January 2010 front pages.pmd

43Journal of Petroleum Geology, Vol. 33(1), January 2010, pp 43 - 66

© 2010 The Authors. Journal compilation © 2010 Scientific Press Ltd

DOLOMITIZATION AND ANHYDRITEPRECIPITATION IN PERMO-TRIASSICCARBONATES AT THE SOUTH PARSGASFIELD, OFFSHORE IRAN:CONTROLS ON RESERVOIR QUALITY

H. Rahimpour-Bonab+*, B. Esrafili-Dizaji*, and V. Tavakoli*

Dolomitization and related anhydrite cementation can complicate the characterization of carbonatereservoirs. Both processes have affected the Permo-Triassic Upper Dalan – Kangan carbonates,the main reservoir at the South Pars gasfield, offshore Iran. The carbonates were deposited in ashallow-marine ramp or epeiric platform and, according to previous studies, underwent intensenear-surface diagenesis and minor burial modification. Detailed petrographical and geochemicalanalyses indicate that dolomitization and anhydrite precipitation can be explained in terms of thesabkha/seepage-reflux models. The early dolomites then re-equilibrated or re-crystallized in ashallow burial setting. Evaluation of poroperm values in different reservoir intervals indicates thatreplacive dolomitization in the absence of anhydrite precipitation or with only patchy anhydritehas enhanced the reservoir quality. Where anhydrite cement is pervasive and has plugged therock fabric, poroperm values are significantly decreased.

As emphasized in previous studies and confirmed here, dolomitization and anhydrite cementation,together with original facies type, are the major factors controlling reservoir quality in the Dalan –Kangan carbonates at South Pars. When associated with minor anhydrite cementation, replacivedolomitization has enhanced reservoir quality by increasing permeability. However, porosity infabric-retentive dolomite was apparently inherited from the precursor rock and therefore reflectsthe original depositional environment.

Low-temperature dolomitization is commonly fabric-selective and partially fabric-retentive. Wholerock stable isotope thermometry indicates that fabric-destructive dolomites in the reservoir rocksformed at temperatures above 22ºC, whereas fabric-retentive dolomites and associated anhydritesformed in surface and near-surface conditions. Fabric-destructive dolomite or dolomite neomorphismpost-date fabric-retentive dolomite and continued to form in deep burial conditions (~1400m).These observations may explain why fabric-retentive dolomite and anhydrite fabrics are traversedby stylolites.

* Department of Geology, College of Science, University of Tehran, Iran.+ Author for correspondence, email: [email protected]

Key words: dolomitization, anhydrite precipitation, SouthPars, carbonates, diagenesis, reservoir characterization,Iran.

INTRODUCTION

Dolomitization and related anhydrite precipitationcommonly affect platform carbonates and can exert a

significant control on reservoir quality. Theseprocesses have been investigated in majorhydrocarbon reservoir rocks such as the PermianKhuff and Jurassic Arab Formations (Cantrell et al.,2004; Lindsay et al., 2006; Ehrenberg, 2006;

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44 Reservoir quality at the South Pars field, offshore Iran

Ehrenberg et al., 2006, 2007; Rahimpour-Bonab,2007; Ehrenberg et al., 2009; Maurer et al., 2009;Rahimpour-Bonab et al., 2009). The importance ofthese diagenetic processes on reservoir heterogeneityis widely recognized but the precise impact ofdolomitization and anhydrite precipitation onporoperm evolution is a matter of debate (e.g. Warren,2000; Lucia, 2004; Machel, 2004; Warren, 2006;Esrafili-Dizaji and Rahimpour-Bonab, 2009).

In most cases, dolomite porosity is inherited fromthe precursor limestone but is frequently reduced dueto cement precipitation (Lucia, 2004), althoughinstances are known in which dolostones have higherporosities than the precursor limestones (Machel,2004). Dolomitization can therefore have a variableeffect on poroperm values (Purser et al., 1994;Machel, 2004) and can enhance or reduce porositydepending on the mode and timing of thedolomitization process (Mazzullo, 1992).

Dolomitization is frequently associated withanhydrite precipitation, a process that commonly hasa negative effect on reservoir quality. However, Luciaand Ruppel (1996) and Lucia (1999) noted thatreplacive nodular and poikilotopic anhydrite had only

a minor influence on poroperm values in the SouthCowden carbonates (West Texas), and Lucia et al.(2004) showed that anhydrite fabrics can enhancereservoir quality. Thus the combined effects ofdolomitization and associated anhydrite precipitationon reservoir quality are complex and variable.

Permo-Triassic carbonates (“Khuff equivalent”)form the most important producing reservoir in thePersian Gulf and surrounding area (Ehrenberg et al.,2007; Bordenave, 2008). More than 25 non-associatedgas reservoirs producing from this interval are locatedon- and offshore Iran. The South Pars gasfield (Table1) was discovered in 1990 and mainly produces fromthe Upper Dalan Member and Kangan Formation(lateral equivalents of the Upper Khuff Formation)on a north-plunging anticline in the Qatar Arch (Fig.1). South Pars together with its extension in Qatar(North field) forms the largest natural gasaccumulation known.

Fig. 1. Location map of the South Pars and NorthDome fields where the world’s largest gasaccumulation is present in the Upper DalanMember – Kangan Formation. The location of 15exploration/appraisal wells in the South Pars field isshown. Data from ten of these wells is used in thispaper.

Table 1. General data for the South Pars gasfield.

General informationArea 37000 sq. km.Field Discovery 1990Proven Gas Reserves 280-500 TcfTrap type AnticlineTotal well drilled More than 80Depth to reservoir 2.5-3.2 km

Petroleum system elementsProducing horizons

Formation Upper Dalan and Kangan Fm.Lithology Carbonate and anhydriteAge Late Permian-Early Triassic

Seal rocksFormation Dashtak Fm.Lithology Shale and EvaporitesAge Middle-Late Triassic

Source rocksFormation Sarchahan Fm.Lithology Black shaleAge Early Silurian

Reservoir CharacteristicsAverage Gross 430 mAverage Net 328 mAverage Porosity (%)Arithmetic 9.72Geometric 5.46

Arithmetic 26.81Geometric 1.44Average Sw 0.14

Average Permeability (mD)

45H. Rahimpour-Bonab et al.

Previous studies have shown that carbonatereservoir rocks can be extremely heterogeneous innature (e.g. Ehrenberg, 2006; Rahimpour-Bonab,2007; Rahimpour-Bonab et al., 2009; Esrafili-Dizajiand Rahimpour-Bonab, 2009). More than 60% of thereservoir rocks at South Pars are dolomitized andanhydrite occurs in close association with thedolomites, reflecting the influence of hypersalinedepositional conditions on both calcium sulphateprecipitation and dolomitization (Ehrenberg, 2006).The purpose of this paper is to investigate the originand nature of dolomitization and anhydriteprecipitation in the reservoir carbonates at South Parsand to assess their impact on reservoir quality.

LOCATION AND REGIONAL GEOLOGY

The Qatar Arch, a NNE-SSW-trending positivetectonic feature of Infracambrian origin, divides thePersian Gulf into two troughs (the eastern and thewestern Hormuz Salt sub-basins) (Fig. 2). Thestructure of the arch was inherited from the Amar andNajd tectonic systems (Al-Husseini, 2000). Both theArch and the adjacent troughs have been rejuvenated

and uplifted repeatedly since the Early Silurian(Murris, 1980; Alsharhan and Nairn, 1997; Konert etal., 2001; Pollastro, 2003) (Fig. 2).

The troughs had different subsidence rates anddepositional histories during the Phanerozoic.Sediments tend to thicken east- and westwards, awayfrom the Qatar Arch. Sediments in the western sub-basin are 6.7 km thick, those in the eastern sub-basinare about 5.2 km thick (Fig. 2). At the present day, thethe eastern flank of the Qatar Arch forms a gentlydipping monocline. The western side is bounded byfaults (Kazerun Fault) and a series of steep-sidedanticlines (e.g. Konert et al, 2001; Alsharhan andNairn, 1997; Ziegler, 2001). Tectonic movementsduring the Late Precambrian–Early Cambrian (Najdfault system) in central Saudi Arabia causedreactivation of pre-existing fault systems resulting inregional uplift, and may have gently elevated structuralfeatures including the Qatar Arch (Al-Husseini, 2000;Murris, 1980). Both sub-basins were rejuvenatedduring the Silurian, resulting in the deposition of thinsource-rock intervals (Bordenave, 2008).

Thinning of Permian sediments may indicate theexistence of a syn-depositional structural high to the

Fig. 2. Sediments thicknesses in the subsurface of the Qatar Arch and Western and Eastern sub-basins, asestimated from regional isopach maps. Permian to Oligo-Miocene data are from Bahroudi and Talbot (2003),and Silurian data is from Bordenave (2008). The map above shows the boundaries of the InfracambrianHormuz salt basin (in white).


SE of the Zagros, or a block-faulted horst in the QatarArch area (Edgell, 1977; Kashfi, 1992). Post-Palaeozoic tectonic activity may have revived thisstructural high, as indicated by the erosion of LateTriassic units (Murris, 1980; Kashfi, 1992). The Archwas a positive structure during the Palaeozoic andgradually subsided during the Jurassic (Saint-Marc,1978). It was then active periodically throughout theMesozoic and Cenozoic, including the late Tertiarywhen sediments currently exposed at the surface weredeposited (Konert, 2001; Alsharhan and Nairn, 1997).The relatively small thickness of the sedimentary coverin the Qatar Arch (some 4 km) compared to adjacentareas (i.e. 7 to 14 km in the Zagros foldbelt) indicatesthat it has been a palaeohigh during most of thePhanerozoic.

The stratigraphic succession in the subsurface ofthe Qatar Arch is well-documented. Analogous rockscrop out in the Zagros Mountains, Central ArabianArch, and the central and northern Oman Mountains(Sharland et al., 2001; Alsharhan and Nairn, 1997).Most of the subsurface succession was deposited inshallow-marine conditions (limestones, dolomites,shales and evaporites) (Fig. 3).

SOUTH PARS GASFIELD

The Upper Dalan Member and Kangan Formation,the reservoir intervals at South Pars and North fields,are predominantly composed of dolomites with somegrain-rich limestone intervals (Alsharhan and Nairn,1997; Ehrenberg et al., 2007; Esrafili-Dizaji andRahimpour-Bonab, 2009). Although Jurassic-Cretaceous oil-bearing reservoirs (Khami andBangestan Groups) are also present in parts of theSouth Pars and North fields, the Upper Dalan –Kangan intervals form the main gas reservoir. AtSouth Pars, the interval can be divided into fourreservoir units: K4, K3, K2 and K1 (Fig. 3). The K1and K3 units are mainly composed of dolomites andanhydrites while K2 and K4, which constitute majorgas reservoirs, comprise limestone and dolomite. Amassive anhydrite (the Nar Member) separates theK4 from the underlying K5 unit which has poorreservoir qualities.

The gross pay zone in the South Pars field isapproximately 450 m thick, extending from depthsof approximately 2750 to 3200 m. Reservoir stratadip gently to the NE. The average thickness of thereservoir units declines from South Pars (some 450

Fig. 3. Generalized stratigraphy of the South Pars field showing lithostratigraphic units and depositionalenvironments. The thickness of the stratigraphic column is approximately 4 km; it is dominated by shallow-marine limestones, dolomites, shales and evaporites. Megasequences I-XI are after Alavi (2004, not to scale).There is uncertainty about the presence of Cambro-Ordovician and Silurian sediments in the Qatar Arch.The stratigraphic position of the Upper Dalan and Kangan carbonates, the focus of this paper, is highlighted.


m) to North field (385 m). As in other reservoirstructures in neighboring areas, the reservoir in theQatar Arch is cut by a set of NNW-SSE trending faults.

Non-associated gas reservoirs hosted by equivalentPermo-Triassic carbonates are located onshore in theFars Platform of Iran and include Aghar, Homa, Nar,Kangan, Day, Sepid Zakhur, Shanul, Lamard,Assaluyeh, Tabnak and Varavi. These fields showsimilar properties and petroleum system elements. Weestimate that the Qatar Arch contains more than 1300TCF of gas reserves, or some 20% of global provennatural gas reserves.

MATERIALS AND METHODS

This study is based on data from 15 exploration/appraisal wells (Fig. 1) in the South Pars gasfield.Analyses of core materials included thin-sectionpetrographic examination, stable isotopemeasurements of bulk samples (δ18O, δ13C and 87Sr/86Sr values), SEM analyses, X-ray and CT scans pluscore poroperm measurements. Core descriptions anddetailed thin section analyses were carried out onseven wells (A, B, C, D, K, F and H), which weresampled at intervals ranging from 0.3 to 1 m. All thinsections were half-stained with Alizarin Red-S.

Geochemical characteristics were obtained fromanalyses of bulk core samples from three other wells.Some 248 core samples from two wells (163 samplesfrom well E and 185 samples from well F) wereselected for carbon and oxygen stable isotope analyses(δ13C and δ18O). All samples were powdered and thensent to Texas A&M University for stable isotopemeasurement. Thirty-two samples were selected forSr isotope analysis from anhydrites and anhydriticdolomites in the K4, K3 and K2 units in well K. Foranalysis of anhydrite cement distribution at a plugscale, X-ray CT scan data (n = 32) were used.Secondary electron and back-scattered images (n =240) were used for detailed investigation of the effectsof dolomitization and anhydrite precipitation onreservoir properties.

For facies analysis, a modified Dunham texturescheme was used together with sedimentary structuresand fabrics, grain size, rock composition, anddiagnostic allochems such as ooids, pelloids andshells. Rock fabric was separated into grain- and mud-dominated fabrics (Lucia, 1999), which are minormodifications of the Dunham (1962) classification.In this study, 14 facies types were distinguished andare described and interpreted in Table 2. Detaileddescriptions of these facies were reviewed by Esrafili-Dizaji and Rahimpour-Bonab (2009).

The depositional environment of thelithostratigraphic equivalent of the studied interval(Khuff Formation) is comparable to that of the present-

day Persian Gulf (Alsharhan and Kendall, 2003). Thecarbonates studied were deposited in the inner part ofan epeiric or ramp-like carbonate system.

DIAGENETIC SEQUENCEAND POROSITY VARIATIONS

The interpreted sequence of diagenetic events, inferredfrom petrographic relationships and geochemicalevidence, is summarized in Fig. 4. These processeshave also been distinguished and discussed by otherworkers such as Ehrenberg (2006), Moradpour et al.,(2008), and Esrafili-Dizaji and Rahimpour-Bonab(2009). Core examinations together with detailed thinsection analyses indicate that many of the diageneticprocesses overlapped in time. Three major diageneticenvironments were recognized (Figs. 4 and 5) and areanalogous to those in Khuff reservoirs in the UnitedArab Emirates (Alsharhan, 2006).

(1) Marine and syndepositional diagenesis:Hypersaline conditions occurred in landward locationson the carbonate platform (e.g. in restricted lagoons).Calcite cementation occurs in lagoon and shoal facies(Figs. 5A, B, 6A).This diagenetic phase ischaracterised by the presence of isopachous calcitecement, micritization and early dolomitization withanhydrite cementation and nodule formation.

(2) Meteoric diagenesis: subaerial exposure duringsea-level lowstands caused extension of a meteoriclens into the near-shore and shallow-marine realms.The effects of this diagenetic mechanism on non-dolomitized deposits (mainly open-lagoon, shoal andoffshoal facies) was more important than ondolomitized intervals. Calcite is more reactive andunstable than dolomite in these conditions (Moore,2001). Meteoric diagenetic features includingdissolution, drusy and blocky calcite cement, togetherwith aragonite stabilization and neomorphismtherefore occur mostly in limestones (i.e. basinwardfacies) (Figs. 5C, D and 6B).

(3) Burial diagenesis: Alteration andrecrystallization of replacive dolomites led to theformation of fabric-destructive (coarse and idiotopic)dolomite bodies (Figs. 6C). Late stage anhydrite andcalcite cement occluded early porosity and latefractures, which are locally important. Fracturing andsaddle dolomite cementation occurred during latephases of burial diagenesis.

In general, diagenetic processes were facies-selective and limited by stratigraphic boundaries at afield scale. Hypersaline and meteoric diageneticrealms can be differentiated clearly (Fig. 5). Thehypersaline diagenetic zone coincides with peritidaland lagoonal facies; pervasive dolomitization,anhydrite cementation and nodule formation are themain diagenetic features in this zone (Figs. 5A and


B). Meteoric diagenetic overprints mainly occur inopen lagoon, shoal and offshoal facies, and includearagonite stabilization, calcite cementation, dissolutionand neomorphic processes.

The hypersaline diagenetic zone passes into themeteoric realm via a narrow transitional zone showingselective replacive dolomitization with or without rareanhydrite cements (Figs. 5B, C and 6A). In this zone,metastable grains (aragonitic bioclasts and ooids) weredolomitized preferentially. Although both zones wereoverprinted by late burial diagenetic processes, theirinfluences on reservoir qualities are important. Ingeneral, the original depositional facies controlleddiagenesis and thus porosity.

In addition to primary pore preservation, twoprocesses were responsible for the creation of porosity

and permeability in the Permo-Triassic carbonates atSouth Pars: dolomitization (discussed below) anddissolution. By contrast cementation and compactionhad negative effects on poroperm characteristics.

Dissolution resulted in the generation of mouldicpores especially in open lagoon, shoal and offshoalfacies (Figs. 5D and 6B). Much of this leaching isthought to have occurred in near-surface conditionsduring sea-level lowstands, with exposure and theinfiltration of fresh waters in the vadose zone.

Based on our detailed visual observations of coresand thin sections and on previous studies (e.g.Ehrenberg et al., 2008), the main cement types includecalcite which occurs in high energy limestones (Fig.6A); and anhydrite which mostly occurs in dolomiticfacies (Figs. 5A; 6D and E). The latter is more

Fig. 4. Diagenetic sequences and porosity evolution in the Upper Dalan-Kangan carbonates in the South Parsfield. Three major diagenetic environments (marine-syndepositional, meteoric and burial) were recognizedfrom the paragenetic sequence and from cross-cutting relationships. Based on petrographic examinations, theevolution of porosity is mainly related to diagenetic processes.


Facies code Name Mineralogy, texture and color

Dominant sedimentary structure

Grain size and sorting

Component, frequency, occurrence

Associated facies Interpretation

F1 Crystalline anhydrite Anhydrite, crystalline, white (light)

Massive, layered, chicken-wire - - F2, F3, F4, F5 Supratidal and

salina (sabkha)

F2 Nodular dolomudstoneDolomite with

anhydrite, mudstone, brown to light

Nodular fabric Calcilutite Pelloid and bioclast (r) F1, F3, F4, F5 Supratidal (sabkha)

DolomiteMudstone, light (brown-yellow)

DolomiteMudstone, light (brown-yellow)

Stromatolite and Thrombolite boundstone

Dolomite and limestone, dark to

brownMassive - Ostracods,

foraminifera, pelloidsF1, F2, F3,

F6, F7 Intertidal

F6 Fenestral dolomudstone Dolomite, mudstone, dark brown Fenestral fabric Calcilutite Bioclast and pelloid (r) F5, F7, F8 Intertidal

F7Pelloid/intraclast

packstone and grainstone

Dolomite, packstone and grainstone, light

Massive, grain grading and orientation

Calcarenite, poor to moderately

sorted

Intraclast, pelloid (a), shell (r) F5, F6, F8 Intertidal

channel and bar

Dolomite, wackestone and Lamination

Mudstone, dark brown (Bioturbation,

Burrowing)Dolomite (limestone)

packstone and wackestone,

Bioturbation,

dark brown Burrowing

Bioturbation,

Burrowing

F11 Ooid mudstone /wackestone

Limestone, mudstone and wackestone, light

yellowMassive Calcilutite, poorly

sorted Ooid and bioclast (c) F8, F9, F10, F11 and F12 Back shoal

F12 Ooid/bioclast grainstone/packstone

Limestone, packstone and grainstone, light

yellow

Cross bedding/ Grain grading and orientation

Calcarenite, well sorted Ooid (a), bioclast (c) F8, F9, F10,

F11, F13Central shoal

body

F13 Coarse bioclast-intraclast grainstone

Limestone, grainstone, light yellow to brown

Cross bedding/ Grain grading and orientation

Calcarenite, calcirudite, well to moderately sorted

Bioclast (a), Intraclast (c), ooid, pelloid (c)

F9, F10, F11, F14 Fore shoal

F14 Very fine bioclast mudstone

Limestone, mudstone, dark brown Lamination Calcilutite, poorly

sorted Bioclast (c) F10, F11, F13 Shallow mid ramp

y: (r) rare; (c) common; (a) abundant.

F7, F8, F10Subtidal

(protected lagoon)

F10 Poorly sorted bioclast packstone

Limestone (Dolomite),

packstone, light yellow

Calcarenite, poorly sorted

Bioclast (a), pelloid, ooid, Green Algea,

Foram (c)

F8, F9, F11, F12

Subtidal (open lagoon)

F9 Pelloid-bioclast packstone/wackestone

Calcarenite, moderatly sorted

Pelloid and bioclast (a), Green Algea,

Foram (c)

F8 Fine grain pelloid wackestone/mudstone

Calcilutite, poorly sorted

Micropelloid (a), Miliolida, Gastropoda, ostracoda (c), shell (r)

F5, F6, F7, F9Subtidal

(protected lagoon)

Biolamination/ layered Shell (r)

F5

Ostracods and Algal filaments (r) F1, F2, F4 Peritidal

(intertidal ponds)

F4 Dolomudstone or Dolomicrite

Massive and Homogenous Calcilutite - F1, F2, F3 Peritidal

(intertidal ponds)

F3 Dolomudstone with sparse anhydrite crystals Massive Calcilutite

Table 2. Facies descriptions and interpretation in the Upper Permian Upper Dalan Member and LowerTriassic Kangan Formation.

Fig. 5. Hypersaline and meteoric diagenetic zones distinguished from core and thin-section analyses. In thehypersaline zone, which coincides with the peritidal and closed lagoon facies, dolomitization, anhydritecementation and nodule formation occur (plates A and B). Meteoric diagenetic features, which occur in openlagoon, shoal and offshoal facies, include aragonite stabilization, calcite cementation, dissolution andneomorphic processes (plates C and D).


Fig. 6. Photomicrographs of diagenetic features in the Upper Dalan - Kangan carbonates which influencereservoir quality.A. Calcite cementation (a), dissolution (b) and dolomitization (c): these features occur in lagoonal and shoalfacies.B. Extensive dissolution occurred during meteoric diagenesis (a) together with drusy calcite cementation (b)and neomorphism (c).C. Alteration and recrystallization of replacive dolomites led to the formation of fabric-destructive (coarseand idiotopic) dolomites.D. Anhydrite cementation mostly occurs in dolomitic grainstone facies; alteration and recrystallization ofreplacive dolomites led to the formation of fabric-destructive dolomites.E. Late-stage anhydrite cement resulted in the partial occlusion of secondary dissolution and fracture porespaces.F. Fenestral porosity in tidal flat peloidal-algal wackstone.


significant as a control on reservoir quality. Anhydriteoccurs as pore-filling cement in tidal-flat and lagoonaldeposits, and anhydrite precipitation appears to haveoccurred at two different stages. Later-stage anhydritesresulted in the partial occlusion of secondarydissolution and fracture pore spaces (Fig. 6E).

In the studied interval, more than 60% of existingporosity in porous lithologies, is diagenetic(secondary) in origin (as indicated by Digital ImageAnalysis: Esrafili-Dizaji and Rahimpour-Bonab,2009). Porosity types in the studied intervals includedmouldic, interparticle and intercrystalline types, whilefracture, stylolitic and fenestral porosity was locallysignificant (Figs 5D, 6F and 7A,B). Assuming thatthe precursor limestones had porosities similar to thosein shallow-marine Cenozoic limestones (average of30–50%; Enos and Sawatsky, 1981; Schmoker andHalley, 1982), its porosity has decreased during

Fig. 7. Thin-section photomicrographs showing examples of different dolomite types.A. Dolomitized grainstone with slightly-compacted fabric (a, point contacts) which is associated withinterparticle porosity filled by anhydrite (b).B. Dolomitized packstone with fine to medium dolomite crystals which is partially fabric retentive.C. Fabric-destructive dolomite with subhedral to euhedral crystals.D. Coarse and anhedral crystals of saddle dolomite cement (a) which appears as void and fracture fills.

progressive diagenesis. In the reservoir rocks,diagenetically altered facies have 9% porosity and 26mD permeability on arithmetic average (Fig. 4).

This paper focusses on the influence ofdolomitization and anhydrite cementation on theporoperm characteristics of the carbonate reservoirrocks at South Pars.

DOLOMITIZATION ANDRESERVOIR QUALITYDolomite petrography

Three dolomite types can be distinguished in theUpper Dalan – Kangan carbonates at South Pars: (1)fabric-retentive, (2) fabric-destructive, and (3)cements. These types can be identified on the basis oftexture (crystal size, shape and distribution) and SEMcharacteristics (Figs 7 and 8).


Fabric-retentive dolomiteCore examinations and thin section analyses indicatethat about 95% of dolomites in the studied intervalsare fabric-retentive. These dolomites are micro-crystalline to finely crystalline (mean size < 0.2 mm)and comprise subhedral to anhedral crystals (Figs 7A, B). Fine-scale depositional structures (e.g.lamination, fenestral fabric and bioturbation) andfossils (ostracods, gastropods, miliolids, green algae)are well-preserved, as are the original pore properties(type, shape and distribution) of the precursorlimestone. This type of dolomitization mostly occurswithin low-energy landward facies (F1 to F9), butfacies F10 to F12 may also be partially to completelydolomitized. Dolomite fabrics do not vary withdepositional facies. Dolostones change gradationallyinto limestones via partially dolomitized limestoneswhich in general contain fabric-retentive dolomites.There is a close association between this type ofdolomite and anhydrite nodules and cement.

The occurrence of slightly-compacted contactsbetween dolomitized grains suggests thatdolomitization took place before significant burial.These dolomites are post-dated by cross-cuttingstylolites and fractures, which in turn are filled by late-stage calcite, anhydrite and dolomite cements.

Fabric-destructive dolomiteThis type of dolomite is volumetrically of minorimportance (<4%) and takes the form of fine, mediumto coarse crystals that range in size from 0.4 to 0.9mm, and which are commonly present as mosaics ofeuhedral or subhedral crystals which may almostcompletely obliterate the precursor limestone textures.Dolomite crystals commonly have cloudy (inclusion-rich) cores and relatively clear outer rims.Identification of depositional features and facies isdifficult.

This type of dolomite is mainly associated withcoarse-grained lagoon facies (such as F9, F10). In mostcases, the boundary between fabric-destructive andfabric-retentive types is continuous (particularly in themiddle part of the K4 unit) (Fig. 7C).

Fractures cross-cut this type of dolomite. Thepresence of stylolite ghosts in this dolomite typeindicates that it post-dated chemical compaction

Dolomite cementsDolomite cements (Fig. 7D) are volumetricallyinsignificant (<1%). Two types were distinguished.Type A cement crystals are clear and euhedral,frequently occurring in mouldic pore spaces (meansize < 0.1 mm) (Fig. 8). Type B is composed of coarse

Fig. 8. Calcite and dolomite cements (A-type) whose composition and morphology are revealed by SEManalysis. These two types of cements are associated with mouldic pores. A-type dolomite cements have aeuhedral fabric and pore filling nature (C = calcite, D = dolomite).


crystals (mean size <1 mm) and is usually present infractures. This type of dolomite cements displaystypical “saddle dolomite” characteristics such assweeping extinction, cloudy appearance, curvedsurfaces and cleavages (Fig. 7D). Dolomite cementsare not discussed further.

Dolomite and calcite geochemistryThe isotopic composition of 248 carbonate samples(excluding dolomite cements) were analysed in termsof their δ18OPDB and δ13CPDB values (Figs. 9 and10). Dolomites show δ18O values ranging from -5.2to 3.9‰ and δ13C values from +1.8 to +6.8‰ in theUpper Dalan Member. In the Kangan Formation,dolomite δ18O values range from -6.4 to -1.8‰ andδ13C from -0.8 to +2.5‰.

Fabric-retentive dolomiteThe Permian dolomites from the Dalan Member (K3and K4 units) have δ18O and δ13C values ranging from-0.3 to 3.9‰ (average = 1.65‰ ) and 1.6‰ to 6.8‰

(average = 4.29‰ ) PDB, respectively (n = 81, Fig.9). Compared with Permian marine dolomites (valuesfrom Veizer et al., 1986), the measured values areeither similar or slightly higher for δ18O and lower forδ13C.

The isotopic values for Triassic dolomites of theKangan Formation (K1 and K2 units) range from -3.4‰ to -1.8‰ (average = -2.61‰) for δ18O, and -0.3‰ to 2.5‰ (average = 0.18‰) for δ13C. The δ18Ovalues for these dolomites, however, are more negativethan for Triassic marine dolomites (from Schauer andAigner, 1997). δ13C values for this interval are similarto the theoretical values (Fig. 10).

Fabric-destructive dolomiteThe thirteen samples of fabric-destructive dolomitefrom the Permian Dalan Member analyzed hadδ18OPDB values of -5.3 ‰ to -1.2 ‰ (average= -3.77‰), and δ13CPDB values of 3.4 ‰ to 5.8 ‰ (average=3.34 ‰). δ13C values of these dolomites are similar tothose expected from Permian marine dolomite, but

Fig. 9. Carbon/oxygen isotope plotfor different types of dolomite vs.calcite in the Permian UpperDalan Member. Values aredisplayed in standard deltanotation relative to the PDBstandard. Isotopic composition ofPermian marine dolomites isbased on Amthor and Okkerman(1998). Estimates for the isotopiccomposition of Permian marinecalcite are calculated usingδ18Odol-calcite = 3‰ (Land, 1980).Equilibrium fractionation of δ13Cbetween calcite and dolomite isnegligible, so both minerals havenearly equal δ13C compositions.

Fig. 10. Carbon and oxygenisotope plot for two types ofdolomite versus calcite in theTriassic Kangan Formation(theoretical values for Triassicmarine dolomites and calcitesfrom Schauer and Aigner, 1997).


their δ18O values are depleted in comparison withtheoretical values (Fig. 9). Likewise, Triassic fabric-destructive dolomites of the Kangan Formation (n =7 samples, average δ18O PDB = -4.83‰ and averageδ13CPDB = 0.53‰) show similar results (Fig. 10).

Calcite or limestoneThe stable isotope analysis of calcites in the DalanMember show positive values for δ13CPDB (average= 5.06 ‰) and negative values for δ18OPDB (average= -4.31 ‰), comparable to the theoretical values ofPermian marine calcites (n = 51 samples, ranging from-1.3‰ to -5.9‰ for δ18O, and from -0.1 to +6.9‰ forδ13C values, respectively) (Fig. 9). Measured δ18OPDBvalues for the Kangan calcites are more negative thanthose expected from Triassic marine calcite, but δ13Cvalues are either similar to, or slightly lower than,theoretical values (average δ18OPDB= -5.68‰ andaverage δ13CPDB= +0.37‰) (Fig. 10).

Dolomite poroperm characteristicsPetrophysical data from core samples was used toassess the texture-related variability of reservoirproperties and the identification of dolomitizationeffects (Fig. 11). There are no significant differencesbetween the poroperm values of the two dolomitetypes (Fig. 11). As mentioned above, most dolomitesin the South Pars field are fabric-retentive, andtextures range from grainstone to mudstone. Theheterogeneity in dolomite poroperm characteristicsappears to be inherited from the precursor limestones.Therefore it is necessary to examine the pre-existingtexture of this dolomite together with the equivalenttexture in the non-dolomitized limestones. Fig. 12 andTable 3 illustrate the poroperm characteristics ofdifferent dolomite textures versus that of equivalentlimestones. As shown there are systematic differencesin the porosity-permeability distribution ofdolomitized and non-dolomitized textures. In generaldolomitized samples have higher permeabilities for a

Porosity (%)

Permeability (mD)

Porosity (%)

Permeability (mD)

Non-dolomitized 23.13 32.22 20.5 3.83 0.2Dolomitized 15.53 108.78 13.82 26.4 0.25




Grainstone

Packstone

Wackestone

Mudstone

Rock texture and lithologyArithmetic mean Geometric mean Coefficient of

determination (r2)

Fig. 11. Permeability versus porosity for reservoir dolomites. Plotting symbols indicate fabric-retentive andfabric-destructive dolomites. As shown these dolomites do not plot within discrete fields.

Table 3. Statistical parameters of each texture according to the means of porosity and permeability. Ingeneral, dolomitized samples have rather higher permeability than the non-dolomitized textures.


Fig. 12. Porosity–permeability cross-plot based on the dolomitized and non-dolomitized texture types.Comparison of the reservoir properties in the various textures indicates that dolomitized textures havebetter reservoir quality than the limestones (non-dolomitized textures).

given porosity compared to limestones but thecorrelation coefficients are variable (r2 = 0.10–0.68).

In carbonate reservoirs, dolomite stable carbon andoxygen isotope compositions can contribute to anunderstanding of porosity development (e.g. Al-Aasmand Azmy, 1996; Saller and Henderson, 1998). Theporoperm values of Triassic calcite and co-occurringfabric-retentive dolomites which clearly showdifferent δ13C and δ18O compositions (from Fig. 10)are plotted and compared in Fig. 13. As shown, in thisformation (particularly in unit K2), heavier δ18O valuesfor dolomite generally correspond with higherporoperm values. In addition, there is a positivecorrelation between low poroperm values and low δ18Ovalues for calcite (Fig. 13). Diagenetic alterations inlimestones are generally associated with the formationof calcite cement with light δ18O values. The calcitecementation occludes primary porosity and decreasesthe original poroperm values. In this way, meteoriccalcite cementation results in a decrease of bulk δ18Ovalues of the rock.

In contrast to the Triassic Kangan Formation, theoxygen isotope compositions of the Upper Dalandolomites are generally lighter (lower) in samples withhigher porosities, particularly in the K3 unit of theUpper Dalan (Fig. 14). This inverse relationshipbetween δ18O values and dolomite porosity (Fig. 14)demonstrates the impact of increasing diageneticalteration on reservoir rock porosity.

Anhydrite: petrography, geochemistry andreservoir qualityPetrographyFour types of anhydrite texture were observed in thecarbonate reservoirs at South Pars (Fig. 15): (1)bedded (depositional) anhydrite (2) pore-occludinganhydrite cements, (3) poikilotopic anhydrite crystals,and (4) replacive anhydrite nodules (c.f. Lucia, 1999).

Anhydrite cements are either simply pore-fillingor poikilotopic in nature (the former does not showpoikilotopic textures). As previously shown byEhrenberg et al. (2008), our thin section and core


Fig. 13. Plot of the porosity versus permeability forcalcite and fabric retentive dolomites of the KanganFormation in the South Pars field. Poroperm valuesfor samples in Fig. 10 are used here. As shown,there is a general positive correlation between low-poroperm and low-isotope values in dolomite andlimestone lithologies (exponential regression lines).

Fig. 14. Plot of porosity versusoxygen stable isotope compositionof dolomites in the Upper Dalanunit, South Pars field. In general,δ18O values decrease with increasein porosity. Correlation coefficient(r2) for δ18O versus porosity is 0.60.

Fig. 15. Four anhydrite textural types are distinguished and classified in the reservoir intervals based on coreanalyses and thin section examinations: (A) pore-filling texture; (B) poikilotopic texture, (a: anhydrite cement)(C) bedded texture and (D) nodular texture.


analyses indicate that anhydrite cements and nodulesare generally associated with dolomitic intervals(supratidal, intertidal and lagoon facies: F1 to F9).Bedded and nodular anhydrite were associated withsupratidal facies (F1 and F2), and pore-filling andpoikilotopic textures were common in lagoonal andintertidal facies (Fig. 15). Anhydrite cementscommonly appear as primary pore fillings in slightly-compacted grainy dolomites. In general, stylolitesshow cross-cutting relationships with dolomite andanhydrite crystals (and anhydrite nodules). However,these crystals were not formed along stylolites. Thesetextures are depositional and early diagenetic(syndepositional). In some cases, anhydrite cementshave occluded solution and fracture pore spaces.

Anhydrite geochemistry and datingThe 87Sr/86Sr ratios of 32 anhydrite and anhydrite-cemented samples (reservoir and seal rocks) plotwithin the range 0.7071 to 0.7081 (Fig. 16), and exceptfor the Kangan (K2) samples show values close tothat of Late Permian – Early Triassic seawater(McArthur, 2001; McArthur and Howarth, 2004).

According to the timescale in Al-Husseini (2008),the lower part of the Dashtak Formation, whichoverlies the carbonate reservoir interval at South Pars,is dated 249.7 Ma. The Upper Dalan – Kanganreservoir interval is therefore dated at 259.1 to 249.7Ma. There is a good correlation between these agesand those derived from Sr isotope dating, particularlyfor the Upper Dalan and Dashtak Formations. TheKangan Formation lies between the Dalan andDashtak Formations in the lithostratigraphicframework, but 87Sr/86Sr ratios for samples of thisinterval overlap with the Dalan samples. Petrographicanalyses and stable isotopic ratios indicate thatmeteoric diagenesis has affected Sr isotopic signatures(Rahimpour-Bonab et al., 2009). The depositional

ages of the reservoir intervals range from 259.1 to250 Ma (Fig. 16).

Strontium isotope values for the reservoir rocksclosely follow the trend of global seawatercomposition for the Late Permian – Early Triassic(except for the Kangan samples). This supports thedirect involvement of seawater as the original fluidfor anhydrite and dolomite formation.

Anhydrite poroperm characteristicsThere is a clear relationship between anhydritic textureand reservoir quality (Fig. 17 and Table 4). Intervalswith a poikilotopic anhydrite texture show relativelyhigh poroperm values (r2 ranges between 0.05 and0.68). If a permeability of 1 mD is used as the thresholdfor a gas reservoir, only samples with a poikilotopictexture in general have higher permeabilities than this.

The anhydrite cement distribution controls theporoperm heterogeneity of the anhydritic units. Onthe basis of the anhydrite cement distribution (at thecore, plug and thin-section scales: Fig. 18), theseintervals can be divided into areas with either patchyor uniform anhydrite cementation. Patchy anhydrite(nodular and poikilotopic) results from localizedmineral precipitation. The distribution of anhydritecement has a significant effect on porosity-permeability relationships (Lucia, 1999), andporoperm values are significantly higher with patchyanhydrite than where cementation is uniform (Fig. 19,Table 5).

ORIGIN OF DOLOMITEAND ANHYDRITE GENERATIONPetrographical evidence

In general, the Late Permian Dalan and Early TriassicKangan Formations consist of a series of cyclic unitsbeginning with non-dolomitized to partially

Fig. 16. Comparison of 87Sr/86Sr data for core samples of reservoir and seal rocks with the secular Sr isotopiccurve of seawater from 200 Ma to 280 Ma (McArthur and Howarth, 2004). 250Ma marks the boundarybetween the Permian and the Triassic.


dolomitized subtidal grainstones to packstones(offshoal and shoal facies), passing up into lagoonal/intertidal dolomites (anhydrite-cemented facies) andcapped by supratidal or dolomitic anhydrite(Ehrenberg et al., 2006; Esrafili-Dizaji andRahimpour-Bonab, 2009) (Table 2). Replacivedolomites commonly occur in restricted peritidal andlagoonal depositional facies, and dolomitized intervalsare associated with non-fossiliferrous facies or facieswith restricted marine biota. Therefore, the main areasfor anhydrite cementation / dolomitization werelandward of the studied carbonate platform. Variationsin the saturation state of dolomitizing fluids arereflected in the diagenetic zonation of the reservoirrocks. This variation ranges from intervals withpervasive dolomite-anhydrite cementation, to partiallydolomitized facies without anhydrite cements, to non-dolomitized intervals (i.e. limestone). The anhydritecementation / fabric-retentive dolomitization occurredduring syndepositional to shallow-burial conditionsbefore significant burial as indicated by the followingobservations:

(1) fine crystals of fabric-selective dolomite occur

Fig. 17. Porosity-permeability cross-plot of various anhydrite textures in the reservoir rocks of the UpperDalan – Kangan succession. Poikilotopic anhydrite texture shows better reservoir quality (generally k >1 mD)than other anhydrite textures (generally k < 1 mD).

Porosity Permeability

(%) (mD)Poikilotopic 15.69 50.31 13.51 9.38 0.05

Nodular 3.06 1.39 2.36 0.04 0.2Pore filling 3.92 0.95 2.5 0.08 0.17

Bedded 2.26 0.19 1.61 0.04 0.68

Anhydrite texture

Arithmetic mean Geometric mean Coefficient of determination

(r2)Porosity

(%)Permeability

(md)

Table 4. Statistical parameters of anhydrite texture according to the means of porosity and permeability.Apart from poikilotopic anhydrite texture, other textures have low reservoir quality in the reservoir rock.

in early pore spaces;(2) this dolomitization preserved depositional and

early diagenetic characteristics (bioturbation,micritization and marine cementation);

(3) stylolitization and fracturing post-date fabric-retentive dolomitization and anhydrite nodules/cements;

(4) anhydrite cementation occurred after orcoincided with fabric-selective dolomites (Fig.15A, B);

(5) anhydrite fabrics associated with replacivedolomites are cut by stylolites.

This petrographic evidence suggests that fabric-retentive dolomite and anhydrite fabrics predatemechanical and chemical compaction. Finelycrystalline dolomites which are traversed by stylolitesformed at relatively low temperatures and shallow-to-intermediate burial depths (around 50 ºC and belowthe 1000 m) (Sibley and Gregg, 1987; Lind, 1993;Kirmaci and Akda, 2005). Therefore, these phases areearly diagenetic. Some late anhydrite cements occurand occlude diagenetic pore spaces (mouldic andfracture porosity).

Compared with fabric-retentive dolomites, fabric-


Fig. 18. Patchy and uniform anhydrite crystals distribution (A and B, respectively) in an X-ray CT scan, SEM(back-scattered) and thin-section photomicrographs.

destructive types probably formed during deeperburial because they post-date stylolitization, as theyshow stylolites “ghosts”. Crystals have cloudy(inclusion-rich) cores and clear (inclusion-free) rims.Euhedral shapes and enlarged crystal sizes result fromincreasing burial depths and temperatures. There areno sharp boundaries, petrographical or geochemical,between fabric-retentive and fabric-destructivedolomites. The transition from fabric-retentive tofabric-destructive fabrics suggests that the latterdolomite formed by recrystallization of the earlierfabric-retentive dolomite (Tucker and Wright, 1990;Melim and Scholle, 2002). Dolomite recrystallizationcan be controlled by heterogeneities in mineralogy

and permeability in lagoonal facies (Machel andHubscher, 2000). Thus, replacive dolomitization hasnot occurred in two separate stages which would showa transitional boundary.

Analogous evidence for dolomite recrystallizationduring burial has been reported from the Khuffcarbonates offshore Dubai (Videtich, 1994), andsimilar dolomite cements occur in reservoir rocks fromthe east of the Qatar Arch (Alsharhan, 2006).

Geochemical evidenceUnder evaporative conditions, dolomite crystals havegrain sizes of less than 5 mm; they are non-stoichiometric and show slightly positive δ18O values


(e.g. McKenzie et al., 1980; Schauer and Aigner,1997). Because most of the Dalan fabric-retentivedolomites show higher δ18O values than their Permianmarine counterparts (Fig. 9) and have a lowtemperature of formation (from assumed values ca.25ºC), they probably formed from concentratedseawaters which have higher δ18O values (Lloyd,1966). The Dalan fabric-retentive dolomites have afine crystal size and positive δ18O values (close totheoretical values for Permian marine dolomites),indicating a seawater origin for the dolomitizing fluidsunder evaporative conditions.

An increase in crystal size, associated withdepletion in δ18O values (negative δ18O) in fabric-destructive dolomites (in both Dalan and Kangansamples) may be explained by two mechanisms: (1)meteoric water input and (2) recrystallization underburial conditions. Measured positive carbon valuesare typical for inorganic precipitates from marine orseawater-rich pore-fluids, suggesting that meteoricgroundwaters were not involved in dolomitization asextensive meteoric influx would have led to depletionin δ13C (Kirmaci and Akda, 2005).

Strong depletion in δ13C, δ18O, and 87Sr/86Sr in theKangan dolomites and calcites could be explained bythe mass extinction at the Permian-Triassic boundaryas well as diagenetic alteration (Rahimpour-Bonab et

al., 2009). The observed isotopic depletions reflectexcursions typical of other P-T sections and the effectsof diagenetic alterations. According to the oxygenisotopic ratio of dolomites (Allan and Wiggins, 1993),three temperature-related dolomitic zones can bedistinguished: a low temperature dolomite zone withδ18O values ranging from positive values to -2.5‰; azone of high temperature dolomite with negative δ13Cvalues down to -6.5‰; and a zone of intermediatetemperature dolomites (δ18O values from -2.5 to -6.5‰). Empirical evidence suggests that temperaturesof 50 to 80ºC mark the approximate boundary betweenhigh and low temperature dolomites (Machel, 2004).

δ18O values of reservoir rocks indicate that hightemperature dolomites are absent from the studiedintervals. Fabric-retentive dolomites in the DalanMember (and a few samples of fabric destructivedolomites), and most Kangan Formation fabric-retentive dolomites, formed in low temperatureconditions. Most fabric-destructive dolomites in bothDalan and Kangan Formations, and some fabric-retentive dolomites in the Kangan Formation, formedin intermediate temperature conditions. Thus,excluding dolomite cements, all the replacivedolomites in the Dalan – Kangan succession formedat low temperatures, between 50 to 80ºC. As pointedout by Machel (2004) and seen in this study, low-

Porosity (%) Permeability (mD) Porosity (%) Permeability (mD)Patchy 15.89 43.34 13.67 5.89 0.08

Uniform 3.28 0.7 2.25 0.1 0.17

Cement distribution

Arithmetic mean Geometric mean Coefficient of determination (r2)

Table. 5. Statistical parameters of anhydrite distribution texture according to the means of porosity andpermeability. Although patchy type have wide range and scatter of poroperm data (Fig.18 and with a lowcoefficient of determination), this type is associated with high poroperm values.

Fig. 19. Plots of permeability versus porosity forpatchy and uniform anhydrite texture in the SouthPars field. A patchy distribution of anhydrite crystalsin the reservoir rock fabrics is associated withhigher poroperm values.


temperature dolomitization (50-80ºC) is commonlyfabric-selective but at least partially fabric-retentive.

The ambient temperature controls the fractionationof oxygen isotopes between water and the dolomitemineral (Friedman and O’Neil, 1977) andtemperatures of dolomitization can be therefore beestimated. For this purpose, the formula of Fritz andSmith (1970) and Dickson and Coleman (1980) wasused:

T ºC= 31.9 - 5.55 (δd - δw) + 0.17 (δd - δw)2

where T is temperature in Celsius; and δd and δw arethe oxygen isotopic compositions of dolomite (PDB)and formation water (SMOW) (modified sea water),respectively (Land, 1985).

Assuming a constant δ18Oseawater of about -0‰(SMOW) (Tucker and Wright, 1990), calculatedtemperatures for the studied replacive dolomites rangefrom 12 to 73 ºC. Considering secular variations inδ18Oseawater, this estimate can be re-evaluated accordingto the δ18O value of Late Permian – Early Triassicseawater, which was isotopically lighter than present-day seawater (Veizer et al., 1999). By assumingaverage δ18O values of about -4.5 ‰ PDB (LatePermian) and -3.5 ‰ PDB (Early Triassic) for calcite(Veizer et al., 1999), the mean δ18O SMOW valuesfor Late Permian – Early Triassic seawater can beestimated, using Friedman and O’Neil’s (1977)formula (δ18Ocalcite(‰SMOW) = 1.03086 δ18Ocalcite (‰ PDB) +30.86). Accordingly, δ18O values for Late Permian -Early Triassic seawater is calculated by using theequation:

103 ln α = 2.78 * 106 T-2(Kelvin) - 2.89 (Friedmanand O’Neil, 1977).

A seawater temperature of about 25ºC is assumed.In this calculation, mean δ18Oseawater values of ~ -3.1‰ (SMOW) and ~ -2.3 ‰ (SMOW) for the LatePermian and Early Triassic, respectively, are used. Ourcalculations indicate the following:

(1) The temperature of formation for Late Permianfabric-retentive dolomite in the Dalan Formationranges from 1.5 to 18ºC. For fabric-destructivedolomite, temperatures were about 22-45ºC.

(2) Early Triassic fabric-retentive dolomitesformed at temperatures of 30-56ºC, but fabric-destructive types ranged from 40 to 56ºC. In general,isotope thermometry indicates that fabric-destructivedolomites formed at temperatures above 22ºC whereasfabric-retentive dolomites and associated anhydritesformed in surface and near-surface conditions.

Timing and depth of diagenetic eventsThe burial depths at which various diagenetic featuresare formed depend on many factors (Machel, 1999).For example, minimum depths of about 500 m

(equivalent to temperatures of more than 30ºC) arerequired for the development of horizontal stylolites(Lind, 1993; Nicolaides and Wallace, 1997; Dugganet al., 2001), although it has been suggested thatgreater depths are required for solution seam andstylolite formation (Dunnington, 1967; Machel, 1999).Qing and Mountjoy (1994) proposed that the activestylolitization zone is situated at depths of 600 to 900m. In this study, a depth range of 300 to 900 m wastaken for solution seam and stylolite formation. Saddledolomites are also a useful depth indicators, pointingto temperatures of ca. 60 ºC (Warren, 2000).

A burial history model for a well in the South Parsfield was reconstructed by Aali et al. (2006) (Fig. 20).Assuming a surface temperature of 25ºC and a bottom-hole temperature (BHT) of 96ºC, the calculatedgeothermal gradient for this field is 22ºC/ km. Thisgeothermal gradient can be used to infer thetemperatures and depths of the various diageneticfeatures, and “windows” for the different diageneticoverprints can be inserted into the modelled geohistory(Fig. 20). The formation of fabric-destructive dolomitebegan just after fabric-retentive dolomite generationand continued to deep burial conditions (~1400 m),explaining why fabric-retentive dolomite and allanhydrite fabrics are cross-cut by stylolites. The modelshows that chemical compaction features formedduring the Triassic to Early Jurassic. According to theburial history model, saddle dolomites started to formduring the Late Jurassic at depths of 1600 m, but themost favourable conditions for the precipitation of thiscement type occurred during the onset of the oilwindow in the Tertiary (Fig. 20).

Dolomitization modelsThe close association of anhydrite with fabric-retentive dolomites suggests that dolomitization mayhave resulted from one of four models: (1) “Coorong”,(2) evaporation-drawdown, (3) seepage-reflux, and (4)evaporative (Flügel, 2004).

Suitable conditions for Coorong-typedolomitization (e.g. ephemeral alkaline lakes andrecharging freshwaters) were absent. In general,petrographic and geochemical evidence suggestdolomitization by seepage-reflux and/or evaporativemechanisms, as has previously been proposed for theDalan-Kangan/Khuff carbonates (Talu and Abu-Ghabin, 1989; Ehrenberg, 2006; Alsharhan, 2006;Insalaco et al., 2006). The well-documented sabkhasoffshore Abu Dhabi are good modern-day analoguesfor the formation of such dolomite-anhydritesuccessions (Purser, 1973). In the sabkha capillaryzone, dolomitization is closely associated with thegrowth of nodular anhydrite due to an increase in theMg:Ca ratio caused by the precipitation of calciumsulphates (Kinsman, 1966; Alsharhan and Wittle,


1995). Anhydrite precipitation takes place in the zonejust below the sabkha surface producing bedded andnodular fabrics (Warren and Kendal, 1985; Warren,2006).

An important feature in the studied reservoirinterval is a tendency for a reduction in the quantityof replacement dolomite, downward from the cyclecaps. This is analogous to the modern sabkha-typedolomitization model (Yoo and Lee, 1998). In thisenvironment dolomitizing fluids may be recharged byseawater flowing through the shallow subsurface(driven by tidal pumping or storms), and evaporativepumping (Yoo and Lee, 1998). These dolomites coulddevelop at some distance, laterally and vertically, from

the sabkha surface. In the rock record, thispenecontemporaneous metastable dolomite(Mazzullo, 2000) is commonly recrystallized. Inrestricted lagoons or platforms, seawater may becomeconcentrated due to evaporation (particularly in aridclimates), forming dense dolomitizing brines whichinfiltrate downwards through the underlyingsediments. The abundance of evaporitic minerals suchas anhydrite in these dolomites supports the movementof saline brines through the strata (Saller andHenderson, 1998). The origin of anhydrite is attributedto the reflux of hypsersaline brines from the overlyingevaporite beds (i.e. post-dolomitization fluids). Thegeneration of extra calcium during dolomitization has

Fig. 20. Burial history model for a well in the South Pars gasfield (Aali et al., 2006). The thickness and lithologyof pre-Permian sediments are reconstructed then modelled using regional correlations. Petroleum systemevents chart is based on Alsharhan and Nairn (1997) and Aali et al. (2006). Assuming a geothermal gradient of22 ºC/ km (25 ºC surface temperatures and 96 ºC BHT in the well), diagenetic event windows were insertedbased on the thermometry of dolomite samples. A depth range of 300 m and 900 m was assumed for theformation of solution seams and stylolites.


also been proposed to explain anhydrite mineralization(Machel, 1986; Leary and Vogt, 1986; Ruppel andCander, 1988; Melim and Scholle, 1999).

Considering the low palaeolatitudes of the PersianGulf (around 17-25º) in the Late Permian to EarlyTriassic and the prevalence of arid climatic conditions,evaporative-hypersaline conditions may have beenwidespread. These conditions are reflected in therestricted faunal assemblage, the presence of anhydriteand of early evaporite-related dolomite. Also, ourobservations indicate that fabric-destructive dolomiteformed after fabric-retentive types, under highertemperatures (~50 ºC) in low to intermediate depthsfrom more isotopically depleted waters.

Dolomitization is favoured at elevated burialtemperatures because kinetic inhibitions are active atlower temperatures (less than 50 ºC) (Machel andMountjoy, 1986; Hardie, 1987; Budd, 1997).Therefore, recrystallization occurred at elevated burialtemperatures which resulted in an increase in crystalsize and a depletion of δ18O values.

DISCUSSION

Previous studies (Saller and Henderson, 2001; Lucia,2002) have suggested that replacive dolomitizationcan cause a reduction in precursor porosity andpermeability in platform carbonates. Replacivedolomitization is due to saturated fluids during brine/evaporative reflux (Saller and Henderson, 2001;Lucia, 2002). This study shows that dolomitizationand anhydrite cementation, along with original faciestype, are major factors controlling the reservoir qualityof the Dalan – Kangan carbonates at South Pars.

When associated with minor anhydritecementation, replacive dolomitization has enhancedreservoir quality by increasing permeability. Porosityparameters (percent, type and shape) in fabric-retentive dolomites are inherited from the precursorrock fabric. However, dolomite recrystallization andneomorphism has generated or enlarged pore spacesas a result of an increase in crystal size. Dolomitesamples with patchy cement fabrics (nodular andparticularly poikilotopic cements) show enhancedreservoir qualities. Similar observations from Permiandolostone reservoirs in West Texas (Lucia et al., 2004)suggests that patchy anhydrite reduces porosity butdoes not reduce pore-throat sizes. Pore-throat sizecontrols permeability and remains nearly constant. Inaddition, the development of micro-fractures in suchmineralogically heterogeneous rocks may be due to arange of mechanisms. In pervasive and highlyanhydrite cemented reservoir rocks (with uniformanhydrite cementation), anhydrite has occluded porespaces, resulting in tight carbonate intervals whichlocally act as barriers to fluid flow (Rahimpour-Bonab,

2007). Bedded anhydrites may act as intraformationalseals.

Although dolomitization is the main diageneticfeature in the studied succession, dolomitecementation (“over-dolomitization”) occurs but is notextensive. This process may strongly reduce dolomiteporosity and permeability (Saller and Henderson,2001; Lucia, 2002, 2004).

This study shows that variations in the calciumsulphate saturation in different parts of a restrictedplatform may have caused differences in the modeand amount of anhydrite precipitation, which, in turn,gave rise to variations in dolomite poroperm values.Depositional fabrics appear to have exerted a strongcontrol on anhydrite infiltration and subsequentporoperm heterogeneities. To further clarify thecontrols exerted by hypersaline diagenesis onporoperm values in the Dalan – Kangan carbonates,more detailed studies of the spatial and temporaldistribution of hypersaline diagenetic products willbe required. However, in spite of strong diageneticalterations, this reservoir has preserved its originaldepositional architecture at a field scale. This is thesubject of future research by the authors.

CONCLUSIONS

1. At the South Pars gasfield, offshore Iran, gas isproduced from Permo-Triassic carbonates of theDalan – Kangan Formations. Previous studies andfacies analysis show that the carbonates weredeposited in supratidal, intertidal, lagoonal, shoal andoffshoal settings on the inner part of an extensiveramp-like or epeiric carbonate system. Faciesdistribution has exerted a primary control on theporoperm heterogeneity of the reservoir. The facieswere overprinted in three separate diageneticenvironments: marine, meteoric and burial.2. Petrographical examinations and geochemicalanalysis indicate that producing intervals weresubjected to extensive dolomitization and anhydriteprecipitation. A key challenge is to evaluate the effectsof these processes on reservoir quality.3. Three dolomite types and four anhydrite fabrics aredistinguished in producing units at South Pars. Fabric-retentive dolomites, the most comon dolomite type,are generally associated with improved reservoirqualities. However, the presence of patchy anhydrite(nodular and poikilotopic fabrics) has had little effecton poroperm values.4. This study indicates that the reservoir rocks weredolomitized just after deposition in a near-surfacesabkha/seepage-reflux system. Minor portions of theseearly dolomitic intervals were then recrystallizedduring burial with neomorphism and formation offabric-destructive dolomites. Anhydrite precipitation


was closely associated with dolomitization. Thesediagenetic processes together with depositional facieswere the major factors controlling the reservoir qualityof the Dalan – Kangan carbonates at South Pars.

ACKNOWLEDGEMENTS

This work was supported by grant no 84/124 fromthe National Research Council of I.R. Iran (NRCI),received by Dr. H. Rahimpour-Bonab. The Universityof Tehran provided facilities for this research, whichwe are grateful. The authors thank Pars Oil and GasCompany of Iran for some data preparation.

We greatly appreciate reviews by StevenEhrenberg and an anonymous referee whosecomments on a previous version greatly improved themanuscript.

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