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12 Middle East Well Evaluation Review

Tethys

Sea

Tethys

Sea

Europe

Arabia

Africa

Tethys

Sea

India

SouthAmerica

NorthAmerica

Transformationof Tethys

The origins of oil-fields in theTurkey to Oman

mountain belt startedlong before the Gond-

wana supercontinent began to break up50 million years ago. To capture a trueunderstanding of how these oilfieldswere created we must go back 250Myears, to the break-up of Pangea - theearths sole land mass at that time.Pangea was splitting apart, formingLaurasia in the north and Gondwana inthe south.

In the late Permian and Triassic, rift-ing occurred in Iran, Turkey and Omanwhich eventually led to the formationof very small fragments of crust, micro-plates, and the creation of two sea-

ways, the northern Palaeo-Tethys andthe southern Neo-Tethys (figure 2.2).These early rifts produced ideal deposi-tional sites for carbonates, clastic reser-voirs and sometimes organic-richsediments in intracratonic basins,marginal basins, shelf and platformmargins.

The last remnants of these Tethysseaways had virtually disappearedwhen subsequent crustal movementsled to the ocean areas being over-rid-den by continental crust. In addition,intense deformation occurred in colli-sion or crush zones where tectonic

forces thrust the plates together.It is extremely difficult to recon-

struct how Laurasia was interlockedwith Gondwana some 250M years ago.In southern Laurasia, a number ofsmall crustal plates joined to form acomplex region running from northernTurkey along northern Iran toAfghanistan, including the Moslemstates of the former Soviet Union. Piec-ing these together is a headache andthe problem is compounded by sub-duction of crust and the additionalintense deformation which created themain mountain belts from the Black Sea

to the Indian Ocean. Fortunately, thedeformation in these frontal edges ofthe mountain belts has been lessextreme.

However, there is a major incentiveto understand what has happenedalong this folded belt as some of theworlds richest oil and gas provinces lieunder the foothills. The micro-plates ofTurkey, northern Iran and the Moslemstates of the former Soviet Union layalong the northern edge of the Tethys

Sea. And hydrocarbon reservoirs inthese areas have similarities with thosealong other margins of the Tethys Seawhere compressional subsidence

occurred.The oil and gas in the Caucasian-Dag

and Zagros-Bitlis-Oman regions arefound in gently folded rocks whichformed on both the northern and south-ern sides of the Tethys. The southernmountain belt (Zagros), which resultedfrom the closing of the Neo-Tethys,extends from south east Turkey throughnorth east Syria, northern Iraq andsouthern Iran and reaches as far as theeastern UAE and northern Oman. Oiland gas is mainly of Cretaceous age inthe northern (Turkey and Syria) andsouthern (UAE and Oman) ends of the

Arabian/Iranian foreland folded zone.In contrast, both Cretaceous andyounger reservoirs are found in Iraqand Iran.

The northern Caucasian-Dag moun-tain belt extends from the Black Sea toAfghanistan and includes the states ofGeorgia, Armenia and Azerbaijan to thewest and Turkmenistan, Uzbekistan andTadzhikistan to the east. This fold beltwas also created when the Palaeo-Tethys Sea closed.

To the east of the Caspian Sea, in theDag fold belt, the reservoirs are primar-ily gas prone. The great oilfields whichmade Russia pre-eminent as an oil pro-

ducer in the last century were found inthe foothills of the Caucasus Mountains,generally on the northern flank. Thesouthern states of the former SovietUnion can be considered geologicallyas part of the Middle East. There areover 180 gas and gas condensate fields,including six giants with reserves morethan 3 tcf. Close to the north east Ira-nian border (east of Serakhs, Iran) isthe giant Dauletabad-Donmez(Sovetabad) Field which could be Turk-menistans largest gas field with 38 tcf ofinitial reserves.

Here, the main reservoirs are Juras-

sic carbonates making them similar inage and lithology to the major reser-voirs found on the Arabian platform. Aswith the Arab reservoirs, these aresealed by overlying Jurassic evapor-ites. However, some deltaic sandstonereservoirs are also present in the Juras-sic. Older and more deeply buried Tri-assic and Middle Jurassic gas-prone,organic-rich rocks are the likely sourcerocks in this region.

Fig. 2.1: THE LOST OCEAN: The vast Tethys Sea was formed when the giant Pangeasupercontinent broke up 250 million years ago, forming Gondwana to the south and Laurasia inthe north. The Black Sea and Caspian Sea are the only remnants of this sea.


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S NOman margin

Continental crust

Hawasina Ocean

Hawasina sediments

Oceanic crust

Volcanicbelt

Mantle

Oceaniccrust

S N

Oman margin

Hawasina sediments

Semail Ophiolite

Volcanicbelt

Mantle

Continental crust

Water

Oceanic crus

t

Oce

anic crust

S N

Water

Mantle

Semail Ophiolite

Continental crustVolcanic

belt

Jebel AkhdarHamratDuru

RangeFahud/Natih

area

Oceanic crust


CYAN MAGENTA YELLOW BLACK

Ophiolitesoutcrops in

Oman

Many of the oilfieldsalong the Oman foldbelt produce from frac-

tured Cretaceous car-bonates - from the Bukha Field in thenorth to Natih Field in the south1.

The growth of the Natih structuralhigh was triggered by movements of thedeeply buried Eo-Cambrian Salt. Com-pressional tectonics subsequentlyaffected the region in the Late Creta-ceous (figure 2.4). The thrusting orobduction of the ocean floor slab andophiolites onto North Omans continen-tal margin occurred during this period(figure 2.5). During Tertiary times, fur-ther compressive movements gener-ated localised structures such as the

Salakh Arch jebels.Production from the Natih Formation

is almost entirely fracture-related andPetroleum Devlopment Oman (PDO) isdeveloping the field using Gas-Oil Grav-ity Drainage (GOGD). During the next10 years, the fracture oil rim (ie thereservoir interval with oil-filled frac-tures) will be lowered by 70 m usingcontinuous gas injection at the fieldcrest, and additional down-dip waterproduction. The gas rapidly invades thefracture system at the top of the field,completely surrounding the oil-filled,less permeable matrix blocks (Middle

East Well Evaluation Review, Number12, 1992).

During the drainage process, gravitycauses gas to be drawn (or imbibed)from the fractures into the oil-filledmatrix. The matrix oil displaced by thisgas moves into the fracture system,where it is partially reabsorbed intoadjacent matrix blocks. Eventually itdrains down to contribute to the frac-ture oil rim, where it is produced. Bylowering the fracture oil rim, theamount of STOIIP exposed to GOGD isgreatly increased, resulting in improvedoil recovery - an approach which has

been successfully applied in the nearbyFahud Field2.

Fig. 2.4: SUFFERING FROM COMPRESSION: During the Late Cretaceous, ocean floor slab andophiolites (see also figure 2.5 below) were thrust onto North Oman's continental margin. DuringTertiary times, further compressive movements generated localised structures such as the Salakh

Arch jebels. (From Oman's Geological Heritage, by PDO).

Fig. 2.5: AUTHORITIVE EVIDENCE: The end of the Tethys Sea occurred when crustalmovements led to the ocean areas being over-ridden by continental crust. Various depositswere left in these crush zones, including these ophiolites which are remnants of the earth'smantle. Here, author Roy Nurmi examines a very fractured ophiolite overlying the top ofthe Mesozoic (Jurassic) carbonates in Oman.

References:

1 Mercadier, C.G.L. and Makel, G.H., 1991.Fracturepatterns of Natih Formation Outcrops and theirimplications for Reservoir Modelling of the NatihField, North Oman. Proc. 7th SPE Middle East OilShow Tech. Conf. & Exhib., 16-19 November 1991,

Manama, Bahrain, SPE Paper 21377, p. 357-368).

2 ONeill, N., 1987.Fahud Field Review: a switchfrom Water to Gas Injection. Proc. 5th SPE MiddleEast Oil Tech. Conf. & Exhibit., 7-10 March 1987,

Manama, Bahrain, SPE Paper 15691, p. 51-66.


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15Number 13, 1992.

Fig. 2.6: LANDSAT photograph of North Oman showing the main structures formed by the compressive plate movementsand the main jebels (mountains) in the region.

30

00

Zagros fold belt

endanfa

ultsy

ste

m

Makran

Accretionaryprism

Batinahpassivemargin

Gulf of Oman

Musan

dam

Prom

onto

ry

OmanMoun

tains

2000

1000

500

4000

Jebel Salakh

SaihHatat

JebelQusaybah


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If recent studies are to be believedthere is much more order to reser-voir faulting and fracturing than peo-ple first realised. The wealth of

structural information gained through3-D seismic surveys and boreholeelectrical imagery is helping scien-tists piece together what seem to befractal relationships between faultsand fractures.

Fractal relationships can be seenin many features, independently ofscale. For example, if you look at arocky coastline from the air, it willappear to have a jagged edge. Even ifyou land and take a walk along thesea shore, it will still have a jaggededge. And, if you put the sea shoreunder a magnifying glass, what doyou see? - youve guessed it - ajagged edge.

Some of the first indications offractal relationships in faults and frac-tures were discovered by comparinglaboratory-scale rock cracking exper-iments with studies of earthquakecharacteristics. More recently agroup in England has found a rela-tionship between fault length andvertical displacement which varieswith the material properties of therock.

Such studies inspired geologists in

California and Japan to look at thecharacteristics of other faults. Thisproved fruitful. It showed the width-to-length ratio of wrench faults oneither side of pull-apart basins(grabens) is independent of scale.The geometrical arrangements ofoverlapping faults are also similar atvarious orders of magnitude. Statisti-cal comparisons have been made toidentify relationships between char-acteristics such as fault frequencyand vertical displacement.

More recently, geologists havebegun to realise that the relationship

between fault length (ie lateral extentin the strike direction) and verticaldisplacement (or throw) differsaccording to the material propertiesof the rock.

An intriguing comparison betweenthe lateral extent and frequency of frac-tures and faults has been drawn up

Glass

Granitic crust rock

Mancos shale

Ocean basalt

Crack

Quartz - feldspar

vein

Minette dike

Ridge1000m

0.001m

100m

0.1m

Increasing

fre

quency

Increasing fracture/fault length (m)

FRACTIOUS RELATIONSHIPS

Fig. 2.8:Log-log plot ofnormalizedlengthfrequency inthe Gulf ofSuez. (Heffer

and Bevan,1990, SPEPaper No20981).

Fig. 2.7: Thegeometry ofoverlappingcracks is

similar fordifferentmaterials,independent ofscale (Pollardand Aydin,1984).


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17Number 13, 1992.

1

10

100

1000

10000

100000

1 10 100 1000 10000 100000

Length(m)

Width (m)

Soft

rock

s

Hard

rocks

1m 10m 100m 1km 10km100km 1000km

1cm

10cm

1m

10m

100m

1km

10km

100km

Displacement

Mapped fault length (width)

Increasing

frequency

-4 -3 -2 -1 0 10

1

2

3

2 3 4

Increasing fault displacement (m)

Kodels Canyon, UK

Onjoko, Japan

Gulf ofMexico,USA

Boso Peninsula,Japan

Fig 2.10: Loglength versuslog width for 70pull-apart basinsaround theworld. (Aydinand Nur, 1986).

Fig. 2.11:Summary ofobservationsshowing therelationshipbetween faultstrike-length anddisplacement(after Walsh andWatterson, 1988).

Fig. 2.9: Log of fault displacementversus fault frequency for variousbasins around the world. Note thesimilar ratios in each of the plots.

after studies of Egypts Ras Budran Fieldin the Gulf of Suez and outcrops in Sinai.Similar work in other parts of the worldappears to support this work but not allgeologists are convinced. Some believethat extensional fractures do not exhibitthe same kind of fractal nature thatappear to be common in shear fracturestudies.

Further work on fractal relationships

may pave the way for modelling reser-voir faults and fractures from small-scaleborehole imagery combined with 3-Dseismic data. This may prove to be aparticularly useful combination of data -recent work in the North Sea has shownthat faults which are too small to bedetected by the highest resolution 3-Dseismic surveys may still have a majorinfluence on reservoir behaviour.

Already in Egypt, fractal relationshipsbetween fracture apertures at the large,medium and small scales have beenseen in borehole electrical imagery.


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Head for thehills

Recent hydrocarbondiscoveries in areassuch as Abu Dhabiand Sharjah, UAE areencouraging explo-

rationists to search forother targets in the mountain fold beltwhich runs along the Musandam Penin-sula.

Amocos new discovery in SharjahsLower Cretaceous Thamama carbonateswas found after reinterpretation of 3-Dseismic data originally acquired in 1984.The discovery well (Amoco, Kahaif-2),located 18 km south of the Sajaa andMoveyeid fields, flowed 73 million cubicfeet per day of gas and 1,615 barrels perday of condensate from a 700ft-thick payzone.

The seismic re-interpretation had

additional spin-offs as it led to a betterdefinition of both the Sajaa andMoveyeid fields, which in turn guidedthe drilling of additional producers. Suc-cess in this region has prompted Amocoto increase its acreage by 50% and carryout new seismic surveys during 1993.

The structures of the fields along thenorthern end of the Emirates/Oman foldbelt are more complicated than thosebeing drilled in Oman. The dip patternsseen in figure 2.12 were created by dragdeformation along two thrust faultswhich pass through the flank of a majorfield in the UAE. Computer analysis ofdip data, using the recently introducedDip Trend* software, enables the geome-try of the thrust faults to be defined pre-cisely (Midd le East Well Eval ua tionReview, Number 8, 1990). The first diptrack in the Dip Trend output shows theoriginal dip recordings made and pro-cessed using a Stratigraphic High Resolu-tion Dipmeter Tool (SHDT*). The DipTrend software groups and colours thedips according to their initial structuralclassification. During this early stage, theprogram defines the structural dips of

Fig.2.12: (Right): Typical Dip Trend output.The original dip recording, made using theSHDT Tool, is shown in the left track. Thedip tadpoles have been coloured accordingto their initial structural classification. In thisprocessing stage, the Dip Trend analysisdefines the structural dips of intervals whichhave consistent magnitude and azimuth.The right-hand track shows the finalprocessing stage. This time, groups oftadpoles which belong to the same structuraltrend and fold patterns have been linked bycoloured bands. These dip clusters can beprojected on a polar plot as shown in figure2.13 (far right). Each of the structural trendsand folds is assigned an identificationnumber.

Structural dip

Dips indicate folding

Dips indicate folding

Structural dip

Original dips and classification Dip trends and structural analysis

401

323

303

312

303

301

100ft


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19Number 13, 1992.

Semail ophiolite

W Fateh Field Margham Field E

0 ft

20 000

40 000Salt

The Gulf

Pre-Permian

Oman mountains

6450

6500

6550

6600

6650

6700

6750

6800

6850

6900

6950

7000

7050

7100

7150

7200

0 Duadip 90

Northwest Southeast

90 0DuadipDepth

m

StrucViewintervals which have consistent magni-tude and azimuth.

In the final Dip Trend analysis, theprogram produces a second output trackwith coloured bands linking the dips ofeach structural trend and each fold pat-tern. An identification number is alsoassigned to each of these trends and isprinted at the base of the dip pattern ona log plot and near the cluster (or circle)of dips when projected onto a polar plot.

In this example, the structural dip

zones fall in the centre of the polar plot,as they have low dip angles, whereas thedips of the folds fall on a circle. The DipTrend software adds identification num-bers to dip populations that define indi-vidual folds on both polar and log plots.

A cross section showing the precisegeometry of the folding is drawn by theStrucView* software program which iscurrently under test in the Middle East.In this case, the angle of the thrust faultwas estimated since the survey wasundertaken using a Dipmeter. Had bore-hole imagery been used, it would havebeen possible to measure the exact dip

and strike angles of the fault planes.Well-to-well correlations in this areashow that parts of the formation arerepeated which confirms the interpreta-tion of thrust faulting. (See Charisma-tique, Issue 1 and WER Structural GeologySupplement, 1990).

0 1020

30

40

50

60

70

80

90

100

110

120

130

140

150160170180190

200210

220

230

240

250

260

270

280

290

300

310

320

330340

350

401

301

Small circle defining a fold(#303)

Small circle defining a fold(#323)

Structural dip trends (301 & 401)

Fig 2.14: Cross section through the UAE showing the structural complexity of the major exploration target, CretaceousThamama carbonates.

Fig. 2.15: Thiscross sectionshows theprecisestructuralgeometry of theexample shownin figure 2.12,based on DipTrend analysisof dipmeter data.However, theangle of the

thrust fault mustbe estimated.Had boreholeimagery beenused, it wouldhave beenpossible tomeasure theexact dip andstrike angles ofthe fault plane.The orientationand verticalscales can bechanged tomatch othercross sections,

such as a seismicsection.


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+2

+1

0

-1

-2

km

Melange NappeOphiolite Nappe

Mardin Limestone

Tertiary

Paleozoic

Upper

Cretaceous

Bitlis extension of the Zagros fold beltN S

Peering intoTurkeysfractures

Borehole imagery, DipTrend analysis and land3-D seismic surveys are

giving a new insight intothe complexity of the reservoirshoused in the fold belt which runs fromOman to Turkey. Structural and frac-ture analysis of borehole images haverevealed that Turkeys reservoirs havebeen put through a complex mixture ofnormal, thrust, reverse and wrenchfaulting.

Rift faulting first affected southeast-ern Turkey along the northern edge ofthe Arabian Plate. Later, the closure ofthe Tethys Sea caused thrust faultingwhich reactivated some of the normalfaults within these rift blocks. Subse-

quent interaction of the Arabian andEurasian plates during the Miocene pro-duced widespread wrench faulting andreactivated both normal and thrustfaults along the Zagros-Bitlis Mountainbelt forming the East Anatolian faultzone.

The dominant structural influenceon the Turkish Petroleum Company's(TPAO) fields in south east Turkey isthe NE-SW Adiyaman wrench-fault sys-tem. This has a left-lateral displacementand has formed structures with imbri-cated and faulted anticlines which areoverthrust from north to south. The

region has also suffered at least twophases of major tectonic deformation,one in the Late Cretaceous and theother during Miocene times.

Studies by TPAO in three fields,Ozan Sungurlu, Karakus and Cendere,give some idea of the complicated geo-logical history of south east Turkey andthe structural geometry of its reser-voirs. Analysis of borehole imageryfrom three wells in Ozan Sungurlu Fieldhas revealed faults, unconformities andfractures. A Dip Trend structural inter-

pretation of one of these wells is shownin figure 2.19. A 3-D structural model ofthe field has since been made andincorporates the various fault move-ments over time.

Similar reservoir complexities andfault types have been highlighted by 3-D seismic surveys of N V Turkse ShellsMardin reservoirs. The seismic data

Fig. 2.16: STRETCHINGTHE IMAGINATION:The dominantstructural influence inthe TPAO fields hasbeen the Adiyamanwrench-fault system.This has produced aleft-lateral

displacement and hasformed complexstructures, with faultedanticlines, which areoverthrust from northto south. Dip Trendstructural analysis ofdipmeter and FMS datahas helped to unravelsome of thecomplexities of thesestructures.

Fig. 2.17: How the

Adiyaman wrench-fault system hasdisplaced many ofthe imbricated andfaulted structures inTPAO's fields.

Fig. 2.18: Typical cross section through TPAO's fields showing the complex faulting that has taken place. (WER Structural GeologySupplement, 1990).

shows that the thrust faults later devel-oped a lateral shear component. It alsoindicated that there was a previouslyundiscovered reservoir fault block tothe south which is separated from Kas-tel Field by a normal fault. (Details ofthe complexity of Shells Beykan Fieldcan be found in Middle East Well Evalu-ation Review Structural Geology Supple-


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21Number 13, 1992.

ment, 1990).Borehole electrical imagery provides

more detailed structural information ona smaller scale than 3-D seismic sur-veys. Using these images, it has beenpossible to investigate the fracture sys-tems and unconformities which sepa-rate the main units in the Cretaceous

Mardin Group (the Areban, Sabunsuyu,Derdere and Karababa formations).The porosity of the Mardin Group

rocks is associated with unconformitiesand fracture-related dolomitization. Infact, fracturing is often critical to the via-bility of these reservoirs which normal-ly have a matrix porosity of less thanfive percent. In these fields, Dip Trendanalysis is proving to be invaluable toboth structural and fracture analysis asit highlights changes in dip associatedwith faults and folds. It also reveals theslight dip changes which are related tothe Mardin reservoir unconformities

which house different fracture types.In Mardin reservoirs, FMS data hasgiven a clear indication of fracture ori-entation. It has shown that there arenumerous changes in direction anddensity of fracturing intersected by thewells. There is also a wide variation infracture porosity and permeability withdepth.

Fig. 2.19: (Left): This structural interpretationof one of the wells in the Ozan SungurluField have revealed a complex mixture offaults, unconformities and fractures. Theanalysis was made using Dip Trend computersoftware and dip data derived from boreholeelectrical imagery (above). Polar projections

(below) of this data have helped in theidentification of structural trends and thisinformation is now being used to make a 3-Dstructural model of the field. This will alsoincorporate the various fault movementsover time.


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The Dip Trend analysis revealedthat the various fracture orientationsseen in each well can be related to spe-cific structural events. In addition, plotsderived from Dip Trend analysis werecorrelated with well logs which provedthat the highest fracture densities werelinked to some major faults, certain

lithologic units and unconformities.Changes in fracture orientations andfracture porosity seem to go hand-in-hand with increased fracture densityand proximity to unconformities. Thefracture widths increase in zones belowthe unconformities in the Mardin reser-voirs and this could be due to stressrelief and leaching along cracks.

In general, the tectonic fractures areindicative of tensional stresses andshow that the rocks probably failedduring uplift or due to deformationclose to faults. The widths of these frac-tures normally decrease with increased

fracture density - the reverse of what isseen in the cracking associated withunconformities.

A close look at drilling-induced frac-tures, coupled with an investigation ofborehole shape, has given a good indi-cation of the principal horizontal stressdirection across the field. Variations inthe stress direction often occurred nearmajor faults.

The Ozan Sungurlu 1 well to thenorth east of the study area containsthe greatest density and widest range offracture orientations. The strike his-tograms in figure 2.22 show that the ori-

entations vary from NW to ENE butthey remain sub-parallel to the majorfaults. The predominant NW fractureset is influenced by the principalregional stress - and this is also reflect-ed in the drilling-induced fractures. Thefracture orientation shows a sharpchange across the fault zone at 2,680 msuggesting that a different, more local-ized, stress regime exits on either sideof some of the main faults.

Fig. 2.21: A study ofthese drilling-induced fracturescoupled with aninvestigation ofborehole shape hasgiven a goodindication of theprincipal horizontalstress across theOzan SungurluField.

Fig. 2.20: TheseFMS imagesclearly reveal thefractured nature ofthe Mardinreservoirs. Thereare numerouschanges indirection and a

wide variety offracture types andsize with depth.The variousfractureorientations can berelated to specificstructural events.


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Supergiant Asmari reservoir Masjid-i-Sulaiman oilfield

Gachsaran Fm Asmari Fm Bangestan Group Khami Group

Agha Jari Fm

Irans fracturedformations

Some of the worldslargest reservoirs arecontained in the gentlefold belt that lies on thesouth western side of

Irans Zagros Mountains. The biggestreservoir is the Oligocene-MioceneAsmari Formation which is made of car-bonate rocks which were deposited justbefore the Tethys Sea closed.

An indication of the highly fracturednature of the carbonate Asmari fieldscame at the turn of the century whenoil was first discovered in Iran. Numer-ous gushers were found and all thewells had high production rates. By1920, pressure measurements at Masjid-i-Sulaiman Field indicated that reservoircommunication could only beexplained by fracturing - an important

observation that radically changed thefield development strategy. Instead ofdrilling many closely-spaced wells,fewer more widely spaced wells wereput down. Without knowing it, the engi-neers were sowing the seeds of modernreservoir management.

Rock fractures in the youngerAsmari reservoirs also provide theroute for fluid and pressure communi-cation with the older and deeper Creta-ceous (Bangestan Group) rocks. Thisexplains why Cretaceous oils are foundin the Asmari rocks.

The fracture systems in many ofthese reservoirs are complex. However,recent studies of a supergiant field,which lies further away from the Zagrosrange, have revealed a surprisinglyorderly fracture distribution. Most ofthe fractures that occur are in the cre-stal portions of the carbonate layersand it is thought that the interlayeredand highly porous reservoir sandstoneshave a dampening effect on the stressesin the anticline.

The distribution of fracture charac-teristics in the same field is neither uni-form, chaotic nor random. However,

the distribution can be investigatedusing FMS/FracView summary logs,and lithology and porosity details can

be obtained from log analysis. Most ofthe fractures are of tectonic origin but afew are karstic. As would be expected,the tectonic fractures are congregatedalong the crest and their orientation isparallel to the axis. There is a system-atic decrease in fracture characteristics(aperture, width, density and length)from the crest to the flanks.

The uppermost carbonate unit con-tains the most fractures. However, inthis and lower layers, the fracture pat-terns do not conform a 3-D fractureblock network that is traditionallyassumed during reservoir simulationand production analysis. Instead, theyhave a preferred orientation but rarelyintersect. This means that they increasethe vertical permeability of these tightrocks and provide a permeabilityanisotropy which runs parallel to thefields structural axis. Using todays hor-izontal drilling technology, it is now pos-

sible to intersect such fractured zonesand significantly improve well produc-tion.

Fig. 2.24: Studies of Asmari outcropsreveal similar fracture characteristics tothose seen in the studied supergiantfields. (WER IranSpecial Supplement, 1991).

Fig. 2.25: Geological cross section acrosssouth east Iran.

Recent FMS imagery has providedquantitative information about thefields fracture distribution. The flanks

of the field contain between 50 % and70 % fewer fractures than the crest andthere seems to be a gradual decreasefrom crest to flank. This suggests thatmost of the fractures occurred at thesame time rather than over a prolongedperiod.

Photo:BP.

SW


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25Number 13, 1992.

Asmari formation outcrops Intensely deformed core

Crest

Top carbonate

Porous sandstone

Second carbonate

60/0.49 mm

42/0.37 mm

94/0.80 mm

155/0.96 mm

Total number of fracturesand their aperture

Fig. 2.26: To the right is a textbook example of simple

extensional fracturing whichresulted from the gentle folding ofa supergiant Asmari reservoir. Thefractures are only present in thelimestone zones interlayered withhighly-porous sandstone zones.The tectonic fracture lengths,density and apertures decreasesystematically from the cresttowards the flanks. Karsticfracturing is also presentimmediately belowuncomformities (top right) formedduring times of low sea level of theNeo-Tethys Sea.

NW

X

Tectonic Karst

Y

0.2ms

0.2ms

0 180 360 0 180 360

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