213Journal of Petroleum Geology, Vol. 35(3), July 2012, pp 213-236

2012 The Authors. Journal of Petroleum Geology 2012 Scientific Press Ltd

FLOW UNIT DISTRIBUTION AND RESERVOIR

MODELLING IN CRETACEOUS CARBONATES OF THE

SARVAK FORMATION, ABTEYMOUR OILFIELD,

DEZFUL EMBAYMENT, SW IRAN

H. Rahimpour-Bonab1*, H. Mehrabi1, A. Navidtalab1 and E. Izadi-Mazidi2

Carbonate sediments within the Mid-Cretaceous Sarvak Formation form an important reservoirat the Abteymour oilfield in the western Dezful Embayment, SW Iran. The poropermcharacteristics of this reservoir were controlled by factors including deposition under tropicalclimatic conditions and early diagenesis, repeated phases of subaerial exposure due to local,regional and global-scale tectonism, and diagenetic modification during burial. From microfaciesanalysis, the Sarvak Formation carbonates in the Abteymour field were interpreted in a previousstudy as having been deposited on a homoclinal ramp-type platform. Three third-order sequenceswere recognized in the middle Cenomanian to middle Turonian part of the formation. Thereservoir quality of the carbonates was enhanced both by dissolution (comprising separatephases of eogenetic and telogenetic meteoric dissolution) and dolomitization (especially stylolite-related dolomitization).

In this paper, a rock/pore type approach was used in order to integrate petrophysical datawith facies and diagenetic models within a sequence stratigraphic framework. Two differentrock-typing methods for the determination of flow units were considered. Hydraulic flow units(HFUs) were identified firstly using flow zone indicators and secondly using a stratigraphicmodified Lorenz plot. The flow units resulting from these two methods are compared, and theirclose correspondence within the sequence stratigraphic framework is discussed. In addition, thepreviously-used large-scale reservoir zonation scheme for the Abteymour field is correlatedwith the defined flow units, and four new Integrated Reservoir Zones are introduced. By integratinggeological information with petrophysical parameters (including porosity, permeability andsaturation) within a sequence stratigraphic framework, field-scale variations and controls onreservoir quality are described.

1 Department of Geology, Faculty of Science, Universityof Tehran, 14176-14411 Tehran, Iran.2 Geological Operation Office, Karoon Industry Area,N.I.S.O.C., Ahwaz, Iran.

* author for correspondence, email: rahimpor@khayam.ut.ac.ir; hrahimpor@gmail.com

Key words: Sarvak Formation, Abteymour oilfield, DezfulEmbayment, SW Iran, carbonate reservoir, integratedreservoir zones, rock typing, karst, Cretaceous.

INTRODUCTION

Facies analysis of carbonate reservoir rocks can beused to study primary (depositional) pore sizedistribution and reservoir quality. Subsequentmodification and pore rearrangement however can

214 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

only be understood by studying the diagenetic history.Determining the paragenetic sequence helps toconstrain poroperm evolution and to identify othercontrols on the quality of a carbonate reservoir rock(Dunnington, 1967; Moore, 2001; Al-Habshi et al.,2003; Rahimpour-Bonab, 2007; Ahr, 2008). In mostcarbonate reservoirs, diagenesis is a major factorcontrolling secondary porosity and reservoir quality(Schlager, 2005; Lucia, 2007; Ahr, 2008). The impactof the diagenetic overprint on reservoir heterogeneityand porositypermeability values is emphasised incarbonate platforms in which deposition occurred ina humid, tropical climate with heavy rainfall and aninflux of meteoric waters (e.g. Sun and Esteban, 1994;Petty, 2005; Ehrenberg et al., 2007; Hollis, 2011).The development of karst features in this context maysignificantly improve reservoir qualities (Mazzullo andChilingarian, 1992).

Unconformity-related reservoirs contain about 20to 30% of known hydrocarbon reserves (Mazzulloand Chilingarian, 1992). Subaerial exposure and relatedmeteoric diagenesis are a major control on the reservoirquality of these carbonate successions (Ehrenberg etal., 2007; Weidlich, 2010, Razin et al., 2010; vanBuchem et al., 2011; Hollis, 2011). Diageneticprocesses can have both positive and negative effectson reservoir quality depending on many factorsincluding the original mineralogy and texture of thecarbonate sediments and the climate (Harris et al.,1984; Sun and Esteban, 1994; Ehrenberg et al., 2007;Razin et al., 2010; Weidlich, 2010). In addition, theduration of subaerial exposure may influence poresystems and poroperm values (Harris et al., 1984;Mazzullo and Chilingarian, 1992; Weidlich, 2010).

In a previous study (Rahimpour-Bonab et al.,2012), the depositional environment and diagenetichistory of the Sarvak Formation, an epeiric platformsuccession formed under a humid climate in the mid-Cretaceous, were discussed with reference toAbteymour oilfield, SW Iran (Fig. 1). The importanceof climatic and tectonic controls on facies types andpatterns, diagenetic history and reservoir quality wereinvestigated and the significance of disconformitieson the diagenetic history was emphasized. In orderto visualize the spatial distribution of depositionalfacies and diagenetic imprints, the data were integratedwithin a sequence stratigraphic framework. Theprevalence of humid tropical climatic conditions inthe mid-Cretaceous in the Arabian platform has beensubstantiated by a range of geochemical modellingstudies (e.g. Huber et al., 2002; Fluteau, 2007; Kelleret al., 2008; Keller, 2008; Steuber, 2010).

The purpose of the present paper is to investigatethe relationship between facies patterns and diageneticevents and reservoir zones (flow units) in the SarvakFormation within a sequence stratigraphic framework.

A two-dimensional model of the Sarvak reservoir atAbteymour oilfield was constructed to illustrate theheterogeneities within, and zonation of, the reservoir.For flow unit determination, two approaches wereadopted based respectively on flow zone indicatorvalues and stratigraphically modified Lorenz plots.First, the proposed flow units determined by the twomethods are compared. Then, they were correlatedwith the large-scale reservoir zones currently usedby reservoir geologists (NIOC) at Abteymour field.Using this approach, the previous reservoir zonationscheme of the Sarvak Formation at this field has beenimproved and a more precise, integrated reservoirzonation is introduced. The main advantage of thisnew scheme is that it is supported by geologicallypredictable features throughout the field and has thepotential to predict and justify variations in reservoirquality.

GEOLOGICAL SETTING AND STRATIGRAPHY

Thick sedimentary successions of Cretaceous age inthe Arabian Platform and Zagros Basin containnumerous economically important hydrocarbonaccumulations (Setudehnia, 1978; Alsharhan et al.,1986; Alsharhan and Nairn, 1988; Ghabeishavi et al.,2009 and 2010; Hollis, 2011; Lapponi et al., 2011).The development of an epeiric platform occurredduring the Early Cretaceous throughout the MiddleEast (Murris, 1980; Koop and Stoneley, 1982). Thestratigraphic record of the Late Albian, Cenomanianand Turonian (89-98.9 Ma) in the Arabian Platformand Zagros Basin includes the Mishrif, Ahmadi andRumaila Formations in Saudi Arabia; the NatihFormation in Oman (van Buchem et al., 1996, 2002);the Derdere Formation in SE Turkey; the MishrifFormation in Iraq; and the Sarvak Formation in theZagros Basin, Iran (Fig. 2). These successions arecharacterized by a marked reduction in siliciclasticinflux, the development of a carbonate platform tointra-shelf basin topography and the deposition ofbasinal source rocks. Rudists are the dominantcomponent of grainy, high-energy platform margin/barrier facies, and are also present in the platform-top sediments (e.g. Aqrawi, 1998; Alavi, 2004; vanBuchem et al., 2011).

Throughout the Cretaceous, the Arabian Platemigrated northwards to tropical and subtropicallatitudes (Murris, 1980; Beydoun, 1991; Beydoun etal., 1992; Alavi, 2007; Heydari, 2008). In addition,local factors including the effects of salt diapirismand movement on basement blocks led to episodicregional uplift and emergence. Interactions betweeneustatic and local sea-level fluctuations and the warmhumid climate are reflected in both the depositionalfacies and diagenetic history (Blanc et al. 2003;

215H. Rahimpour-Bonab et al.

Fig. 1. (A). Location map of the study area and Abteymour oilfield in the transition zone between the DezfulEmbayment (SW Iran) and the Mesopotamian Basin.(B). UGC map for the top-Sarvak reservoir at Abteymour field, OWC and location of studied wells(contours in m).(C). Principal geological and structural sub-divisions of SW Iran; the Dezful Embayment and Abteymour fieldare labelled.

Abteymour Oilfield

Iran

Iraq

Mesopotamian basin

Turkey

Saudi ArabiaA

bu J

ir fault z

one

South Euphrates fault zone

Persian Gulf

Dezful embayment

General information Abteymour Oilfield

Discovery year 1969

Trap type Anticline

Drilled wells 31

Reservoir units

- Sarvak Formation(late Albian-mid Turonian)

- Illam Formation(Santonian-early Campanian)

Hydrocarbon characteristics

API

Sulfur content (percent)

26

>3

Abteymour Oilfield

33003250320031503100

31003150320032503300

OWC: 3317

OWC: 3317

3050

Studied wells

A

B

C

N

N

0 2km.

0 100km.

Main Zagros Reverse Fault

Zagros Front F

.

Mountain Front F.

Balar

ud F

ault

Mountain Front F.

Zagros Front F.

IRAN

HIGH ZAGROS

IZEHLURESTAN

FARS

QATAR

Minab F.

Qata

r-K

aze

run F

ault

DEZFULEMBAYMENT

WESTERN

PERSIAN GULF

EASTERN

PERSIAN GULF

QATAR

ARCH

BA

ND

AR

AB

BA

S

HIN

TER

LA

ND

Gulf of Oman

Abteymour Oilfield

0 300km.

AT-1

AT-2

AT-3

AT-4

216 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

McQuarrie, 2004; Sepehr and Cosgrove, 2005; Sepehret al., 2006; Sherkati and Letouzey, 2004; Sherkati etal., 2005; Verges et al., 2009; Razin et al., 2010; Hollis,2011; Rahimpour-Bonab et al., 2012).

In oilfields in south and SW Iran, the SarvakFormation of the Bangestan Group (Fig. 2) togetherwith the Oligo-Miocene Asmari Formation are the mostimportant reservoir intervals. However, at Abteymourfield (Fig. 1) only Bangestan Group reservoirs are oilproducing and the Asmari produces water. Here, theSarvak consists of shallow-marine limestones whichare clay-rich, micritic and sparitic in parts, and whichinclude grainstones, rudist-bearing packstones andstromatoporid wackestones alternating with intervalsof shale and marl. At its type section at Tang-e-Sarvak(Bangestan Mountain), the Sarvak Formationconformably overlies the Kazhdumi Formation with atransitional contact; its upper contact with the shalesand marls of the Gurpi Formation is sharp (Motiei,1993). However, at Abteymour field, the SarvakFormation is disconformably overlain by the carbonatesof the Illam Formation, and the upper boundary isdifficult to recognise (Fig. 2).

The Sarvak Formation has been dated as late Albian(?) to middle Turonian (James and Wynd, 1965;Setudehnia, 1978; Motiei, 1993). The complex tectonichistory of the Zagros Basin and Arabian Platform(Sharland et al., 2001; Bahroudi and Talbot, 2003;Sepehr and Cosgrove, 2005; Alavi, 2004; Sharp et al.,2010; Casini et al., 2011) led to wide variations in thedepositional and reservoir characteristics of the SarvakFormation and its equivalents (Hollis, 2011; Rahimpour-Bonab et al., 2012). These include formation ofintrashelf basins and palaeohighs in the SW sector ofthe Zagros Basin (including the Dezful Embayment).The influence of these tectonically induced featuresare reflected in the sedimentary successions in the areaboth as particular sedimentary facies and as the severalunconformities which are associated with significantdiagenetic events (Rahimpour-Bonab et al., 2012). Forexample, the combined effects of sea-levelfluctuations, salt diapirism and movement on basementblocks in the CenomanianTuronian resulted in threephases of emergence dated as mid-Cenomanian, lateCenomanian early Turonian, and mid-Turonian,respectively (Blanc et al. 2003; Emami et al. 2010;Verges et al. 2009; Aqrawi et al., 2010; Sharp et al.,2010; Razin et al., 2010; Hollis, 2011). The mid-Cenomanian and mid-Turonian phases of emergencecan be recognized in terms of regional-scaledisconformities (Alsharhan and Nairn, 1986; Alsharhanand Nairn, 1988; Motiei, 1993; Aqrawi, 1998; Aqrawiet al., 2010). However, the CenomanianTuronianphase is restricted to SW Iran and the MesopotamianBasin (Razin et al., 2010; Aqrawi et al., 2010,Rahimpour-Bonab et al., 2012). In the studied field,

all three unconformities are present and the SarvakFormation can be divided into three portions: (i) thelower Sarvak (late Albian to mid-Cenomanian); (ii)middle Sarvak (mid-Cenomanian to the Cenomanian-Turonian disconformity); and (iii) upper Sarvak,comprising the interval between this hiatus and themid-Turonian surface which is marked by extensivekarstification.

In this study, subsurface sections of the SarvakFormation from the Abteymour field in SW Iran areinvestigated. The field is located in the SW marginof the Zagros Basin, in the transition zone betweenthe Mesopotamian Basin and the Dezful Embayment(Fig. 1). Since its discovery in 1969, 31 wells havebeen drilled in the NW-SE trending Abteymouranticline for oil production, and have targettedBangestan Group reservoirs (Illam and SarvakFormations). This symmetric anticline, located on apalaeohigh, measures about 23 by 5 km at the levelof the oil-water contact. Currently, on the base ofdrilling and production data, the Bangestan Groupinterval at this field is divided into nine large-scalezones (Zones 1 through 9). In general, zones 2 and4 are productive at this field.

In previous investigations, zones 1 to 3 wereconsidered to correspond to the Illam Formation,and zones 4 to 9 to the Sarvak Formation (Table 1).However, Rahimpour-Bonab et al. (2012) concludedthat the mid-Turonian disconformity separating theIllam Formation from the underlying SarvakFormation is located above zone 3. Thus zone 3,which was previously erroneously considered as thelowermost interval of the Illam Formation is in factthe uppermost Sarvak interval (Table 1). In this study,reservoir zones 3 and 4, located respectively belowthe mid-Turonian disconformity (i.e. just below therevised Sarvak-Illam boundary) and the CenomanianTuronian disconformity (Table 1, right), wereconsidered. The study attempts to show whyreservoir zone 3 (with an average thickness of 55m), which is intensely karstified as a result of long-term subaerial exposure during the mid-Turonian, isof poor reservoir quality. In addition, the study showsthat because of important heterogeneities, reservoirzone 4 (with an average thickness of 171 m) shouldbe considered as two independent flow zonesseparated by a tight interval.

MATERIALS AND METHODS

This study is based on data from four wells in theAbteymour oilfield (wells AT 1-4, Fig. 1). Highresolution petrographic analyses together with imageanalyses and quantitative analysis of rockcomponents were used to determine the depositionalfacies (and microfacies) and diagenetic features of

217H. Rahimpour-Bonab et al.

Burgan 4th sand

S

S

S

KazhdumiBalambo

SarvakMishrif

AhmadiRumaila

Maudud

Burgan Arch

Burgan 3rd sand

Wara

Laffan

Halul

Southwest Northeast

Continent Shelf

CHRONOSTRATIGRAPHY

AGE

MFS/TMS

Preservation Age

(Ma)123

EUSTACY andTECTONICS

200 m 100 0

Late

Sant-onianCon-iacian

Tur-onian

Ceno-manian

Albian

Saudi Arabia Kuwait Zagros

Aruma

S

Was

ia

TTST

HST

AP 9

K150

K140

K130

K120

K110

AP 8

K100

K90

92

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tiv

en

ort

he

as

tm

arg

in

Cre

tace

ou

s

Pa

ss

ive

marg

in

1

2

3

A

Limestone Shale Conglomerate Shaly limestone

Se

rie

s

Stage Lurestan Khuzestan Coastal Fars Interior Fars

Maastrichtian

Campanian

Santonian

Coniacian

Turonian

Cenomanian

Albian

Aptian

Neocomian

Lo

we

rU

pp

er

Amiran

Tarbur

Surgah

Ilam

AhmadiAhmadi

MauddudSarvakSarvak

Ga

rau

KazhdumiKazhdumi

Garau

Gadvan

Dariyan

FahliyanFahliyan

Gadvan

Ba

ng

est

an

gro

upIlam

Sarvak Sarvak

B

Fig. 2. (A). Generalized chronostratigraphy of the Cretaceous successions in the Zagros region (SW Iran),Kuwait and Saudi Arabia together with eustasy and regional tectonics (after Sharland et al., 2001).(B). Detailed stratigraphy of the Cretaceous successions in different parts of Iran, including the SarvakFormation of the Bangestan Group showing lateral facies and thickness variations. The studied subsurfacesection of the Sarvak Formation at Abteymour field is located in the western Dezful Embayment, in Khuzestanprovince.

the Sarvak Formation. For facies description, amodified Dunham (1962) scheme was used togetherwith sedimentary structures and fabrics, grain sizeand rock composition, as well as diagnostic allochems.Facies analysis was carried out using standard modelsand microfacies descriptions (e.g. Buxton and Pedley,1989; Wilson, 1975; Flugel, 1982 and 2004). Adepositional model of the Sarvak Formation atAbteymour oilfield was presented in a previous study(Rahimpour-Bonab et al., 2012) (Fig. 3), and the roleof facies distribution in shaping the reservoir qualitywere evaluated. Petrographic analyses of 300 thin

sections (cores and cuttings) from two wells (AT-1and AT-3) were used to investigate depositional anddiagenetic controls on the distribution of reservoir(flow) units and non-reservoir units (barriers/baffles)in the Sarvak Formation. Core porosity andpermeability data for three wells (AT-1, -2 and -3),together with log data from four wells (AT-1, -2, -3and -4) were also available for this study. Flow unitdetermination was carried out using two approaches:firstly hydraulic flow units were identified using flowzone indicator values; the alternative method wasbased on stratigraphic modified Lorenz plots.

218 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

Finally, by integrating both geological data (faciesanalysis, diagenetic and sequence stratigraphicinterpretations) and petrophysical data, a two-dimensional reservoir model was constructed for theSarvak Formation at Abteymour oilfield. Factorscontrolling the distribution of reservoir (flow) and non-reservoir units were investigated.

GEOLOGICAL FRAMEWORK

Facies analysis of the Sarvak Formation (Rahimpour-Bonab et al., 2012) resulted in the recognition of 12microfacies which can be grouped into five faciesassociations. An epeiric platform model is proposedfor the formation at Abteymour field (Fig. 3). Theleeward nature of the ramp was indicated by theprevalence of muddy facies. This model suggests thatreservoir quality distribution in this formation isprimarily controlled by depositional facies. Talusdeposits (rudist debris floatstones/rudstones), patchreef (rudist-algal) boundstones and shoal facies(peloidal-bioclastic grainstones/packstones) have the

best primary reservoir qualities (Rahimpour-Bonabet al., 2012).

However, the Sarvak carbonates have undergoneintense diagenetic modification (Fig. 4) and fourdiagenetic processes have been identified: eogeneticand telogenetic meteoric dissolution; dolomitization(especially stylolite-related dolomitization);cementation; and fracturing (Hollis, 2011; Casini etal., 2011; Lapponi et al., 2011; Rahimpour-Bonab etal., 2012).

The reservoir zonation scheme presented in thispaper is based on previously-published studies(Rahimpour-Bonab et al., 2012). Thus, only asummary of geological events in the studied fieldwhich shaped the geological framework of thereservoir zonation is presented here. Mid-Cenomanianemergence resulted in the formation of an importantbasin-scale disconformity within the SarvakFormation. A second phase of uplift and emergencein the late Cenomanian early Turonian resulted inleaching of the middle Sarvak. Subsequently, relativesea-level rise or basin subsidence beginning in the

Illa

m F

orm

atio

nS

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ak F

orm

ati

on

Fo

rma

tio

n

Sta

ge Old reservoir

zonation(by NIOC)

Revised zonation scheme

(this study)

Ce

no

ma

nia

n-T

uro

nia

nC

on

iacia

n?

- S

an

ton

ian

Th

ickn

ess

Zone 1

Zo

ne 2

Zone 3

Zo

ne 4

33

104

55

171

Th

ickn

ess

(m

)

Th

ickn

ess

(

m)

Sta

ge

(Ra

him

po

ur-

Bo

na

b e

t al., 2012)

Fo

rma

tio

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lam

For

mat

ion

Upper

Sarv

ak

Form

ati

on

Ce

no

ma

nia

n

Turonian

Sa

nto

nia

n

Not studied

IRZ1

IRZ2

IRZ3

IRZ4

Not studied

55

43

48

80

Mid-Turonian disconformity

C-T disconformity

Table 1. The previous reservoir zonation scheme for the upper Sarvak and lower Illam Formations atAbteymour field (below, left) was established by NIOC geologists and reservoir engineers. This scheme iscompared with the new zonation scheme (below, right) presented in this study which is based on detailedgeological and petrophysical investigations. The positions of the CenomanianTuronian (C-T) and mid-Turonian disconformities are shown. See text for details.

219H. Rahimpour-Bonab et al.

Faciescode F

1

F2

F3

F4

F5

F6

F7

F8

F9

F10

F11

F12

Na

me

Ca

lcis

iltite

with

S

po

ng

e S

pic

ule

s

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dst

on

e w

ith p

ela

gic

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ra

Pe

loid

al W

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on

e

Wa

cke

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to

Mu

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on

e w

ith b

iocl

ast

s

Bio

cla

stic

F

loa

tsto

ne

to R

ud

sto

ne

Pa

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on

e to

wa

cke

sto

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w

ith b

en

thic

an

d p

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F

ora

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oundst

one

Wack

est

one w

ith la

rge

Fora

min

ifera

Ru

dis

t d

eb

ris

be

arin

g

Flo

ats

tone

Mu

dst

on

e w

ith b

en

thic

Fora

min

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Benth

ic F

ora

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l W

ack

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-Pe

loid

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Gra

inst

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e to P

ack

sto

ne

Mid RampInner Ramp Intrashelf basin(outer ramp)

FWWB

SWB

Supra Tidal

MF 5

MF 6 MF 1

MF 2

MF 3

MF 4

MF 7

MF 8

MF 9MF 10

MF 11 MF 12

Faciesbelts

Mid rampOpen marine

lagoonRestricted lagoonShoal Intrashelf basin (outer ramp)

Fig. 3. Ramp-type depositional model for the Sarvak Formation at Abteymour oilfield together with microfaciesand facies associations occurring in different parts of the model (for details, see Rahimpour-Bonab et al., 2012).

early Turonian led to inundation and the reinstatementof the carbonate factory, and the deposition of theupper Sarvak succession. Burial of the middle Sarvakat this time to shallow depths resulted in cementationand mechanical compaction. A prolonged mid-Turoniansea-level fall and basin-wide uplift resulted in a majorregional disconformity at the top of the upper Sarvakwith intense dissolution and karst formation. Downwardpercolation of meteoric waters led to a renewed phaseof leaching and karstification beneath the Cenomanian Turonian disconformity. These consecutive phasesof diagenesis resulted in significant enhancement of themiddle Sarvak reservoir at Abteymour. In general,diagenetic features show a good correlation withdepositional facies, and are predictable within a sequencestratigraphic framework (Fig. 5). Porosity increasesbeneath sequence boundaries due to meteoric dissolutionand karstification. However, during lengthy periods ofsubaerial exposure (e.g. beneath the mid-Turoniandisconformity in the uppermost Sarvak), porosity isdestroyed by over-mature karstification, e.g. by theformation of collapse breccias resulting in porosityocclusion.

Petrophysical attributes are one-dimensional andcannot in general be predicted in inter-well locations orat distance from the well bore (e.g. Lucia, 2007; Ahr,2008). However, integration of this type of data(including wireline logs, core and production data) with

three-dimensional geologic data (e.g. facies anddiagenetic data compiled in a sequence stratigraphicframework) permits detailed 3D reservoir modelling(Al-Habshi et al., 2003; Roger, 2006). In this study,the sequence stratigraphic framework establishedfrom previous studies (Fig. 5) was used as a basisfor reservoir zonation and lateral correlation.

Sequence stratigraphic analysis resulted in therecognition of three third-order sequences in themid-Cenomanian to mid-Turonian interval (Fig. 5).Sequence boundaries were identified by rapidchanges in depositional environment and distinctdiagenetic effects related to sea-level fall, and weredated on the basis of biostratigraphic analysis(Rahimpour-Bonab et al., 2012). The sequences canbe correlated over the Abteymour oilfield and otherareas in the Zagros Basin and Arabian Platform(Grelaud et al., 2010; Razin et al., 2010; vanBuchem et al., 1996 and 2011; Rahimpour-Bonabet al., 2012).

In the following sections, controls exerted byfacies patterns and diagenetic features within thesequence stratigraphic framework on thedistribution of the reservoir (flow) and non-reservoirunits in the Sarvak reservoir are discussed. A two-dimensional reservoir model is presented for theSarvak Formation at Abteymour oilfield.

220 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

B: upper part of the Sarvak Formation (above C-T disconformity)

A: middle part of the Sarvak Formation (below C-T disconformity)

Marine diagenetic environment

(middle to late Cenomanian)

Meteoricdiagenetic environment

(at mid-Turonian)

Eogenetic meteoricdiagenetic environment(at C-T disconformity)

Telogenetic meteoricdiagenetic environment

(at mid-Turonian)

Shallow burialdiagenetic environment

Shallow burialdiagenetic environment

(early Turonian)

Deep burialdiagenetic environment

Deep burialdiagenetic environment(Santonian-present day)

Marine diagenetic environment

(early Turonian)

Micritization

Isopach cements

Dissolution(Eogenetic)

Mosaic/drusyCement

Dolomitization(Mixing-type)

Recrystallization(Neomorphism)

Shallow burial Cementation

Dolomitization(Stylolite-related)

Stylolitization

Silicification

Dedolomitization

Deep burial Cementation

Solution-collapsebrecciation

Dissolution(Telogenetic)

Isopach cements

Micritization

Dissolution(Eogenetic)

Mosaic/drusyCement

Dolomitization(Mixing-type)

Silicification

Dedolomitization

Dolomitization(Stylolite-related)

Stylolitization Deep burial Cementation

Recrystallization(Neomorphism)

Shallow burial Cementation

Fig. 4. Schematic cartoons illustrating the diagenetic history of the Sarvak Formation from its time ofdeposition to the present day. A (above) refers to the middle part of the Sarvak Formation between the mid-Cenomanian and the CenomanianTuronian (C-T) disconformities; B (below) refers to the upper Sarvakbetween the C-T and mid-Turonian disconformities. This diagenetic history comprises the transition frommarine to meteoric diagenesis (eogenetic and telogenetic phases) and subsequent burial (shallow to deep).See Rahimpour-Bonab et al. (2012) for more details.

221

H. R

ahim

pou

r-Bon

ab et a

l.

Fig. 5. Correlation of third order sequences in the four studied wells at Abteymour field. The sequences were determined using facies, diagenetic and palaeontologicalobservations; the time framework for the disconformities and sequences was based on the results of biostratigraphic analysis. The CenomanianTuronian (C-T) andmid- Turonian unconformities which had a major impact on the reservoir quality evolution of the Sarvak Formation (and its equivalents over the Arabian Platform)are shown.

3300

3400

3500

3600

Se

qu

en

ce

2S

eq

ue

nce

1S

eq

ue

nce

3

50 80

SGR20

Well#4

Dep

th 3rd Seq.

MFS1

MFS2

MFS3

Disc. 1

Disc. 2

Dep

th Well#2

3230

3250

3300

3350

3400

3450

20 40 60

3rd Seq.GR

3293

3400

3433

20 40 60GR

AT#14 3rdSeq.

Sedimentological Characteristics

P/B: Pelagic to Benthic forams ratio

TextureM W P G B

Dep

th Well#1Facies

Associations

Lg Pr Tl Sh Bs

Diagenetic Features

Ds Dl St Cm Fr M W P G B

Texture

De

pth Well#3

GR3rd

Seq.20 40 60

3300

3350

3400

3450

Diagenetic Features

Ds Dl St Cm FrFacies

Associations

Lg Pr Tl Sh Bs

Sedimentological Characteristics

3293

SB 1

SB 2

SB 3

SB 4

Mid

dle

Sa

rva

kU

pp

er

Sa

rva

k

Disc.3

Disc. 1: Mid Turonian unconformityDisc. 2: Cenomanian-Turonian boundary unconformity

SB: Sequence boundaryMFS: Maximum Flooding Surface

Highstand systems tract

Transgressive systems tract

M: Mudstone

W: Wackestone

P: Packstone

G: Grainstone

B: Boundstone

Lg: Lagoon

Pr: Patch reef Tl: Talus

Sh: Shoal

Bs: Basin (Outer ramp)

Ds: Dissolution

Dl: Dolomitization

St: Stylolitization

Cm: Cementation

Fr: Fracturing

Disc. 3: Middle Cenomanian unconformity

Bio

zo

ne

No

.

Biozone

26

25

29

Ne

zza

za

tin

ella

-Dic

yclin

a a

sse

mb

lag

e z

on

e(W

yn

d,1

96

5)

Ne

zza

za

ta-A

lve

olin

ida

e A

sse

mb

lag

e z

on

e(W

yn

d,1

96

5)

Oligostegina Interval zone(Wynd,1965)

Sa

rva

k

Fo

rmati

on

Sta

ge

Tu

ron

ian

Cen

om

an

ian

Albian-Cenomanian

Se

rie

s

Sy

ste

m

Mid

dle

Cre

tac

eo

us

Cre

tac

eo

us

P/B

>11

222 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

FLOW UNIT IDENTIFICATION ANDRESERVOIR MODELLING

Rock typing concepts can be used to establish therelationship between petrophysical data from differentsources (such as log, core and production data) andgeological descriptions (Amaefule et al., 1993; Porrasand Campos, 2001; Soto and Garcia, 2001; Granier,2003; Bagheri et al., 2005; Asgari and Sobhi, 2006;Gomes et al., 2008). Rock typing involves theidentification of particular facies types on the basisof their dynamic behaviour (Varavur et al., 2005).The dynamic behaviour is determined from studiesof textures, diagenetic alterations and rock-fluidinteractions (Bear, 1972; Gomes et al., 2008). Thus,rock typing integrates 3D geological data withnumerical 1D petrophysical data.

Petrophysical parameters and dynamic calculationsextracted from core analyses are combined withgeological facies to determine flow unit behaviour andspatial distribution. The relationship between faciesand rock types in a carbonate reservoir is oftencomplex due to interference between faciesdistribution, diagenetic processes and rock-fluidinteractions (wettability variations) (Varavur et al.,2005; Gomes et al., 2008). In this study, two differentapproaches were used to differentiate flow units fromnon-reservoir units in the Sarvak Formation. In thefirst approach, hydraulic flow units were identifiedusing flow zone indicator values (Ebanks, 1987;Abbaszadeh et al., 1996; Porras and Campos, 2001;Tiab and Donaldson, 2004; Uguru et al., 2005). Thesecond approach was based on a stratigraphicmodified Lorenz plot (Gunter et al., 1997; Chopra etal., 1998; Gomes et al., 2008). These methods wereapplied in three wells (AT-1, -2 and -3) for whichwell log and core poroperm data are available. Themethods are detailed below and the results analyzedand compared.

Hydraulic flow unit determination usingflow zone indicator valuesA flow unit is defined as a volume of rock in whichthe pore-throat properties of the porous medium whichgovern the hydraulic character of the rock are bothconsistently predictable and significantly differentfrom those of other units (Amaefule et al., 1993;Abbaszadeh et al., 1996; Porras and Campos, 2001;Soto et al., 2001). A reservoir may be divided intoflow units to describe its performance during differentproduction schemes, and the division can be madefrom either geological or engineering standpoints. Inorder to understand and model the spatial distributionof reservoir characteristics, 1D engineering datashould be integrated with 3D geological data (e.g.Gomes et al., 2008).

Discretizing the reservoir into units, such as layersand blocks, and assigning values of all pertinentphysical properties to these units, will improve theunderstanding of the reservoirs heterogeneity. TheHydraulic Unit concept (e.g. Amaefule et al., 1993)can be used to divide a reservoir into distinctpetrophysical types, each of which has a unique flowzone indicator (FZI) value (Al-Ajmi and Holditch,2000).

Kozeny (1927) and Carmen (1937) simulated aporous medium as a bundle of capillary tubes,combining Darcys law for flow in a porous mediumand Poiseuilles law for flow in a tube. In order todescribe the relationship between porosity andpermeability, a tortuosity factor was included becausein reality connected pores are not straight capillarytubes. Thus:

(1)

where k is permeability,e is effective porosity, is

tortuosity, and rmh is mean hydraulic radius. The mean

hydraulic radius can be related to the surface area perunit grain volume (Sgv) and the effective porosity eby the following equation:

(2)

Combining these two equations gives thegeneralized Kozeny-Carmen equation:

(3)

where k is in m2, e is a fraction and F is formation

resistivity. The term Fs2 is known as the Kozeny

constant, and usually has values of between 5 and100 in most reservoir rocks. The term F

s2 Sgv2 is a

function of the geological characteristics of the porousmedium and varies with changes in pore geometry.The determination of F

s2 Sgv2 is the focal point of the

HFU classification technique.Amaefule et al. (1993) addressed the variability

of Kozenys constant by dividing Eq. 3 by effectiveporosity,

e:

(4)

where the constant 0.0314 is the permeabilityconversion factor from m2 to mD. A flow zoneindictor value, FZI, is defined as:

(5)

The reservoir quality index, RQI, is defined as follows:

(6)

kr r re e e mh 8 2 2 22 2 2 22( )

kF S

e

e s gv

(( ) ) 3 2 2 21 1

FZIF Ss gv

( )1RQI

k

e

0 0314.

Sr

gv

mh

e

e

1 1( )

0 03141

1. (

( ))( )

k

F Se

e

e s gv

223H. Rahimpour-Bonab et al.

where z, the normalized porosity, is:

(7)

Eq. 6 then becomes:

(8)

Taking the logarithm of both sides of Eq. 8 gives:

(9)

Input data for this study include core porosity (e)

(c.f. Tiab and Donaldson, 2004), and permeability

0.001

0.01

0.1

1

10

0.001 0.01 0.1 1

z

RQ

I

AT-3 well

2R =0.7843

2R =0.8529

2R =0.5931

HFU 1

HFU 2

HFU 3

Fig. 6. Log-log plot of RQI versus z for

well AT-3 (see text for details). Datadistribution allows three hydraulicflow units (HFUs) to be distinguished.

Depth

(m)

Porosity

(fraction)

K air

(md) R QI ? z LOG FZI FZI zone

101.13 0.1089 0.762 0.08306 0.12221 -0.16771 0.67966 2

103.34 0.1555 4.022 0.15969 0.18413 -0.06185 0.86727 2

103.6 0.1044 1.402 0.11507 0.11657 -0.00563 0.98711 2

104.76 0.1645 2.461 0.12145 0.19689 -0.20982 0.61685 2

104.84 0.164 3.829 0.15172 0.19617 -0.11159 0.77342 2

106.18 0.1955 11.584 0.2417 0.24301 -0.00234 0.99464 2

106.64 0.1784 6.38 0.18778 0.21714 -0.06309 0.86478 2

106.87 0.1853 7.243 0.19631 0.22745 -0.06393 0.86312 2

106.94 0.1922 3.945 0.14226 0.23793 -0.22337 0.5979 2

107.1 0.1692 3.222 0.13702 0.20366 -0.17211 0.6728 2

107.26 0.1533 1.736 0.10567 0.18106 -0.23388 0.58361 2

108.15 0.1995 3.197 0.1257 0.24922 -0.29725 0.50437 2

108.24 0.2149 2.259 0.10181 0.27372 -0.42954 0.37193 2

108.59 0.192 2.641 0.11646 0.23762 -0.30973 0.49009 2

108.66 0.2049 1.85 0.09435 0.2577 -0.43638 0.36612 2

108.93 0.2048 5.003 0.1552 0.25755 -0.21997 0.6026 2

109.22 0.1587 1.121 0.08345 0.18864 -0.35418 0.4424 2

109.56 0.1666 1.248 0.08594 0.1999 -0.36662 0.42991 2

109.83 0.1856 2.24 0.10908 0.2279 -0.31998 0.47866 2

110.23 0.1835 2.215 0.10909 0.22474 -0.31388 0.48542 2

110.46 0.1861 2.49 0.11486 0.22865 -0.29902 0.50232 2

110.88 0.1781 1.374 0.08721 0.21669 -0.39525 0.40248 2

110.93 0.185 1.039 0.07441 0.22699 -0.48436 0.32782 2

from three wells measured in 0.3 m to 1 m intervals.An example of the dataset used for the FZI method isshown in Table 2. Ideally, on a log-log plot of RQIversus

z (e.g. Fig. 6 data from well AT-3), samples

with similar FZI values will lie on a straight line witha unit slope, and data samples with significantlydifferent FZI values will lie on parallel unit-slope lines.Samples that lie on the same straight line have similarpore throat attributes and thereby constitute a uniquehydraulic flow unit. Each line represents a particularHFU, and the intercept of this line with

z = 1 gives

the mean FZI value for that HFU.Based on this method, three HFUs were determined

in the studied wells (wells AT-1,-2 and -3); poroperm

Table 2. Example of dataset used for the hydraulic flow units determination using the FZI method in well AT-3.The RQI, normalized porosity (

z) and FZI values are calculated using formulae presented in the text.

RQI FZIz log log logRQI FZIz

z e e (( ))1

224

Flow u

nits in

the C

retaceou

s Sarva

k Formation

carb

onates, A

bteym

our field

, SW Iran

Porosity (decimal)

P

e

r

m

e

a

b

i

l

i

t

y

(

m

d

)

0 0.1 0.2

0.0001

0.001

0.01

0.1

1

10

1002

R = 0.9593

2R = 0.7901

2R = 0.8384

Porosity(decimal)

P

e

r

m

e

a

b

i

l

i

t

y

(

m

d

)

0 0.1 0.2 0.3

0.001

0.01

0.1

1

10

100

1000 AT#3 Well

2R = 0.5993

2R = 0.8742

2R = 0.8612

0.15 0.30.2 0.4

0.1

1

10

100

1000

Porosity (decimal)

P

e

r

m

e

a

b

i

l

i

t

y

(

m

d

)

2R = 0.9608

2R = 0.5213

2R = 0.7182

AT#2 Well

LOG FZI Values

HFU 1 HFU 2 HFU 3

< - 0.5 - 0.5 to 0 > 0

HFU 1 HFU 2 HFU 3

-1.5 to - 0.5 - 0.5 to 0 0 to 0.5

HFU 1 HFU 2 HFU 3

< - 0.5 - 0.5 to 0.5 > 0.5

S

t

r

a

t

i

g

r

a

p

h

i

c

m

o

d

i

f

i

e

d

L

o

r

e

n

z

p

l

o

t

(

S

M

L

P

)

H

y

d

r

a

u

l

i

c

F

l

o

w

U

n

i

t

(

H

F

U

)

d

e

t

e

r

m

i

n

a

t

i

o

n

m

e

t

h

o

d

AT#1 Well

Percent Storage Capacity (%PHIH)

P

e

r

c

e

n

t

F

l

o

w

C

a

p

a

c

i

t

y

(

%

K

H

)

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Inflection points

12

33 4

5

5

Percent Storage Capacity (%PHIH)

P

e

r

c

e

n

t

F

l

o

w

C

a

p

a

c

i

t

y

(

%

K

H

)

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Inflection points

12

4

5

Percent Storage Capacity (%PHIH)

P

e

r

c

e

n

t

F

l

o

w

C

a

p

a

c

i

t

y

(

%

K

H

)

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

1 23

4

Inflection points

A

B

Fig. 7. (A) Porosity-permeability cross-plots for wells AT-1, -2 and -3 from which hydraulic flow units (HFUs) can be identified. The limits of log FZI for each HFU arealso shown. (B) Cross-plots of storage capacity ( h%) versus flow capacity (k.h%) for wells AT-1 to AT-3 (see text for details).

225H. Rahimpour-Bonab et al.

diagrams and FZI log values for these wells are shownin Fig. 7A.

HFU determination using astratigraphic modified Lorenz plotThe best way to assess the minimum number of flowunits in a reservoir uses a technique based on astratigraphic modified Lorenz plot (SMLP) (Gunteret al., 1997; Tiab and Donaldson, 2004; Gomes etal., 2008). This method is a graphical tool which usesvarious data including the geological framework,petrophysical rock/pore types, storage capacity andflow capacity. By integrating these data, rock-typebased zonations can be transformed intopetrophysically-based flow units.

To construct the SML plot, continuous (ft-by-ft)core porosity and permeability values and therespective k/ ratios are arranged in stratigraphic order(Gomes et al., 2008). The products of k*h and *hare calculated, the partial sums are computed and totalsare normalized to 100%. *h and k*h are referred asstorage capacity and flow capacity, respectively. AnSML plot is a cross-plot of cumulative flow capacityversus cumulative storage capacity (e.g. Fig. 7B).

The equation for obtaining a single value ofcumulative flow capacity is as follows (Maglio-Johnson, 2000):

(10)

where k = permeability (mD), and h = thickness ofthe sample interval.

A similar equation can be used to determine a singlevalue of cumulative storage capacity:

(11)

where = fractional porosity.An example of dataset used for the stratigraphic

modified Lorenz plot is shown in Table 3. Plots of*h versus k*h values for the studied wells arepresented in Fig. 7B. The slope of the segments onthese plots is indicative of the flow performance ofthe reservoir. Preliminary flow units (speed zones,tight/baffle zones and seals) are interpreted byselecting changes in slope or inflection points. Usingthis method, the main flow units are illustrated in theircorrect stratigraphic position. Segments with steepgradients have a greater percentage of flow capacityrelative to storage capacity, and by definition, have ahigh reservoir process speed. They are referred to asspeed zones (Chopra et al., 1998) or sometimes ashydraulic units (in this paper, flow units, speed

Depth

(m) ? (%) v/v K (md) kh% phi phih% logr35 r35

281.05 1.44 0.0144 1.308 0.66374 4.610416

281.71 1.36 0.0136 27.02 0.639414 0.008976 0.052738 1.458453 28.73779

282.29 3.08 0.0308 0.244 0.005074 0.017864 0.104959 -0.05032 0.890589

282.47 6.85 0.0685 0.535 0.003453 0.01233 0.072444 -0.14976 0.70833

282.53 4.28 0.0428 0.33 0.00071 0.002568 0.015088 -0.09668 0.800422

282.69 6.13 0.0613 13.02 0.074694 0.009808 0.057626 0.707025 5.093607

283.56 13.49 0.1349 4.542 0.141683 0.117363 0.689559 0.142131 1.387174

283.87 13.83 0.1383 1.758 0.01954 0.042873 0.251898 -0.1096 0.776964

283.93 9.75 0.0975 3.134 0.006742 0.00585 0.034371 0.169206 1.476407

284.53 3.71 0.0371 1.575 0.033883 0.02226 0.130787 0.356062 2.270188

284.59 3.07 0.0307 2.124 0.004569 0.001842 0.010823 0.503479 3.187714

284.83 2.08 0.0208 0.743 0.006394 0.004992 0.02933 0.381335 2.406217

285.3 0.76 0.0076 12.752 0.214896 0.003572 0.020987 1.485057 30.55323

285.78 1.77 0.0177 0.078 0.001342 0.008496 0.049918 -0.1337 0.735026

285.84 2.55 0.0255 0.036 7.74E-05 0.00153 0.008989 -0.46814 0.340295

286.1 0.46 0.0046 0.278 0.002592 0.001196 0.007027 0.696476 4.971364

287.67 2.88 0.0288 84.726 4.769459 0.045216 0.265664 1.468759 29.42786

289.67 2.98 0.0298 0.013 0.000932 0.0596 0.350176 -0.78672 0.163409

289.95 7.06 0.0706 0.647 0.006496 0.019768 0.116146 -0.11256 0.771693

289.97 1.77 0.0177 6.119 0.004388 0.000354 0.00208 0.980319 9.556948

290.37 15.32 0.1532 4.587 0.065787 0.06128 0.360047 0.096915 1.250015

290.42 21.47 0.2147 88.284 0.158272 0.010735 0.063073 0.725476 5.314663

290.88 12.56 0.1256 21.145 0.348753 0.057776 0.339459 0.561695 3.644979

290.95 20.38 0.2038 52.469 0.13169 0.014266 0.083819 0.61215 4.094024

Table 3. Example of dataset used for the stratigraphic modified Lorenz plot method for well AT-3. The kh%,h% and R

35 values are calculated using formulae presented in the text.

( ) ( ) ( ) ( ) h h h h h h hcum ki i i 1 1 0 2 2 1 1

( ) ( ) ( ) ( )kh k h h k h h k h hcum i i i 1 1 0 2 2 1 1

226 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

zones and hydraulic units are synonymous). Theyshow high flow and storage capacities (for examplesegments 2, 4 and 5 in wells AT-1 and -2; and segments4 and 5 in well AT-3 in Fig. 7B).

Segments with lower gradients, i.e. relatively lowerstorage and flow capacities, are known as baffle units(for example, segments 1 and 3 in well AT-1 andsegment 3 in well AT-3 in Fig. 7B). These are typicallytight or dense stylolitic zones within the reservoirsuccession. Segments with neither flow nor storagecapacity are seals or barriers (e.g. segments 1 and 3in well AT-2 and segment 1 in well AT-3 in Fig. 7B).

Based on these plots, the studied intervals in eachwell can be divided into three types of unit (Fig. 8):

1. Reservoir units (R.Us), with high h% and kh%values; (h% from 25 to 40, and kh% from 20 to35);

2. Baffle units (Bf.Us), with high h% but lowkh% values (h% from 15 to 25, and kh% below10); and

3. Barrier units (Br.Us), with low h% and kh%values (h% below 10; kh% mostly below 5), whichblock (horizontal or vertical) flow through thereservoir and result in compartmentalization.

Stratigraphic modified Lorenz plots were compiledfor wells AT-1, -2 and -3, and flow units were ientified(Figs. 8 and 9). According to this interpretation (Fig.9), there are three flow units and two baffles in wellAT-1; three flow units and two barriers in well AT-2;and two flow units, two baffles and one barrier unitin well AT-3 (Fig. 9).

H. D. Winland developed an empirical equation todelineate commercial hydrocarbon reservoirs and todefine flow units (Gunter et al., 1997; Aguilera, 2002;Tiab and Donaldson, 2004). The equation is based onthe correlation between porosity, permeability, and porethroat radius at the point of 35% mercury saturation(R35) in capillary pressure measurements and wasderived from formations ranging widely in age andlithology. The equation is (Aguilera, 2002):

Log R35 = 0.732 + 0.588 (Log kair) 0.864 (Log e)

The correlation is generally reliable for rocks withonly intergranular porosity (such as sandstone) wherepore and pore throat geometry are related closely torock texture. However, porosity in carbonate rocksis not always intergranular and the Winland method isnot therefore as reliable for assessing reservoir qualityin carbonate reservoirs. Therefore samples from theSarvak Formation were tested for reservoir qualitywith use of a modified Winland R35 equation (Pittman,1992):

Log R35 = 0.255 + 0.565 Log k 0.523 Log e

This equation was found to provide a moreaccurate graphic solution (e.g. Fig. 8). As shown inFig. 8 and as indicated by other researchers (e.g.Gunter et al., 1997), changes in R35 values can beused to define major flow units. The figure showsthat R35 values are closely correlated with defined flowunits and show corresponding variations.

A NEW INTEGRATED RESERVOIR ZONATIONSCHEME AT ABTEYMOUR

As discussed above, on the base of drilling andproduction data, the Bangestan Group interval at theAbteymour field is divided into nine large-scale zones:zones 1 to 2 are in the Illam Formation and zones 3 to9 are in the Sarvak Formation (Table 1, right). Onlyzones 2 and 4 are productive, and zone 3, in spite ofextensive karstification, shows poor reservoirqualities. In this study, reservoir zones 3 and 4, locatedrespectively below the mid-Turonian disconformity(i.e. just below the SarvakIllam boundary) and theCenomanianTuronian disconformity (Table 1), wereconsidered. Considering flow units determined by thevarious methods discussed in the last section, thestudy investigates why reservoir zone 3, which isintensely karstified as a result of long-term subaerialexposure during the mid-Turonian, is of poor reservoirquality. In addition, the study shows that because ofthe presence of important heterogeneities, reservoirzone 4 should be considered as two independent flowzones separated by a tight interval (Table 1).

In Fig. 8, reservoir, baffle and barrier unitsdetermined on the basis of the stratigraphic modifiedLorenz method for well AT-3 are compared with HFUsresulting from the FZI method. There is a relativelygood correspondence between the results of thesetwo methods. For example, barrier and baffle unitsidentified from the SMLP method in general correlatewith low and medium quality HFUs identified usingthe FZI method (yellow and blue colours, respectively:see key in Fig. 10 below). However, differences inthe resolution and scale of two methods have resultedin some inconsistencies.

As regards the spatial distribution of reservoir andnon-reservoir (baffle or barrier) units and high to lowquality HFUs in the three studied wells (Fig. 9), fournew Integrated Reservoir Zones (IRZs) are introducedin this study. They are numbered IRZ1 to 4 in Fig. 9.These zones are correlatable throughout the field andtheir occurrence at inter-well locations is geologicallyjustifiable.

IRZ1: This zone corresponds to non-reservoirunits and low-quality HFUs in the uppermost intervalof all the studied wells (i.e. at depths of 3230 to 3290m in well AT-1, 3225 to 3295 m in well AT-2; and3280 to 3320 m in well AT-3). This interval coincides

227

H. R

ahim

pou

r-Bon

ab et a

l.

Fig. 8. Hydraulic flow unit (HFU) determination using flow zone indicators (FZIs) and stratigraphic modified Lorenz plot (SMLP), correlated within a framework offacies and sequences at well AT-3. There is a relatively high correspondence between the results of the two methods used for HFU determination; minorinconsistencies are due to differences in resolution and scale of the two methods.

r35 k phi k/phi %KH %PHIH

10 20 30 40 50 10 20 30 40 50Depth

3451

3300

3350

3400

Well#3

GR

3rd Sequences

20 40 60

Texture

M W P G B

Br.U

R.U

Bf.U

R.U

Bf.U

10100 10 10010100.1 100.1101 10110

HST TST

m

f

s

HFU 1HFU 2HFU 3

FZI method 3rd order sequence R.UBr.U

Bf.U:

:

: Reservoir Unit

Barrier Unit

Baffle Unit

HFUfromFZI

method

S

e

q

u

e

n

c

e

1

S

e

q

u

e

n

c

e

2

S

e

q

u

e

n

c

e

3

S

a

r

v

a

k

F

o

r

m

a

t

i

o

n

S

t

a

g

e

T

u

r

o

n

i

a

n

C

e

n

o

m

a

n

i

a

n

High qualityMedium qualityLow quality

228 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

with the reservoir zone 3 of previous studies (Table 1and Fig. 9).

IRZ2: This zone comprises the high quality HFUsand reservoir units developed just below theCenomanian-Turonian disconformity in the studiedwells. Depths are 3290 to 3350 m in well AT-1, 3295to 3330 m in well AT-2, and 3320 to 3350 m in wellAT-3. This interval coincides with the uppermost partof high-quality reservoir zone 4 of previous studies(Table 1 and Fig. 9).

IRZ3: This zone corresponds to the medium tolow quality HFUs and baffle/barrier units developedin the middle parts of the studied intervals at the threewells (depth ranges of 3350 to 3390 m in well AT-1,3330 to 3370 m in well AT-2, and 3350 to 3375 m inwell AT-3). This interval coincides with the middlepart of high quality reservoir zone 4 of previousstudies (Table 1 and Fig. 9), but in contrast to theprevious zonation scheme includes very low reservoirquality layers.

IRZ4: This zone comprises the high quality HFUsand reservoir units developed in the lower parts ofthe studied intervals at the three well locations (depthranges of 3390 to 3435 m at well AT-1, 3370 to 3450m at well AT-2, and 3375 to 3451 m at well AT-3).The zone coincides with the lowermost portion ofhigh-quality reservoir zone 4 of previous studies(Table 1 and Fig. 9).

Factors controlling reservoir quality variations andtheir spatial distribution are discussed in the nextsection.

DISCUSSION

Carbonate reservoirs are often characterized byconsiderable heterogeneity ranging from the pore tothe reservoir scale (Aplin et al., 2002; Lucia, 2007;Ahr, 2008; Gomes et al., 2008). Identifying reservoirrock types and their vertical and horizontalheterogeneities is important in reservoircharacterization and the construction of three-dimensional geological and flow simulation models(e.g. Beiranvand et al., 2007). Two frequently usedmethods were applied in this study, and there is closecorrespondence between the respective results. Fromthe available data (i.e. well logs and core poropermdata from three wells) using both methods, rock typegroups (flow units) were determined and four newintegrated reservoir zones (IRZ #1 to 4) wereidentified. Below, the origins and spatial distributionof this new reservoir zonation scheme for Abteymourfield are discussed in a sequence stratigraphicframework (Figs 10 and 11).

IRZ1: Relatively short episodes of subaerialexposure (10 to 400 ky) can result in high porosity

development (and to some extent permeabilitydevelopment) in comparison to longer periods ofexposure (1 to 20 million years) (Mazzullo andChilingarian, 1992). A widespread mid-Turonian fallin sea level and basin-wide exposure lasting severalmillion years led to the formation of an importantregional disconformity in the upper Sarvak unit atAbteymour field (Rahimpour-Bonab et al., 2012),represented by major diagenetic dissolution and anover-mature karst profile. This interval correspondswith the non-reservoir (barrier) units in IRZ1 in thestudied wells and is highly compacted and includescompletely cemented horizons (Figs. 11A and 12A).Contrary to expectation, intense meteoric diagenesisunder a warm, humid climate has resulted in karstover-maturation and thus low reservoir quality in theSarvak Formation carbonates in the studied wells.

IRZ2: The highest quality flow units occur (Figs.9 to 11) in the upper parts of sequence 2, includingthe interval below the Cenomanian-Turonianunconformity; and in the underlying succession whichmainly comprises sequence 1. During theCenomanian-Turonian hiatus, local uplift causedsubaerial exposure of the Sarvak Formation in thestudy area. Under a tropical climate, extensivedissolution led to development of karst and microkarstnetworks in the formation (Taghavi et al., 2006;Hajikazemi et al., 2010; Sharp et al., 2010; Razin etal., 2010; Rahimpour-Bonab et al., 2012). Karstdevelopment depends on a number of factorsincluding the duration of exposure. Despite acomparable climate and similar sediments, a shorterperiod of exposure in the Cenomanian Turonian (lessthan 1 Ma, compared with mid-Turonain exposureof about 4 to 6 Ma) resulted in better developmentand preservation of microkarst networks at Abteymourfield (Fig. 11B).

Two generalizations can be made about therelationship between flow units and sequencestratigraphic position in IRZ 2. Firstly, high qualityflow units are located in the uppermost parts ofhighstand system tracts, especially below theCenomanian-Turonian unconformity (Figs. 11B and12B). Secondly, high quality flow units occur insequence 1, and with less importance in sequence 2,in transgressive system tracts below maximumflooding surfaces and, with less importance, in theearly HST. High reservoir qualities in the latter (mud-dominated) intervals (IRZ4) are a result of stylolite-related dolomitization (Figs. 11D and 12C). Dissolutionduring mesogenetic diagenesis was a secondary factorresulting in the improvement of reservoir quality inbasinal and outer ramp oligosteginid facies in IRZ2. Amajor part of this reservoir zone correlates with thetalus facies containing abundant rudistid bioclasts

229

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Fig. 9. Correlation of hydraulic flow units (HFUs) determined by the FZI method with reservoir, baffle and barrier units resulting from the SMLP method for wellsAT-1 to AT-3 within a sequence-stratigraphic framework. There is a relatively high correspondence between the results of the two methods. The new IntegratedReservoir Zones (IRZ1-4) together with the previously-used gross reservoir zonation scheme (zone 3, zone 4) (see Table 1) are also shown and correlated.

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230 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

Fig. 10. Correlation of HFUs resulting from the FZI method and sequence stratigraphic positions, faciesassociations and diagenetic features (including dissolution and dolomitization) in well AT-1. Gamma-ray and oil-water saturation logs and parameters resulting from microscopic image analysis are also included.

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Fig. 11. Main panel shows the correlation of hydraulic flow units (HFUs) identified using the FZI method and integrated reservoir zones (IRZs 1 to 4) within asequence stratigraphic framework for wells AT-1 to -3 at Abteymour field. Schematic cartoons (A, B, C and D) illustrate processes and features which controlledreservoir qualities of these zones in different parts of the model:(A) IRZ1 Barrier unit (tight interval): reservoir quality destruction below mid-Turonian disconformity due to extended exposure and over-mature karst profile.(B) IRZ2 Flow unit: high reservoir quality below Cenomanian-Turonian disconformity due to karstification in shorter exposure.(C) IRZ3 Barrier unit (tight interval): domination by low reservoir quality mud-dominated facies without secondary porosity enhancement, compartmentalizing thereservoir.(D) IRZ4; Flow unit: high reservoir quality resulting from burial (stylolite-related) dolomitization.

232 Flow units in the Cretaceous Sarvak Formation carbonates, Abteymour field, SW Iran

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composed of aragonitic and high magnesium calcite.Dissolution of these bioclasts during early diagenesisresulted in mouldic pores which were completely orpartially filled by meteoric and shallow burial cements.During subsequent uplift of the Sarvak carbonatesand infiltration of meteoric waters during mid-Turonian exposure, these cements were dissolvedresulting in either vuggy porosity or well-connectedtouching pore spaces. Higher values of permeabilityindicate the development of microkarst networks andmicrofractures in this zone. Thus, flow units withintermediate reservoir qualities show a relatively goodcorrelation with talus facies associations (Fig. 10).

As detailed by Rahimpour-Bonab et al. (2012) forAbteymour field, extensive fabric-selective dissolutionduring eogenetic meteoric diagenesis and non-fabricselective dissolution during telogenetic meteoricdiagenesis have enhanced the reservoir properties ofthe Sarvak Formation. These diagenetic processesoccurred at the end of highstand system tract

development (especially in the late HST of Sequence2, below the Cenomanian-Turonian unconformity),and were observed in all the studied wells (Figs. 11Band 12B).

IRZ3: Domination by mud-dominated facies, lackof dolomitization and karst-related features make thisinterval a non-reservoir unit (baffle zone in wells AT-1 and -3, and a barrier in well AT-2). Pelagicwackestones and mudstones (outer ramp and basinaldeposits) are the main facies associated with this IRZ.They occur principally in the transgressive systemtract of sequence 2 in all the studied wells (Figs. 9and 11C). Contrasting with the results of previouszonation schemes, this IRZ has resulted incompartmentalization of the Sarvak reservoir (Figs.9 and 11).

IRZ4: Facies types are wackestones to mudstoneswhich underwent stylolitization and related

Fig. 12. Core photos and thin section images illustrating the three principal controls on reservoir quality of theSarvak Formation at Abteymour field.(A) Collapse brecciation and cave-filling due to karst over-maturation; this occurs below the mid-Turoniandisconformity. Some of these breccias are marked as a1 and a2 in the figure.(B) Macro- and microscopic karst networks were preserved as a result of short term subaerial exposure undertropical climatic conditions during the Cenomanian-Turonian hiatus. Some of these solution vugs are markedas b1 and b2 in the figure.(C) Stylolite-related dolomitization controlled the final reservoir quality of the Sarvak carbonates (especially inthe lower parts of the formation). Stylolite related dolomites and oil staining in stylolite paths are marked asc1 and c2.

233H. Rahimpour-Bonab et al.

dolomitization. Dolomitization resulted in formationof high quality reservoir intervals in the early (andespecially the late) TST and early HST of sequence 1and the TST of sequence 2 (labeled as IRZ4, Figs.11D and 12C). Lapponi et al. (2011) also describedvolumetrically significant dolomitization of latest Albianto Turonian carbonates in the Lower and UpperSarvak Formation. Intermediate and low quality flowunits in these sequences show a close correlation withfacies associations and diagenetic features (Fig. 10).

There is a close relationship in the studied wellsbetween reservoir and non-reservoir units, sequencestratigraphic position and facies characteristics (Figs.9 to 11). Logs of porosity (from image analysis) versusrock-type groups in well AT-1 (Fig. 10) show thathigh quality rock-type groups (i.e. high quality flowunits) occur in the lower parts of sequence 1.

Fractures occur in various states (from open topartially and completely filled) and have, at least partly,influenced the reservoir quality of the SarvakFormation, as has also been reported from other areas(e.g. Casini et al., 2011).

CONCLUSIONS

This study has focused on carbonates in the mid-Cretaceous Sarvak Formation at Abteymour field, SWIran. Microfacies analyses, diagenetic studies andsequence stratigraphy from a previous investigationwere integrated with petrophysical data to develop anew reservoir zonation scheme. In order to visualizethe spatial distribution of depositional facies anddiagenetic imprints, the data were integrated within asequence stratigraphic framework.

Flow (reservoir) units in the Sarvak Formationwere modelled by two independent methods: hydraulicflow units (HFUs) were identified firstly using flowzone indicators (FZIs); and secondly, usingstratigraphic modified Lorenz plots. The results ofthe two methods show reasonable correlation.Integrating the rock types resulting from these twomethods, a new reservoir zonation scheme is identifiedfor the Sarvak Formation at Abteymour field. Thisnew scheme is an improvement on the reservoirzonation which has previously been used. IntegratedReservoir Zones (IRZs) 1 to 4 are identified andillustrated within a sequence stratigraphic frameworkfor the upper Sarvak Formation. Compared to theprevious zonation scheme, the new IRZs show: higherresolution; greater predictability within a sequencestratigraphic framework; greater correlation betweenwells; and an origin which is more geologicallyjustified. The new IRZs thus provide a better basisfor 3D reservoir modelling.

This study showed that in various horizons atAbteymour field, reservoir distribution in the Saravk

Formation is governed by different factors. In IRZ1,sequence stratigraphic position associated withtectonic movements (emergence during the mid-Turonian), influenced the reservoir characteristics andflow unit structures. But in IRZ2, despite comparableconditions (e.g. emergence and karstification under ahumid climate at the Cenomanian-Turonian boundary),difference in the duration of the exposure led to higherflow unit quality, forming the most productive zoneof the Sarvak reservoir. The low quality mud-dominated facies association within the transgressivesystem tract, in the absence of secondary porosityenhancing processes (such as karstification ordolomitization), formed the low-quality IRZ3.However, in IRZ4, the reservoir quality of the mud-dominated facies in the early and late TST and earlyHST improved as a result of extensive dolomitization.

This study emphasises the importance of detailedgeological studies, including facies analysis anddiagentic history reconstructions, integrated within asequence stratigraphic framework, for the predictionof controls on reservoir development. Establishing apredictive model for the reservoirs structure enableda definition of flow units using two separate methods,within a sequence stratigraphic framework. Theresults of this study will be applicable to similar SarvakFormation reservoirs in SW Iran.

ACKNOWLEDGEMENTS

The University of Tehran provided facilities for thisresearch for which the authors are grateful. NIOC isthanked for their support and data preparation. JPGeditorial staff assisted with the English languagepresentation. Journal reviews were by Cathy Hollis(Manchester University) and Farid Taati (NIOC)whose comments on a previous version areacknowledged with thanks.

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