inhomogenous finely bioclast allomicrite matrixcharacterizedthis facies. The bioclasts are shell debris

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HUSSAIN AND AL-RAMADAN

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Lithology Composition Classification

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trfj,~;~i;1J,i;i~;t~11etri:n==.:mica with associated fewbioclasts ofmicrobialite &calcite cement.

Figure 6. Stratigraphic column showing vertical distribution ofthe microfacies of Wadi Waqb Member at mouth ofWadiAynunah, near Al Khuraybah (28°04'15.2"N; 35°11 '36.0"£).

dominated by rhodophyte fragments. Subordinatefragments are smaller benthic foraminifers, ostracods,small mollusk mould. The bioclastic matrix isgradational in size with sand-size components. Thisindicates strong reworking of lithified and semi­lithified sediments.

DEPOSITIONAL ENVIRONMENT

The depositional environment is evidently consideredto be an open shallow marine environment with waterdepth of about 15m. The abundant of encrusting redalgae indicate a high energy zones particularly at thecrest of the reef front that have been subjected to high

wave activity. The encrusting red algae were abundantprobably, not only because of the wave-resistantframework formed to withstand the wave action butalso due to the high level tolerance to light encounteredin shallow water. The abundance of calcareous algaein many shallow marine environments and theirsensitivity to changes in water depth and energy, andlight penetration is well known (cf. A.E. Adams andW.S Mackenzie 1998). Intraclasts in microfacies 4are commonly found in shallow marine environmentthat are characterized by wave-dominated regimesand tides that continuously rework carbonate deposits(cf. Erik FliigeI2004).

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MICROFACIES ANALYSIS OF WADI WAQB MEMBER IN WADI AYNUNAH, SAUDI ARABIA

Figure 7. Thin section photomicrographs showing A) Microfacies J, quartz-peloid packstone and Red algal bioclasticboundstone consists ofpoorly sorted detrital quartz grains (black arrow), microboring (ME) and microbial peloids within situ encrusting and binding red algae (RA). Note the sparite cement (white arrow); B) Microfacies 2, Bioturbatedrhodophyte-bioclastic grainstone characterized by grain-supported encrusting rhodophyte (RA) with rare ostracod andscatteredangular detrital quartzfragments (black arrow) as well as pyritepigments (white arrow); C) Microfacies 3, Calc­allochemic sandstone, with abundant angular grains ofdetrital quartz, feldspar, and mica with associatedfew bioclastsofmicrobialite fragments that had been cemented by calcite; D) Microfacies 4, Intraclastic wackestone. lnhomogenousfinely bioclast allomicrite matrix characterized this facies.

Figure 8. Field photograph showing an entrapped mixed carbonate-siliciclastic layer (Hammer as scale). Note the bigvuggy hole at the bottom-centre below the hammer.

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HUSSAIN AND AL-RAMADAN

Figure 9. Thin section micrographs showing: A) Vuggy porosity; B) Solution vugs and microfracture lined withmicrocrystalline calcite; C) Interparticle porosity in the calc-arkosearenite which comprises ofpoorly sorted detritalquartz grains, feldspar and mica. Carbonates grains are lithoclasts with fragments coatings; D) Primary intergranularporosity; E) SEMmicrograph showing monodispersed character ofcalcite cement; F) Close-up view ofFigure 9E.

Microfacies I and 2 may have formed in the upperreef front and reef crest sediments that were beingtransported episodically by storms up and over the topand accumulate in the lee ofthe reefcrest. Microfacies3 represents a progradational deposit characterized bya strong siliciclastic influx derived from erosion ofthe granitic basement. Abrupt transitions betweenthe terrigenous sedimentation and carbonate withtrapping of the siliciclastic sediments (Fig. 8) pointto periodical flash floods that usually affect nearshoreshallow marine environment. Mixtures of siliciclasticand carbonate rocks in shelf settings are a result oflateral facies mixing, or by sea level changes and!

or variations in sediment supply causing verticalvariation in the stratigraphic succession. Microfacies4 may have formed in the lower reef front deposits.

DIAGENESIS

Diagenetic alterations encountered in this studyinclude micritization and boring of carbonategrains, cementation by fibrous rim and formation ofsecondary porosity. Algae and microbial encrustationon skeletal grains resulted in a total occlusion of mostof the interskeletal pores. Due to rapid and pervasivecementation of synsedimentary organic encrustation,

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MICROFACIES ANALYSIS OF WADI WAQB MEMBER IN WADI AYNUNAH, SAUDI ARABIAa limited mechanical compaction was encountered in histories of Wadi Waqb member.the carbonate units.

ACKNOWLEDGMENTSInmicrofacies 1and 2, obliteration ofthe early porosityis common by deposition ofvadose silt and formationof phreatic sparite cement in the microfractures (Fig.9a,b). Secondary porosity is represented by enlargedmoldic to vuggy porosity that has resulted frompartial to complete dissolution of the bioclasts (Fig.8). In microfacies 3, porosity is dominated by primaryintergranular type that may have been preserved dueto limited cementation and compaction (Figs. 9c,d).Such preservation is usually promoted by absenceof water in the pores (Flugel 2004). However,microfacies 4 posses no visible porosity in thin section.In this case, the primary porosity is represented byminute inter- and intra-particle pore spaces as wellas secondary porosity correspond to biomolds whichhave been occluded by microsparite cement (Fig. 9E& F; Carozzi 1989).

CONCLUSIONS

Four microfacies are recognized at the outcroppingWadi Waqb carbonate member of the Jabal KibritFormation (latest early to earliest middle Miocene) atmouth ofWadi Aynunah, near Al Khuraybah, where itis unconformably seated on the Proterozoic basement.High energy open shallow marine characterized thedepositional environment. This environment wassubjected to a strong influx of siliciclastic sedimentsthat were eroded from the granitic basement complexand deposited in form oftrapped sedimentation.

Microfacies 1 and 2 were probably upper reef frontand reef crest sediments that were being transportedepisodically by storms up and over the top andaccumulate in the lee of the reef crest.

Microfacies 3 represents a progradational depositcharacterized by a strong siliciclastic influx derivedfrom erosion of the granitic basement. Whilemicrofacies 4 is probably a lower reef front deposits.

Excellent preservation of intergranular primaryporosity in the siliciclastic facies suggests existenceof good reservoir potential in Wadi Waqb member.Interestingly, the potentiality of Wadi Waqb memberhas been initially focused on the secondary porosity.With this finding, there is a scope for refinement andimprovements. An improved microfacies analysisshould therefore, lead to a better understanding ofalmost all aspects of depositional and diagenetic

The authors would like to express their thanks to KingFahd University ofPetroleum and Minerals (KFUPM)in Saudi Arabia for support and access to analyticalfacilities. Thanks are also due to Mohammed Aqeel,Azizulah Khan and Mushbab Asiri for their help inthe field, thin section preparation and SEM analysis,respectively. The valuable discussion and commentsby Wyn Hueges are also acknowledged as well asthe Magna coastal guards' logistics assistance. Themanuscript benefited greatly from the comments ofCarbonates and Evaporites referees.

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