1 3

FaciesDOI 10.1007/s10347-014-0414-4

OrIgInal artIcle

Generation and evolution of oolitic shoal reservoirs in the Permo‑Triassic carbonates, the South Pars Field, Iran

Behrooz Esrafili‑Dizaji · Hossain Rahimpour‑Bonab

received: 25 March 2014 / accepted: 11 august 2014 © Springer-Verlag Berlin Heidelberg 2014

grainstone (Mg). On the other hand, it has been reduced by cementation and compaction in tightly cemented and compacted grainstones (ceg and cOg rock types). the primary porosity has been relatively well preserved in grainstone with interparticle porosity (IPg rock type). the later porosity reduction during burial was also controlled by rock type. this study shows that despite the great burial depth, a significant amount of porosity is still preserved in oomouldic grainstone.

Keywords Ooid grainstone · Ooid shoal reservoirs · Dalan and Kangan formations · Khuff reservoirs · South Pars Field

Introduction

Significant quantities of the world’s hydrocarbon reserves have been discovered in oolitic grainstone reservoirs. these reservoirs are common in the Permian and Jurassic periods (see roehl and choquette 1985). they commonly show better reservoir properties compared to reefal, tidal-flat, and other reservoir types (see roehl and choquette 1985; Jor-dan and Wilson 1994). the most familiar and well-docu-mented examples of these reservoirs are the Permo-triassic Khuff and upper Jurassic arab formations in the Middle east (ehrenberg et al. 2007; al-awwad and collins 2013; alsharhan 2014; Pöppelreiter 2014), the middle Jurassic great Oolite Formation in the southern england (Heasley et al. 2000; gluyas and Hichens 2003) and the upper Juras-sic Smackover Formation in the southern USa (Heydari 2003; Mancini et al. 2003; al Haddad and Mancini 2013).

Studies of modern oolitic shoals show that these geo-bodies are highly heterogeneous in nature (Major et al. 1996; Kendall and alsharhan 2011; rankey and reeder

Abstract Ooid grainstone is the main reservoir rock in the Permo-triassic Dalan and Kangan formations (Khuff equivalents) in many gas fields of the Persian gulf and neighbouring areas. Ooids with a dominant aragonite min-eralogy accumulated in a series of linear shoals and sand banks parallel to the shoreline, on the shallow parts of a vast epeiric carbonate platform. On the basis of the sequence stratigraphic analysis, these reservoir facies mainly devel-oped during relative sea-level rises and increases in accom-modation space. Integrated petrographic and geochemical studies reveal that ooid grainstone was altered through a complex diagenetic history, largely controlled by water chemistry, as a result of relative sea-level fluctuations. two main types of ooid grainstone are the result: dolomitised grainstone or type H and oomouldic grainstone or type M. Dolomitised grainstone is commonly associated with the transgressive systems tract (tSt), and developed under hypersaline conditions. In comparison, oomouldic grain-stone is predominant in the early highstand systems tract (HSt), which was affected by intensive meteoric diagen-esis. Petrophysical study indicates that the reservoir prop-erties of these rocks are largely a function of diagenesis. On the basis of their dominant pore types and diagenetic modifications, the ooid grainstone facies of the studied for-mations are grouped into five reservoir rock types. the spa-tial and temporal distribution of these rock types and their diagenetic evolution can be predicted within the sequence stratigraphic framework. among these rock types, poros-ity has been greatly enhanced by dolomitisation and disso-lution in the dolomitised grainstone (Dg) and oomouldic

B. esrafili-Dizaji (*) · H. rahimpour-Bonab School of geology, college of Science, University of tehran, 14176-14411, tehran, Irane-mail: esrafily@khayam.ut.ac.ir; esrafilidizaji@ut.ac.ir

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2011). Moreover, they are extremely susceptible to diage-netic modification. consequently, reservoir characteristics of the oolitic reservoirs are commonly heterogeneous.

according to previous research, oolitic reservoirs can be classified into three main sub-types based on their dominant pore type and diagenesis; oolitic reservoirs with dominant interparticle pore (e.g., great Oolite Formation, see Mcli-mans and Videtich 1989 and Smackover reservoirs in the USa, see Heydari 2003; Mancini et al. 2003), oomouldic grainstone reservoirs (e.g., Khuff reservoirs in the Middle east, ehrenberg et al. 2007), and dolomitised oolitic reser-voirs (e.g., arab reservoirs in the Middle east, cantrell et al. 2004; cantrell 2006). Several studies have indicated that the primary mineralogy, palaeoclimate, and nature and intensity of diagenetic alteration imparted the main controls on pore characteristics in these reservoirs (ehrenberg et al. 2007; tedesco and Major 2012). apparently, oolitic reservoirs with predominant mouldic pores were commonly developed in aragonitic seas (e.g., during the Permian). the mouldic pores were formed during the dissolution of mineralogically

metastable ooids (aragonite-dominated ooids, Sandberg 1983). generally, these oomouldic grainstone reservoirs show high porosity, but relatively low permeability, as a result of poorly connected pore spaces (lucia 2007). In contrast, during the periods of calcitic seas (especially in the Jurassic), primary pores and textural details were com-monly preserved in oolitic reservoirs (Swirydczuk 1988; Sellwood and Beckett 1991; Heydari 2003; cantrell 2006). Stable primary mineralogy (i.e., low-magnesium calcite) in these ooids has led to their minor diagenetic alteration. as a result, fabric preservation during diagenesis is common in these facies (Heydari 2000). Dolomitised oolitic reservoirs are found in both the Permian and Jurassic reservoirs of dif-ferent basins (cantrell and Hagerty 2003; ehrenberg et al. 2007). Seemingly, palaeoclimatic conditions (particularly an arid climate) played a major role in the generation and development of the reservoirs.

this paper discusses the genesis and evolution of poros-ity in the oolitic reservoir rocks of the South Pars gas Field in the Persian gulf. In this field, the Permo-triassic

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legend

limestone

dolomite

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a

b

c

Fig. 1 a location map of the South Pars Field (SP) in the central Persian gulf. b location of some wells in the field. c Stratigraphic column of studied intervals, upper Dalan member and Kangan Formation (K4 to K1 units)

Facies

1 3

Tabl

e 1

Sum

mar

y of

fac

ies

char

acte

rist

ics

in th

e re

serv

oir

rock

(D

alan

and

Kan

gan

Form

atio

ns)

of th

e So

uth

Pars

gas

Fie

ld

Faci

es c

ode

nam

el

ithol

ogy

com

pone

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xtur

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cont

ent

Dom

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ition

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cond

ition

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nhyd

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sc

ryst

allin

en

one

chi

cken

-wir

e, m

assi

ve,

lam

inat

edSa

bkha

, eva

pora

tive

cond

ition

s

F2n

odul

ar d

olom

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/wac

kest

one

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ydri

tic d

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itet

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nd

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en

one

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ular

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icSa

bkha

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pora

tive

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ition

s

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nest

ral d

olom

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kest

one

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, anh

ydri

tic

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mite

tra

ces

of b

iocl

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, for

a-m

inif

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and

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rom

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ces

of p

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Var

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for

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ding

Shoa

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), h

igh

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n-/p

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tone

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era,

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Facies

1 3

Dalan and Kangan carbonates (Khuff equivalents) are the major gas-producing intervals (Insalaco et al. 2006; esra-fili-Dizaji and rahimpour-Bonab 2013). these carbonates host the world’s largest gas accumulation in the combined north Dome (Qatar) and South Pars (Iran) fields on the Qatar arch (Fig. 1). Previous studies have indicated that the porous oolitic shoal units are the main reservoir rock in this field (ehrenberg 2006; van Buchem et al. 2014). the results of this study provide some important insights into the oolitic reservoirs and aim to improve the reservoir char-acterisation and prediction in these reservoir types.

Geological setting

Several supergiant and many giant gas fields have been explored from the Permo-triassic Dalan-Kangan carbon-ates (Khuff equivalents) in the Persian gulf Basin (Borde-nave and Hegre 2010; esrafili-Dizaji and rahimpour-Bonab 2013; alsharhan 2014). this prolific basin is located in the ne of the arabian Plate. the geological evolution, stratig-raphy, and hydrocarbon habitat of this basin have been dis-cussed by many authors including Murris (1981), edgell (1996), alsharhan and nairn (1997), Sharland et al. (2001), Pöppelreiter (2014), and van Buchem et al. (2014). the dis-tribution of hydrocarbon reservoirs in the basin is mainly controlled by two types of structural elements: north-trend-ing arches (Qatar-South Fars and Khafji-nowruz arches) and salt diapiric structures (alsharhan and nairn 1997).

From the Infracambrian onward, a nne-SSW-trending palaeohigh (Qatar arch) divided the Persian gulf Basin into two troughs (nW-W sub-basin and e sub-basin; see Fig. 1a). as a result of Hercynian movements during the late Palaeozoic, the sedimentary cover of the Persian gulf Basin was warped in the central part by a regional anticline, the Qatar–South Fars arch (alsharhan and nairn 1997; Perotti et al. 2011; van Buchem et al. 2014). Subsequent erosion after uplift led to considerable removal of the Pal-aeozoic sequences.

Owing to the formation of the neotethys Ocean during the middle Permian through middle triassic, an extensive epicontinental platform was developed in the ne part of the arabian Plate (Persian gulf area, alsharhan and nairn 1997; Ziegler 2001; Pöppelreiter 2014). Under an arid cli-mate (sub-tropical and tropical palaeogeography), a thick shallow-water carbonate succession with evaporite inter-calation was deposited on this wide carbonate platform (al-Jallal 1995; Insalaco et al. 2006; Maurer et al. 2009; Pöppelreiter 2014). a seaward-dipping and thickening wedge of the Permo-triassic succession covered the pas-sive southern part of the neotethys. the resulting units are known as the Kangan and Dalan formations (Szabo and Kheradpir 1978) in Iran, which are the lithostratigraphic

equivalents of the Khuff Formation in neighbouring coun-tries (alsharhan 2006).

Ooid generation and evolution occurred in the inner part of this carbonate platform (ehrenberg et al. 2007; esrafili-Dizaji et al. 2014). regionally, gas was sourced from the lower Silurian hot shale (Sarchahan/Qusaiba Formation; Bordenave 2008; van Buchem et al. 2014) and then charged these carbonates. these reservoirs are capped by the Dashtak or Sudair Formation (Bordenave and Hegre 2010). these Permo-triassic oolitic carbonates of the Qatar-South Fars arch host the largest gas and condensate field in the world, which is shared between Iran and Qatar (Fig. 1a, b). For general geological information about this field, refer to esrafili-Dizaji and rahimpour-Bonab (2009), rahimpour-Bonab et al. (2010) and references therein.

Materials and methods

the present study is based on the integration of petrog-raphy with geochemical, petrophysical, and well-log data from the South Pars gas Field. cores from five wells (wells a, e, H, n, J, in Fig. 1b) were examined in detail and then sampled at 30-cm to 1-m intervals. More than 1,200 thin-sections were prepared from the samples, all stained with alizarin red S. all thin-sections were described on the basis of components, texture, structures, fossils, pore types, cements and other sedimentary and diagenetic features. collected data were recorded in stand-ard core-logging sheets. these sedimentological logs are supplemented by petrophysical data (gamma ray, density logs and core poroperm values). By integration of core and thin-section studies, several facies were identified. then, a depositional model was constructed for the studied units. Subsequently, considering the vertical stacking of the facies and well-log data, the studied units were subdivided into depositional sequences (modified after Insalaco et al. 2006).

to characterise the main reservoir rock, particular atten-tion was paid to the oolitic grainstone and its characteris-tics. Distribution, diagenesis, and reservoir quality of the grainstone was analysed within the sequence stratigraphic framework. On the basis of petrographic examination, sam-ples were chosen for geochemical analysis (carbon and oxygen isotopes) and scanning electron microscopy (SeM). the carbon and oxygen stable isotope values were deter-mined at the texas a & M University’s laboratory for 162

Fig. 2 Detailed sedimentologic log based on cores from the Upper Dalan member (K4 and K3 reservoir units) in the South Pars Field. Graphic log indicates that there are several thick oolitic intervals in the K4 unit with low-density log responses (and relatively cleaning in the gr log). Different oolitic rock types are colour coded. the posi-tion of well e (logged cores) is shown in Fig. 1

▸

Facies

1 3

lith

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1000 0.010 2010

porositypermeability(md) (%)

lithology

anhydrite

dolomite

limestone

component

intraclast

peloid

ooid

structures/fabric

nodular fabric

geopetal fabric

bioturbation

desiccation crack

types of ooid grainstone

oncoid

bioclast

fenestral fabric

upward fining

upward coarsening

seq

uen

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knes

s (m

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rvo

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3K

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rmat

ion

(U

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Lat

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age

20

40

60

80

100

120

140

160

180

200

220

240

260

280

0 100

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GAPI

Formation Density

3G/C32

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rock

typ

e

sequence

TST

HST

grainstone with interparticle pores (IPG)

oomouldic grainstone (MG)

tight grainstone (TG)

dolomitised grainstone (DG)

MFS

SB

SB

KS

4

KS

4aK

S4b

KS

4cK

S3a

KS

3bK

S2

Facies

1 3

lith

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1000 0.01 2010

porositypermeability(md) (%)

10

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nit

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K2

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Formation Density

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sic

age

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rock

typ

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2

Facies

1 3

bulk samples. analysed samples were screened for diage-netic alteration (such as stylolites and microfractures) using a binocular microscope. In addition, the porosity (Φ) and permeability (K) were routinely measured on plug samples.

according to heterogeneity characterisation, ooid grain-stone is classified into five rock types based on its petro-graphic and petrophysical criteria. then, distribution of the rock types is elaborated within the sequence stratigraphic framework of the studied wells. Finally, a conceptual model is proposed to explain the development and distribution of various rock types in the studied reservoir.

Facies and depositional sequences

ten main facies are recognised within the Dalan and Kan-gan formations in the South Pars Field. the lithology, texture, fossil content, environmental setting, and other characteristics of these facies are summarised in table 1.

the facies are interpreted to be deposited mainly in open-marine, shoal, lagoon to intertidal depositional settings. the facies are cyclically repeated constituting metre-scale (up to 10 m), shallowing-upward cycles. each cycle largely begins with a thick cross-bedded ooid grainstone, followed by bioturbated lagoonal facies and capped by microbialite (stromatolite and thrombolite) and anhydrite-dominated peritidal facies.

Figure 4 shows a generalised depositional model and an idealised shallowing-upward cycle of the investigated intervals. the stratigraphic and spatial distribution of facies associations in the reservoir are shown in Figs. 2 and 3 (upper Dalan member and Kangan Formation, respec-tively). Similar facies and sedimentary characteristics have been described in greater detail by alsharhan (2006), Insa-laco et al. (2006), Knaust (2009), Maurer et al. (2009), Koehrer et al. (2011, 2012).

On the basis of Sharland et al. (2001) and alsharhan (2006), Dalan and Kangan carbonates, the stratigraphic equiv-alents of the Khuff Formation, represent a second-order trans-gressive–regressive sequence. the upper Dalan carbonates together with the Kangan Formation constitute the regressive hemi-sequence of the mentioned sequence. according to Insa-laco et al. (2006), the studied carbonates can be divided into four major depositional cycles (KS1 to KS4; see Figs. 2, 3).

Fig. 3 a sedimentological log based on cores from the Kangan For-mation (K2 and K1 reservoir units) in the South Pars Field. there is a missing core interval at the top of the Kangan Formation. note thick oolitic intervals are present in the K2 unit. Various oolitic rock types are presented as coloured code. a key for the symbols is provided in Fig. 2. the position of well e (logged cores) is shown in Fig. 1

ooid shoalback-shoal fore-shoal

M W P G

F9)

/packstone

bioclastic ooid grainstone

F8) ooid grain-/packstone

F7)

oncoid, peloid, ooid grain-/packstone

F6) oncoid bioclast packstone

F5) bioclast, peloid pack-/wackestone

F2 & 3) nodular and fenestral dolomud-/wackestone

F1) anhydrite

F4) stromatolite/thrombolite boundstone

shal

low

ing

up

war

d c

ycle

lithology

ooid

oncoid

peloid

bioclast

intraclast

anhydrite nodule

desiccation crack

lagoonperitidal open marine

anhydrite

anhydritic dolomite

dolomite

limestone

legend

Fig. 4 Idealised depositional model for the Dalan and Kangan carbonate reservoir in the South Pars gas Field, showing the major shoaling-upward cycle with lithological and textural properties

◂

Facies

1 3

these sequences can be correlated from the Zagros Moun-tains and the Persian gulf to the Musandam Mountains (Maurer et al. 2009) and al Jabal al-akhdar (Koehrer et al. 2010) in the north of Oman. likewise, as shown by earlier workers, the recognised sequences consist of several smaller-scale sequences. these small-scale sequences are composed of a series of stacked shallowing-upward cycles.

each depositional sequence (KS1 to KS4) is bounded by a thick sabkha anhydrite facies, which is a potential inter-formational barrier unit (alsharhan 2006). In most of the studied locations, the sequences were commonly altered by dolomitisation (Pöppelreiter 2014). the following sections focus on the sedimentary characteristics, diagenesis, and reservoir properties of ooid grainstone, which was devel-oped in these depositional sequences as the main reservoir target of the upper Dalan-Kangan formations.

Ooid shoal facies: characteristics and development

Petrographic study of the ooid shoal facies indicates that ooids vary in size between 0.3 and 0.5 mm (mean 0.35 mm). In most cases, the original microfabric is not well preserved due to later diagenetic modification. How-ever, locally well-preserved ooids are present and exhibit concentric (type 1 in Strasser (1986) or Bahamian-type) or micritic internal fabrics. In some cases, ooids show more than five cortical laminae. Broken, half-moon, superficial and compound ooid types are very rare. their nuclei, where preserved, have been generally micritised or recrystallised during diagenesis. rarely, a bioclast or quartz grain forms the nucleus to the ooid.

associated grains include oncoid, peloid and bioclast. large micritic-coated grains (larger than 0.5 mm) with irregular and ellipsoidal shape, referred to here as oncoid (type 1 in Védrine and Strasser 2009), are associated with ooid grainstone facies. Peloids with a fine size (less than 0.3 mm) are also present. the most common skeletal grains in this facies are mollusc fragments, bryozoans, echino-derm debris, and foraminifera.

From a palaeo-environmental point of view, the shoal bodies in the studied successions can generally be subdi-vided into three internal facies or subfacies based on their components and depositional characteristics. they include back-shoal, central shoal and fore-shoal subfacies.

the back-shoal subfacies is characterised by oncoid- and peloid-bearing ooid grainstone and packstone. In com-parison, the fore-shoal subfacies is dominated by bioclastic, ooid grainstone and packstone. cross-bedded, well-sorted oolitic grainstone is the common subfacies of the central shoal. grain grading is common in the shoal subfacies.

evaluation of the ooid shoal bodies within a sequence stratigraphic framework (modified after Insalaco et al.

2006) shows that they are largely developed in the KS4, KS2 and, to a lesser extent, in the KS1 depositional sequences (see Figs. 2, 3). these facies are not present in the KS3 sequence (or K3 reservoir unit). the thickness of these units reaches more than 20 m, particularly in KS4. the thickest ooidal grainstone beds are found around the MFS of KS4 and KS2 sequences (late tSt and early HSt). In the early tSt, the peloid- and oncoid-bearing ooid sub-facies (back-shoal subfacies) is common but it is replaced by the ooid and skeletal-ooid grainstone (fore-shoal subfa-cies) in the late tSt and early HSt.

Interpretation

Studies have shown that after the platform was established, an active ooid factory was developed parallel to the shore-line that led to linear shoal formation on the windward side (see also alsharhan 2006; Maurer et al. 2009; Koehrer et al. 2010). the shoals were locally cut across by tidal channels. these ooid shoals are interpreted to have developed under high-energy conditions, above fair weather wave base. Seemingly, the shallow depth of the platform caused by the Qatar arch, favoured extensive ooid shoal accumulation (esrafili-Dizaji and rahimpour-Bonab 2013).

considering the sequence stratigraphic framework of the studied successions, it can be concluded that sea-level fluctuations had a major control on the stacking pattern and development of ooid shoal facies. accordingly, there are systematic relationships between the shoal facies and depositional sequences of the studied formations. In the early tSt, peritidal and lagoonal facies were dominant and only thin intercalations of shoal facies occurred. However, through sea-level rise (mid to late tSt), and creation of

Fig. 5 thin-section photomicrographs of various ooid fabrics in the studied reservoir intervals (Upper Dalan and Kangan carbonates), at the South Pars gas Field. a Ooids with micritised, concentric corti-cal laminae and micritic nucleus; b dolomitised ooids with rhombic crystals of dolomite, not stained by alizarin red S associated with isopachous cements; c recrystallised ooid (filled by calcite-spar) with giant micritic nucleus, ghost of laminae are still preserved; d mic-ritic ooid; e dissolved ooid (oomould) with remains of original con-centric fabric and micrite envelop; f partial dissolution in ooid with concentric cortical part, nucleus also dissolved; g dissolved ooid with large recrystallised nucleus (selective dissolution of cortical part vs. nucleus); h spherical mould of ooid (whole dissolution); i dissolved and dolomitised ooid with ghost of concentric fabric and nucleus; j completely dolomitised ooid, internal fabric are not preserved; k ooid with deformed external laminae and neomorphic nucleus; l large ooid with well-developed concentric laminae and selective dissolution of nucleus and internal laminae; m distorted and dissolved ooids (zig-zag chains); n ooid with micritised laminae and dissolved nucleus; o selectively dissolved cortical fabric in the ooid internal parts; p ooid with replaced by anhydrite; q dolomitised ooid with large crystalline dolomite. crossed polars. Scale bar is 100 µm in all photographs

▸

Facies

1 3

excessive accommodation space, an active ooid factory was developed and the space filled by ooid shoal bodies (partic-ularly in the KS4 and KS2 sequences). thick oolitic facies

usually accumulated during the late tSt and early HSt, when maximum accommodation space was created (Insal-aco et al. 2006). It seems that the development of large ooid

a b c

d e f g

h

o

j

l m n

k

p q

i

Facies

1 3

shoals (late tSt and early HSt) was associated with the occurrence of a relatively wide lagoon behind. During the late HSt, when sea level tends to fall, the shoal bodies’ vol-ume decreased. Subsequently, in the vertical stacking, the lagoonal and peritidal facies capped the ooid shoal facies. It appears that the reduced development of ooid shoal facies in the KS3 and KS1 sequences was due to the lower ampli-tude of the relative sea-level fluctuations (low sea-level stand).

Diagenesis and geochemical characteristics

Petrography

Petrographic study indicates that ooid grainstone in the considered intervals was subjected to a range of diagenetic processes. It was affected by cementation, dolomitisation, micritisation, dissolution, neomorphism, and compaction. Ooid microfabrics were obliterated during diagenesis and show various diagenetic fabrics in thin-sections (Fig. 5). additionally, in some cases, the cortex and nucleus of an ooid are differentially altered (e.g., Fig. 5c, g). Some ooids show selective dissolution in the cortical layers (e.g., Fig. 5i, o).

Detailed diagenetic examination within the sequence stratigraphic context indicates that the ooid grainstone can be divided into two main groups including: (1) type H, pointing to dominant hypersaline diagenesis; (2) type M, indicating dominant meteoric diagenesis.

Type H (Dolomitised ooid grainstone): this is commonly found in the tSt within the sequence stratigraphic frame-work. In the lower KS4 and KS1 sequences, it shows vari-able degrees of dolomitisation. Dolomitisation is usually fabric-selective (see Fig. 6a–c) and rarely fabric-retentive. Isopachous aragonite and sparry calcite cements, which are common in this grainstone, were dolomitised (Fig. 5q). In some cases, ooids were selectively dolomitised in a ground-mass of calcitic or originally aragonitic cements (Fig. 5b). anhydrite, as a pore-filling cement and also nodules, is usually present (Fig. 5p).

Type M (Oomouldic grainstone): thick oomouldic grain-stone is recorded in the HSt and also late tSt of the KS4, KS2, and KS1 sequences. Most of ooids are partially or completely dissolved during diagenesis (e.g., Fig. 5g, h, m, n). the rock is often limestone in lithology and well cemented by isopachous aragonite and sparry calcite cement. Interparticle spaces are usually occluded com-pletely by cement (Fig. 6d–f). Pervasive neomorphism and aragonite stabilisation also occur in this ooid grainstone. locally, oomoulds are filled by various cements (calcite, dolomite, anhydrite, and bitumen).

Physical compaction and grain-overpacking fabrics are less common in these two ooid grainstone types. Spo-radically, distorted and deformed ooids (or voids) occur in highly compacted deposits (see Fig. 5m). High-amplitude stylolites are frequently observed in core samples of the oomouldic grainstone, whereas in the dolomitised grain-stone, low amplitude stylolites are dominant.

Geochemical characteristics

Ooid grainstone shows δ18OVPDB values ranging from −6.27 to + 3.26 % (average = −3.48 %) and δ13cVPDB values from −0.64 to +6.89 % (average = +2.98 ‰) in the reservoir rock. Measured δ18OVPDB values for the con-cerned intervals are more negative than those expected for Permo-triassic marine calcite (approximately −1 to −3 % δ18OVPDB, based on Korte et al. 2005, 2008; Koehrer et al. 2010), but δ13cVPDB values are either similar to, or slightly lower than, the theoretical values of coeval marine carbon-ates (approximately 0 to +5 % δ13cVPDB, based on Korte and Kozur 2010; clarkson et al. 2013).

a chemostratigraphic curve is established using stable carbon and oxygen isotopic composition of bulk samples (Fig. 7). In comparison with δ13c values, δ18O values show greater variability through the reservoir rock. generally, a strong covariance between δ18O and δ13c values is not found. the calculated correlation coefficient for the entire data is 0.40. this statistical coefficient is approximately 0.20 for all ooid grainstones (dolomitised and oomouldic).

generally, two major negative shifts in δ18O values (less than −3 ‰) are recorded in the K4 and K2 units. these δ18O depletions show a positive correlation with the oomouldic grainstone (type M) and with limestone lithol-ogy, commonly occurred in the late tSts and early HSts. additionally, these intervals are characterised by relatively high porosity and permeability values (Fig. 7). In contrast, intervals with dolomitised grainstone (type H) are corre-lated with relatively heavier δ18O values, dominantly asso-ciated with the tSt.

apparently, there is no significant difference between δ13c values of the dolomitised and oomouldic grainstones. except for the Permo-triassic transition interval and sequence boundaries, δ13c data show a slightly negative shift in comparison to the isotopic composition of coeval marine carbonates.

there is an invariant trend between δ18O and δ13c values at the late HSts, in facies above the oomouldic grainstone (type M), whereas δ13c values show a lighter-upward trend and δ18O values display a positive shift in the interval just below the sequence boundaries (particularly in the KS4 and KS2).

Facies

1 3

a b

c d

e f

200 umI

100 umI 100 umI

500 umI

500 umI50 umI

Fig. 6 SeM photomicrograph (a–e: secondary electron images; f: back scattered image) of ooid grainstone in the Permo-triassic reser-voir, the South Pars gas Field. Dolomitised ooid with rhombic crys-

tals and well-preserved intergranular pore (a–c), and mouldic pore (c). Oomoulds associated with isopachous early cement (d–f)

Facies

1 3

litho

logy

Anh

y.M

dst.

Wks

t.

Pkst

.G

rst.

Bds

t.

sedimentary texture

thic

knes

s (m

)

rese

rvoi

r uni

tsK

3K

2

form

atio

nD

alan

For

mat

ion

(Upp

er M

embe

r)

Late

Per

mia

nag

e

20

40

60

80

100

120

140

160

180

200

220

240

260

280

K4

K1

Early

Tria

ssic

Kan

gan

0-2-4-6-8 2 4 6 8 sequ

ence

3rd

geochemical profile (per mil, VPDB)

300

320

340

360

380

permeability (md) porosity (%)

30201000.11101000 100

exposure surface

exposure surface

exposure surface

PTB

H

M

H

H

H

M

H

H

Mdi

agen

etic

zo

nes

KS 4

KS 3

KS 2

KS 1

Facies

1 3

Interpretation

Porous ooid grainstone with an unstable mineralogy (arag-onite) is extensively susceptible to diagenetic modifica-tions. although, petrographic evidence itself is not a con-fident criterion for the determination of an ooid’s original mineralogy (Sandberg 1983), the concentric cortical micro-fabric (Bahamian-type), typical of aragonite ooids, along with extensive ooid dissolution, all suggest an original aragonitic mineralogy for the grains. Similar ooid fabric with the same mineralogy has been described from mod-ern shoal deposits in the Bahamas and Persian gulf (Kend-all and alsharhan 2011; rankey and reeder 2011). On the other hand, in some cases, there is a selective dissolution in the internal fabric, which may indicate a bimineralic cortex for the ooid.

In general, petrographic and geochemical evidence shows that the diagenesis of the recognised ooid grain-stones (dolomitised and oomouldic) was strongly con-trolled by the stratigraphic situation and their position within the relative sea-level curve. there is a difference between the diagenetic modifications in the transgressive and regressive ooid grainstones. Ooid grainstone in the

tSt generally shows extensive dolomitisation associated with anhydrite precipitation (type H). the slightly heavier δ18O composition of this grainstone reflects dolomitisa-tion under hypersaline conditions (Warren 2000), which occurred during or soon after deposition (Maurer et al. 2009; rahimpour-Bonab et al. 2010).

generally during the HSt, the prograding ooid shoal became emergent and was subjected to meteoric diagenesis and type M (oomouldic grainstone) was formed. In view of the short time of subaerial exposure or arid climate (christ et al. 2012), features of subaerial exposure, such as karsti-fication and calcrete horizons, are lacking at the top of the oomouldic grainstone. Some evidence of pedogenetic fea-tures, (rhizoliths) and aeolianite, has been reported from the top of oomoldic grainstone units, beneath the sequence boundary of KS4 (e.g., Frebourg et al. 2010). Subsequently, the meteoric water from the emergent land around the stud-ied area may have penetrated down into the permeable parts of the regressive sequence (HSt’s ooid grainstone). During this stage, extensive dissolution (oomould creation) associated with neomorphism and cementation occurred. It led to δ18O depletion in the oomouldic grainstone as a result of the freshwater recharging (Heydari 2003; ritter and goldstein 2013).

the invariant trends between oxygen and carbon iso-topic values (slight δ13c depletion and δ18O enrichment) just beneath the sequence boundaries is interpreted as the effects of fresh water charged from soil-derived cO2 in the vadose realm (Heydari 2003; christ et al. 2012; railsbak et al. 2013). In general, a weak correlation between δ13c and δ18O reflects the intensive impact of early diagenesis in comparison to burial alteration in the reservoir rock (Shen and Schidlowski 2000; Hamon and Merzeraud 2007).

Table 2 Petrographical descriptions for ooid shoal rock types in the reservoir rock of the South Pars Field

there are five major rock types based on pore modifying processes. Mineralogy, dominant pore types, and main pore modifying processes of each rock type are presented

reservoir properties Ooid grainstone

Porous (Φ > 5 %) tight (Φ < 5 %)

rock types grainstone with inter-particle pores

Mouldic grainstone Dolomitised grainstone cemented grainstone compacted grainstone

code Ig Mg Dg ceg cOg

Dominant pores Interparticle Mouldic Interparticle intercrystal-line

Variable Variable

Pore-modifying pro-cesses

less rim cementation Dissolution, cementa-tion

Dolomitisation cementation compaction

lithology limestone limestone Dolomite limestone, dolomite limestone, dolomite

Schematic view

Fig. 7 chemostratigraphic profile (oxygen and carbon isotopes) of the studied intervals in the well (n), the South Pars gas Field. the profile is supported with sedimentological log, poroperm values, and depositional sequences. a certain relationship is visible between the oxygen and carbon isotopic values and the depositional sequences as well as ooid shoal bodies. two main diagenetic zones are hypersaline (H) and meteoric (M). generally, dolomitised ooid grainstone (type H) shows heavier δ18O values, in comparison to oomouldic grainstone (type M). there are invariant trends between these isotopic data in the sequence boundaries

◂

Facies

1 3

1 mm1 mm

1 mm

1 mm1 mm

a b

c d

e

Fig. 8 thin-section photomicrographs of five ooid grainstone rock types in the Upper Dalan and Kangan formations, the South Pars gas Field. a compacted grainstone (cOg); b cemented grainstone

(ceg); c dolomitised grainstone (Dg), d oomouldic grainstone (Mg); e grainstone with dominant interparticle pore (IPg)

Facies

1 3

Poroperm characteristics

Petrographic examination and petrophysical data indicate that the ooid shoal facies show a wide range of porosity and permeability values (Fig. 9). the porosity varies from nil to 35 % and the permeability ranges from 0.01 to 1,000 mD. the pore systems of these facies are interparticle, intercrys-talline and mouldic, along with microfracture and stylolitic, as well as micropores of various sizes.

On the basis of the dominant pore type and major dia-genetic modifications, ooid grainstone in the studied inter-vals (upper Dalan and Kangan formations) can be divided into two main groups (porous and tight grainstones) and five rock types. Porous rock types are the grainstone with

dominant interparticle pores (IPg), oomouldic grainstone (Mg), and dolomitised grainstone (Dg). tight rock types (tg) include tightly cemented grainstone (ceg) and highly compacted grainstone (cOg).

In table 2 and Fig. 8, characteristics and thin-section photomicrographs of these different rock types are pre-sented. the porosity vs. permeability values of the various rock types are plotted in Fig. 9 and their mean poroperm values are presented in table 3. the dolomitised grain-stone (Dg) is characterised by high permeability (high flow capacity), whereas oomouldic grainstone (Mg) shows high porosity values with relatively low permeability (high stor-age capacity). For the class IPg, porosity and permeability values range between 5–30 % and 1–100 mD, respectively. In general, the tight rock types (ceg and cOg) have poor reservoir properties. Porosity of these rock types ranges between nil and 10 %, and permeability is usually less than 1 mD (Fig. 9; table 3).

In Fig. 10, the spatial distribution of these rock types in the KS4 sequence is displayed in three wells (e, H, J in Fig. 1). this figure indicates that the Dg and ceg rock types are predominant in the dolomitised ooid grainstone (type H) of the tSt, whereas the IPg and Mg rock types are commonly associated with the type M grainstone in the HSt and late tSt intervals (see also Figs. 2, 3). In the ceg rock type, tightly cemented dolomitised grain-stone with anhydrite cement is more common than calcite-cemented grainstone. In addition, the examination showed that the cOg rock type is not common, particularly in the KS4 sequence. regarding this rock-typing scheme within the sequence stratigraphic context, it can be concluded that transgressive hemi-sequences are characterised by ooid grainstone with a high flow capacity, but the regressive hemi-sequence by a high storage capacity.

the ooid grainstones of the studied intervals are cur-rently buried to a depth of over 2.7 km and exposed to temperatures in excess of 100 °c. Previous studies have indicated that the carbonate porosity tends to decrease exponentially with depth due to progressive burial cemen-tation and compaction (Schmoker and Halley 1982; ehrenberg et al. 2009). In Fig. 11, porosity-depth values

porosity (%)

per

mea

bili

ty (

md

)

DG

IPG

MG

TG

Fig. 9 Poroperm cross-plot of five ooid grainstone rock types in the Permo-triassic carbonate reservoir at the South Pars gas Field. (dolomitised grainstone, grainstone with dominant interparticle pore, oomouldic grainstone, tight grainstone; abbreviated as DG, IPG, MG, TG, respectively)

Table 3 Statistical parameters of each rock type according to porosity and permeability means in the Permo-triassic reservoir of the South Pars gas Field

rock groups rock types arithmetic mean geometric mean

Porosity (%) Permeability (mD) Porosity (%) Permeability (mD)

Porous grainstone (Pg) Dolomitised grainstone (Dg) 16.07 309.70 14.99 145.31

grainstone with interparticle pores (IPg) 15.60 73.36 13.78 20.61

Oomouldic grainstone (Mg) 23.40 15.23 22.61 3.59

tight grainstone (tg) cemented grainstone (ceg) 2.83 0.15 1.83 0.08

compacted grainstone (cOg) 1.51 0.12 0.85 0.03

Facies

1 3

for various rock types are compared with the porosity-depth trend in shallow-water carbonates of South Florida (Schmoker and Halley 1982). the range of porosity for the ceg and cOg rock types is approximately 1–3 % (arith-metic and geometric averages, respectively). For the IPg and Dg rock types, mean porosities are 13–15 % and 14–16 %, respectively. the oomouldic grainstones (Mg) have 22–24 % average porosity (see table 3; Fig. 11). In com-parison to the South Florida porosity-depth curve, exclud-ing the class Mg, all rock types show normal porosity with regard to their burial depth. Porosity of the Mg rock type is abnormal and relatively high with respect to its depth, com-pared to the South Florida data (Fig. 11).

Discussion

all ooid grainstones in the studied reservoirs are extremely heterogeneous because of their potential sensitivity to diagenesis. although, porosity in these rock types was greatly enhanced by dolomitisation and dissolution (in the Dg and Mg rock types), it was lost via cementation and compaction (in the ceg and cOg rock types). Primary porosity has been moderately preserved in the IPg rock type.

the generation and occlusion of porosity in the intro-duced rock types are genetically related to early diagenetic modifications, which were strongly controlled by relative sea-level fluctuations. a conceptual model is proposed here to explain development and spatial distribution of various rock types in the studied reservoir (see Fig. 12). this model is composed of three main stages:

Stage A during the early tSts, ooid sands were being deposited in the high-energy zone of the shallow carbonate factory, and scattered ooid bars developed along the wind-ward side of the platform. the influx of hypersaline brines from the sabkha toward and through marine carbonates (lagoon and shoal deposits) led to extensive dolomitisation and cementation (plugging by anhydrite) in the submerged ooid bars. anhydrite cements and nodules were also widely precipitated within these carbonates (Fig. 12, stage a). the ceg rock type (tightly cemented by anhydrite) was gener-ated in this stage.

With progressive sea-level rise during the middle tSt, detached ooid bars merged and formed a series of linear shoals, parallel to the coastline. relatively shallow and pro-tected lagoons extended behind these well-developed ooid shoals. thus, hypersaline diagenesis (seepage-reflux model) prevailed in peritidal and lagoonal settings of this platform.

rock

type

s

rock

type

s

litho

logy

Anh

y.

Mds

t.

Wks

t.

Pkst

.

Grs

t.

Bds

t.

sedimentary texture

sequ

ence

3rd4rd

20

40

0

Scal

e (m

)

litho

logy

Anh

y.

Mds

t.

Wks

t.

Pks

t.

Grs

t.

Bds

t.

sedimentary texture

sequ

ence

3rd4rdlit

holo

gy

Anh

y.

Mds

t.

Wks

t.

Pkst

.

Grs

t.

Bds

t.

sedimentary texture

sequ

ence

3rd4rdrese

rvoi

r uni

tsK

4fo

rmat

ion

Dal

anLa

te P

erm

ian

age

rock

type

s

1000 0.010 2010

porositypermeability(md) (%)

10 30 1000 0.010 2010

porositypermeability(md) (%)

10 30

Well-EWell-HWell-J

30 km10 km

lithology

anhydrite

dolomite

limestone

component

intraclast

peloid

ooid

structures/fabricnodular fabric

geopetal fabric

bioturbation

desiccation crack

types of ooid grainstone

oncoid

bioclast

fenestral fabric

upward fining

upward coarsening

sequence

TST

HST

grainstone with interparticle pores (IPG)

oomouldic grainstone (MG)

tight grainstone (TG)

dolomitised grainstone (DG)

MFS

SB

SB

Fig. 10 Stratigraphic distribution of various oolitic rock types in the sequence stratigraphic framework for the K4 unit (KS4), in three wells in the South Pars gas Field. Based on this correlation panel and third-order sequences, the tight grainstone (tg) is found in the tSt,

near the sequence boundary. Dolomitised grainstone (Dg) are com-mon in the early tSt, but Mg and IPg rock type in the late tSt and early HSt (near the MFS)

Facies

1 3

anhydrite cementation and nodularisation were also common in these environments. anhydrite precipitation and associated calcium depletion in the ambient water led to a high Mg/ca ratio and subsequent dolomitisation in the mostly submerged ooid shoals. the Dg rock type (also ceg) is the main prod-uct of this process during the middle tSt.

Stage B during maximum accommodation space around the MFS, extensive ooid shoal bodies were accumulating in the inner part of the platform. generally, marine cementa-tion was the predominant diagenetic process in the shoals, whereas bioturbation was common in the back-shoal lagoons. With marine flooding and increasing water circu-lation, dolomitisation was limited to the coastal area.

Stage C during the marine regression (HSt), ooid shoals were prograding seaward; accommodation space decreased and shoal expansion was interrupted. the oolite shoal was subaerially exposed and subjected to meteoric diagenesis (dissolution and cementation), resulting in the formation of the oomouldic grainstone (Mg). at the same time, by sea-ward shifting of depositional systems, buried ooid shoal facies were subjected to hypersaline diagenesis (generation

of the Dg rock type). these brines gravitationally perco-lated from the overlying anhydrite brine pools or hyper-saline lagoons. the ooid shoal bodies were progressively overlain by prograding facies (lagoonal and peritidal facies).

after deposition and early diagenesis, the ooid grain-stones were buried and subjected to further diagenesis. although porosity values show a reducing trend with increasing burial depth, a comparison with the porosity-depth curve (Schmoker and Halley 1982) reveals that a great amount of porosity still remains in the oomouldic grainstone (Mg). there are two possibilities to explain the origin of this relatively abnormal high porosity. First, some of mouldic pores may have been produced during mesodi-agenesis. Second, progressive decline in the porosity ver-sus depth in this rock type was relatively less than the other rock types. Mesogenetic dissolution and porosity creation has been questioned by several authors (e.g., ehrenberg et al. 2012). Furthermore, there is no petrographic or geo-chemical evidence within the ooid grainstones for mesoge-netic dissolution. all petrographic observations and geo-chemical data indicate that the second mechanism is more likely. Seemingly, the porosity loss versus burial depth is a function of the degree of pore connectivity. In the oomoul-dic grainstone (Mg), there is poor pore-network connec-tion. therefore, it seems that the porosity loss versus depth in ooid grainstones is controlled by the rock types and their pore system.

Conclusions

In the Permo-triassic reservoir rocks of the South Pars Field, ooid grainstone is the main producing interval. as a result of diagenetic modifications, this reservoir rock exhibits a great variation in porosity types and reservoir characteristics. to characterise heterogeneity in the reser-voir rock, the generation and evolution of porosity in the ooid grainstone has been investigated by integration of pet-rographic, geochemical, and petrophysical data. Sequence stratigraphic analysis shows that deposition and distribu-tion of the ooid grainstones were significantly influenced by sea-level fluctuations and they were developed mostly during relative sea-level rises. Diagenetic characteristics and porosity types in the oolite reservoirs are quite different between the transgressive and regressive facies.

as a result of the heterogeneous nature of these facies, they can be classified into five rock types based on the dom-inant pore types and diagenetic characteristics. Dolograin-stone (Dg), oomouldic grainstone (Mg) and grainstone with interparticle pore-space (IPg) are the major porous rock types, whereas cemented and compacted grainstone (ceg and cOg, respectively) are recognised as the main tight rock types (tg).

dep

th (

km)

porosity (%)0 10 20 30 40 50

1

2

3

South Florida Dolomite

(Schmoker and Halley 1982)

TG MGDGIPG

South Florida Limestones

(Schmoker and Halley 1982)

Fig. 11 comparison of porosity values of recognised rock types ver-sus depth in the South Pars Field with porosity-depth curves (from Schmoker and Halley 1982)

Facies

1 3

In general, the dolomitised grainstone (Dg) and tight grainstone (tg) are common in the transgressive systems tract (tSt), whereas the oomouldic grainstone (Mg) pre-dominantly occurs in the highstand systems tract (early

HSt and also late tSt). Petrographic and geochemical data suggest that the diagenetic changes may have been caused by sea-level fluctuations, with the transgressive hemi-sequence being characterised by hypersaline diagenesis and

early TST

HST

MFS

A

B

C

llaflevel-aes

evitaler

dolomitisationanhydrite cementation

dissolutioncementation

marine cementation

marine cementation

dolomitisationanhydrite cementation

maximum flooding surface

formation of ooid shoal

development and retrogradation of ooid shoal

progradation of ooid shoal

MG

DGTST

HST

TST

HST

TGperitidal facies (sabkha)

lagoon facies

hypersaline flow

shoal facies

open-marine facies

dolomitised intervals

anhydrite nodule

fresh water recharge

facies-diagenetic model of KS4 sequence

Facies

1 3

Fig. 12 a conceptual depositional-diagenetic model that explains development and distribution of various ooid grainstone’s rock types in the KS4 sequence of the South Pars gas Field. this model illus-trates controls exerted by sea-level changes and related early diagen-esis on the generation of various oolitic grainstones. a By sea-level rise, ooid shoal production started. During the early and middle tSt, when ooid shoals were developed in the agitated sea water, hypersa-line fluids percolated through the submerged ooid facies led to gen-eration of the ceg and Dg rock types. b By extensive accommoda-tion space creation during maximum sea-level rise (MFS), large ooid shoals with wide lagoons behind were formed in the platform margin. c During the HSt, ooid shoals prograded seaward and exposed suba-erially. as the result, significant dissolution occurred and oomoulds were formed (Mg rock type). In addition, by seaward progradation of lagoonal and sabkha systems and sea-level fall, hypersaline fluids penetrated down into the ooid grainstone causing dolomitisation in these intervals (Dg formation)

the regressive hemi-sequence being predominantly influ-enced by freshwater meteoric diagenesis.

this study shows that during sea-level falls, when the regressive ooid grainstone (Mg) was periodically subjected to leaching by meteoric water percolation, the transgres-sive ooid grainstone (Dg) was dolomitised by gravitational seepage of Mg-enriched hypersaline brines from the over-lying anhydrite brine-pool. these brines and waters satu-rated with respect to calcite could be the possible source of cements in the tight rock types (tg).

During burial diagenesis, each rock type was affected by compaction to different degrees. although porosity in the main reservoir rock was reduced, a significant amount of porosity is still preserved in the regressive oomouldic grainstone (Mg).

Acknowledgments the vice-president of research and technol-ogy of the University of tehran provided financial support for this research, for which the authors are grateful. the authors also extend thanks to the POgc (Pars Oil and gas company of Iran) for spon-soring, data preparation, and permission to publish this paper. We acknowledge the laboratory of the texas a & M University for geo-chemical analysis. Special thanks are extended to Professor Maurice e. tucker for his careful and constructive reviews. We are grateful to two anonymous referees for reading the manuscript and providing very useful comments and suggestions.

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