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Middle East Well Evaluation Review

Ancient coral reefs, long buried insedimentary rocks, are an impor-tant source of oil in the Middle

East. These carbonate rock formations,created from the skeletal remains of diverse marine species, have beenaround for at least 600 M years. Somereefs formed in shallow coastal waters,while others developed around vol-canic islands a long way from any largecontinent. What factors controlled theoccurrence of these coral reefs, andwhich have the best reservoir potential?

In common with modern coral reefs,ancient reefs (figure 1.1) thrived inwarm, shallow water with very littleassociated sediment. Over geologicaltime many different types of organismhave dominated the reef environment,but all have achieved their best growthrates in shallow equatorial waters. Con-sequently, the diversity and abundanceof coral species decrease with distanceaway from the equator.

The typical island reef, developedaround an oceanic volcano, has littlepotential for oil and gas accumulation.In these reef ecosystems almost all theorganic material present is in the formof living organisms. Nutrients from deadorganisms are recycled very rapidly. Asa result, reefs do not generally produceenough ‘excess’ organic material to gen-erate oil and gas. However, some reefsdo contain hydrocarbons, and the best oil and gas targets are reefs whichdeveloped close to ancient continentalmargins. Here, organic input from anearby continental shelf may have gen-erated hydrocarbons or been incorpo-rated in shales which can act as traprocks.

The Arabian Peninsula spans a rangeof latitudes, so that in the south - theGulf of Aden and the Red Sea - condi-tions are, currently, favourable for coralgrowth, while the Eastern Mediter-ranean and The Gulf are marginal areas,with few coral species. The Red Sea isparticularly rich in modern coral reefs,especially along the edges of fault blocks.

Fig. 1.1: HIGH AND DRY: Anancient (Triassic) reef,thrust up through the rock

sequence, makes up thecliffs of Jebel Misht inOman. Oil-bearing reefs,ranging in age from Triassicto Cretaceous may lieburied east of the Arabian

peninsula, beneath thick,overpressured Tertiary

shales. This image is takenfrom the cover of O m a n '

Over geological time the ArabianPeninsula has passed through the equa-torial belt a number of times (figure 1.2).Optimum conditions for reef growthoccurred during the Precambrian, Juras-sic, Cretaceous and Middle Tertiary.

During the Miocene, between 5 M and20 M years ago, abundant coral structuresformed at shallow depths on the highest points of rotated fault blocks in the Gulf of Suez. Oil is produced from these reefsbut the complex pattern of faulting pre-cludes the development of supergiant (over 1 billion bbls) reservoirs.

Exploration drilling in the eastern part of the Mediterranean, on similar fault blocks, may lead to more Miocene reef discoveries. For example, in the Red Sea,Miocene oil accumulation occurredwhen evaporites were deposited on thereef, forming an excellent seal, trapping hydrocarbons in the porous reef rock.However, in some cases the evaporitelayers developed too early, preventing oil and gas migration into the carbonatehighs.

Tertiary rocks contain many of themost productive reefs found in the Mid-dle East. The reefs of the Precambrianalso contain important oil accumulations,while the reservoir potential of the Juras-sic is still under investigation.



Number 15, 1994.

A number of Tertiary reef prospectsare being developed across the MiddleEast. There are Eocene-Oligocene (earlyTertiary) reefs in northern Syria and Iraqand part of the Asmari reefs facies in Iranproduces hydrocarbons. More Tertiaryreefs have been discovered in Oman andin the Gulf of Aden. These discoveriesmay develop into important reservoirs inthe future, although they are not, as yet,in commercial production.

The oldest reefal reservoirs in Arabiaare located in the Precambrian rocks of southern Oman which contain algal stro-matolitic reefs. Stromatolites are largeaccumulations of carbonate sediment andskeletal material bound together by algae.They are often dome-shaped and have adistinctive, finely-laminated appearance.In addition, deep drilling projects in mar-ginal areas of the Ara and Hormuz saltsare expected to reveal additional oil andgas in stromatolitic reservoirs.

Recently, reef- forming stromatoliteshave been discovered in Belize and Hon-duras, in Central America. Studies of these stromatolites should clarify the rolewhich these organisms play in the devel-opment of modern reef communities.

During the Jurassic, environmentalconditions in Arabia meant that ooliticcarbonate sands were more commonthan reefs. Oolites are rocks composed of

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

C e n o z o i c

M e s o z o

i c

P a

l a e o z o

i c

Precambrian

Presentsea level

0 0.5 1.0

Relative changes of sea levelFalling Rising

1st &2nd - Order Cycles

600

500

400

300

200

100

10 oN 10 oS 30 oS 50 oS

E q u a

t o r

Position of Arabia withrespect to the Equator

Sahara glaciation

Gondwananglaciation

P r e

d o m

i n a n c e o

f c a r b o n a

t e s

P r e

d o m

i n a n c e o

f c

l a s

t i c s

M

y e a r s

Reefs absentIncipient reefsReefs fully developed

World reef potential through time

Can haveoomouldic

Aragonitethreshold

Can haveoomouldic

(Leached oolite)moulds

Ocean chemistry

Holocene

Fig. 1.2: Reefs have come and gone throughout geological history. The presence of well- developed reef facies (a) is linked to sea level fluctuations. The timeswhen reefs have been almost absent from the geological record

show a rough correlation with periods of rapid sea level change(b). Another factor controlling thedevelopment of coral reefs is themovement of Arabia relative tothe equator through time (c).

Although close to the equator during the Devonian, Arabia wascompletely above sea level at that time, so no coral reefs developed on the Arabian peninsula. Thedevelopment of ooids (which arechemical rather than biological carbonates) is controlled by

seawater chemistry. As oceanchemistry fluctuated through time(d) the changes influenced therelative stability of calcite and aragonite cements and thedissolution of ooids. This diagramis based on concepts developed by

David Raup, M.W. Hughes Clarke, Peter Vail and Bruce Wilkinson.

carbonate grains (ooids) which form intidal deltas and other shallow environ-ments or ‘shoals’. They are not pro-duced by biological processes, ratherthe carbonate in each ooid precipitateddirectly from calcium carbonate(CaCO 3 ) saturated seawater.

However, some ‘patch reefs’ didoccur in the Jurassic. These small car-bonate structures were scattered acrossthe shallowest parts of the continentalshelf. The patch reefs found to the west of Riyadh, in Saudi Arabia, providedgood reservoirs on salt domes and anti-clinal structures in The Gulf and along its southern shore.

The hydrocarbon potential of the rel-atively large Jurassic marginal reef inthe Gotnia Basin, has not been exam-ined in detail. However, a recent discov-ery in the Kuwait/Saudi Neutral Zone(South Umm-Guadir DW-1) by theKuwaiti Oil Company (KOC) and SaudiArabia Texaco should renew interest inthis potential reef target. Good dolomiti-zation of the shelf-edge reef trend hasbeen found, with one of the best source-rock sequences down dip in the GotniaBasin. However, effective seals are pre-sent which may separate source fromreefal facies, and could have hinderedoil and gas migration.

(a) (b) (c) (d)




Darwin’s other discovery

Charles Darwin, the naturalist whosetheories revolutionized our understand-ing of biological science, made a signifi-cant contribution to the study of reefs. In1842, while engaged on a scientific voy-age on the research ship HMS Beagle ,Darwin proposed his subsidence theoryfor the development of coral atolls. Hespeculated that reef atolls had, at sometime in their development, been mar-ginal or fringing reefs (figure 1.3a)around an island. Darwin believed that as the island started to subside, the fring-ing reefs continued to grow upwards at arate equal to, or greater than subsi-dence. Consequently, as the area of thecentral island decreased, a shallowlagoon formed between the island andthe growing reef (figure 1.3b). When theisland finally dropped below sea level,

all that remained was a large lagoonfringed with upbuilding reef material (fig-ure 1.3c). Seismic surveys and drilling car-ried out as part of the 1991 Ocean Drilling Program (Leg 143) appear to have con-firmed Darwin’s theory on atolls.

Darwin also speculated on the char-acteristics of a specific group, the Mal-dive atolls. He suggested that subaerialerosion and subsidence were the most important factors in their development.However, recent studies suggest that Darwin’s sea level fluctuations alone cannot account for all the complexities of carbonate deposits in the Maldives.

Seismic surveys and drilling in boththe Maldives and the Seychelles haveconfirmed existing plate tectonic recon-structions of the Indian Ocean. In theMaldives, carbonate deposition began inthe early Eocene with shallow water sed-iments resting on hot-spot basalts (figure1.4) related to the slightly older DeccanTrap basalt rocks of India. Very little isknown about the early development of the islands, but the limited evidence sug-gests that early rifting coincided withdevelopment of a graben system along the transform fault on which the Mal-dives carbonate platform and atolls

grew. The overall structure of the plat-form was probably influenced by crustalcooling effects.

Island reefs and reef platforms arenot potential exploration targets, sincethey generally lack a source rock or seal-ing layer, or both. Reef environmentssuch as the modern Maldives platformcontain large volumes of porous andpermeable rock, but the Maldives aretoo far from sources of organic materialto generate hydrocarbons and too smallto generate the sealing layers necessaryfor oilfield development.

Arabian Sea

Indian Ocean

India

Africa Maldives

Saudi

Arabia Reefs

Island records

The subsidence history of a small vol-canic island and subsequent growth of

an Eocene coral reef has been recon-structed using geochemical logging car-ried out as part of the Ocean Drilling Program. Experts from Columbia Uni-versity’s Borehole Research Group stud-ied a composite volcanic-carbonatesequence in the Indian Ocean, using aGeochemical Logging Tool (GLT*) tounravel the geological history of the area.

The volcanic rocks proved to bemainly vesicular olivine basalts whichshowed weathering effects. This sug-gested that they may have formed thesurface of a volcanic island. These rockswere overlain by plagioclase basalts

with high concentrations of titanium (Ti)and iron (Fe). Basalts can be assigned togeochemical ‘families’, with the chemi-cal composition of the rocks indicating

Fig. 1.3: GOING,GOING, GONE:

Darwin’s theory of volcanic island

subsidence hasbeen confirmed by geochemical analysis carried out during theOcean Drilling

Project. According to Darwin, before subsidence thevolcanic island would have had afringing reef (a).When the island

started to subsidecoral growthcontinued at arate greater thanor equal to

subsidence. Eventually, alagoon appeared behind the reef

(b) and, as subsidencecontinued, theisland dropped below sea level leaving an empty lagoon ringed with coral (c).

Fig. 1.4: HOT SPOTS: The Maldives are oceanislands produced by movement of the Indian

Plate over a mantle ‘hot spot’ which pushesbasalt through the crust to form volcanicislands. As each island moves away from thehot spot the volcano loses its source of magma

and the island starts to subside.

(a)

(b)

(c)



Number 15, 1994.

probable tectonic setting. High concen-trations of iron, aluminium and silicon(figure 1.5) are typical of basalts foundin volcanic island settings. The alu-minium ‘spikes’ in the lower part of thelog correspond to weathered ‘soil’ lay-ers deposited between the basalt flows.

A thin calcarenite zone, interpreted

from core as a beach deposit, is fol-lowed by a distinctive, titanium-richbasalt layer which marks the end of vol-canic activity on the island.

The reef proved more difficult tosample, with a core recovery rate of only 5%. However, this was enough toindicate a transition from high-energygrainstones to low-energy packstones.This, together with successive faunalchanges, confirmed that the waterabove the reef was getting deeper. Car-bonates, in stark contrast to basalts, arecharacterized by low concentrations of aluminium, iron and silicon and, of course, a high calcium content.

Fig. 1.5: WHERE’S THE BEACH? Geochemical logging is proving a

powerful tool for stratigraphicinterpretation. The geochemical data retrieved from this borehole

show a calcium-rich carbonate(reef) deposit overlying an iron- rich basalt (volcanic complex). A

separate calcium-rich layer,identified from core material as abeach deposit, occurs in themiddle of the volcanic sequence.Columbia University, Borehole

Research Group, 1990.

0 TiO 2 5

100 SiO 2 5 100 CaO 0

0 FeO 20 0 S 10

50 Al2O3 0

8 0 0 0

7 9 0 0

8 1 0 0

8 2 0 0

8 3 0 0

D e p t h

b e l o w t h e r i g

f l o o r

Ti Si Fe Ca S Al

ft

Benthicforaminifera,gastropods,bryozoans,molluscs,solitary andcolonial corals

Gr a instone s

Subaerialvesicular olivinebasalt lava flows(1-5m thick)with lateriticweathering

Calcarenite(beach)

High Ti/Feplagioclasebasalt

Volcanic island

Fringing reef

Pack s tones

The sulphur curve in this log showszones of high sulphur concentrationwithin the reef. These sulphur peakshave been interpreted as sulphate evap-orites. Experts have speculated that thelog shows sea level changes with lowstands marked by relatively high sul-phur content. Sulphur peaks at wave-lengths of 25ft and 50 ft could be relatedto the Eocene low sea level stands (at 36M, 40M, 42M, 49M and 54M yearsago) recorded in the Vail eustatic sealevel curve. This example illustrates thevalue of geochemical techniques indetermining the geological history of asequence from raw element datarecorded direct from logs.

The karst (subaerial weathering of limestone) features found in manyancient reefs underline the importanceof fresh water diagenesis, which causedleaching of less stable carbonate miner-als. Fresh water zones fluctuate throughtime as sea level rises and falls in rela-

tion to the atoll islands and the platform.Geothermally-heated fluids, which risealong crustal fracture and fault systems,mix with cooler seawater and fresh waterlenses which are scattered within upperparts of the atolls and the platform.

Permo-Triassic ‘exotics’ - large rockmasses which have been structurally

emplaced into a sedimentary sequence -were thrust onto the slope/shelf at theeastern edge of Arabia during the Creta-ceous. These may be fragments of ancient atolls and platforms similar tothose in the Maldives. Although theexotics seen at the surface contain nohydrocarbons, there may be oil and gas-filled reef blocks beneath Tertiary shalesoff the east coast of Arabia. If thesedeeper blocks could match the produc-tivity of similar reefs in the Gulf of Mex-ico, they would represent a major newexploration target in the Middle East.However, the overpressured Tertiaryseal, which may also be the source rock,is a serious obstacle to deep drilling operations.

Sorting out the salt

The salt dome structures which underlieprolific reservoirs in The Gulf - including the Permo-Triassic Khuff carbonates, theJurassic grainstone reservoirs and Creta-ceous grainstone/reefal reservoirs -sometimes form islands, but have verylittle in common with the ocean atolls.Formations overlying the salt domes are

closely interlinked and continuous withformations which surround the dome.Hydrological studies suggest that thesereefs often form part of regional aquifers.The numerous evaporite layers found inPermo-Triassic and Jurassic formations,and the thick clay-rich shales depositedduring phases of low sea level, are wide-spread and form effective seals: ideal forthe development of giant oil and gasreservoirs.




The Egyptian experience

The Belayim carbonate facies, and theequivalent Gemsa Formation, developedalong Egypt’s Gulf of Suez as scatteredand separate carbonate deposits. In thenorthern part of the Darag Basin, theBelayim carbonates were deposited in avery shallow marine sabkha environment.In the west central part of the Gulf of Suez,in the Ras Gharib and Ras Fanar fields,the Belayim was deposited as a reef com-plex. In the southern Gulf of Suez thesecarbonates are represented by reefal lime-stones, such as those in Gemsa Field, andalso sabkha carbonates.

Structural factors control the distribu-tion of the Belayim carbonate facies,which were deposited on tilted anderoded pre-Miocene fault blocks (figure1.7). These fault blocks developed during the opening of the Gulf of Suez and areformed by NW-SE oriented Clysmic faults.

Fractures associated with the structureshave enhanced secondary porosity, per-meability and hydrocarbon potential.

Fig. 1.6: ALL SHOOK UP: The dolomite replaces limestone layer-by-layer and its small-scale distribution through the reservoir rock sequence is difficult to predict in this Miocene reservoir sequence in the Gulf of Suez. Only detailed geochemical logging techniques can provide a quantitative view of dolomitediagenesis in this complex sequence; a mixture of calcite, dolomite, quartz,

pyrite, glauconite and two types of clay.

Diagenesis, dissolution and dolomite

Porosity evolution in rocks is a com-plex, but vital part of reservoir develop-ment and a clear understanding of thisprocess is crucial in the search for oil

and gas. Porosity varies within rock lay-ers. Where the porosity of a reservoirlayer falls below a threshold, or cut-off,it ceases to be a viable reservoir. Thiscut-off value varies from reservoir toreservoir.

A picture of the porosity distributionin each reservoir zone depends on aclear understanding of reservoir geo-chemistry (figure 1.6).

Other elements in the porosity equa-tion are sedimentary geochemistry,pressure and temperature of burial, fluc-tuating sea level and changing porefluid composition. Reservoir analysts

must build a composite picture of porosity, extrapolating and interpolating data between wells from the start of drilling.

Dolomite makes a difference

Dolomite mineralization can play amajor part in influencing reservoir prop-erties such as porosity and permeabil-ity. The conversion of pure limestones(CaCO 3 ) to dolomite (CaMg (CO 3 ) 2 ) is a gradual processwhich can start almost as soon as the

carbonate sediments have beendeposited. Dolomite crystallization iscaused by seawater interacting withfresh water lenses or pore water in car-bonate rocks. This dolomitizationprocess can take place in hypersalineponds where there are freshwaterlenses in the sediment or in coastallakes which are subjected to intenseevaporation.

While dolomites can be produced ina number of ways, the chemicalchanges involved do not vary. Magne-sium from the seawater replaces someof the calcium present in the original

limestone. The concentration of magne-sium in dolomite is much higher than inthe seawater from which it was derived.

Dolomite crystallization and dissolu-tion processes often control porositydevelopment in carbonate reservoirs.Early dolomitization can preserveporosity which might be lost by com-paction effects and calcite cementation.

Dolomitization often occurs as aresult of repeated sea level changes andthe mixing of hypersaline basinal brinesand normal seawater which accompa-nies these changes. At the same time,leaching of less stable skeletal compo-

nents (aragonite) occurs, along the plat-form margins, increasing porosity. Dur-

ing long periods of rising sea level(marine transgressions) dolomite miner-alization may spread to carbonates at the centre of the platform. The extent of dolomitization, and its effect on reser-voir properties, depends on the volumeand salinity of the hypersaline brines.Large volumes of mouldic, vuggy andintercrystalline porosity can be createdby marine transgressions.

Changing sea level and water chem-istry also influence the composition of common pore- filling cements. Calcitecementation is retarded because the cal-cium carbonate in solution is incorpo-rated into the precipitating dolomite.When calcite cementation is inhibited,the development of anhydrite cementsis the most important porosity-reducing mechanism. In some cases, both pri-mary porosity and early-generated sec-ondary porosity have been filled byanhydrite cements.

Calcite

Dolomite

MontmorilloniteQuartz

Oil

WaterKaolinite

Pyrite

x400ft

x500ft



Number 15, 1994.

Impermeable salt and anhydrite

which surround carbonate deposits inthe southern Gulf of Suez, for example at Gebel al Fessayan, prevented hydrocar-bon migration into potential reservoirzones. Clearly, the position of salt andanhydrite layers is crucial in any evalua-tion of reservoir potential in carbonates.Miocene carbonate facies vary through-out the Gulf of Suez. The supratidalsabkha deposits at the northern end of the basin, around Ras Fanar Field and inthe Darag Basin, generally have poorreservoir potential. In contrast, the reef complexes of Nullipore facies found inthe Ras Gharib, Ras Fanar, Ras Bakr and

Gemsa fields are excellent reservoirs.

Fig. 1.7: The reef reservoirswhich developed in Egypt’sGulf of Suez are found in avariety of positions along thetrend of Miocene fault blocks.

Fig. 1.8: Dolomite mineralizationdevelops gradually, spreading

grain by grain through thereservoir rock. The progressive

growth of dolomite crystalswithin a Miocene reservoir is

shown in these microscope photos (a-c). In the final stages of dolomitization pore space can befilled by dolomite (d).

Abu Shaar el QibliZeit Bay Gemsa

Miocene Sea Level

Modern sea level

15-20mYounger reef

Older reef

Dolomite

Fig. 1.9: The interaction of seawater withfresh water (arrows) provided an ideal environment for the replacement of calciteby dolomite after deposition of the older reef. The upper limit of dolomitedevelopment coincides with maximum sealevel. The younger reef is presently undergoing dolomitization. Radioactivedating indicates the older reef cycle isbetween 350,000 and 270,000 years old.The younger reef cycle was deposited between 140,000 and 60,000 years ago(Strasser et al., 1992).

These images provided by Denise Stone, Amoco Prod. Res., Houston

(a) (b)

(c) (d)

Early dolomitization and subsequent

dissolution of the dolomite crystals werevital steps in porosity development in Gulf of Suez carbonates. Other factors favouring the development of high-porosity rocksincluded; skeletal aragonite dissolutioncoupled with late corrosion of anhydrite,and fine grained sediments. The porosity of Miocene carbonates could reach valuesbetween 15% and 30% following deep bur-ial, and associated fracturing and late disso-lution of anhydrite cements, carbonategrains and even, in some cases, the rockmatrix. The corrosive fluids capable of alarge-scale, late-stage dissolution wereprobably associated with source rock mat-

uration or basinal shale compaction.

Dolomite mineralization develops

gradually (figure 1.8) and chemicalchanges can halt the process at anystage. Unfortunately, some of the majorfields (e.g. along the Shoab Ali Trendand the Kareem Formation of the Zeit Bay Field) contain chalky microporosityand are only partly dolomitized. Marly-shaly units, which overlie potentialreservoir zones, probably kept thedolomitizing fluids out of the carbonate.

Modern dolomitization effects can beseen in the Gulf of Suez (figure 1.9)where freshwater from the surface andfrom the basement mix with seawater.




Dolomite close up

At any given depth, dolomitesequences seem to havegreater porosity than a lime-stone sequence. Most of thisporosity difference is due to‘porosity retention’ in thedolomite.

The factors which encourage dolomi-tization are reduced sulphate content inseawater (which typically occurs during gypsum and anhydrite precipitation);dilution of seawater where the ionic con-centration is lowered while the molarMg:Ca ratio is maintained; raising of theMg:Ca ratio by evaporation; and temper-ature increases during burial. Climate isa factor, since dolomitization commonlyoccurs in arid depositional systems. Sealevel fluctuations also mix fresh waterand marine fluids in subsurface pore sys-

tems - another cause of dolomitization.Studies in the Khuff, Arab and Asmariformations and in Miocene carbonatesfrom the Gulf of Suez, indicate that all of the factors mentioned above played apart at some stage in the development of dolomite in these major reservoirs.

In most Cretaceous reservoirs thedominant factors were groundwater andseawater mixing.

Pleistocene sea level fluctuations inthe Gulf of Suez and Red Sea are believedto be the main factors in stratigraphicvariations of reef dolomites, although theclimate and tectonics of any area will

always influence dolomite mineralization.Age dating of dolomitized sequences

indicates that major cycles of dolomitedevelopment correlate well with the100,000 year cycle of eccentricity in theEarth’s orbit (Milankovitch cycles). Thiseccentricity affects the amount of solarradiation reaching the Earth and, therefore,has a profound effect on global climate. Cli-matic variations may, in turn, controldolomite development. The smallersequences which comprise the individualreef sequences are believed to be con-trolled by sea level fluctuations every21,000years, a cycle which relates to move-ment of the Earth’s axis. These smallerdepositional sequences have reefal andlagoonal facies which represent transgres-sive stages and coral rubble and siliciclas-tics associated with sea level highstands.

At the southern end of Sinai, aroundSharm el Sheikh, studies of Pleistoceneand younger reefs by Andre Strasser et al. (1992) underline the importance of seawater and groundwater mixing in thedolomitization of the reefs and associ-ated sediments (figure 1.10).

The carbon and oxygen isotope val-ues in the older (Pleistocene) reefs (fig-ures 1.11 and 1.12) indicate a fresh waterinfluence on carbonate mineralization,whereas the younger reef samples showvalues typical of dolomite mineralizationin normal marine waters. Results fromthe Sinai reefs resemble findings fromPacific atolls and reefs in Latin Americawhere mixing zone dolomitization is themost important mechanism.

However, evidence for other mecha-nisms has emerged recently. Thermalpumping - hot water rising from depth tomix with seawater is the focus of current research - while small-scale seawaterfluctuations, such as tides, may also pro-mote dolomite mineralization.

T h i s i m a g e

a n

d t h o s e o p p o s i t e w e r e p r o v i d e

d b y

A n

d r e

S t r a s s e r ,

I n s

t i t u t o

f G e o

l o g y ,

U n

i v e r s i t y o

f F r i b o u r g ,

S w

i t z e r l a n

d .

Fig. 1.10: Pointed aragonite crystals growing into pore space in the younger reef, Red Sea, southern end of the Sinai Peninsula.

Andre Strasser et al. (1992): Sequential Evolution and

Diagenesis of Pleistocene Coral Reefs (South Sinai, Egypt).

Sedimentary Geology , 78, pp. 59-79.



Number 15, 1994.

Fig. 1.11 (above): This Scanning Electron Microscope (SEM) image shows rhombs of dolomite growing over crystals of calcite which contain highconcentrations of magnesium. This rock is from the Pleistocene (older) reef.

Fig. 1.12 (below): Dolomite and high-magnesium calcite crystals are invaded by small needle-like crystals of aragonite. This change in the older reef is in response to changing water compositions as seawater and groundwater mix.



Middle East Well Evaluation Review16

2400

2425

2450

2475

2500

2525

2550

2575

2600

2625

D e p

t h f t

0 25 50 75 100 0 25 50

Permeability(md)

Block calcite cement(%)

Fig. 1.13: NOW YOU SEE IT, NOW YOU DON'T: Permeability comes and goes in

Kirkuk Field, Iraq. This plot of dolomite layers shows that the zones of low permeability in the

sequence correlate with

layers where late stagecalcite cements havedeveloped. It is difficult to reconcile the

presence of this calcitecement with burial diagenesis. Fresh water lenses preserved in the

sediment seem themost likely cause of this patchy calcitecementation.

Developing dolomite

In the Cretaceous and Tertiary rocks of the Middle East there is patchy develop-ment of algal and foraminiferal lime-stone, with some coral and associateddetrital limestones. These were not con-nected with true fringing reefs, barrierreefs or reef banks. Some modern exam-ples can be seen in The Gulf today.

F.R.S. Henson, in his 1950 AAPGpaper, suggested the term ‘reef-shoals’for these small reefs which lack rigidfore-reef walls. He also recognized that there were massive rudist accumulationsmaking up banks which he observed out-cropping in north east Iraq (Upper Creta-ceous between Bekhme Gorge and Aqra,and Late Middle Cretaceous at Pir-i-Mugrun). Dolomite mineralization devel-oped in a variety of tectonic settings innorthern Iraq and in Syria (figure 1.12).

The Shuaiba reef of Bu Hasa Field inAbu Dhabi may be a rudist bank, but Ibrahim Marzouk, Supervisor of Reser-voir Geology at the Abu Dhabi NationalOil Company (ADNOC), has indicatedthat wrench faulting may have affectedthe topography of the reef buildup.

Opportunities in Oman

Occidental has recently announced thediscovery of a Lower Cretaceous reser-voir in northwest Oman, near the flank of the Middle Cretaceous high situated west of Safah Field. This should lead to a re-evaluation of this reef-bearing region.

Reef facies were deposited aroundthe Kirkuk Field during both the earlyCretaceous and the Oligocene, along with Eocene nummulitic shoals. The min-eralogy of the reef facies was originallycalcite (limestone) but many zones werelater dolomitized. Recent studies indicatethat dolomitization of the Eocene bankwas related to falling sea level. Thisoccurred before development of themajor unconformity which precedesdeposition of the Fars evaporites.

Fig. 1.14: The porosity and permeability of dolomite(green) and limestone(blue) samples from thecore indicate a closerelationship between these

properties. The graph shows two dolomite

trends. One (a) follows thelimestone trend, showing that dolomitization had little effect on the pore

system. The second trend (b) indicates that dolomitization led to ahigher permeability for a

given porosity. In this casethe leaching associated with dolomitemineralization hasimproved pore systemconnectivity. This coredata is taken from theCretaceous sequence

shown in figure 1.16.

0Core porosity (pu)

10 20 30 400.01

.01

1

10

100

1000

10000

C o r e p e r m e a b i l i t y

( m d )

(a)

(b)

Sea Level

Evaporites &associated deposits

Globigerinal chalks,marls & limestone

Reef, back-reef &fore-reef limestones

Sublittoral shales &marls

Sands,conglomerates, etc.

T e c t o n i c s i n k A (G r a b e n a t d e p t h )}}}

BC Barrier or

fringing reefSubmerged high

with bank reefSubmerged high

with disconformity

A......Deposited horizontally before uplift

B......Deposited during and after upliftC......Deposited during subsidence

Fig: 1.12: Dolomites developed in a variety of tectonic settings in Syria and northern Iraq.



Number 15, 1994. 17

Fig. 1.15: Dolomite - related porosity typically develops in one of twoways. In the early stages individual dolomite crystals appear in thelimestone matrix (a). Dolomitemineralization continues until individual crystals come intocontact (b), and a framework may emerge (c). Alternatively, chemical changes may favour the dissolutionof the dolomite crystals and therock may develop leached mouldic

porosity (d). If dolomitemineralization continues, thedolomite framework may provemore durable than the limestonehost. Dissolution of the limestoneleaves a dolomite framework withinter-crystalline porosity (e).

(a)

(b)

(c) (d)

(e)

Fig. 1.16: Mineral analysis of thisCretaceous carbonate reservoir inthe Emirates used geochemical data collected by a Geochemical

Logging tool (GLT). The best porosity is found in the lower part of the sequence, but the high

permeability values correlate withdolomitization.

Core mineralogy, isotopic variationsand rock examinations suggest theremay have been seven stratigraphic dis-continuities caused by sea level fluctua-tions. Detailed mineral analysis indicatesthat cementation variations from one sealevel fluctuation to the next account forthe permeability variations found in the

sequence. Changing sea level alsoresulted in the development of calcitecementation following the dolomitizationphase. The best permeability is found indolomite intervals where blocky mete-oric calcite cements have not developed.Data from the Kirkuk Field in northernIraq (figure 1.13) shows this clearly.

Dolomitization can have a very pro-found influence on permeability andporosity (figure 1.14). This makes a clearunderstanding of the process crucial toreservoir development. Dolomitization isa complex process and dolomite - relatedporosity can develop in two ways (figure1.15). Dolomite crystals appear in thelimestone matrix and, as dolomitizationcontinues, may coalesce to form a frame-work. At this stage chemical changes candissolve the dolomite leaving leachedporosity, or may dissolve the remaining limestone, producing inter-granularporosity and high permeability in a puredolomite rock.

Studies from around the Middle East show that dolomites retain their porositylonger than interbedded or associatedlimestones. There are a number of rea-sons for this, perhaps the most important being less physical and chemical com-paction and reduced cementation associ-ated with dolomites. In mixed carbonatesequences dolomites often show thehighest permeability values (figure 1.16).However, shallow dolomite reservoirswith relatively high porosity values canhave lower permeabilities than grain-stones with similar porosities.

Susan Herron et al. (1992), Geochemical Logging of a

Middle East Carbonate Reservoir. Jour. Pet. Tech .

November 1992.

Oil

Moved HydrocarbonSecondary Porosity

Core permeability

Permeability

(Core)0.01 (md) 10000

Water

Calcite

Dolomite

SPI

0 (PU) 50

x200ft

x300 ft




Subtle traps revealed in the Middle East

Most of the giant anticlines and largereef reservoir bodies in the Middle East

have been surveyed and drilled. Newreef and carbonate shoal reservoirs arelikely to be smaller than those in exist-ing fields, and will only be foundthrough careful processing andinformed interpretation of 3D seismicsurveys. In The Gulf region, many Creta-ceous reservoir zones are not dolomi-tized. Consequently, depositional char-acteristics are the most important factorin understanding oil and gas accumula-tions. Seismic surveys are thereforebeing evaluated for depositional charac-teristics as well as reservoir structure.

A team of seismic experts from Geco-Prakla/GeoQuest recently summarizedan integrated seismic processing strat-egy which can be applied in carbonate

exploration and reservoir characteriza-tion. Figure 1.17 (a to c) shows theirwork on a prograding carbonate plat-form and aggrading shoals similar tothose seen in northern Iraq and Syriaand northern and eastern Arabia. Thefirst step (figure 1.17a) is a preliminaryinterpretation of the structure, seismicsequence analysis and interpretation of depositional facies. The next stage (fig-ure 1.17b) produces a complete inter-pretation of depositional environment,using all available data from systemstracks and depositional sequences. Inthe third and final stage (figure 1.17c),synthetic modelling is carried out tocheck the interpretation and to give anindication of the geophysical risk factorsin the area.

Risk evaluation is a vital step in newexploration areas where the seismic,structural and depositional interpreta-tions are usually based on limiteddatasets.

Fig. 1.17: STEP BY STEP: After preliminary interpretation had beencarried out (a) theinterpreters brought together all existing data from system

tracks and sequences to present an integrated pictureof the reservoir (b). This waschecked and the potential risks for development assessed (c) before any major productioncommitment was made.

(a)

(b)

(c)

Klaus Fischer et al. (1993) Remarks on Exploration Tools:

Integrated Exploration Strategy being applied to Carbonate

Environments. SPE Middle East Oil Show.

Basin

Shelf interior / marginal mounds

Shelf interior

Slope upper ramp

1.6 sec

1.8 sec

2.0 sec

2.2 sec

2.4 sec

2.0 sec

1.9 sec

5000 m 10000 m



Number 15, 1994. 19

M a r k

E l l i o t t , G e o

Q u e s t ,

L o n

d o n

C h r i s

t o p

h e

M .

RUDISTS, REEFS AND RESERVOIRS During the Cretaceous an aberrant group of large bivalves, the rudists,moved into the reef environment.These particular organisms filled the

high-energy shoal so successfully that many geologists think of rudists asreef builders.

For 40M years rudists dominatedthe tops of shoaling highs and theedges of carbonate platforms. Theseunusual bivalves have one long cylin-drical valve hinged with a flat ‘lid’(figure 1.18). Rudists, like recent bivalves, filtered seawater for food.The elongate valve helped keep therudist’s feeding mechanism highabove the sediment-rich layer whichwould have clogged their food gath-ering system. This adaptation

allowed them to feed almost continu-ously, stopping only when verystrong currents lifted muddy sedi-ment from the sea floor.

Rudists did not replace coralscompletely, they simply took overpart of the environmental nichewhich corals had exploited in thepast. As rudists moved into the envi-ronments where corals had been lesssuccessful, their shape evolved toovercome the soft mud problemswhich had faced the corals.

The hippuritids and radiotitidsformed the most striking of all in-

place rudist congregations, with indi-viduals sometimes so denselypacked together that they resembled

colonial organisms (figure 1.19). Thesedense clusters were most common inquieter water. In high energy facies, thecaprinids were dominant.

The best reservoirs in the Creta-ceous are typically carbonate sandgrainstones or rudist shallow-marinecarbonate deposits. Of the latter, themost significant are the Middle Creta-ceous rudist facies which form banks,thickets and biostroms (fossil rich lay-ers). The rudists did not build reefs, nordid they form large bioherms, but theyare a vital component of many Creta-ceous reservoir rocks.

The best rudist reservoir facies arethose which contain a high proportionof skeletal aragonite (from Caprinid)shells. The leaching (dissolution andremoval) of aragonite, an unstable car-bonate, has produced important, sec-ondary porosity in the form of large‘vugs’ or cavities in the limestone. The

reservoir potential of a horizon is oftenenhanced if the aragonite intervals weresubaerially exposed after deposition.Increased porosity related to this typeof exposure can be seen in the Natihreservoir, in Oman. The leaching associ-ated with fresh water lenses during sub-aerial exposure is often high in the car-bonate reservoir, but not necessarily at the very top of the sequence.

Fig. 1.18: IDENTITY CRISIS: It looks like acoral, but it's a bivalve. Rudists grew one long valve to keep their filter feeding mechanismclear of muddy sediments.

Fig. 1.19: FAMILY TREE: The rudists, unlikecorals, were not colonial organisms. However,they normally crowded together to formmounds, with successive generations building on top of their parents.




Fig. 1.20: This seismic line shows the facies changes which occur across the platform edge and into the basin. The sedimentary lobesdeveloped during the High Stand Systems Tract (HST) of the Natih ‘e’ Member are remarkably clear on this seismic section. During this

period of high and stable sea level sediment was prograding from the NE towards the SW. This sedimentation was terminated by a sea level drop, creating a sequence boundary (SB).

Pictures of the prospect* Explorationists often have to deal withvery complex sedimentological andstructural problems in prospective areas.Their aim is to understand the detail of reservoir variations, while drawing all of the information together into a compre-hensive picture of reservoir develop-ment and overall hydrocarbon potential.

Petroleum Development Oman (PDO)carried out an evaluation study of theSirat structure, making use of sequencestratigraphic techniques.

The Sirat Prospect, in the Natih For-mation of Oman, has been the focus of intensive seismic and geological model-ling. This formation consists of stackedlimestone cycles separated by relativelythin shaly beds. The depositional envi-ronment of the Natih Formation has var-ied from deep water shales, with charac-teristic marine fossils such as ostracodsand planktonic foraminifera, to very shal-low marine packstones, grainstones andrudstones with abundant largerforaminifera and rudists.

In the upper part of the Natih ‘e’ Mem-

ber a number of sedimentary lobesdeveloped (figure 1.20). These pro-graded from the shallowest parts of theshelf, building out to deeper water at theedge of the shelf. Maximum water depthduring this progradation was probablyno more than 100 m.

Each lobe contains a cycle of rocktypes changing from deep waterdeposits at the base to shallow sedi-ments at the top.

Sequence stratigraphy attempts toclassify sediments and sedimentarypackages by their relationships tochanging sea levels (rise, fall, rate of change) for local and worldwide (eusta-tic) changes. This allows us to define dif-ferent packages or sequences consist-ing of a Transgressive Systems Tract (TST), a Highstand Systems Tract (HST)and, in deeper areas, a Lowstand Sys-tems Tract (LST). Sequences are sepa-rated by sequence boundaries (SBs)created by sea-level fall. During times of maximum rate of sea-level rise, a Maxi-mum Flooding Surface (MFS) isdeposited. Sequences with their sys-

tems tracts and surfaces can all be recog-nized on seismic lines, giving vital cluesto the structure and likely compositionof sediments. Micropalaeontology pro-vides important, additional informationabout the sequences.

Sequences are ranked, according totheir importance and the type of changes which they represent. The 1st-order sequence boundary is moreimportant than a 2nd-order boundaryand so on.

The cyclic response from theGamma-Ray log has been used to definetwo 2nd-order sequences (Sequence Iand II) and a number of smaller 3rd-order sequences (figure 1.21). The top of the Natih ‘e’ Member is identified as animportant sequence boundary. Theshorter period cyclicity defines the vari-ous members (a - g) which constitute theNatih Formation. In shallow areas onlythe TST and the HST are present. In thebasin at the southwestern end of theseismic section (figures 1.20 and 1.21) aLST developed.

*Taken from: - Sequence Stratigraphy and Hydrocarbon Habitat of the Natih Formation in Oman . Presented by WytseSikkema (Petroleum Development Oman) at the 1993 AAPGInternational Conference, The Hague, The Netherlands.

SW NE



Number 15, 1994. 21

Fig.1.21: Sequence stratigraphic analysis of the area revealed two 2nd-order sequences (Sequence I and Sequence II) and a number of 3rd-order sequences. Two maximum flooding surfaces have been identified and the top of the ‘e’ Member is an important sequenceboundary. By correlating seismic lines with this analytical approach to sedimentary structures, experts can assess the structural history of the area and determine the risks associated with any given prospect.

The sequence stratigraphic recon-struction has a number of implicationsfor the prospectivity of the Sirat struc-ture. The prograding lobes of Sequence I,as seen on the seismic section, containexcellent reservoirs. Porosity and reser-voir permeability were enhanced by theexposure of the sediments whichoccurred during low sea level phases.This reservoir quality, coupled with theclear images available using seismictechnology, suggest that this would be anexcellent prospect. However the sedi-

ments at the top of the Natih ‘e’ levelwere deposited in a shallow environ-ment and are of poor sealing quality,thereby downgrading the prospect.

LST deposits are present but willprobably be low value reservoirs. Thesesediments normally contain a high pro-portion of fine clastic sediment whichwould reduce porosity. In addition, theabsence of rudist fragments suggests that initial porosity was low.

Three important reflectors relating tothe sequence stratigraphy can be seenon the seismic line:• the maximum flooding surface of Sequence I (basal ‘e’ Member)• the sequence boundary between I andII (near top of ‘e’ Member)• the maximum flooding surface of Sequence II.

The seismic view

Explorationists integrated seismic lineswith well data in the sequence strati-graphic model, to reconstruct the depo-sitional environment of the Sirat Prospect.• Deep water sediments occur aroundthe maximum flooding surface. The rela-tively deep limestone-shale alternationsare represented by a ‘reflective’ seismicfacies containing a number of continu-ous, high-amplitude reflectors.• The rudist accumulations are some-times visible as high-amplitude discon-tinuous reflectors.• Thick deposits of shallow marine car-bonates appear as low amplitude ‘trans-parent’ seismic facies.

The study concluded that despitethe excellent reservoir qualities of theHST lobes, the limited sealing capacityof overlying sediments made develop-ment of the Sirat Prospect a high-riskproject. A further conclusion from thestudy was that sequence stratigraphicmethods could be used to reconstruct the detailed sedimentary history of thearea and to predict the character of therocks in the sequence.

x300m

x500m

x500m

x300m

SB

mfs

mfs




Cyclic sequences

The Middle Cretaceous is one of themain hydrocarbon producing horizonsin Oman and offshore Dubai. Sedimentssuch as the Middle Cretaceous NatihFormation were part of a Mesozoic plat-form carbonate succession, accumulat-ing around intrashelf depressions on theeastern edge of the Arabian peninsula.To the north west, in the Emirates, theequivalent reservoir rocks are known asthe Mishrif Formation (figure 1.22). Elf has recently discovered oil in the sameformation offshore Qatar.

The Natih limestones are separatedfrom the deeper Shuaiba reservoir car-bonates by the Nahr Umr Shale. This,and the Fiqa Shale which overlies theNatih Formation, act as regional seals.

The Natih Formation is cyclic, com-prising a succession of coarsening-

upward sequences. Each cycle consistsof deep marine shales and mudstonesgrading up to shallow marine rudist packstones and grainstones. Emergencesurfaces occur at the top of each cycle.The cyclic sequence was caused byeustatic sea level changes, although it appears that deposition of the Natih For-mation was halted by tectonic uplift.

Away from the local highs, typifiedby shallow water deposits, the lime-stones interfinger with two deepermarine shales. These have significant organic content and a rich fauna of planktonic foraminifera. The cycles have

formed the basis of a scheme of subdivi-sions (members labelled ‘a’ to ‘g’) forthe Natih Formation.

Regional uplift during the Jurassiceffectively reduced average sea leveland led to the deposition of evaporitesover much of the Arabian carbonateplatform. In some areas uplift raised thesediment above sea level and there isevidence of subaerial erosion. Compres-sion, as the Arabian and Eurasian plateswere forced together, caused rapid sub-sidence along the plate boundaries. Thiswas followed by the spread of transgres-sive seas across the Arabian platform

(figure 1.23).On two more occasions during theCretaceous, uplift pushed topographichighs to a position where they wereeroded. Both phases were followed byrapid subsidence and shale deposition.The transgressive seas which developedafter these events became areas of deposition for the three main carbonatemegasequences which covernortheastern Arabia namely; theThamama/Kahmah, Wasia and Arumagroups. Each megasequence contains

NW SEDubai Oman

70

80

90

100

110

MillionYears

?

?Simsima

Shuaiba

Fiqa shale

Juweiza

MutiHalul-Ilam

Laffan

MishrifKhatiyahAhmadi

Mauddud

Nahr Umrshale

ac

b

efg

A r u m a g r o u p

W a s i a g r o u p

U p p e r

L o w e r

C r e t a c e o u s

N

a t i h

Eroded

Eroded

??

?

?

?

M i d d l e

Fig. 1.22: TIME ZONES: The study of rudist assemblages and the discovery of ammonites withinthe Natih Formationhave provided a

precise correlationof time lines withinthe sequence.Correlationbetween outcrop

sections allowed explorationists todevelop aconceptual

sequence stratigraphic model which includes the

subdivisions(‘a’ to ‘g’) used in

subsurface studies.

Fig. 1.23: PRIME SITES: The best locationsfor rudist buildups were in the shelf setting as shown in this Middle Cretaceous map of The Gulf area.

numerous depositional cycles (3rd-order or parasequences) related tosmall-scale sea level fluctuations.

Understanding the cycles, and defin-ing which areas were most suitable forreef and shoal development, is an essen-tial part of the interpretation. Thesedepositional factors control the natureand location of the Cretaceous carbon-ate reservoirs.

The best reservoirs are generallyfound in the upper part of each megase-quence. This is due to upward shallow-ing, the abundance of coarse grain car-bonate particles, leaching caused bysubaerial exposure and the presence of particularly effective seals immediatelyabove the uppermost carbonate units ineach megasequence.

Moderncoastline

BasinShelf



Number 15, 1994. 23

Fig. 1.24: Typical mottled fabric of the Upper Cretaceous Ilam carbonate reservoir faciesrevealed by FMI imagery and in core (inset) from

Fateh Field.

Fig. 1.27: Karst surfaces seen in borehole imagesand core within the Mishrif reservoir provideclear evidence of repeated subaerial exposure.

Fig. 1.26: Electrical imagery and equivalent Mishrif Reservoir core sample (inset) of a rudist shoaling buildup in Fateh Field. An arrow indicates the seal mark of the MDT pressure probe, the tool used todefine reservoir pressure and permeability. A stylolite

seam can be seen just beneath the seal mark.

Fig. 1.25: FMI image and core interval (inset) of the unconformable contact at the top of the

Middle Cretaceous Mishrif reservoir. This unit isoverlain by Upper Cretaceous Laffan Shale.

Mishrif reservoirs

Rudist reefal-shoaling deposits com-prising the Cretaceous Mishrif Forma-tion, which is partially equivalent tothe Natih Formations of Oman, are themajor reservoirs in many fields inDubai and eastern Abu Dhabi. Thedomal Fateh Field is the largest off-shore Mishrif-age field in the Emirates.It was discovered in 1966, despite theabsence of Mishrif rocks from the dis-covery well - the result of pronouncedpost-Cenomanian erosion on the crest of the structure. Typical structures andfabrics from wells in Fateh Field areshown in figures 1.24 to 1.27.

Other fields in the region, including the Shah Field in Abu Dhabi and theAwali Field in Bahrain, are character-ized by erosion of Mishrif and equiva-lent rocks.

Fluid inclusion data, maturation cal-culations and burial history modelling indicate that cementation by blockycalcite crystals and oil migration hap-pened about the same time, between theLate Miocene and Early Eocene. TheKhatiyah Shale, which lies directlybeneath the Mishrif, is believed to be themajor source rock for these reservoirs.

A number of depositional cycles,locally bounded by erosional uncon-formities, have been identified by geol-ogists of the Dubai Petroleum Com-pany. These unconformities arebelieved to have been caused by

global sea-level fluctuations and uplift of the deep, Eocambrian Hormuz Salt.The combination of sea level fall

and uplift probably led to the develop-ment of new, tectonically-controlledislands and erosion of these structures.Anticlinal fold belts and deep-seatedsalt deposits were raised to the surfaceof the Cretaceous sea which coveredmuch of the Middle East. Having reached the surface, they were sub-jected to the mixing of fresh water andmarine water. The results of this mix-ing process can be seen along themountain fronts from Turkey to Oman.

Subsequent transgression over thesubaerially exposed islands was asso-ciated with deposition of the LaffanShales which seals the Mishrif andother, slightly younger, Cretaceousreservoirs.

x043.0 ft

x047.0 ft

x317.0 ft

x324.0 ft

x343.0 ft

x340.0 ft



P r o

g r e

s s i v

e s

o l u

t i o n

Initial particle

Mould

Solution-enlargedmould

Vug

Number 15, 1994. 25

Fig. 1.32: MAKING THE MOULD (AND THE VUG):When a particle, sedimentary

grain or organic fragment dissolves from the

surrounding rock matrix it leaves a mould. If dissolutioncontinues the original shapeof the mould is lost,

producing a vug.

Fig. 1.30: (above) Analysis of themould ‘population’ in a short section of core can be carried out using the

Formation MicroScanner (FMS)* tool.This example, from offshore Bombay,

India, shows a plot of vug size and area, giving an indication of vug density in the rock sample.

Fig. 1.31: (left) Concentrations of coral/algal moulds and vugs in thisTertiary coralgal boundstone fromoffshore India have been revealed and quantified using FMS imagery and core by N.R. Devrajan and R.S.

Iyer of The Oil and Natural Gas

Commission of India.

Detecting vugs and moulds

The petroleum industry devotes a lot of time to mould and vug evaluation (figures1.30 and 1.31). However, strict definitionsof moulds and vugs are often ignored andusing the two terms synonymously canlead to confusion.

Moulds are pores formed by the selec-tive removal, normally by solution, of anexisting rock particle such as a shell frag-ment, crystal or grain. The resulting poros-ity is referred to as mouldic porosity and isdescribed according to the type of particleremoved; e.g. oomouldic for an oolitic rockwhere ooids have been dissolved.

If the leaching of the original particle goesbeyond the point at which it can be identi-fied the hole is a referred to as a vug (figure1.32). The condition of the hole, not its size,determines whether it is a mould or a vug.

The authors of the basic reference on

carbonate porosity, Philip Choquette andLloyd Pray, suggested that a vug which islarge enough to be examined from theinside should be referred to as a cave.They also defined micropores as thosewhich have a diameter or cross-sectionwhich averages less than 1/16 mmwhether the pores are equidimensional,platy or tabular.

The full capabilities of the ModularDynamic Tester (MDT)* tool include thedefinition of vuggy reservoir zones whichcannot be characterized by core or bore-hole imagery even when combined withother well logs. Even the RFT tool has lim-

ited applications for vuggy intervals. Testsoften fail due to lack of seal or the pres-ence of a tight patch resulting in a drytest. Fractures in low porosity patches fur-ther complicate the situation. However,the MDT tool has inflatable packers whichcan be placed above and below the vuggyzone to isolate it. The zone can be definedby FMI/FMS tools or core data.

While testing vuggy zones the MDTtool can be configured to include a con-ventional probe and an inflatable packermodule. The tool can then provide probemeasurements, and allows the operator touse the inflatable packers when a seal is

not possible in the best fractured orvuggy interval.

The MDT tool’s pumpout module canbe used for the dual packer approachwhich often succeeds where RFT attemptsfail. Packer spacing can be set to matchthe small intervals defined in FMI/FMS (orUBI in oil-base muds) or core data, to aminimum of 3ft. This minimum size actu-ally provides a surface area thousands of times greater than the standard RFT orMDT probe. In this respect it can bethought of as a small-scale DST-type test which provides a pressure buildup with aradius of investigation just under 100 ft into the formation. This figure varies withthe pore system in the formation.

x752 ft

x753ft

x751ft

x373.4 ft

x373.0 ft




Evaluation of isolated zones is nor-mally achieved by pressure tests. How-ever fluid samples for evaluation can betaken from vuggy zones or even low per-meability or thin bed intervals. This ben-efit is derived from the large seal andsample area created by the dual pack-ers. Tough sampling situations require

use of the MDT tool’s pumpout module,fluid analyzer and sample throttling; anapproach which relies on the tool’s mod-ular design.

Since pressure and fluid content read-out is done at the surface, the test needonly continue until the formation fluid isdetected. This appears after the flow of drilling fluid which invaded the forma-tion has been pumped out. This type of arrangement can replace the moreexpensive drill stem test and offers ahigh degree of safety. The MDT tool hasbeen used for production testing forwells with high hydrogen sulphide (H 2S)concentrations.

Revealing reservoir permeability

Measured slowness, derived from lowfrequency Stoneley waves, can be usedto evaluate the permeability of hydrocar-bon reservoirs. At low frequencies theStoneley wave produces fluid flow whichis related to the connectivity of porespace. By comparing observed slownesswith elastic slowness computed for a for-mation with no fluids we can calculatepermeability (figure 1.33). Elastic slow-

ness is calculated using three factors whichhave a direct effect on Stoneley wave prop-agation- formation density, borehole fluiddensity and shear slowness.

Stoneley attenuation provides analternative to permeability estimatesbased on slowness. In permeable forma-tions Stoneley waves are attenuated byfluid moving in the pore space (figure1.34) to a degree proportional to fluidmobility in the formation. From thisvalue engineers can derive the qualityfactor, Q (inverse attenuation), which isdirectly related to reservoir permeabil-ity. The calculation used to derive per-

meability from the quality factorinvolves values for pore fluid and bulkelastic moduli, and for porosity andborehole diameter.

The technique was tested on a dataset collected from a high porosity, pure car-bonate reservoir in Saudi Arabia. Theslowness and attenuation techniqueswere applied to data gathered using aDipole Shear Sonic Imager (DSI*) tool.

The predicted permeability was mod-ified to simulate a synthetic flowmeterprofile. Agreement between slowness-derived permeability and the flowmeterprofile was very good.

Squirting flow

Pore throatto pore

Edge to centre

Crack lubricationfacilitatingfriction

Biotic fluid flowwith boundaryshear

Fig. 1.34: SLOWING THE FLOW: Thereare several mechanisms whichcontribute to the attenuation of shear waves. The lubricating effects of liquids in cracks absorb energy. Themechanisms involved are fluid flowwith boundary shear effects and

squirting flow (which occurs whenfluids are forced through narrow porethroats between grains). By measuring energy absorption we can estimaterock permeability. From Johnston,Toksoz and Timur (1978).

Fig. 1.33: Stoneley permeability values can be calibrated by RFT tool testsand confirmed by core data where available. The MDT tool provides a

greater range of permeability than the RFT tool used here to calibrate the Stoneley energy-derived permeability. The lithology/porosity/fluid columnis an ELAN-computed result.

x950ft

x000ft

x050ft

M. Petricola and B. Frignet (1992) A Synergetic Approach to

Fracture and Permeability Evaluation from Logs. 5th Abu

Dhabi Petroleum Conference.

D. Johnston, M. Toksoz and A.Timur (1978) Attenuation of

Seismic Waves in Dry and Saturated Rocks. Geophysics 44.

Permeability

(Core)0.01 (md) 1000

Permeability(Stoneley)

0.01 (md) 1000

ShearSlowness170 (us/f) 90

StoneleySlowness

250 (us/f) 200

Permeability(RFT)

0.01 (md) 1000

• Thick deposits of shallowmarine carbonates appear aslow-amplitude ‘transparent’seismic facies.



Number 15, 1994. 27

Imagery + Stoneley analysis + OH Logs Reservoir data for well testing strategyand optimum MDT tool configuration(FMI) (DSI) (ELAN)(UBI) Lithology(ARI) Porosity

Saturation

Permeability progress

Permeability measurements in carbon-ate reservoirs present a major challengeto well logging analysts. A group of expert analysts and geophysicists inDubai, Abu Dhabi, Egypt, Saudi Arabiaand at the Oil and Natural Gas Commis-sion (ONGC)-Schlumberger Joint Research Council in India, have testedcarbonate reservoir permeability using Stoneley wave data. Present efforts areconcentrated on sample analysis and

Stoneley frequency using the DSI toolwhich samples at lower frequencies thanthe earlier Array Sonic tool (figure 1.35).

The permeabilities found in shoaling sequences, where coarse particles over-lie fine chalky facies with micropore sys-tems, have been characterized using theDSI tool. The tool has also found suc-cess in reservoirs where there are avariety of secondary porosity types.

RFT tool permeability data can beused to calibrate permeability profilesdefined by Stoneley wave data. In theexample, core permeability data fromone inch diameter plugs, taken at one

foot intervals, compare favourably withthe DSI tool and RFT tool profiles. How-ever, in carbonate reservoirs where thepore system is heterogeneous, thematch is often poor, despite accuratepermeability measurements.

This situation typically arises wheneach measurement relates to a different rock volume. Whole core analysis isrecommended for permeability charac-terization in the heterogeneous poresystems found in many carbonatereservoirs.

The MDT tool has already succeededin defining pressure, permeability andfluid content within complex carbonatereservoirs in the Middle East. The tool’s

30

20

10

0

S t o n e l e y o b s e r v e d - e l a s t i c

( µ s / f t )

0 1000 2000 3000 4000 5000

Fre uenc Hz

3000md1000md

300md

30md

10md

100md

Fig. 1.35: At low frequencies the Stoneley wave produces fluid flowwhich is related to the connectivity of pore space (permeability). This

plot shows the sensitivity of Stoneley slowness to frequency - in therange measured by the DSI tool - for a water-saturated sandstone.

From Cheung and Liu (1988).

Electric power

Pump-out

Sample

Optical analyzer

Hydraulic

Single probe

Packer

Fig. 1.36: The MDT tool can define

pressure, permeability and fluid content withincomplex carbonatereservoirs. The tool’smodular designallows the operator to select the optimumconfiguration for each task. The MDT tool is reliable inreservoirs, where

permeability rangesfrom hundreds of millidarcies tohundredths of amillidarcy.

Thin layered porosity

Interwoven porosity

Isolated non-porous

Fractured porosity

Shale barriers & baffles

Isolated porosity

Porosity

Non-porous (or low-porosity)rock

Uniform high porosity

Layered porosity

Thin porous layers

Uniformly non-porous

Thin non-porous layers

modular design allows the operator toselect the optimum configuration foreach task (figure 1.36). The MDT tool isreliable in challenging reservoirs, suchas those where permeability ranges fromhundreds of millidarcies to hundredthsof a millidarcy. To devise a high-qualityMDT tool test we require informationfrom several sources (e.g. electricalimagery and Stoneley). Only by combin-ing data from several sources can we besure of maximizing test efficiency.

Fig. 1.37: The main types of carbonate porosity heterogeneity revealed by borehole imagery.

Heterogeneities defined by imagery




0

100

200

300

400

T i m e

( M i l l i o n y e a r s )

N

P

UCr

I

JTr

P

C

D

S

O

0.7070 0.7080 0.7100

(after BP 1992)

E .

C r e

t

L a

t e J u r a s s

i c

M i d d l e J u r a s s

i c

E .

J u r a s s i c

Valanginian

Barriasian

Tithonian

Kimmeridgian

Oxfordian

Callovian

Bathonian

Bajocian

Aalenian

Toarcian

Pliensbachian

145.6

152.1154.7

157.1

161.3

166.1

173.5178.0187.0

194.5

Sr seawater curve for the Jurassic(modified after Smalley et al , 1989)

87 Sr/ 86 Sr0.7065 0.7070 0.7075Ma Age

87 Sr/ 86 Sr0.7090

Ratio

Global seawater strontium curve

Fig. 1.38: NAME THE DATE: This simplified curve for global Sr isotope ratios in seawater illustratesthe principle of the isotopic dating methods. For a known 87 Sr/ 86 Sr ratio a vertical line can bedrawn. Wherever this line crosses the curve, the sample ratio matches the seawater ratio for that

particular time. However, some isotopic ratio values occur at two or more places in the curve.When this happens age must be defined by alternative dating methods.

Fig. 1.39: This series of logs (a) from the Upper Jurassic shows the Asab Oolite at well A. Strontium isotope dating indicates that rocks of the same age are also found in well B, but havebeen lost from the sequence at well C to thenorth east. The unconformable contact between

the Jurassic and Cretaceous beds in the third well marks a period of erosion or non-deposition.Core taken from this level (b) confirms theunconformity.

Chemical timing

Geochemistry is finding new applica-tions as a tool for explorationists andreservoir analysts. A few years ago,most geochemical surveys weredirected at identifying source rock. Thisled to new applications in maturationand migration studies. Today, laborato-ries are using petroleum geochemistryto tackle reservoir problems such asassessing heterogeneity.

Geochemical methods include deter-mining ‘biomarkers’ in a sequence andthen using isotopes to ‘fingerprint’ dif-ferent oils present in a reservoir. Recent studies have investigated hydrocarbonvariation within reservoirs and clarifiedthe extent of compartmentalizationcaused by tar mats, shale barriers andsealing faults. This data is vital in estab-lishing models for development and

production phases.Rock geochemistry is especially use-ful where the reservoir rocks are not composed of the usual quartz, lime-stone or dolomite lithologies on whichlog interpretations are based. Problemscan even arise where mixtures of thesebasic lithologies are being investigatedfor basic formation evaluation. This hasencouraged the spread of geochemicalwell logging and core studies.

Well-to-well correlation can beenhanced by applying geochemical tech-niques to core or well log data. In thisway, we can identify geochemical varia-

tions in major lithologies or the presenceof minor minerals in adjacent wells.Log analysts who routinely use the

lithology indications from density/neu-tron variations in simple lithology mix-tures are defining lithology by compar-ing a single element, hydrogen, to thebulk density of the formation. Interpre-tations from geochemical logs are basedon much more information.

At present, the gamma ray log is most widely used for correlation in carbonatesequences: the elements identified areuranium (U) thorium (Th) and carbon(C). Geochemical logging analyses use a

further nine elements for correlation.

Ocean chemistry

Ocean chemistry influences the compo-sition of minerals being deposited on thesea floor. However, the chemical com-position of the oceans varies throughtime and these variations control min-eral stability. Ocean chemistry is crucialin determining the proportions of arago-nite or calcite present on the sea bedand, consequently, in the accumulatedsediment which reservoirs contain. Thisproportion influences ultimate reservoirporosity and permeability.

U p p e r

J u r a s s i c

L o w e r

C r e t .

SW A B C70km105km

HabshanFm

HithEquiv

AsabOolite

LowerAsab

R a y d a

F m

A s a

b F o r m a t i o n

NE(a)

Weighing the evidence

Isotopes are atoms of the same element having different numbers of neutrons inthe nucleus and, therefore, different atomic weights. The weight difference isimportant, and useful, because naturalprocesses such as evaporation, conden-sation and photosynthesis cause signifi-cant variations in the distribution of iso-topes within the various geochemicalcycles.

For example, the light oxygen iso-tope, 1 6 O, is concentrated inwater vapour when seawater evapo-rates. The 1 6O-enriched vapourtravels through the atmosphere towardsthe poles where it condenses and isincorporated in the polar ice sheets. Thedifferential evaporation of oxygen atomswhich occurs at the equator means that the 1 8 O / 1 6 O ratio inpolar ice caps is much lower than in sea-

(b)



N b 15 1994 29

1330

Core dataafter demagnetization

-50 0 50

Reverse Normal

Log data

NMRT SUMT

Normal

1335

1340

1345

D e p t h , m

Magneticcolourcode Normal

Reverse

0 1 2-1-2

Normal

water at the equator.These stable isotope ratios have varied

systematically over time and can, there-fore, be used to date rock samples andcorrelate sequences. The stable isotopicratios of strontium (Sr) and sulphur (S)are used in chronostratigraphic studies,confirming time gaps at unconformities

and determining sedimentation rates.They can even be used to date diageneticevents such as dolomitization.

The principle of strontium dating relieson changes in 8 7 Sr / 8 6 Srthrough time and the assumption that theratio within seawater is uniform worldwideat any given time. Seawater curves forstrontium ratios have been plotted and cal-ibrated against the geological time scale(figure 1.38). This was done by analyzing the Sr isotope ratio in carbonate and phos-phate from fossils of known ages.

Strontium dating, and correlations basedon strontium ratios, can be used when thereare few fossils and when biostratigraphiczonation is poor. Independent of facies andfossil occurrence, this technique can evenbe used to date evaporite sequences.

Very small samples are required (as lit-tle as 0.1 mg) to provide a reliable age,with uncertainty normally being +/-1 mil-lion years, or less. The technique can beapplied worldwide and, unlike fossil cor-relations, is completely objective.

While the benefits of this technique areobvious, there are some limitations.Weathering affects all isotopic systems,and Sr isotope ratios can be modified bycontamination from meteoric / mixing zone diagenesis and burial cements.Depending on the modifying mechanism,the 8 7Sr/ 8 6Sr ratios can beshifted towards values typical of younger,or older, rocks. If unaltered carbonatesamples are not available for Sr isotopestudies then adjustments must be made toaccount for sample impurities.

Brachiopods and belemnites, withtheir low-magnesium calcite skeletons,are little affected by diagenesis and thebest samples come from these and fromthe phosphates which make up fish andconodont fossils. Whole rock samples,with the exception of anhydrites, usuallygive less accurate dates since their8 7 Sr/ 8 6 Sr values have fre-quently been altered by diageneticprocesses.

The final problem occurs when one8 7Sr/ 8 6Sr value correspondswith two or more ages in the seawatercurve. Ratios recorded from rocks in theKimmeridgian have the same8 7Sr/ 8 6 Sr ratio as found inBajocian rocks which are 10 M yearsolder. This problem occurs on both largeand small scales throughout the geologi-cal record. Isotope values recorded in the

Upper Permian can be identical to thosein Cretaceous rocks, although there is lesschance of confusion between these units.Matching the isotope ratio to a position on

Fig. 1.40: POLE POSITION: Over geological time scales, the Earth's magnetic field switches polarity.These changes are recorded in rock sequences. This forms the basis of a new logging tool which cancorrelate polarity changes between wells, offering a high-resolution magnetic log. The magneticreversals shown here were revealed by combining data from the Nuclear Magnetic Resonance well logging tool (NMRT*) with the induced field as measured by the Susceptibility Measurement tool (SUMT*). Modified from Arnaud Etchecopar et al., Oilfield Review , October 1991.

the curve is normally a problem only if the age of the sample layer is verypoorly constrained.

In 1991, the Abu Dhabi Company forOnshore Oil Operations (ADCO) carriedout an isotopic pilot study to resolvesome of the uncertainties in Jurassicstratigraphy. Early results were encourag-ing and the study expanded. Today, thedatabase consists of Sr isotope analysesfrom ooid grainstones, belemnites, lime

mudstones, anhydrites and bivalves.Sr dating has provided evidence of adirect stratigraphic correlation betweenthe pelagic transgressive belemnite lag deposit and the unconformity (a type IIsequence boundary) of the Jurassic-Cre-taceous contact and the intraclasticbelemnite horizon in the Asab Oolite (fig-ure 1.39a and b). This type of geochemi-cal correlation has helped to refine

regional stratigraphy. For example,anhydrites which had been included inthe lowermost Cretaceous Habshan For-mation, were re-assigned to the UpperJurassic while the Manifa Member(150.5 M years) was correlated with theAsab Formation (151.2 M years) and withthe lateral equivalent Qatar Formation(150.5 M years) using this technique.

A new logging technique (figure 1.40)which relies on reversals of the Earth's

magnetic field through geological time,has proved very successful for cross-well correlation. Rocks can retain themagnetization from previous magneticfields, a phenomenon called naturalremnant magnetism (NRM). The logging techniques which record this magnetic‘memory’ are very accurate and can beused worldwide. Absolute age correla-tions derived from reversals have beenmade between three wells drilled byTotal in the Jurassic sediments of the

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