Carbonate-Evaporite Sequences of the Late Jurassic

8/9/2019 Carbonate-Evaporite Sequences of the Late Jurassic

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Carbonate-Evaporite Sequences of the Late}urassic,Southern and Southwestern Arabian Gulf1

A. S.Alsharhan and G. L.Whittle2

ABSTRACT

The carbonate-evaporite sequences of the UpperJurassic Arab and overlying Hith formations in thesouthern and southwestern Arabian Gulf formmany supergiant and giant fie1dsthat produce fromthe Arab Formation and are exceIlent examp1es of ac1assic reservoir/seal relationship. The present-daysabkha depositional setting that extends along most

of the southern and southwestern coasts of theArabian Gulf provides an analog to these UpperJurassic sedimentary rocks. In fact, sabkha-relateddiagenesis of original grain-supported sediments inthe Arab and Hith formations has resulted in fivedistinct lithofacies that characterize the reservoir/seal re1ationship: (1) oolitic/pe1oidal grainstone, (2)dolomitic grainstone, (3) dolomitic mudstone, (4)dolomitized grainstone, and (5) massive anhydrite.Interpartic1e porosity in grainstones and dolomiticgrainstones and intercrystalline porosity in dolomi-tized rocks provide the highest porosity in thestudy area. These sediments accumulated in fourtypes of depositional settings: (1)supratidal

sabkhas, (2) intertidal mud flats and stromatoliticflats, (3) shaIlow subtidallagoons, and (4) shallowopen-marine she1ves.

The diagenetic history of the Arab and Hith for-mations in the southern and southwestern ArabianGulf suggests that the anhydrite and much of thedolomitization are a result of penecontemporane-ous sabkha diagenesis. The character and timing ofthe paragenetic events are responsible for theexceIlent porosity of the Arab Formation and thelack of porosity in the massive anhydrites of theHith, which together result in the prolific hydrocar-

bon sequences of these formations.

Copyright 1995. The American Association0 1 Petroleum Geologists. AIIrights reservad.

1Manuscript received July 7, 1994; revised manuscript received May 8,1995;linal acceptance June 30, 1995.

2Desert and Marine Environment Research Center, United Arab EmiratesUniversity, P.O. Box 17777, Al Ain, United Arab Emirates.

The authors would like to thank the Desert and Marine EnvironmentResearch Center and the United Arab Emirates University lor support0 1 thisprojecl. In addition, we extend our gratitude to Christopher G. SI. C. Kendall,Mark Longman, Kevin Biddle, Charlotte Schreiber, and Kinji Magara lorcritically reviewing the manuscript, and to J. A. Antar lor drafting the figures.

1608

I NTRODUCTI ON

The Upper Jurassic in the Arabian Gulf, knownas the Arab and Hith formations, comprises a thicksection of shallow-water carbonates interbeddedwith evaporites and capped by an anhydrite sec-tion (Figure 1). This sequence was formallydescribed by Steineke and Bramkamp (1952), whodefined the formation names for these rocks,

which were adopted by Aramco following thework of Steineke (cited in Powers et al., 1966).Thetype sections for the Jurassic, in most cases, have

been described from the outcrops in central Arabiain the vicinity of the Riyadh area, with the excep-

.tion being the Arab Formation. The type section forthe Arab is the Dammam well 7 (located inDammam field in eastern Saudi Arabia), whereasthe type section for the Hith Formation is at DahlHith in central Saudi Arabia.

The Arab Formation itse1f was subdivided bySteineke et al. (1958) into four members, labeledA-D from top to bottom. These members exhibit acyc1ic pattern of rock types. From the base up,

each member (except A) comprises a limestoneand dolomitic limestone section and an overlyinganhydrite section. Member A is overlain by theanhydrite of the Hith Formation (Powers, 1962,1968).The limestone-anhydrite sections are c1earlydistinguished and very little interdigitation occurs.The sedimentology and diagenetic features of theArab Formation in Saudi Arabian oil fie1ds werestudied in detail by Steineke et al.(1958), Powers(1962), Wilson (1985), MitcheIl et al. (1988), andMeyer and Price (1993) and will not be repeatedhere; however, these features are shown in the gen-eralized section in Figure 2. We will concentrateour study on the southern and southwestern parts

of the Arabian Gulf. The Arab Formation is themajor Mesozoic producing interval in the ArabianGulf oil fields (Figure 3).

In the onshore Qatar area, the Upper Jurassic(Arab-Hith) is subdivided into the Fahahil, Qatar,and Hith formations (Figure 4), which have beengrouped informally into the Riyadh Group, a termoriginally used in Saudi Arabia, but now discarded(Sugden and Standring, 1975). This grouping has

MPG Bulletin, V. 79, No. 11 (November 1995), P.1608-1630.


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Alsharhan and Whittle

Figure l--Stratigraphic columnfor the Upper Jurassic Arab andbasal Hith formations showingrock type, llthology, depositionalenvironment, compensatedneutron log (CNL),andcompensated formation density(FDC)log responses for eachmember. Note that only the baseof the Hith Formation is shown

because its character is similarthroughout. For lithologysymbols, see Figure 7.

been considered useful for areas where the distinc-tion between these formations is poorly defined.In offshore Qatar, the Qatar and Fahahil forma-tions are grouped informally as the Arab zone toequate them with the Arab Formation defined inSaudi Arabia. The four carbonate units of the Arabzone are known as limestones 1, 11,111,and IV,orArab A-D, from top to bottom. The top limestoneI to top limestone IV sections (or top Arab A totop Arab D) have been designated the QatarFormation, and limestone IV(or Arab D) is formal-ly designated the Fahahil Formation. The Fahahil

Formation was distinguished from the overlyingsection because it marks the transition from deep-er to shallower water conditions. Its type sectionis described from the Qatar Petro1eum Company(QPC) 66 Dukhan well. The Qatar Formation (thatis, the Arab I-III or A-C), the type locality of theQPC 28 Dukhan well, comprises the three porouscarbonate units and the intervening dense anhy-drite sections. The formation also inc1udes the. .lowest significant anhydrite development withinthe Arab zone, which immediately overlies theFahahil Formation.


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1610 Arabian GuJf, LateJurassic

: :z :N o:1 : . . . ~ u ' " GENERAL DESCRIPTIONo


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Figure 3-Fields producing ollor gas from the Jurassic in thesouthern and southwestern ArabianGulf. 1 = UmmShaif, 2 = Nasr,3 =Bu Dana, 4 = ADNOC1-B,5 = ADNOC1-C, 6 = Belbazem,7 =Umm Al Dholou, 8 = Umm AlSalsal, 9 =Saath Al Raaz8001,10 = Albateel, 11 = UmmAl Anbar,12=Hail,13=BuT~14=Ghas~15 =Hair Dalma, 16 =Dalma,17 =Arzanah, 18 = BuJufair,19 =Jarnain, 20 = Satah,21 = El Bunduq, 22 = Abu AlBukhoosh, 23 = BuI Hanine,24 =Idd El Shargi, 25 =MaydanMahzam, 26 = Dukhan,27 = Mubarraz, 28 =WestMubarraz, 29 =Bab.

Alsharhan and WhittIe 1611

QMAN

ARAB FORMATION (MEMBERS D-A)(KIMMERIDGIAN-EARLY TITHONIAN)

Steineke et al. (1958) described a trend of theAfab D Member from the fmer grained (calcarenitic)limestones at the Dukhan field (Qatar) passing later-

ally to coarser sediments at the Ghawar fie1d(SaudiArabia). These latter grain-supported sediments of ashallow, high-energy setting grade into finergrained lagoonal sediments toward Riyadh in cen-tral Saudi Arabia and, stratigraphically, they gradedownward into a thick anhydrite. The porous lime-stone intervals are quite distinct from the interven-ing massive anhydrite that separates each memberof the Arab Formation. Virtually no interfmgering. ofthe limestone and anhydrite facies occurs and thetransition from one depositional setting to theother is very abrupto :rhese limestone/anhydrite

based on lithology (Figure 1). The Arab A-C arecomposed mainly of alternating anhydrite and car-

bonates (predominantly dolomite and partIy lime-stones). The Arab D comprises mainly carbonatesthat are predominantly limestones, partly dolomiteand, locally, anhydrite.

This study was compiled from the literature and

a series of cores and thin sections from wells locat-ed in the southern offshore Arabian Gulf. The Araband the Hith formations are the prime subjects ofthis paper and their occurrence in the southernand southwestern Arabian Gulf and their importantreservoir/seal relationship are described in detail.

GULFARABIAN

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Jurassic Oil Fld

() Jurassic Gas Field

OOther Oil and Gas Fields

facies changes at the top of the Upper ]urassic,with the disappearance of the anhydrite to the east,cause problems in its correlation, and in the west-central areas, the distinction and corre1ation isquite clear (Figure 5). However, a Hith equivalent(Asab Oolite) is commonly recognized in adolomitic facies even though the distinction

between this facies equiva1ent and the underlyingArab Formation is unclear and cannot be satisfacto-rily traced westward to the Hith anhydrite proper(Alsharhan, 1989;de Matos, 1994).

In onshore Abu Dhabi, the Fahahil Formation,about 140m (470 ft) thick, consists of crystallineaphanitic lime mudstone that grades upward towackestones and packstones containing dolomiteand nodular anhydrite. The depositional setting isthought to have been shallow marine to supratidal.The Qatar Formation, about 120m (400ft) thick, iscomposed mainly of dense, dolomitic lime mud-stones interbedded with anhydrite and dolomite.Dolomite layers are common and are believed to

have been deposited under extreme1y shallow tointertidal conditions. Dolomites are either finegrained and tight or medium to coarse grained,sucrosic, and porous. The relationship betweenthese two types is probably related to subsequentdiagenetic changes. Laminated or brecciated limemudstones are sometimes associated with dolomitesand correspond to variants of the same tidal-flat set-ting. Fossiliferous wackestones containing commonostracods are also found (Alsharhan, 1989).

In offshore Abu Dhabi, the Arab Formation isdivided into four zones, A-D fram top to bottom,


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1612 ArabianGulf,LateJurassic

LATE JURAS SIC EPOCH

OXFORDIAN - KIMMERIDGIAN - TITHONIAN ST AGE

H A N I F A !D I Y A B / ARAB HI TH F O R M A T I O N S A U D I A R A B I AJ U B A I L A H

B A H R A I N

e/m

I B / I I

I I M A N I F AOFFSHOREQATA

D/IV A II MEMBER O F F S H O R E A B U D

D I Y A B F A H A H I L Q A T A R HITH F O R M A T I O N

O N S H O R E Q A T A

LIMESTONE LIM);S('()NEI LlMESTONE~ LlMESTONE M E M B E R IV " m II [

D U K H A N F A H A H I L Q A T A R HI'IH I ~ F O R M A T I O N O N S H O R E A B U DMENDER

SOURCE RESERVOI R SEAL H Y D R O C A R B O NH A B I T A T

Figure 4-Nomenclature for the LateJurassic in the Arabian GuH.

eyc1es are re1ated to the transgressive (shoal) andregressive (supratidal) phases of eaeh depositionalcyc1e. The total thickness of the Arab Formation inthe study area ranges from 151m (500 ft) in the off-shore Qatar-Abu Dhabi area to over 257 m (850 ft)in onshore Abu Dhabi (Figure 6).

In these areas (southern and southwesternArabian Gult) the Arab A Member ranges from 30 to34 m (100 to 11Oft) thick and is primarily com-

posed of alternating anhydrite and carbonates.Because of this anhydrite, it is difficult to differenti-ate the Arab A from the overlying Hith Formation,which changes its depositional character eastward;

carbonate facies pinch-outs become common (deMatos, 1994). The Arab A anhydrite is fme1y crys-talline, generally exhibiting a chickenwire texturewith no porosity. The carbonate facies are mainlydolo mitic grainstone and dolomitized grainstone.Pe10ids are highly glauconitized and the associateddolomite is fine1y rhombic (subhedral to euhedral).Allochems show poor sorting with many botryoidaland intrac1astic grains in association with oolitesand pe1oids, suggesting a variable-energy deposi-tional setting. Rare chertification occurs in thedolomitic grainstones, as well as evidence of glau-conitization, suggesting chertification is preglau-conite. Dolomitic grainstones give way to com-

plete1y dolomitized grainstones at the top of thesection. The dolomite fabric is anhedral sucrosic oraphanocrystalline, with occasional streaks of sub-hedral rhombs. Porosity is very low due primarilyto original mudstone textures, anhydrite formation,and late cementation by coarse calcite spar andmosaic dolomite.

The Arab B Member ranges from 20 to 21 m (65to 70 ft) in thickness and is also composed of

dolomitic grainstone and dolomitized grainstoAnhydrite is less common than in the Araboccurring as a chickenwire texture. This redtion in anhydrite cement has left excellent ondary porosity. Calcite spar is very limited this absence, combined with later dissolution, resulted in some very good secondary porosiDolomitized grainstone occurs as euhedrrhombs that become anhedral upward in the tion. These completely dolomitized samples low original oolitic grainstone/packstone textuthat are rounded to subrounded, well sorted, with some sparry to microgranular matri

Allochems are commonly leached, providing merate moldic porosity. Those that are cementhave been filled by euhedral dolomite rhombs more rare1y, by chickenwire anhydrite. Thallochems that are not leached retain a micritizcalcitic texture with some evidence of glauconzation. The dolomite is grayish brown, moderathard, locally argillaceous, and contains anhydrin places. Fractures are uncommon, but remunfilled where occurring, enhancing permeabilslightly.

The Arab C Member ranges from 38 to 41(125 to 135ft) in thickness and, similar to the AA and B, is composed of carbonates alternatinwith anhydrite. The anhydrite occurs more cmonly in association with dolomite as opposed dolomitic limestone. The anhydrite is crystalliwith a chickenwire texture, containing streaksdolomite and limestone. The carbonate facinc1ude dolomitized grainstone with subordinadolomitic mudstone and dolomitic grainstone. Tdolomitic mudstones are moderate1y harddense, commonly in association with crystalli


6/23

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AlsharhanandWhittle 1613

WELL o

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Figure 5--Well-log correlation of four offshore Abu Dhabi wells showing the generallithology and the gamma-rayand neutron logs for the Diyab (Dukhan), Arab, and Hith fonnations in the studyarea. Note that the Hith Fonnationthins eastward and eventualIy disappears farther eastward ofwell D. For lithology symbols, see Figure 7.

anhydrite. Porosity in these mudstones is poor andlimited to small vugs, a few of which are oil stained.

Porosity in the dolomitic grainstones is verygood as a result of a decrease in anhydrite cementa-tion. Early ca1cite spar has been dissolved to a largeextent in the Arab B, and coarse dolomite rhombs,late diagenetic coarse ca1cite spar, and rare chertnodu1es occ1ude porosity only to a very smalldegree. Intrapartic1e porosity is present inmanyforaminifer tests within the grainstones. Dolomitic

limestone is micritic, commonly with anhydriteand local packstone/grainstone. Allochems of thelatter are primarily oolites and peloids, which areglauconitized. Those oolites and peloids that have

been leached are filled by anhydrite cementoCrystals in the dolomitized grainstones, which

dominate the Arab C, are anhedral to subhedrallower in the section, commonly sucrosic oraphanocrystalline, changing to rhombs, which arecoarser and more euhedral, and grade upward intodolomitic limestones before finally becominganhedral and sucrosic again at the top of the section.

Intercrystalline porosity within the dolomitizedgrainstone is moderate and moldic porosity, whichsuggests original grain-supported lithologies, is com-monly occ1udedby anhydrite. Subordinate packstone/grainstone is fme to medium grained and coarse in

places, rare1yoolitic, and cornmonly anhydritic. Thedolomite rhombs show some glauconitization, prob-ably incorporated into the crystal structure as aresult of the replacement of the glauconitized

peloids of the originallimestone. Dedolomitization

occurred in rare instances as indicated by coarse calcite crystals in which the extinction pattern showsengulfed rhombs identical to the surroundingdolomite rhombs.

The Arab D Member ranges from 91 to 168 m(300 to 550 ft) in thickness and differs from ArabA-C by an increasing bioc1astic dolomitic grainstone and dolomitized grainstone exhibiting extensive glauconitization. Mudstones and wackestonesin the lower Arab D grade to moderately hard,dense, microgranular packstone-grainstone faciethat are sub~ngular, well sorted, and locally oolitic


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Haradh

SAUDI

o' " K mI C.I = 50Ft

GULF

IR AN

,Fateh

fSWFateh

Figure 6-Isopach map for the

Upper Jurassic ArabFormation

in the southern aud southwestern .

Arabiau GuIf.onfields thatproduce from the Arabare shown

in bIack spots.

or crystalline, with thin dolomite beds and someanhydrite. The uppermost part of the Arab D iscomposed of dark brown to brownish-graydolomite and dolomitic limestone with a micritic tomacrogranular matrix that is commonly sparry andslightly argillaceous. The glauconite has primarilyaffected pe10ids and oolites; however, dolomiterhombs sometimes contain glauconite inc1usions,

suggesting that glauconite occurred prior to thedolomitization process. Early cementation in a mix-ing zone environment by sparry calcite prohibitedcompaction of grains, and later dissolution of thisearly cement resulted in well-deve1oped intergranu-lar and intercrystalline porosity. Late diagenetic

poikilotopic coarse calcite spar reduces porosityonly locally in some of the deeper Arab D samplesand also ftlls some fractures.

Kawaguchi (1991) divided the Arab D in Satahfield (western offshore Abu Dhabi) into eight litho-logic layers and fifteen reservoir layers based on

petrophysical characteristics. He found the Arab Dto be bottom sealed by a "bitumen mat," which

filled pores created through leaching. AlthoughKawaguchi (1991) defmed eight lithofacies in Satahfie1d, each fits into the depositional framework ofthe present study.

Higher permeability in the aphanocrystallinedolomite is due to its idiotopic and sucrosic tex-tures, as well as the development of goodenhanced intercrystalline porosity. The dolomi-tized lime mudstones and wackestones, which are

often argillaceous and bioturbated, generally havemoderate to good porosity. The peloidal pack-stones and grainstones have good porosity, whichis expressed by the occurrence of interpartic1e,vuggy, and moldic pores. In Qatar, Wilson (1991)defined two facies of the Arab D that are suitab1ereservoirs in low-relief structural traps: a porousgrainstone facies and a massive, highly permeable

dolomite. These two facies correspond to theoolitic/pe1oidal grainstone and dolomitized grain-stone facies recorded in this study; these facies alsoshow the best reservoir quality.

HITH FORMATION (TITHONIAN)

The upper porous zone of the Arab Formation issucceeded by the thick anhydrite section of theHith Formation, which represents the final regres-sive supratidal stage of the major depositionalcyc1e. Some thin porous carbonate sections in the

base of the Hith Formation in the offshore AbuDhabi area, however, do indicate local develop-ments of minor lagoonal and intertidal transgres-sive phases, which thin and disappear towardthe west.

The type section for the Hith Formation at DahlHith near Riyadh in Saudi Arabia has a measuredthickness of approximate1y 91 m (300 ft). The sec-tion was thought at one time to be niuch thicker. Inthis section, the anhydrite has afine chickenwire


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Figure 7-Schematic lithostratigraphic section and stratigraphic column of the Upper Jurassic ArabFormation inthe southern Arabian Gulf for (A)offshore areas (Qatar, AbuDhabi, and Dubai) and (B)onshore areas (Qatar and

AbuDhabi). Notto scale.

" 1\ 1\ I\-r:z:: A 1\ 1\ 1\ I\;:Z=,., 1\ A 1\ I\~AI\ HITH '" A A '" 1\ A ti. 1\ / /

ASAB

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~ Oolomitic timestone ~ Anhydr-ite

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1615

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SOUTHEASTERN ABU DHABI

t~7' J Glauconitic limestone

~ Oolitc timestone

Alsharhan and Whittle

E AS TE RN A BU DHA BI - DUBA I

I

I

variety, and the interbedded dolomites are light todark brown and sucrosic. In the lower beds, lightto dark brown pellety lime packstones and wacke-stones replace the dolomite. The dolomite in theHith is finely rhombic, anhedral, and contains abun-dant poikilotopic, nodular, and blocky anhydrite.Late diagenetic poikilotopic calcite spar occurs

rarely, occluding intercrystalline pores in a fewdolomite samples. Fractures within the dolomiteare partially to completely filled by laths of anhy-drite. Porosity is very poor as a result of the tightlyinterlocking fine rhombs and the extensive anhy-drite cementation and fracture fill.

In eastern Abu Dhabi and toward Dubai, the Hithis known locally as the Asab and Fateh members(Figure 7A). The Fateh consists of 107 m (350 ft) ofgray to dark brown sucrosic dolomite and is medi-um to coarse grained with minor oolitic grain-stones. Anhydrite occurs throughout the section in

CENTRAL - E ASTERN ABU DHABI

~ Dolomitized limestone

r=T1ArOlllaceous and deep-- water limetane

CENTRAL ABU DHABIQATAR - WESTERN ABU DHABI

QATAR - WESTERN ABU DHABI

( A) OFFSHORE AREAS

HITH

( B) ONSHORE AREAS

-1-

L 1/'FAH- ~iL I I

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o: z . . . : < : . :> = > 1

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~ T r

structure with some associated dolomite in its basalpart and grades upward into a finer but similarnodular texture. Toward the top of the section it

becomes finely laminated (on the order of millime-ters) and appears to contain some fine-grained car-

bonate material (Alsharhan and Kendall, 1994).In Qatar and Abu Dhabi, the anhydrite gradually

thins toward the east. The edge of the Hith marksthe eastern limit of massive anhydrite develop-ment and, hence, the final transition from suprati-dal to lagoonal and intertidal dolomites. Thisfacies change is clearly recognized from west toeast across central Abu Dhabi, where the anhy-drite progressively gives way to dolomite from the

base up.. In Qatar and western Abu Dhabi, the Hith ranges

from 140 to 183 m (462 to 600 ft) thick, whichincludes massive anhydrite, anhydritic dolomite,and dolomite. The anhydrite is of the chickenwire


9/23

1616 ArabianGuIf,Late]urassic

very minor quantities, but comprises clear, trans- grain type impossib1e (Figure 9A); however, relictparent crystals forming cement within the concentric laminae were observed and grain shape .dolomite. This facies reflects a supratidal setting indicates that these grains were primarily ooids orwith intervals of shallow-water shoaling. pe1oids. These grainstones are well sorted, but intra-

The Asab Oolite (Figure 7A, B), about 91 m (300 clasts and composite botryoidal grains do occur,ft) thick, comprises dolomitic lime mudstone grad- some of which appear to have originally beening upward to fine to medium, well-sorted oolitic grapestones (Figure 9B).

grainstone and peloidal-bioclastic wackestone/ The grainstones may contain a few dolomitepackstone with cha1cedony replacing some grains. rhombs, but what distinguishes them from theThe fossil content consists of common to abundant dolomitic grainstone lithofacies is the relativeforaminifers, dasycladacean algae, echinoids, and paucity of dolomite and the presence of some earlyshell fragments. Gastropods, stromatoporoids, marine cement (Figure 9B, C). Allochems arecorals, and be1emnites are uncommon and restrict- encrusted by bladed calcite cement, probably origi-ed to a few levels (de Matos, 1994). The Asab was' nally high-Mg calcite in mineralogy. Although somedeposited at the open-marine edge of a very shal- late diagenetic coarse poikilotopic ca1cite sparlow platform as an oolitic bar deposit (shoallime occurs (Figure 9A, D), porosity is very goodsand). Occasional coated grains indicate lower because of extensive dissolution. Also, the encrust-energy, protected conditions (AIsharhan, 1989). ing bladed cement does not extensively occludeDue to environmental and facies changes in south- pore throats (Figure 9E). Red algae, brachiopods,eastern Abu Dhabi (Mender region) toward Oman, foraminifers, and oyster shells were ole"rved in aAIsharhan (1989) introduced the term "Mender few samp1es and create some intraparticle porosity.

Glauconite Limestone Member" (Figure 7B). It has Grainstones that are interbedded with the Hitha maximum thickness of 59 m (192 ft) and consists anhydrite are more poorly sorted, containing abun-of light-gray to buff-brown stylolitic, pelletal mud- dant coated grains and intraclasts in addition tostone and wackestone that is well cemented with ooids and pe1oids. Porosity is reduced by extensivetraces of shale lamination, glauconite, skeletal anhydrite cementation, which occludes moldicdebris, and chert. Anhydrite and, to a lesser extent, porosity. Allochems are commonly glauconitized togypsum are common. A few dolomitic grainstones some extent.occur at the top of the section, but porosity is very In the southeastern part of the study area, grain_

poor due to glauconitization and little dissolution stones occur in the facies-equivalent Menderof early ca1cite spar cemento The original deposi- Glauconite. These grainstones are dominated bytional setting is thought to have been a deeper, off- lO-15-J.lm olive-green glauconite pellets. In mud-shore low-energy setting that graded upward to a stones, some of these smaller pellets nucleate intoshallow shoal or channe1levee, and fmally to inter- 25-50-J.lm pellets (Figure 9F). Together withtidal/supratidal conditions through a slow regres- cementation by anhydrite, these pellets have the

sion of sea level, causing dolomitization followed effect of occluding most porosity.by glauconitization.

ARAB/HITH LITIlOFACIES AND ASSOCIATEDPOROSITY

Based on our study of thin section and core data,the carbonate-evaporite sequences of the Arab andHith formations contain five general rock types: (1)oolitic/peloidal grainstone, (2) dolomitic grain-stone, (3) dolomitic mudstone, (4) dolomitizedgrainstone, and (5) massive anhydrite. Variationswithin lithofacies include faunal associations(Figure 8), as well as glauconitization and chertifi-cation.

Oolitic/Peloidal Grainstone

This subordinate lithofacies is found throughoutthe Arab Formation. The highIy micritized grains inthis facies commonly make identification of original

Dolomitic Grainstone

Dolomitic grainstones were observed primarilyin the Arab B, C, and D members. Anhedral to sub-hedral rhombs selectively replace the matrix in thisfacies, leaving highIy micritized allochems as evi-dence ofthe precursor (Figure lOA). Relict marinefibers were noted partially encrusting ooids and

peloids on several occasions, but most of themarine cement in this facies has been removed by

dissolution. These samples have escaped extensivefreshwater diagenesis and have retained excellentprimary porosity. Early ca1cite spar that cementedthe matrix of many grainstones has been virtuallycomp1etely dolomitized. Dolomite occurs as fine,medium, and coarse rhombs and may indicate sepa-rate dolomitization events. The fme rhombs (thoseranging from 30 to 50 J.lm)and medium rhombs(those ranging from 60 to 90 J.lm)are much morecommon and could be associated with a sabkha


10/23


11/23


Chertification is local; only a few grainstones showevidence of chertification and even then it is limit-ed to one or two allochems (Figure lOD).

Samples from the Mender Glauconite show glau-conites that appear to folIow original dolomiticgrainstone textures. These dolomitic grainstonesare commonly highly fractured and stylolitized and

filled by glauconite, anhydrite, or a bituminousresidue. Aphanocrystalline dolomite cemented thematrix prior to glauconitization and occ1udes inter-

partic1e porosity. Laths of anhydrite and patches ofchickenwire anhydrite are uncommon. Likewise,chertification is uncommon.

Dolomitic Mudstone

A thick sequence of dolomitic mudstone wasfound at the base of each of the Arab B- D mem-

bers. The thickest of the mudstones by far was inthe Arab D and exhibits various stages of dolomiti-zation. The dolomite is primarily finely rhombic

and euhedral (Figure 11A) with subordinateaphanocrystalline textures. Poikilotopic anhydriteand late diagenetic coarse calcite spar rarely engulfrhombs in the dolomitized areas. Rare leached andrecrystallized sponge spicules occur. Chertificationis locally deve10ped because microquartz se1ective-ly replaces allochems. Porosity is very poorIy deve1-oped except in a seam of extensive vuggy porosity.Pyrite crystals appear to be a late diagenetic addi-tion to the samples.

Dolomitic mudstones of the Mender Glauconiteshow heavy glauconitization by olive-green pellets.Although it is characteristicalIy similar to thedolomitic mudstones of the Arab, the dolomite is

more commonly aphanocrystalline than rhombic,and anhydrite (and rarely gypsum) cements aremore abundant (Figure 11B).

Dolomitized Grainstone

Completely dolomitized samples in the ArabFormation follow original grainstone rock types.

Micritized and dolomitized remnant ooids and. pe10ids are visible as ghosts within the replacement

dolomite fabrico This lithofacies is the completdydolomitized end member as opposed to theoolitic/peloidal grainstone as the opposite (com-

pletely undolomitized) end member, with thedolomitic grainstone lithofacies being partialIy

dolomitized and intermediate. Intercrystallineporosity in the Arab C and D is very good (Figure12A) and is enhanced by postdolomitization disso-lution. Rhombohedra are the most common formof dolomite (Figure 12B), but an aphanocrystallinetexture was also observed (Figure 12C). Poikilo-topie and blocky anhydrite occurs much more com-monly in the Arab A and B members, as does latediagenetie coarse calcite spar, reducing the inter-crystalline porosity (Figure 12D). Glauconitie inclu-sions are common in the dolomite, probably rem-nants of the glauconitization of the originalallochems.

Dolomitized grainstone witmn the Hith

Formation shows very poor intercrystalline porosi-ty and anhedral rhombs. The sabkha-related anhy-drite formation is the reason for the low porosity.

Dolomitization is extensively higher in the sec-tion and generally decreases with depth. This sug-gests dolomitization may be related to sabkha dia-genesis as Mg-charged flood waters dolomitize thelimestones from the surface downward, althoughinfiltration by meteoric groundwaters also mayhave assisted in the process (Magaritz and Peryt,1994).

Massive Anhydrlte

In the southern and southwestern Arabian Gulf,massive anhydrite occurs almost exc1usive1yin theHith Formation and between Arab members with achickenwire fabric, the associated porosity ofwhieh is virtually ni! (Figure 13A); however, patch-es of rhombic dolomite within the anhydrite bedsare common and these exhibit some intercrys-talline porosity (Figure 13B). Poikilotopie anhydrite

Figure 9--Photomicrographs ofthe oolitic/peloidal grainstone facies. (A)Highly micritized peloid encased in poik-ilotopic calcite. Despite late calcite cementation, excellent interparticle porosity has been retained (crossed nicols;

scale bar = 300pm). (B) Bladed to flbrous low-Mgcalcite cement encrusting intraclasts, ooids, and peloids. These

were originally marine cements of probable aragonite or high-Mgcalcite mineralogy (crossed nicols; scale bar =

300pm). (e) Former marine cements in an oolitic/peloidal grainstone ofthe ArabDMember. Despite early cemen-tation, most ofthe primary porosity has been retained (crossed nicols; scale bar = 120pm). (D)poikilotopic calcite

cement in an oolitic/peloidal grainstone of the Arab DMember. This late diagenetic feature has not signiflcantlyreduced porosity, which shows enhancement by dissolution, possibly after poikilotopic cement (crossed nicols;

scale bar = 300pm). (E) Dolomitized grainstone with excellent interparticle porosity due to dissolution of early

marine cements and limited dolomitization (crossed nicols; scale bar= 300pm). (F) Glauconitizedpellets (dark-col-ored aggregates) in a highly micritized mud matrix in the Arabequivalent known as the Mender Glauconite (in the

southeastern part ofthe studyarea). Smaller pellets(40-15pm) were observed inMender Glauconite grainstones;

the presence of scattered small pellets in this photomicrograph suggests the larger peUets(""25-50 pm) are aggre-gates ofthese small pellets (crossed nicols; scalebar =300pm).


12/23

A

e

E.

Alsharhan and Whittle 1619

B

D

F


13/23

1620 Arabian Gulf, Late Jurassic

A

e

B

oFigure 10--Photomicrographs of the dolomitic grainstone facies. (A) Selective dolomitization of the matrix of an

Arab Cgrainstone. Allochems remain undolomitized and intercrystalline porosity has been enhanced by dissolu-

tion (crossed nicols; scale bar =300 pm). (B) Coarse rhombs in a dolomitic grainstone of the Arab D. These

anhedral to subhedral rhombs have low intercrystalline porosity as opposed to the finer euhedral rhombs. Note

glauconite inclusions within rhombs (crossed nicols; scale bar = 120 pm). (C) Dolomite rhomb showing glauconite

inclusions. Glauconitization probably occurred prior to dolomitization and this glauconite was subsequendy incor-

porated in the dolomite crystallattice (crossed nicols; scale bar = 120 pm). (D) Partial chertification of an intraclast

in a dolomitic grainstone. The source of silica was probably from leached sponge spicules; however, the source is

difficult to determine because chertification was not well developed within the study area (crossed nicolsj scale bar= 300 pm).

crystals within the dolomite patches can make thisintercrystalline porosity very low. The texture ofthe anhydrite is occasionally felty, but this is a sub-ordinate fabrico Some of the anhydrite follows theemplacement of dolomite in the form of anhydriticrhombs, which are evident locally. Microfracturingis common, but this does not appreciably increase

permeability. Oil staining and glauconitization arerare and relict peloid ghosts are uncommon.

DEPOSI TI ONALSETTI NG

The Arab Formation is the major oil-producinginterval in the Arabian Gulf (Murris, 1980; Ayres etal., 1982; Alsharhan and Kendall, 1986). The forma-tion shows considerable regional variation in faciesand reservoir characteristics and represents thetransition from deep-water shelf conditions at the

base, through shoal, lagoonal and intertidal to


14/23

A

Alsharhan and Whittle 1621

B

Figure l1-Photomicrographs of the dolomitic mudstone facies. (A) Dolomitic mudstone showing characteristicselective dolomitization of the matrix by scattered euhedral rhombs (crossed nicols; scale bar =300 }1m). (B)

Dolomitic mudstone of the Mender Glauconite showing aphanocrystalline texture and abundant anhydrite nodules.

The anhydrite has occluded most ofthe secondary moldic porosity (crossed nicols; scale bar = 300 }1m).

supratidal conditions at the top (Figure 14). Thedepositional features present vary considerably,although certain major distinctions are apparent.The area from Saudi Arabia to eastern offshoreQatar and western offshore Abu Dhabi is large1ydeve10ped as a shallow deposit (Figure 15)com-

posed mainly of a grainstone fabricoTransition fromdeeper she1f sediments at the base to lagoonal and

supratidal sediments at the top is rapid, and thereappears to be very little interdigitation of deposi-tional facies. At Dukhan, a thin interval of anhydriteassociated with dolomite is present just be10wthe main anhydrite, which overlies the FahahilFormation. In offshore Qatar fie1ds,grainstone sedi-ments become more prominent and probably forman extension to the high-energy shoal deposits(AIsharhan and Nairn, 1994). Shoal deposits do notform the total Arab D section here because they areunderlain by finer grained, muddier she1f depositsand are overlain by similarly fine-grained lagoonaldolo mite s and limestones similar to Umm Shaiffie1dof Abu Dhabi.

Based on previous studies by Murris (1980),Wilson (1985), and Alsharhan and Nairn (1994), inaddition to the present study, the deposition of theArab and Hith formations is believed to haveoccurred in four types of settings (Figures 1, 14,15): (1) supratidal sabkhas, (2) intertidal mud flatsand stromatolitic mats, (3) shalIow subtidallagoons,and (4) shallow open-marine shelves.. During deposition of the Arab Formation, thesupratidal environment was dominated by sabkhasdeposited in an essentialIy arid climate. Storm activ-ity and exceptionally high tides resulted in flooding

of the sabkha surface and deposition of storm sedi-ments as described from the present-day ArabianGulf sabkha by Kendall and Skipwith (1968, 1969),Kendall (1969), Butler et al. (1982), Warren (1990),and Alsharhan and Kendall (1994). Because.of thevery flat surface of the sabkha, such sedimentscould be deposited over extensive areas, and theseareas are characterized by the early diageneticgrowth of nodular anhydrite and dolomite, suchthat the depositional fabrics of the supratidal sedi-ments are often destroyed. Sabkha-re1ated anhy-drite growth may also take place in intertidal andlagoonal sediments, wherever the sabkha pro-grades over such underlying strata.

In Abu Dhabi, the Arab A-C and Hith formationsare the best examples of sabkha cycles; these arewide1y recognized in most offshore reservoirs inwestern Abu Dhabi. Porosity varies dependingupon depositional grainstone preservation anddolomite texture. This porosity occurs in shallowsubtidal grainstones and dolomites, although per-meability is commonly restricted by anhydrite

cemento Wood and Wolfe (1969) described ninesabkha cycles from Umm Shaif throughout theArab, and similar cycles occur across the easternAbu Dhabi region, but deteriorate to the west. Ineastern Abu Dhabi and Dubai the sequence isreduced to a Hith equivalent that lacks the beddedanhydrite, with the sequence consisting of anhy-dritic dolomite and dolomitized grainstone.LaPointe (1991) suggested a mode1 that attributesthe evaporite formation in this area to submarinedeposition within a basin. Among other things, alack of a classic sabkha sequence and poor lateral


15/23

1622 Arabian Gulf, Late Jurassic

A

e

B

D

Figure 12--Photomicrographs of the dolomitized grainstone facies. (A) Dolomite of the Arab D showing well-devel-

oped intercrystalline porosity (crossed nicols; scale bar = 300 pm). (B) Rhombohedral dolomite of the Arab eshow-ing partial cementation by anhydrite (center). This late diagenetic cementation has not significantly reduced the

intercrystalline porosity (crossed nicols; scale bar = 300 pm). (e)Aphanocrystalline dolomite texture of the Arab B.

Note anhydrite completely occludes fracture. Anhydrite cementation was more pronounced in the Arab A and B

members, reducing much of the secondary porosity. The increase in anhydrite suggests the evaporite formation

was probably surface related (crossed nicols; scale bar = 120 pm). (D) Poikilotopic anhydrite in an Arab Adolomite.

Anhydrite in these upper Arab members is both displacive and replacive and significantly reduces porosity(crossed nicols; scale bar = 120 pm).

corre1ation are cited as evidence for this mode1(LaPointe, 1991); however, stromatolitic mats gen-erally show poor preservation. The Holocene stro-matolitic mats of the Abu Dhabi coast, buried to adepth of only a few tens of centimeters, areextreme1y thin and commonly pinch out laterallyaltogether. Evidence of their presence is sometimesestablished by a dark-green to black, highly organicgypsum mush layer above or be10w their former

positions. A typical sabkha sequence may show

great lateral variation not only in facies changesperpendicular to shore, but paralle1 to shore,where lensoid geometries cause pinch-out (Peebleset al., 1995). Thus, the lack of lateral correlation isnot uncommon in a sabkha setting.

The intertidal environment occurs in an areabetween low and high mean tides, and thus is anarea subjected to both marine flooding and subaeri-al exposure. Background sedimentation in thisenvironment is characterized by the formation of


16/23

A

Alsharhan and WWttIe 1623

B

Figure 13--Photomicrographs of the massive anhydrite facies. (A) Vein of chickenwire anhydrite within a dolomiteof the Hith Formation. Much of the dolomite is aphanocrystalline with low intercrystalline porosity. The chicken-

wire anhydrite texture is a result of compaction and is extremely nonporous and impermeable (crossed nicols;

scale bar = 300 pm). (B) Although some intercrystalline porosity occurs in the Hith Formation dolomites, massive

chickenwire anhydrite is much more common and creates an excellent seal above the Arab reservoir (crossed

nicols; scale bar = 300 pm) ..

mud flats and algal mats, with periodic floodingthat produces influxes of shallow subtidal innerlagoon sediments. Beach bars may develop towardthe seaward margins and shoals may form at thelandward limit by high storm tides. Minor channelsare also developed in this zone, but are of local

extent and deposits tend to be very thin. However,large-scale tidal channels commonly migratethrough and across the normal background sedi-ments, within which significant sand bodies weredeposited. Primarily, however, intertidal sedimentscomprise mudstones and boundstones. Variabilityoccurs if there are frequent incursions of coarsergrained lagoonal sediments or if the mud flat com-prises peloidal sediments. The proximity of thisdepositional setting to the supratidal environmentmeans that the intertidal facies are susceptible tosabkha diagenesis.

The lagoonal, shallow, subtidal environmentextends from the low mean tide line seaward to

shoal bodies that define the outer lagoon limitoDepth may be only a few meters or up to tens ofmeters, with sedimentation dominated by tidal cur-rents. This environment is thus defined as beingcontinually covered by seawater. Lagoons withinthe Arab Formation can be divided into two types:(1) an inner (nearshore) lagoon with hypersaline

biota and background sediments becoming progres-sively coarser grained landward, and (2) an outer(seaward) lagoon with predominantly normalmarine salinity biota and fine-grained backgroundsediments. Within this broad environment, shoaling

above wave base may create intralagoonal bars withassociated coarse-grained sediments from spillovers.Further coarse-grained sediments may occur pro xi-mal to offshore bars in the outer lagoon. Channelsrunning into the lagoon from the intertidal zonemay also produce an influx of coarse-grained sedi-ments (Le., channel outwash sands).

High-energy shoal environments comprise mainlygrainstone bodies (shoals or bars) and channels, andorganic banks comprise grainstone and algal-stro-matoporoid boundstones. The shoals are dependenton wave energy for their formation and thus occuras beach bars dividing lagoons from intertidal zonesor as offshore bars built up above wave base bothwithin and at the seaward margin of lagoons. Theseshoal deposits are in contrast to channel sands,which are dependent on tidal energy and thus tend

_to be restricted to lagoons and intertidal settings.The shelf environment extends seaward from the

outer reaches of the lagoon, with the inner bound-

ary generally delineated by offshore bars. In someinstances the shelf may pass directly into a lagoonwithout delineation by offshore bars, but suchinstances are rare. The characteristic feature of theshelf environment is open-marine normal-salinitywaters with sedimentation taking place at or belowwave base. Shelf environments can be broadly divid-ed into inner and outer shelf zones on the basis ofwater depth (Le., photic zone is inner shelf andbelow photic zone is outer shelt) or energy index(at or marginally above wave base is inner shelf and

below wave base is outer shelt).


17/23


Figure 14-Depositional model

for the Arab and Hith formations

in the southern and southwestern .

Arabian Gulf. Deposition of the

Arab and Hith formations is

interpreted to have occurred in

four distinct settings: supratidal

sabkha, intertidal mud/algal flat,

shallow subtidallagoonal, andopen-marine shelf.

- - - -

DIAGENETIC FEATURES

Calcium Carbonate Cementation

Cementation occludes pores throughout the

Arab carbonates and the cements formed in severaldiagenetic settings. Primary pores are partiallyoccluded by ca1cite spar precipitated in a mixingzone or meteoric setting (Harris et al., 1985).Despite micritization and dolomitization, ghosts ofallochemical grains may still be distinguished insome cases; however, most lithologies have under-gone dolomitization to some extent, obliteratingearlier cement fabrics. Relict mar in e bladesencrusting ooids and pe10ids in grainstones do notsignificantly occlude pore throats, suggesting that

primary porosity may have been rather high. Dueto dissolution, relict marine cements are uncom-mono The generallack of marine cements suggests

that secondary porasity was high toward the end ofmarine phreatic diagenesis (cf. Loucks and Budd,1981)prior to ca1cite spar precipitation. Ca1citespar was common in the study area (composing asmuch as 50% of the rack in some cases), but a sig-nificant amount has been removed by dissolution.The result is excellent interparticle porosity inoolitic/pe1oidal and dolomitic grainstones.

Meniscus cements are uncommon in theoolitic/pe1oidal grainstone facies, indicating somevadose diagenesis. However, the paucity of thiscement texture suggests dissolution was moredominant than precipitation and that perhaps thesesediments had a very limited residence time above

the water table.

Leaching

While leaching of the secondary calcite matrix isminor in the Arab Formation, leaching of originalaragonitic grains is important in reservoir deve1op-mento This process takes place in the mixing zone

and meteoric phreatic and vadose diagenetic set-tings, commonly followed by subsequent moldic

pore cementation by calcite.Grain leaching within the Arab Formation takes

two forms: (1) leaching of sponge spicules during

marine phreatic diagenesis, and (2) leaching ofaragonitic grains and ca1cite matrix. The formerprovides a possible silica source for the chertifica-tion observed in the Arab C and D. Preferentialleaching of allochems was observed locally in theArab and some completely dolomitized sampleshave moldic porosity (commonly filled by anhy-drite), which is interpreted as the result of leachingof allochems prior to dolomitization of the matrix.

Neomorphic Overgrowth Cementation

Neomorphic ca1cite cements fill primary and

moldic pores, the result of recrystallization of origi-nal marine cement fabrics. Lime mud fabrics showovergrowth cementation and neomorphismincreasing in occurrence with depth. Recrystal-lization of oolitic and micritic grains creates matrix-size pores within grains that are then subject toocclusion by overgrowth cementation. Poikilotopicand coarse sparry calcite cement appear to begenetically re1ated, the sparry texture precipitatingin open pores and the poikilotopic texture engulf-ing grains. They probably formed as burialcements, as suggested by their large crystal size andtheir common association with anhydrite (alsooccasionally poikilotopic) and dolomite textures.Whereas most neomorphosed allochems anddolomite rhombs contain glauconitic inclusions,the sparry and poikilotopic ca1cite cements lackglauconite, suggesting this calcite is a late diagenet-ic feature. The large crystal size is also commonlyassociated with burial diagenesis (Meyers, 1974;Choquette and ]ames, 1990). These fabrics com-monly appear throughout the Arab Formation, butrare1yin great abundance.


18/23

-


I RAa. .

Anhydrite Growth

The initiation of anhydrite precipitation is wide-ly recognized (Kinsman, 1966; McKenzie et al.,1980;Warren, 1990) to take place in the sabkhacapillary zone just below the sabkha surface.

sediment characteristics, pore-water enrichment inMg2+,and mobility of such fluids. Totally dolomi-tized rack types tend to vary in petrophysical char-acteristics. Dolomitization was initially controlled

by facies (Le., scattered rhombs in mudstones,

pseudomorphing of grainstones), but developmentof purely crystalline textures was controlled bypore water composition during sabkha phreaticdiagenesis similar to that described by Magaritz andPeryt (1994), where evaporative and meteoricwaters combine to dolomitize evaporite-related car-bonates.

The dolomitization process occurred in two sep-arate stages (early and late) in the Arab. A sabkhasetting is indicated for the early dolomite by its

presence in association with anhydrite and gyp-sumo Marine water was supplied to the sediments

beneath the sabkha by flood recharge (Butler,1965; Butler et al., 1982). The waters became Mg-

charged through precipitation of gypsum and fil-tered down thraugh the sediment, causing dolomi-tization. Isolation of the sabkha from tidal watersoccurred behind manmade causeways that parallelthe coastline in Abu Dhabi, resulting in the pres-ence of anhydrite in the upper sabkha. However,gypsum is present in front of the causeways, wheretidal flow is unhindered. This evidence is contraryto the idea of evaporative pumping (Hsu andSiegenthaler, 1969; Hsu and Schneider, 1973;McKenzie et al., 1980), suggesting that it is surticialwaters that are driving evaporite formation.Dewatering of gypsum results in anhydrite for-mation.

Although most of the dolomite observed in theArab sediments can be attributed to sabkha-associ-ated diagenesis, dolomitization could have taken

place whenever the Mg concentration was suffi-ciently high. This is the case for a second genera-tion (late diagenetic) dolomite that occurs ascoarse crystalline (rhombic and mosaic) void fill.On rare occasions the mosaic crystals have slightlycurved crystal faces giving them an appearancesimilar to saddle dolomite. Radke and Mathis(1980) found saddle dolomite to form at elevatedteinperatures, generally in association with sulfates(Le., anhydrite and gypsum). The coarse crystal

size suggests significant depth of formation and aslower growth rateo These void-fill fabrics show noevidence of glauconitization and so probably post-dated glauconite formation.

100 kmoI

IR A N

~ AP PROX IMATE EASTERN

EXTENT OF ARAS D

rr:::n lIGHT-COLORED LIMEMUDSTONE

. .

U l Z Z J OPEN-MARINE SHELF

EEDOLDMITE

[ : - - - - -1 LAGOONAlISHALLOW-SHELF- - - DOLOMITIC LIMESTONES

...... " lurlAn A

~ .KUWAIT

. .

Dolomitization

Dolomitization is closely associated with thegrowth of nodular anhydrite in the sabkha capillary

zone (Kinsman, 1966) due to an increase in theMg:Ca ratio caused by the precipitation of calciumsulfates. The dolomitizing fluids may also migrateto much greater depths, particularly where highly

permeable strata provide suitable conduits, thuscreating dolomitized layers within predominantlylimestone sequences. Such dolomites may be devel-oped at some distance laterally and vertically fromtl1esabkha.

Dolomitization produces a wide variety of fab-rics that may enhance or reduce reservoir quality.Such fabrics depend for their formation on original

Figure 15-Paleogeographic facies map of the Upper

Jurassic Arab D Formation. Locations of selected oil

fields (black) are also shown.


19/23


Where anhydrite nodules are sufficientIy numer-ous, they coalesce to form a dense impermeablelayer of chickenwire anhydrite upon compaction.Scattered anhydrite, however, causes a more localreduction in porosity and permeability. Nodularanhydrite is blocky or consists of laths, whereas

poikilotopic crystals commonly cement dolomitesand, less frequentIy, dolomitic limestones through-out the Arab Formation. Mitchell et al. (1988) simi-lady interpreted the nodular anhydrite as depositedin a sabkha setting in the Arab D in Saudi Arabia.Most of the original gypsum that precipitated hasdewatered to anhydrite through compaction.Remnant gypsum fabrics are blocky or in the formof euhedrallaths.

Stylolitization

Compaction of sediments occurs to some degreethroughout all diagenetic environments, with

increasing compaction directIy proportional tooverburden pressure. Numerous examples showcompaction and pressure solution reducing porosi-ty and permeability (Dunnington, 1967; Coogan,1970; Mossop, 1972; and many others). Com-

paction is, in part, inhibited by cementation, butincreasing overburden pressure leads to the deve1-opment of stylolites.

In c1ose1yspaced swarms, stylolites will affectvertical permeability due to matrix cementationand insoluble residue formation in the seams.Individually, they only locally reduce this quality.Frequency of pressure solution phenomena has lit-tIe effect on the upper Arab reservoirs and only a

minor effect in reducing vertical permeability atcertain zones in the lower Arab reservoir. Stylolitesare all filled with insoluble residues that inc1uderesidual bitumens, as well as some anhydrite andcoarse1y crystalline calcite cements. Stylolites aredeve10ped during deep-burial diagenesis.

DIAGENETIC HISTORY

The earliest events to affect the sediments weremicritization, recrystallization, and cementation(Figure 16). During algal micritization the outer lay-ers of grains on the sea floor are riddled by a sys-tem of tubes, 6-15 Jlm in diameter in Holoceneexamples (Bathurst, 1975), thus producing a rindof damaged material. Upon the death of the algaethe bores are vacated and later become inf1lled bymicrocrystalline calcite or aragonite. The effect onthe grains is threefold: (1) the structure of thegrains is destroyed centripetally, (2) the grains

become more susceptible to abrasion, and (3) theyare more easily rounded.

AIso assigned to the early diagenetic periothe process of neomorphic recrystallization ofmudo The micritized grains (formed by mitroaction) now consist of this fme microspar, aneady micritic calcite filling the microbial borthought to have recrystallized at this time.

Early cementation has led to the lining or f

of intragranular pores, especially foraminifer cbers, by micrite or drusy cemento These drusyof radially oriented crystals on grains were reed in certain grainstones. This cement was pr

bly originally aragonite or high-Mg calcite, busince inverted to low-Mg calcite. The rims appear to be locally distributed at or about gcontacts, suggestive of meniscus cements; hoer, they lack the dassic curved outline of the wmeniscus noted by Dunham (1971). If these wmeniscus cements, then the original composiwas low-Mg calcite.

Following the deposition of drusy calcite cem(Figure 16), epitaxial cement rims deve10ped

echinoderm fragments (Evamy and Shearm1965), locally engulfmg the earlier druse cemenadjacent grains. Microcrystalline calcite cemmay also have been precipitated in intergranu

pores just after the formation of drusy calcite. cement is so fme grained that examination is

plicated by the thickness of the thin sectionsJlffi).Epitaxial cement rims about echinoderm ments poikilotopically endose the microcrystalcement and, therefore, postdate it.

All of the cementation processes described place at a re1ative1yearly stage in the diagenetictory of the sediments, and their timing overlaps leaching of aragonitic biodasts (Figure 16). Som

the molds produced by this leaching have thse1ves been partially cemented, and the calcicarbonate-saturated solutions derived by leachmust be the source of some of the calcite or arnite cemento The current opinion is that leachof aragonite takes place in both the vadose

phreatic zones due to percolation of meteowaters (Bathurst, 1975). This suggests that disstion of aragonite is probably re1ated to the emgence of the sediments. Note, however, that 1eaching is not intense in the studied cores long periods of subaerial exposure may not hoccurred. Meteoric water may have migrated ward for long distances through the sediments,

placing former marine phreatic waters. The enpoint for these meteoric solutions, therefore, have been at some distance from the strata taffected.

In the normal marine facies, the presence molds and calcite-cemented molds after sponspicu1es is commonly recorded. These sponspicules were probably originally siliceous becacalcareous spicules would not have dissolved

~----------------t


20/23

MICRITIZATION

RECRYSTALLIZATION

MARINECEMENTATlON

DRUSYCALCITE

EPIT AXIAL CEMENT

LEACHlNG

DOLOMITIZATION

EV APORITES

SPARRYCALCITE

MOSAIC DOLOMITE

STYLOLITES

FRACTURES

MINOR MINERAL REPLACEMENT

EMPLACEMENT OF BITUMEN

MARINE

RE

MIXED

M AR IN FJ SA BK HA D EE P S AB KH A

- - - - -- - - - -

BRIAL

- -

--


Figure 16--Diagenetic sequence

for the Upper Jurassic Arab

Formation in the southern and

southwestern Arabian Gulf.

they were siliceous, the spicules would be conve-nient for explaining the silica source for chertifica-tion in the deeper Arab C and D.

Minor 1eaching of lime mud has also taken placelocally. Most of the vugs so produced are small andtend to form in intergranular areas of packstones.Large solution vugs are rare, and those that are

present are probably solution-enlarged molds.Many of the originallimestones have been affect-

ed to some extent by dolomitization. Most com-monly, only small amounts of secondary dolomiteare present (as scattered rhombohedra) and thesehave little effect on original fabrics and porosityand permeability characteristics.

The zones of dolomite probably relate to periodswhen supratidal conditions prevailed in surfacesediments. In such conditions, Mg-rich brinescould percolate down into the underlying or more

seaward sediments, resulting in dolomitization(Kinsman, 1966; Patterson and Kinsman, 1982).The precipitation of anhydrite and dolomite is

commonly associated, but in these instances thereplacement or local cementation by various formsof anhydrite probably occurred at depth ratherthan at the sabkha surface (Figure 16).The amountof anhydrite, however, is small compared to thatdeveloped in the Holocene sabkha of the UAE.Conditions conducive to the growth of anhydrite atdepth were probably also responsib1e for thecement filling the fractures in the upper part of thesection.

Just below the sediment surface in the capillary

zone, anhydrite developed as nodules of very finegrained laths with a complex parallel, radiating, ordecussate growth (Shearman, 1966). These nodulesdisplace the host sediment and, where they areabundant, chickenwire nodular anhydrite is pro-duced. The formation of anhydrite is mainly sec-ondary, forming through the dewatering of gypsumcrystals. During gypsum precipitation the Mg:Ca(atio is increased by the removal of Ca2+ from thepore water. As this ratio increases, dolomite crystal-lizes in preference to aragonite or magnesium cal-cite (Kinsman, 1966; Bathurst, 1975). Dolomite

also forms by replacement of aragonite (Illing et al.,1965). As the dolomitization process continues, theMg:Ca ratio is pushed in the other direction (super-saturated with respect to Ca2+) and favors evapo-rite precipitation. Thus, anhydrite and earIy

dolomite both form at almost the same time in thesupratidal environment, although dolo mitizatio ncontinues even after the formation of nodular anhy-drite. Initially, idiotopic textures resulted, but withcontinued dolomitization overgrowth rims were

precipitated on the rhombs, 1eading to hypid-iotopic and xenotopic textures.

Aggregate grains are occasionally present in thelimestones studied. The most common type is thegrapestone of Illing (1954); this grapestone wasformed by interpartic1e cementation at points ofcontact by cryptocrystalline aragonite or ca1cite,followed by further reworking and abrasion.Intrac1asts are sometimes involved, and these repre-sent torn up substrates that have been partIy lithi-fied prior to erosion and redeposition. In bothinstances, the lithification and cementation pro-cesses were very early, indeed syndepositional.

As the ca1cium-carbonate-saturated waters fil-tered through the sediments, they caused localcementation in the form of sparry calcite and,where Mg ions were present in high concentra-tions, mosaic dolomite (Figure 16).

Minor diagenetic effects recognized inc1udereplacement by celestite, chert, quartz, fluorite,ahd pyrite. Working out the timing of these eventsis often impossible because they cannot be related

to other diagenetic events. Celestite probablyformed fairly early, but after early dissolution andca1cite/aragonite cemento Chert, quartz, and fluo-rite are generally found replacing calcitic grains,and in rare instances chert nodules have formed.Fluorite has an affinity for echinoderm grains,whereas chert and quartz more commonly replacefibrous pelecypods. Pyrite is rare and its relation-ship to other diagenetic events is not c1ear.

Stylolites of varying size and geometry have beenfound throughout the studied section, but tend tobe more common in the mud-bearing limestones


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1628 ArabianGulf,lateJurassic

than in the grain-supported sediments. To someextent they are less common in dolomites anddolomitic limestones, although this is not alwaysthe case. The stylolites formed by dissolution ofrock under the directional control of pressure,most commonly overburden, such that insolublematerial was concentrated at the interface of thetwo interpenetrating rock masses. This processoperates on lithifted rock during deep burial.

Emplacement of residual tar is seen to be a latediagenetic process because the tar coats all diage-netic minerals, although the presence of thickresidues of tar along some stylolite seams showsthat tar emplacement postdated stylolite formation(Dunnington, 1967). Dunnington (1967) also com-mented on the preferential development of stylo-lites in porous, fine-grained, low-permeability lime-stones where residual water saturation remainedhigh, but in the more coarsely porous and highly

permeable units pressure solution was large1yinef-fective because of the oil ftlling.

HYDROCARBON OCCURRENCE

The anhydrite units between each zone (A-D) inthe Arab Formation act as intraformational seals inthe same field. The anhydrite of the Hith has

proved to be a regional seal for the oil and gas accu-mulation in Abu Dhabi, Qatar, Bahrain, and SaudiArabia (Murris, 1980; Ayres et al., 1982; Alsharhanand Kendall, 1994). When the facies in the Hith are

breached through faulting (as at Abu Al Bukhooshand Idd El Shargi South dome) or facies changefrom anhydrite to dolomite or dolomitic limestone

(as in eastern Abu Dhabi), the Arab oil can escapeupward (Murris, 1980; Alsharhan, 1989; Alsharhanand Kendall, 1994).

The Hanifa in offshore Qatar was deposited in anintrashelf basin setting. It consists of fine1ylaminat-ed, dark-gray, argillaceous lime mudstone and cal-careous shales; the lower part contains more car-

bonaceous, laminated, argillaceous limestone(AIsharhan, 1989; Droste, 1990). The facieschanges gradually in eastern offshore UAEtowardDubai, becoming shallower, and is composed ofdolomitic limestone and c1ean, suero sic dolomite.The Diyab/Hanifa (in Abu Dhabi), in addition to

being the major source rock for the ]urassic andLower Cretaceous reservoirs, forms a good reser-voir unit in the oolitic-pe1oidal packstone and grain-stone of the porous section (Alsharhan, 1989). Oilhas been produced in this zone at Bu Dana,ADNOC 1-B and ADNOC 1-C fields (Lutfi andElbishlawy, 1986; Alsharhan, 1989).

In the lower part of the Arab Formation at HairDalma and Umm Shaif, there is an organic enrich-ment corresponding to an increase in argillaceous

contento In this basal unit, minor oil-source potetial may be deve10ping in the Arab D.

The Arab Formation forms the principal oil resvoir (Figure 3). Oil accumulations occur in QatarDukhan, Idd El Shargi (North dome), Bul Haninand Maydan Mahzam (Alsharhan and Nairn, 1994In Abu Dhabi, oil has be en found at Umm Sha

(with a well-deve1oped gas cap) Ghasha, Nasr, Tini, Satah Al Raaz Boot, Abu Al Bukhoosh, Sata]arnain, Dalma, El Bunduq, Arzana, Hair DalmHail, Umm Al Dholou, Belbazem, ADNOC 1-ADNOC 1-C, and Umm Al Salsal. Mubarraz, weMubarraz, and Bab fields have proved to be g

bearing (Figure 3).

CONCLUSIONS

The Upper ]urassic Arab and Hith formations the southern and southwestern Arabian Gulf m

be divided into five lithofacies: (1) oolitic/pe1oid

grainstone, (2) dolomitic grainstone, (3) dolomitmudstone, (4) dolomitized grainstone, and (5) masive anhydrite. The best reservoirs occur in assocition with interpartic1e porosity in grainstones andolomitic grainstones and intercrystalline porositin dolomites and dolomitic limestones.

The Arab Formation comprises four membersArab A-D (or I-IV) that were cyc1ically depositedas transgressive and regressive carbonate-evaporiteunits. The Arab D is the most prolific hydrocarbonreservoir in the Arab Formation and is characterized by mudstones and wackestones in the bas

part of the section that grade upward into bioc1atic dolomitic packstone/grainstone and sucrosi

dolomite.The best porosity in the Arab C was found i

dolomitic grainstones due to a decrease in anhydrite cementation. Mud-supported lithologies within this member have low porosity, and thdolomites at the base and top of the section hav

poor intercrystalline porosity due to anhydritecementation.

The Arab B consists of dolomitic and dolomitized grainstone. Anhydrite cementation is greatethan in the Arab C or D members, but dissolutionof the calcite spar matrix has created some excelent secondary porosity.

The Arab A contains primarily dolomitized grainstone and poorly sorted dolomitic grainstone, suggesting variable-energy depositional conditions.Porosity is genera1ly low due to anhydrite cementation and original mudstone textures.

The Arab A is succeeded by the massive anhydritof the Hith Formation, the final regressive stage othis sequence that forms the seal over the underlyinArab reservoirs. The anhydrite has a chickenwiretexture with some dolomite in the lower part of th


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a.-----------... ~Alsharhan and Whittle 1629

Alsharhan, A. S., 1989, Petroleum geology of the United ArabEmirates:Joumal ofPetroleum Geology, v. 12, p. 253-288.

Alsharhan, A. S., and C. G. StoC. Kendall, 1986, Precambrian toJurassic rocks of Arabian Gulf and adjacent areas: their facies,depositional setting, and hydrocarbon potential: AAPG

Bulletin,v. 70,p. 977-1002.

Alsharhan, A. S., and C. G. StoC. Kendall, 1994, Depositional set-ting of the Upper Jurassic Hith Anhydrite of the Arabian Gulf:an analog to Holocene evaporites of the United Arab Emiratesand Lake MacLeod of Western Australia: AAPGBulletin, v. 78,

p.1075-1096.Alsharhan, A. S., and A. E. M. Nairn, 1994, Geology and hydrocar-

bon habitat in the Arabian basin: the Mesozoic of the state ofQatar: Geologie en Mijnbouw, V. 72, p. 265-294.

Ayres, M.G., M.Bilal,R.W.Jones, 1.W. Slentz, M. Tartir, andA. O.Wilson, 1982, Hydrocarbon habitat in main producing areas,SaudiArabia:AAPGBulletin,V. 66, p. 1-9.

Bathurst, R. G. c., 1975, Carbonate sediments and their diagenesis:New York, Elsevier, 658 p.

Bouroullec, J., and A. Meyer, 1995, Sedimentological and diagenet-ic model of the Arab Formation (Qatar): Reservoir implications,

section, grading upward to a similar nodular textureand becoming laminated at the topo

The Arab and Hith formations in the southernand southwestern Arabian Gulf represent fourtypes of depositional settings: (1) supratidal

sabkha, (2) intertidal mud flat, (3) shallow subtidallagoonal, and (4) shallow, open-marine shelf(Figures 1, 14). These depositional settings were aresponse to the fluctuating sea level conditionsduring the Late]urassic.

Cementation, leaching, and dolomitization werethe primary diagenetic processes affecting the orig-inal carbonate and evaporite lithologies of the Araband Hith formations. Leaching of grain-supportedlithologies created excellent secondary porosity,most notably in the Arab D. Where cementation byanhydrite and calcite spar occurred, porosity wasreduced. Stylolites locally reduce permeability dueto cementation and residual bitumens within the

seams.The diagenetic sequence of events that affected

the Arab and Hith formations includes (1) micritiza-tion and inversion of original aragonite and high-Mg calcite to low-Mg calcite, recrystallization, andearly cementation in a marine phreatic setting; (2)dolomitization and evaporite formation concomi-tant with leaching; (3) late cementation by sparrycalcite and mosaic dolomite; (4) formation of stylo-lites in the burial stage of diagenesis; and (5)emplacement of residual tar and bitumen in open

pores and stylolitic seams. The timing of minorreplacement minerals, such as celestite, chert,

quartz, pyrite, and fluorite, is variable. Celestiteprobably formed after early dissolution and cemen-tation, but before dolomitization and evaporite for-mation. The other cements appear to postdatedolomitization.

REFERENCES CITED

in M. W. AI-Husseini, ed., The Middle East PetroleumGeosciences, v. 1: Bahrain, GulfPetrolink, p. 236-246.

Butler, G. P., 1965, Early diagenesis in the Recent sediments of theTrucial Coast of the Persian Gulf: Master's thesis, University of

London, U.K., 162 p.Butler, G. P., P. M. Harris, and C. G. StoC. Kendall, 1982, Recent

evaporites from the Abu Dhabi coastal flats, inC. R. Handford,R. G. Loucks, and G. R. Davies, eds., Depositional and diagenet-ic spectra of evaporites: SEPMCore Workshop 3, p. 33-64.

Choquette, P. W., and N. P. James, 1990, Limestones-the burialdiagenetic environment, in 1.A. McIlreath and D. W. Morrow,eds., Diagenesis: Geoscience Canada, Reprint Series 4,

p.75-111.Coogan, A. H., 1970, Measurements of compaction in oolitic grain-

stone: Joumal of Sedimentary Petrology, V. 40, p. 921-929.de Matos, J. E., 1994, Upper Jurassic-Lower Cretaceous stratig-

raphy: The Arab, Hith and Rayda formations in Abu Dhabi, inM. D. Simmons, ed., Micropalaeontology and hydrocarbonexploration in the Middle East: London, Chapman and Hall,

p.81-111.Droste, H., 1990, Depositional cycles and source rock develop-

ment in an epeiric intraplatform basin: the Hanifa Formation oftheArabianPeninsula: SedimentaryGeology, V. 69, p. 281-296.

Dunham, R. J., 1971, Meniscus cement, in

O. P. Bricker, ed.,Carbonate cements: Baltimore, Johns Hopkins UniversityStudies in Geology 19, p. 297-300.

Dunnington, H. V., 1967, Aspects of diagenesis and shape changein stylolitic limestone reservoirs: proceedings of the 7th WorldPetroleum Congress, Mexico, V. 2, p. 339-352.

Evamy, B. D., and D.J. Shearman, 1965, The development of over-growths from echinoderm fragments: Sedimentology, V. 5,

p.211-233.Harris, P. M., C. G. StoC. Kendall, and 1. Lerche, 1985, Carbonate

cementation-a brief review, inN. Schneidermann and P. M.Harris, eds., Carbonate cements: Society of EconomicPaleontologists and Mineralogists Special Publication 36,

p. 79-95.Hsu, KJ., andJ. Schneider, 1973, progress report on dolomitiza-

tion hydrology of Abu Dhabi sabkhas, Arabian Gulf, in B. H.Purser, ed., The Persian Gulf-Holocene carbonate sedimenta-

tion and diagenesis in a shallow epicontinental sea: New York,Springe~Verlag,p.409-422.

Hsu, KJ., and C. Siegenthaler, 1969, preliminary experiments onhydrodynamic movement induced by evaporation and theirbearing on the dolomite problem: Sedimentology, V. 12,

p. 11-25.Illing, 1. V., 1954, Bahamian calcareous sands: AAPGBulletin,

V. 38, p. 1-95.Illing, 1.. V., A. J. Wells, and J. C. M. Taylor, 1965, Pene-

contemporary dolomite in the Persian Gulf, in 1. C. Pray andR. C. Murray, eds., Dolomitization and limestone diagenesis-asymposium: Society of Economic Paleontologists andMineralogists Special Publication 13, p. 89-111.

Kawaguchi, K-l., 1991, Geological controls on reservoir quality of'Arab Formation in Satah field: Proceedings of the 7th MiddleEast Oil Show, Society ofPetroleum Engineers, p. 933-945.

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the Permian evaporite/carbonate shelf sediments of theGuadalupe Mountains: Geological Society of America Bulletin,

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morphology of a Recent shallow-water province; Khor alBazam, Trucial Coast, southwestem Persian Gulf: GeologicalSociety of America Bulletin,V. 80, p. 865-892.

Kinsman, D. J. J., 1966, Gypsum and anhydrite of Recent age,Trucial Coast, Persian Gulf, inJ. 1. Rau, ed., Second sympo-sium on salt: Cleveland, Northem Ohio Geological Society,

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1630 Arabian GuIf, Late Jurassic

LaPointe, P. A., 1991, Sabkha vs. saIt basin mode1 for the ArabFormation understanding in the Umm Shaif field, U.A.E.:Proceedings of the 7th Middle East Oil Show, Society ofPetroleum Engineers, p. 523-534.

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ABOUT THE AU1HORS

A. S. Alsharhan

A. S.Alsharhan received his M.S.degree (1983) and Ph.D. (985)from the University of SouthCarolina. PresentIy, he is the assis-tant deputy vice chancellor for aca-

demic affairs at the United ArabEmirates University. His currentresearch interests include theHolocene coastal sabkha of theUnited Arab Emirates and geologyand hydrocarbon habitat in theArabian Gulf and adjacent areas. He is currently co-authoring his first book with A. E. M. Naim.

habitat: AAPGBulIetin, v. 64, p. 597-618.Patterson, R.]., and D. J. J. Kinsman, 1982, Formation of diag

ic dolomite in coastal sabkhas along the Arabian (persia) AAPGBulIetin, v. 66, p. 28-43.

Peebles, R. G., M. Suzuki, and M. Shaner, 1995, The effects oflterm shalIow burial diagenesis on carbonate-evaporite susions: Proceedings of the Middle East Geosciences Geo '94ference, Bahrain, p. 761-769.

Powers, R. W., 1962, Arabian Upper Jurassic carbonate reserrocks, in W. E. Ham, ed., Classification of carbonate roAAPGMemoir 1, p. 122-197.

Powers, R. W., 1968, Saudi Arabia: Lexique Stratigraphiqlnternational, Paris, Centre National de la RechercScientifique, v. 3, Asie, part IOb, 180 p.

Powers, R. W., 1. F. Ramirez, C. D. Redmond, and E. 1. Elb1966, Geology ofthe Arabian Peninsula, sedimentary geoof Saudi Arabia: U.S. Geological Survey Professional Pa560D, 127 p.

Radke, B. M., and M. 1.Mathis, 1980, On the formation occurrence of saddle dolomite: Journal of SedimentaPetrology, v. SO,p. 1149-1168.

Shearman, D.J., 1966, Origin of marine evaporites by diageneTransactions of the lnstitute of Mineralogy MetalIurgy, SecB,v. 75,p. 208-215.

Steineke, M., and R. A. Bramkamp, 1952, Mesozoic rocks of eern Saudi Arabia (abs.): AAPGBulIetin, v. 36, p. 909.

Steineke, M.,R. A. Bramkamp, and N. J. Sander, 1958, Stratigrare1ations of ArabianJurassic oil, in 1. G. Weeks, ed., Habitaoil: AAPGSymposium, p. 1294-1329.

Sugden, W., and A. J. Standring, 1975, Qatar Peninsula: LexiStratigraphique lnternational; Paris, Centre National dRecherche Scientifique, v. 3, Asie, part IOc, 120 p.

Warren, J. K., 1990, Evaporite sedimentology. lmportance hydrocarbon accumulation: New Jersey, Prentice H285 p.

Wilson, A. O., 1985, Depositional and diagenetic facies in Jurassic Arab C and D reservoirs, Qatif field, Saudi ArabiaP. O. Roehl and P. W. Choquette, eds., Carbonate petroleureservoirs: New York, Springer-Verlag, p. 319-340.

Wilson, E. N., 1991, Evaluation of]urassic Arab D reserVoir quain low-relief traps in Qatar: Proceedings of the 7th Middle EOil Show, Society of Petroleum Engineers, p. 925-932.

Wood, G. V., and M. H. Wolfe, 1969, Sabkha cycles in Arab/Darb Formation off Trucial Coast of Arabia: Sementology, v. 12, p. 165-192.

Gregory LWhittle

Gregory WhittIe completed hisgraduate work at the University ofSouth Carolina, where he studiedHolocene carbonate sedimentationand cementation in the Exumas,

Bahamas, for his M.S.degree (1991),and helped develop a graphicalcomputer simulation to model car-

bonate and clastic depositionalsystems for his Ph.D. (1993).CurrentIy, he is on a postdoctoralfellowship at the United Arab Emirates University woring on the carbonate diagenesis of ancient sequenceand the Holocene coastal sabkhas of the United ArEmirates.

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Carbonate-Evaporite Sequences of the Late Jurassic

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