Microfacies correlation analysis of the Oligocene-Miocene Asmari Formation, in the central part of the Rag-e-Safid anticlinal oil field, Zagros Basin, south-west Iran

Turkish Journal of Earth Sciences
/>
Research Article

Turkish J Earth Sci
(2013) 22: 204-219
© TÜBİTAK
doi:10.3906/yer-1201-6

Microfacies correlation analysis of the Oligocene-Miocene Asmari Formation, in the
central part of the Rag-e-Safid anticlinal oil field, Zagros Basin, south-west Iran
Mahnaz AMIRSHAHKARAMI*
Geology Department, Payame Noor University, Iran
Received: 15.01.2012

Accepted: 14.07.2012

Published Online: 27.02.2013

Printed: 27.03.2013

Abstract: The Oligocene-Miocene Asmari Formation was deposited in a carbonate ramp setting at the margin of the Zagros Basin in
south-western Iran. The subsurface sedimentary successions of the Asmari Formation have been studied using cores from the Rag-eSafid oil field, in order to determinate their microfacies and sedimentary palaeoenvironments. Based on texture analysis and faunal
assemblages, 10 microfacies types have been recognised and interpreted. They indicate different depositional settings: open marine,
oolitic and bioclastic shoal, lagoon, tidal flat and beach. The microfacies have been interpreted as indicative of the inner and middle
ramp. In accordance with the temporal and spatial correlation model for the Asmari Formation across the south-western part of the
Zagros basin, deposition of the Asmari Formation in the south-west had started in a deeper environment and continued in a shallower
high energy environment.
Key Words: Asmari Formation, Oligocene-Miocene, microfacies correlation, carbonate platform ramp, larger benthic foraminifera,
Zagros Basin, Iran.

1. Introduction
The Oligocene-Miocene shallow marine carbonates of
the Asmari Formation are one of the most important oil
reservoirs at the margin of the Zagros Basin in southwestern Iran. This formation has been studied on both
outcrop and subsurface layers. The type section of the
Asmari Formation, measured in the Tang-e-Gele Torsh
outcrop in Khuzestan province by Richardson (1924),
consists of 314 m of limestones, dolomitic limestones,
and argillaceous limestones (Motiei 1993). Generally,
the Asmari Formation conformably overlies the deeper
microfacies of the Palaeocene–Oligocene Pabdeh
Formation. The Gachsaran Formation unconformably
overlies the Asmari Formation in most places.
The Asmari Formation was originally named after the
Kuh-e-Asmari outcrop in Khuzestan province by Busk
and Mayo (1918) as a sequence of Cretaceous-Eocene
age. The first publications on the Asmari Formation
(Richardson 1924, Boeckh et al. 1929) were revised by
Lees (1933), who considered the Asmari Formation to be
Oligocene-Miocene in age and Thomas (1948), who dated
it as Oligocene–Burdigalian. Biostratigraphic data on the
Asmari Formation were established by Wynd (1965) and
reviewed by Adams and Bourgeois (1967) in unpublished
reports. Ehrenberg et al. (2007) and Laursen et al. (2009)
*Correspondence:

204

have applied the method of strontium stratigraphy to date
the Asmari Formation biozones (Table 1).

More recent studies about palaeoecology, microfacies
and sequence stratigraphy of the Asmari Formation
were carried out by Seyrafian and Hamedani (1998,
2003); Seyrafian (2000); Vaziri-Moghaddam et al. (2006);
Amirshahkarami et al. (2007a, 2007b); Rahmani et al.
(2009); Vaziri-Moghaddam et al. (2010) and Seyrafian
et al. (2011). The Asmari Formation is widespread, with
very varied characteristics of lithostratigraphy, biozones
and microfacies in different locations in the Zagros
Basin (Motiei 1993). Most researches of the Asmari
Formation involve outcrop sections. Also, there has been
no correlation between subsurface and outcrop sections
of the Asmari Foramation in previous works. Therefore
the Asmari formation needs more detailed microfacies
analysis for its correlation model and palaeoenvironment
reconstruction.
Also, studies of the subsurface sedimentary
successions of the Asmari Formation are necessary for
exploration of the oil reservoirs. The Rag–e–Safid oil field
is one of the most important oil fields in Iran, and it needs
comprehensive research about microfacies analyses and
palaeoenvironment reconstruction.
This paper has two objectives: (1) the interpretation
of the depositional settings of the Asmari Formation in


AMIRSHAHKARAMI / Turkish J Earth Sci
Table 1. Biozonation of the Late Oligocene–Early Miocene using the distribution of larger benthic foraminifera (Laursen et al. 2009).
Standard Chronostratigraphy
Age (Ma)

Epoch

Biozonation of the Asmari Formation

Stage

Borelis melo curdica-Borelis melo melo

Burdigalian
20

Miocene
Aquitanian

25

Miogypsina
Elphidium sp. 14, Peneroplis farsensis

Chattian

Archaias asmaricus
Archaias hensoni
Miogypsinoides complanatus

Rupelian

Nummilites vascus
Nummulites fichtelii

Oligocene
30

subsurface sections at the Rag–e–Safid oil field, using
the microfacies analysis of cores and (2) the microfacies
correlation of subsurface sections of the Asmari Formation
in the Rag-e-Safid oil field with some of the previously
studied outcrop sections. The Asmari Formation contains
numerous species of larger benthic foraminifera that
thrived in the photic zone of tropical to subtropical
seas and these provide additional useful tools for the
reconstruction of the sedimentary palaeoenvironments
(Vaziri-Moghaddam et al. 2006; Amirshahkarami et al.
2007a, 2007b; Rahmani et al. 2009).
The field investigations of the Asmari Reservoir in the
Rag-e-Safid oil field were carried out by Shirmohammadi
et al. (1974), Wiley and Habibi (1978) and Zahrabzadeh
(2007). The biostratigraphy of the Asmari Formation in the
Rag-e-Safid oil field has been studied by Amirshakarami et
al. (2010), who recognised four assemblage zones based on
the distribution of the larger benthic foraminifera (Table
2).
However, the present work is the first full study of the
microfacies analysis and palaeoenvironment of the Asmari
Formation in the Rag-e-Safid oil field.
2. Study area and geological setting
Iran and some adjacent countries were detached from the
Arabian Plate in the Permian (Berberian & King 1981).
From the Middle Eocene to Early Miocene, the Arabian

Plate began to impact the southern Asian Plate border and
the Zagros belt orogeny began. The Zagros Basin extends
from Turkey, north-eastern Syria and north-eastern Iraq
through north-western Iran and continues into southeastern Iran (Figure 1). The Zagros Mountains of Iran are
divided into three principal tectonic units (Stocklin 1968;
De Jong 1982) namely the Zagros fold–thrust zone, the

Indeterminate

Lepidocyclina
Operculina
Ditrupa

GlobigerinaTurborotalia cerroazulensis-Hantkenina

imbricated zone and the Urumieh–Dokhtar magmatic
zone (Alavi 2004).
The study area is in the fold–thrust zone of the Zagros
Basin (ZFTB in Figure 1) and is located in the Rag-e-Safid
oil field, about 150 km south-east of Ahvaz in southwestern Iran (Figure 2a). The Rag-e-Safid oil field extends
from 30°30′N, 49°4′E to 30°10′N, 50°25′E at the surface,
and is an asymmetric anticline, so the most comprehensive
and deepest wells have been drilled almost in the centre
of the oil field. This study involves wells numbers 13 and
21 at the Rag-e-Safid oil field (Figure 2b), but cores from
the boundary between the Asmari Formation and the
underlying Pabdeh Formation (Palaeocene–Oligocene)
are not available because the contact lies much deeper. The
Asmari Formation is overlain by the Miocene Gachsaran
Formation.

3. Materials and methods
Two well sections of the Asmari Formation have been
studied in the Rag-e-Safid oil field. Well number 13 is 2576
m deep and well number 21 is 2702 m deep (Figure 2b). The
samples include both cores and cuttings but the majority
of thin sections have been prepared from the cores (Table
3). The microfacies characteristics were described in more
than 1100 thin section of the cores and 170 thin sections
of the cuttings samples.
The classification of carbonate rocks followed the
nomenclature of Dunham (1962) and Embry and Klovan
(1971).
4. Microfacies description
Based on the study of the textures, allochems and skeletal
components in thin sections of the cores, ten microfacies

205


AMIRSHAHKARAMI / Turkish J Earth Sci
Table 2. Biozonation of the Asmari Formation using distribution of larger benthic foraminifera in the Rag-e-Safid oil field
(Amirshahkarami et al. 2010).
Stage

Assemblage biozone
IV

Borelis melo, Borelis curdica

III

Peneroplis thomasi, Peneroplis evolutus, Peneroplis sp., Miogypsinoides deharti

Chattian

II

Archaias kirkukensis, Archaias hensoni, Archaias operculiniformis, Archaias asmaricus,
Borelis pygmaea, Miogypsinoides complanatus, Austrotrillina asmariensis, Austrotrillina
howchini, Austrotrillina striata

Rupelian

I

Nummulites vascus, Nummulites fichteli, Nephrolepidina sp., Eulepidina sp., Eulepidina
dilitata

Burdigalian

Barren Zone

Aquitanian

* This table was mistakenly omitted from the print version.

were identified (FR or Microfacies of the Rag-e-Safid oil
field, Figures 3-4).
Microfacies FR1 –Bioclastic Nummulitidae-Lepidocyclinidae
packstone-grainstone (Figure 5/a-b)

This microfacies type is a grain-supported texture
(packstone-grainstone) with densely packed, flat larger
benthic foraminifera. The foraminiferal assemblage
comprises numerous perforated larger foraminifera
50°

46°

such as Lepidocyclinidae and Nummulitidae. The
Nummulitidae are represented by Nummulites,
Operculina, Heterostegina and Spiroclypeous. Other
skeletal grains include bryozoans, corallinaceas, Ostrea,
gastropoda, echinids, ostracods and small benthic
foraminifera. This facies occurs in well number 13 in the
lower part of the Asmari Formation and is Rupelian in
age (Figure 3).
58°

54°

62°

EN

M

AR

TURKMENISTAN

Caspian
Sea

IA

TURKEY

AZERBAIJAN

I

38°

R

A

N
AFGH

34°

AN

ANIST

Zagros Orogen
IRAQ

N

Ahvaz
U
A

KI

Z

AN

TB

ST

ZF
ian

rs

Pe

T

AI

OL

f

ul

G

Study area
300 km

PA

M

D

W

ZI

KU

ARABIA

30°

ZDF

26°

Figure 1. Subdivisions of the Zagros orogenic belt and geological setting of the study area: OL, Oman line; UDMA, Urumieh-Dokhtar
magmatic arc; ZDF, Zagros deformational front; ZFTB, Zagros fold-thrust belt; ZIZ, Zagros imbricate zone; ZS, Zagros suture (After
Alavi 2004).

206


50° 40ʹ

14Km

N

50° 40ʹ

50° 30ʹ

50° 20ʹ

50° 10ʹ

50° 00ʹ

49° 50ʹ

49° 40ʹ

49° 30ʹ

49° 20ʹ

49° 10ʹ

AMIRSHAHKARAMI / Turkish J Earth Sci

31° 10ʹ

To Ahvaz

31° 00ʹ
30° 50ʹ

Omidiyeh

30° 40ʹ

Mahshahr

Behbahab

30° 30ʹ
Rage Safid
Oil Field

Hendijan

Bandar Daylam

Persian Gulf

30° 20ʹ
30° 10ʹ
30° 00ʹ

50 20

50° 00ʹ

49° 50ʹ

49° 40ʹ

50° 10ʹ

(a)

N

30° 30ʹ

0m

180

Rag-e-Safid Field
Asmari Reservoir
Core Compendium Data
Top Asmari Formation 2450 m
1-15

00

15

240

m

0m

3000

m

2450-2702 m
100% Roc.
T.D. 2702 m

21

D

1-12

2141, 6-2575.7 m
86% Roc.
T.D. 2576 m

3000

13

Top Asmari Formation 2203 m

30° 20ʹ

U

4 km

1800

m

m

U D

30° 10ʹ

(b)
Figure 2. (a) Location of the study area at the Rag-e-Safid oil field in south-western Iran. (b) Location
of wells numbers 13 and 21 from the Rag-e-Safid oil field (After Amirshahkarami et al. 2010).

Interpretation
Microfacies FR1 was deposited in a medium-high
energy open marine environment. This interpretation
is supported by the abundance of typical open marine
skeletal fauna including large and flat Nummulitidae,
Lepidocyclinidae, bryozoans, and echinoids (Romero
et al. 2002). The presence of those fauna, in comparison
with analogues in modern platforms (Hottinger 1983;
Reiss & Hottinger 1984; Leutenegger 1984; Hohenegger

1996; Hottinger 1997; Hohenegger et al. 1999), suggests
that this microfacies type has been deposited in the lower

photic zone. This microfacies has also been reported
from the lower parts of the Asmari Formation in other
sections, such as Chaman-Bolbol and Tang-e-Gurgdan
(Amirshahkarami et al. 2007a, 2007b).
Microfacies FR2 –Bioclastic bryozoans-coral floatstonerudstone (Figure 5/c-d)
This microfacies is characterised by abundant and densely
packed skeletal grains. The texture is floatstone-rudstone
with coarse-grained fragments of coral colonies and
bryozoans. Other bioclasts are small benthic foraminifera,
miliolids, fragments of mollusca and corallinacean algae.

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AMIRSHAHKARAMI / Turkish J Earth Sci
Table 3. The sample data from wells numbers 13 and 21 at the Rag-e-Safid oil field, Zagros Basin, south-west of Iran.

Total Depth: 2529.7 m
Top of the Asmari Formation: 2203 m
Depth (m)

7150–55 to 7995–8000

(below sea level)

2203 to 2445

8021 to 8100

2448.6 to 2475

8101 to 8200

2448.9 to 2505

8201 to 8312

2505.3 to 2529.7

(Driller depth in feet)
Cuttings

(Driller depth in feet)

Interpretation
This facies is interpreted as an open marine facies that
formed between the fair weather wave base and the storm
wave base (Wilson 1975; Flügel, 2004). Coarse-grained
debris of corals and bryozoans and the floatstone-rudstone
texture of Facies FR2 suggest the absence of an effective
barrier. A similar microfacies has been also reported by
Amirshahkarami et al. (2007a) and Vaziri-Moghaddam et
al. (2010) from outcrop sections of the Asmari Formation.
Microfacies FR3 –Bioclastic grainstone (Figure 5/e-f)
This microfacies is characterised by clean calcareous
rounded and coated skeletal grains in a depositional
texture of grainstone. Common biota are large bivalves,

gastropods, echinoids, dasycladacean algae fragments
and small benthic foraminifera and abraded biota such as
corallinacean algae.
Interpretation
Dasycladacean algae indicate shallow marine conditions
within the euphotic zone and the grainstone texture
suggests sufficient energy to winnow away the fines in this
microfacies. In accordance with the standard microfacies
types described by Wilson (1975) and Flügel (2004), this
microfacies suggests a bioclastic sandy shoal. A similar
microfacies has been also reported by Amirshahkarami et
al. (2007a) from the Chaman-Bolbol section.
Microfacies FR4 – Bioclastic-ooidal packstone-grainstone
(Figure 6/a)
This microfacies is characterised by rounded ooids in a
packstone-grainstone depositional texture. Other grains
are miliolids, micritised skeletal grains and bioclasts.
Minor elements are foraminifera such as Miogypsinoidae
and Dendritina. The ooids are well sorted, small with

208

Total Depth: 2700.52 m
Top of the Asmari Formation: 2450 m
Sample no.

Cores

Sample no.

Well no. 21

Depth (m)

(below sea level)

8039 to 8210

2450 to 2500

8211 to 8300

2500.3 to 2532.5

8301 to 8400

2532.8 to 2562.5

8401 to 8500

2562.8 to 2587.5

8501 to 8600

2563.1 to 2620

8601 to 8700

2620.3 to 2650

8701 to 8800

2650.3 to 2680

8801 to 8864

2680.3 to 2700.5

100% Cores

Well no. 13

multiple concentric laminates and exhibit distinct
tangential structures. Some of them are micritised and a
few are dissolved.
Interpretation
The rounded ooids of this facies suggests an ooid shoal,
with a depositional environment located in the high
energy shoals of the outer platform margin (Flügel 2004).
A similar microfacies has been reported from the Asmari
Formation in other sections, such as the outcrop Khaviz
section (Kimiagari 2006) and well number 30 from the
Aghajari oil field (Yazdani 2006).
Microfacies FR5 – Bioclastic perforate foraminifera miliolid
wackestone-packstone (Figure 6/b-c)
The major components of this microfacies are benthic
foraminifera and micritised bioclasts with a wackestone–
packstone texture. The larger benthic foraminifera
include both perforate and imperforate forms. Common
foraminifera with perforate walls are small-medium

sized Nummulitidae, Miogypsinidae, Neorotalia and
Amphistegina. Imperforate forms are miliolids, Borelis
and Austrotrillina. Minor components are small benthic
foraminifera, Dendritina, fragments of molluscs, echinoids
and corallinacean algae.
Interpretation
The coexistence of perforate benthic foraminifera
(Nummulitidae, Miogypsinidae and Amphistegina,
Neorotalia) and imperforate foraminifera (miliolid,
Borelis and Austrotrillina) of microfacies FR5 indicate
that deposition took place in an open shelf lagoon. The
small Nummulitidae were reported from open marine
conditions by Romero et al. (2002). Miogypsinoids lived
in shallow waters of normal salinity (Geel 2000) and


7400

Aquitanian

M

I

O

C

2240

Ti d a l f l a t

Beach
Fr10

Fr6

Fr8
Fr9

Shoal

Fr3
Fr4
Fr5

Open marine
Fr1
Fr2

No thin
section

E

7300

Lagoon

Barren Zone

2220

Biozone

Lithology

Sample no.
(Driller depth in feet)

Depth (m)
(Below sea level)

Formation

Stage
?

Microfacies and
sedimentary paleoenvironment

Gachsaran
2200

N

E

Series

AMIRSHAHKARAMI / Turkish J Earth Sci

2260
2280

7500

III

2300
7600

r

i

2320

7700

A

i
t

a

s

2360

2380

II

a

7900
2420
2440

8000

L

I

C

G

h

O

7800

2400

C

t

N
E

E

n

m

a

2340

O

2460

Rupelian

2480
2500

8100

8200

I

2520
8312
Pabdeh Total Depth: 2529.7

Figure 3. Vertical distribution of the microfacies of the Asmari Formation at the
Rag-e-Safid oil field, well no. 13, (Zagros Basin, SW Iran). For Biozones see Table 2.
Fr: Microfacies of the Rag-e-Safid oil field. (For lithology symbols see Figure 4).

209


8050

2460

Tida l fla t

Beach

Lagoon

Fr10

Shoal

Fr8
Fr9

Biozone

Fr2 Open marine
Fr3
Fr4
Fr5
Fr6
Fr7

Lithology

Sample no.
(Driller depth in feet)

Depth (m)
(Below sea level)

Stage

Formation
Gachsaran

2450

Microfacies and
sedimentary paleoenvironment

IV

2480

8150

2490

Barren Zone

2470

2500

i

2520

r

2530

a

2510

2540

8250

III

8350

2550

2570
2580

a
i

2600

o

t

2610

t
a

2620

h

2630

C

O

l

i

c

e

2590

g

n

e

2560

n

A

s

m

A q u i t a n i a n

M i

o

c

e

n

e

Burdigalian

Series

AMIRSHAHKARAMI / Turkish J Earth Sci

2640

8450

8550

II
8650

2650
2660
2670

8750

2680

2690
2700
Pabdeh

8850

Total Depth: 2700.52

Bioclastic limestone with diagenetic recrystallized skeletal and dolomitization
Limestone with abundant larger benthic foraminifera
Anhydrite

Mudstone with anhydrite
Quartzarenite and Quartz mudstone

Figure 4. Vertical distribution of the microfacies of the Asmari Formation in the Rag-e-Safid oil field, well no. 21,
(Zagros Basin, SW Iran). For Biozones see Table 2. Fr: Microfacies of the Rag-e-Safid oil field.

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AMIRSHAHKARAMI / Turkish J Earth Sci
500 µm

500 µm

b

a

500 µm

500 µm

c

d

500 µm
e

500 µm
f

Figure 5. (a-b) Microfacies FR1: (a) Bioclastic Lepidocyclinidae packstone–grainstone; Rag-e-Safid oil field; Well no. 13 (Sample
No. 8105’6’’). (b) Bioclastic Neorotalia Nummulitidae packstone–grainstone; Rag-e-Safid oil field; Well no. 13 (Sample no.
8308’6’’). (c-d) Microfacies FR2: (c) Bioclastic coral floatstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8765’). (d) Bioclast
bryozoan floatstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8622’). (e-f) Microfacies FR3: (e) Corallinacea Dasycladacea
bioclastic grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8531). (f) Bioclastic packstone–grainstone. Rag-e-Safid oil
field; Well no. 21 (Sample no. 8227).

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AMIRSHAHKARAMI / Turkish J Earth Sci
recent Amphistegina and Neorotalia live in the photic zone
of shallow water (Romero et al. 2002). The occurrence of
the imperforate foraminifera such as miliolids, Borelis
and Austrotrillina was reported from restricted lagoon
conditions by Hallock and Glenn (1986); Geel (2000)

and Romero et al. (2002). The open lagoon depositional
setting is characterised by microfacies types that include
mixed open marine bioclasts and protected environment
bioclasts (Vaziri-Moghaddam et al. 2010).
Microfacies FR5 has also been reported in other
sections of the Asmari Formation by Vaziri-Moghaddam
et al. (2006); Yazdani (2006) and Amirshahkarami et al.
(2007a, 2007b). A similar microfacies with abundant
Neorotalia was also identified in well sections of the Asmari
Formation from the Aghajari oil field by Yazdani (2006).
Microfacies FR6 – Bioclastic peloid imperforate foraminifera
packstone-grainstone (Figure 6/d-f, Figure 7/a)
The main components of this microfacies are larger
benthic foraminifera with imperforate walls, such
as Archaias, Peneroplis, Dendritina, Meandropsina,
Borelis, Austrotrillina and miliolids. Other bioclasts are
corallinacean algae, corals, bryozoans, molluscs and shell
fragments. Peloids are also present. The minor components
are small benthic foraminifera, Ostracode, peloids and
micritic skeletal grains. The poorly–medium sorted grains
of this facies are fine–medium in size and subangular to
round in shape. The depositional texture is represented by
packstone–grainstone.
Interpretation
The existence of abundant larger benthic foraminifera with
imperforate walls in Microfacies FR6 indicates deposition
in a restricted shelf lagoon. Imperforate foraminifera such
as miliolids, peneroplids, alveolinids and soritids lived in
a restricted shelf lagoon (Hallock & Glenn, 1986). The
restricted conditions are suggested by rare to absent normal

marine biota and abundant restricted biota (imperforate
foraminifera) (Geel 2000; Romero et al. 2002). This
microfacies has also been recognised in other sections of
the Asmari Formation (Yazdani 2006; Amirshahkarami et
al. 2007a, 2007b; Rahmani et al. 2008; Vaziri-Moghaddam
et al. 2006, 2010).
Microfacies FR7 –Intraclastic bioclastic miliolid packstonegrainstone (Figure 7/b)
The main skeletal grains consist of miliolids, echinoids
and intraclasts. Minor skeletal grains are corallinaceans,
Neorotalia; Miogypsinoides and small benthic foraminifera
such as Discorbis. The depositional texture is a poorly
sorted packstone–grainstone. Some grains have been
partially micritised.
Interpretation
Because of the coexistence of biota, including miliolids
and peneroplids with intraclasts, the depositional setting
of Microfacies FR7 is recognised as being at the lagoonal

212

end of the platform margin. Miliolids and peneroplids
lived in restricted lagoon conditions (Geel 2000; Hallock
& Glenn 1986). This microfacies has also been identified
in other sections of the Asmari Formation, such as the
Chaman–Bolbol outcrop section by Amirshahkarami et
al. (2007a).
Microfacies FR8 –Mudstone (Figure 7/c)
This microfacies consists of homogeneous micrite with
a low diversity of foraminifera and very rare carbonate
and non-carbonate grains. These sediments are mainly

composed of 90% to 100% of lime mud. In some samples,
subordinate detrital quartz grains are also present.
Interpretation
There is no evidence of subaerial exposure (such as a
vesicular fabric, microcodium, birdseye and fenestral
fabric) in lime mudstone in microfacies FR8. However,
those unfossiliferous homogeneous micritic limes are
interbedded with the lagoonal facies. Therefore lime
mudstone with a paucity of fauna in microfacies FR8 was
deposited in a protected lagoon (Tucker 1985; Flügel 2004).
Microfacies FR9 –Quartz mudstone (Figure 7/d-e)
This microfacies is lime mudstone with grains of detrital
quartz. Fenestrate structures and evaporite materials such
as anhydrite can be found in some thin sections (Plate 7/e).
These sediments are mixed siliciclastic-carbonate rocks or
alternating layers of sandy limestone, lime sandstone and
carbonates.
Interpretation
Mixed siliciclastic-carbonate rocks can be common in
near-coast environments (Flügel 2004). The input of
terrigenous materials into the carbonate environment
can take place by erosion of the underlying sediments in
a tidal zone (Flügel 2004). The microfacies characteristics,
including its fine grained nature, lack of fauna and the
presence of fenestrate fabric, are common in tidal flat
sediments (Vaziri-Moghaddam et al. 2010).
Microfacies FR10 –Quartzarenite (Figure 7/f)
This quartzarenite microfacies is mature sandstone
composed primarily of subangular to angular grains of
quartz containing more than 95% detrital quartz grains

in a clay matrix. Minor components are bioclastic grains
such as mollusc shell fragments. The quartzarenite facies
has been recognised in the Ahvaz Member sandstones
from the Asmari Formation and is interbedded with the
Chattian-Aquitanian carbonate layers (Motiei 1993).
Interpretation
The occurrence of detrital quartz with skeletal grains
suggests a beach facies on a coastal environment. Mixed
siliciclastic-carbonate rocks can be common on the
shoreline (Flügel 2004). A similar microfacies has also been
reported from the Asmari Formation in the subsurface
sediments from wells 30 and 66 in the Aghajari oil field
(Yazdani 2006).


AMIRSHAHKARAMI / Turkish J Earth Sci
200 µm
a

500 µm
c

500 µm
b

500 µm
d

500 µm
e

500 µm
f

Figure 6. (a) Microfacies FR4: Bioclastic ooid packstone–grainstone. Rag-e-Safid oil field; Well no. 21 (Sample no. 8252’).
(b-c) Microfacies FR5: (b) Bioclastic miogypsinidae miliolids packstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8238’).
(c) Bioclastic miliolids Neorotalia packstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8229). (d-f) Microfacies FR6 (d)
Bioclastic miliolid corallinacean packstone–grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8733). (e) Bioclastic
miliolid Peneroplidae packstone–grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8727). (f) Miliolid peloid bioclastic
packstone-grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8240 6’’).

213


AMIRSHAHKARAMI / Turkish J Earth Sci
500 µm
a

500 µm
c

500 µm
e

500 µm
b

500 µm
d

500 µm
f

Figure 7. (a) Microfacies FR6: Miliolid peloid bioclastic grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8224). (b)
Microfacies FR7: Intraclastic bioclastic Peneroplidae miliolid grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8730). (c)
Microfacies FR8: Mudstone; Rag-e-Safid oil field; Well no. 13 (Sample no. 7975). (d-e) Microfacies FR9: (d) Quartz mudstone;
Rag-e-Safid oil field; Well no. 21 (Sample no. 8670). (e) Quartz mudstone with anhydrite; Rag-e-Safid oil field; Well no. 21
(Sample no. 8382). (f) Microfacies FR10: Quartzarenite; Rag-e-Safid oil field; Well no. 21 (Sample no. 8128).

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AMIRSHAHKARAMI / Turkish J Earth Sci
Larger benthic foraminifera are important to
recognition of the palaeoecology of the Cenozoic
carbonate platforms and ramps. The palaeoenvironmental
distribution of foraminiferal assemblages depends on
intrabasinal conditions, including nutrient, temperature,
salinity, depth, light, substrate and water energy (Geel
2000; Hottinger 1983). The gradient of the light is the most
important factor in the distribution of species because it is
effective on symbioses and nutrient (Hottinger 1983). The
size, degree of flatness and the wall of the larger foraminifera
tests, can also provide environmental information (Hallock
& Gleen 1986; Geel 2000). Larger and flatter individuals
become more common at the lower limit of the euphotic
zone (Geel 2000). For example, lepidocyclinidae and large
and flat nummlitidae occur in the lower photic zone of an
open marine but small to medium-sized and lens-shaped
nummulites lived together with alveolina in the upper part

of photic zone in an interior platform (Hallock & Gleen
1986; Geel 2000). The occurrence of abundant imperforate
foraminifera (e.g. miliolids in Microfacies FR5-FR7)
is generally taken as evidence for a relatively nutrientrich restricted lagoon with a slightly hypersaline habitat
(Hallock & Gleen 1986; Geel 2000). Perforate foraminifera
with symbiotic algae (e.g. lepidocyclinidae, large and flat
nummlitidae in Microfacies FR1) occur in open marine
shallow water conditions (Hallock & Gleen 1986).

XRF analysis and the CIW΄ Index in the Ahvaz Member
sandstones from the Asmari Formation in the Ahvaz oil
field indicate a marginal marine or coastal environment
depositional setting (Hosseinibarzi et al. 2008).
5. Sedimentary model
Microfacies analyses have allowed the interpretation of
several carbonate marine system environments including
open marine, oolitic and bioclastic shoal, open lagoon,
restricted lagoon, tidal flat and shoreline or beach in the
Asmari Formation in the Rag-e-Safid oil field.
The palaeoenvironments of the Asmari Formation
can be reconstructed by means of the arrangement of
the facies belts and the distribution of the larger benthic
foraminiferal assemblages.
By comparing the textures, lithofacies and biofacies
with those of modern carbonate depositional settings,
such as the Persian Gulf, a very low gradient homoclinal
carbonate ramp model is suggested for the Asmari
Formation in the Rag-e-Safid oil field (Figure 8). It seems
that the Asmari Formation depositional environment was
similar to the modern homoclinal carbonate ramp of the

Persian Gulf (Read 1985; Jones & Desrochers 1992).
A carbonate ramp is separated into inner ramp, middle
ramp and outer ramp in classical facies models (Burchette
& Wright 1992).
Key to symbols
Bioclasts
Bryozoan fragments
Coralline algae detritus
Corals fragments
Echinodermata fragments and spines
Quartz mudstone with anhydrite
Imperforate foraminifera

Inner ramp

Middle ramp
Distal

Proximal
Tidal flat
Beach

Lagoon

Oolitic
and
bioclastic
shoal

Outer ramp

?

Skeletal grains
and
flat Nummulitidae

Nummulitidae and
Lepidocyclinidae-rich
grainstone and
packstone
Ooid
Peloid
Rotalia
Sandstone

?
FR10

FR9

FR8
FR7

FR6
FR5
FR4
FR3

FR2

FR1

Figure 8. Depositional model for the platform carbonates ramp of the Asmari Formation in the central part of the Rag-e-Safid oil
field, Zagros Basin, SW Iran.

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AMIRSHAHKARAMI / Turkish J Earth Sci
Planktonic foraminifera are individuals indicative of open
marine conditions with slope and basin facies (Hallock &
Gleen 1986; Geel 2000).
In the Asmari Formation sediments in the Rage-Safid
oil field, the middle ramp is recognised by open marine,
oolitic and bioclastic shoal environments and the inner
ramp is recognised by lagoon, tidal flat and shoreline or
beach environments.
The shoreline and beach facies is characterised by
mixed siliciclastic-carbonate rocks and detrital sediments
affected by a high energy environment (Microfacies
FR10). The tidal flat setting is identified by fenestral lime
mudstone and quartz mudstone (FR9).
The most common microfacies of the inner ramp
is medium-coarse-grained imperforate foraminifer–
bioclastic wackestone-packestone (Microfacies FR5FR7). The restricted shallow sub-tidal environment or
restricted lagoon is commonly dominated by imperforate
foraminifera (Geel 2000; Romero et al. 2002) such as
Archais, Meandropsina, Peneroplis, Borelis and miliolids.
The mudstone microfacies is interbedded with these

sediments (Microfacies FR10). The coexistence of
imperforate and perforate foraminifera indicates that
the depositional setting was a semi-restricted lagoon
(Microfacies FR5).
The middle ramp can be divided into a proximal
and distal middle ramp. The proximal middle ramp is
characterised by oolitic and bioclastic shoals in a skeletal
packstone–grainstone texture (Microfacies FR3 and FR4).
Skeletal grains are dominated by echinoids, mollusca,
bryozoan, corallinacean and benthic foraminifera. This
evidence suggests a shallow water depositional setting
near the fair weather wave base.
The distal middle ramp is identified by the foraminiferal
assemblage with perforate walls such as Nummulitidae
and Lepidocyclinidae and the textures that correspond
to an increase in the water depth (Microfacies FR1 and
FR2). These faunal associations adapt to a symbiontbearing strategy (Hottinger 1983; Hallock & Glenn
1986; Leutenegger 1984; Hottinger 1997). The most
common facies of the outer ramp environment in the
Asmari Formation is bioclastic planktonic foraminifera
wackestone (Vaziri-Moghaddam et al. 2006, 2010;
Amirshahkarami et al. 2007a; Rahmani et al. 2009). In
accordance with the studies of the palaeoenvironment and
sequence stratigraphy by Vaziri-Moghaddam et al. (2010)
an outer ramp environment can mostly be recognised in
the lower part of the Asmari Formation.
Because of the undrilled strata in the lower part of the
Asmari Formation in the Rag-e-Safid oil field, no cores of
the Asmari Formation reach the underlying Palaeocene–
Oligocene Pabdeh Formation. Therefore, the existence of

an outer ramp environment cannot be discussed.

216

6. Correlation of depositional environments
Following correlation of the Palaeocene–Oligocene
Asmari Formation depositional environments across the
study area and with other sections such as the ChamanBolbol and Tange-Gurgdan sections (Amirshahkarami et
al. 2007a, 2007b), tidal flat and beach environments are
more extensive in the Rage-Safid oil field (Figure 9). The
sandstones of the Ahvaz Member of the Asmari formation
have been recognised in the Aquitanian stage in the RageSafid oil field too. Patch reef, identified in the Chattian
stage in the Tange-Gurgdan section, comprises coral
boundstone.
Correlation diagrams (Figure 9), in the ChamanBolbol section identify a bioclastic shoal without any ooids
in the Chattian stage but in the Rag-e-Safid oil field, the
bioclastic ooids shoal facies is widespread in the Chattian
and Aquitanian stages (Figure 9). Sandstone of the Ahvaz
Member of the Asmari Formation in the Aquitanian stages
has been recognised with a beach facies in the subsurface
layers at the Rag-e-Safid oil field.
Based on the chronostratigraphic scheme (Figure
9), outer ramp platform microfacies with a planktonic
foraminiferal fauna has been recognised in the Rupelian
stage of the lower part of the Asmari Formation in the
Chaman-Bolbol section.
Therefore, in accordance with temporal and spatial
correlation model for the Asmari Formation across
the south-western part of the Zagros basin (Figure 9),
deposition of the Asmari Formation in the south-west

(Chaman-Bolbol section) was in a deeper environment
and continued in a shallower higher-energy environment
(Rage-Safid oil field).
7. Conclusions
This investigation shows that the Asmari Formation
was deposited with varied microfacies on a homoclinal
carbonate ramp, so that the Asmari Formation formed
in a deeper environment with lower energy in the southwestern parts of the Zagros Basin (e.g. Chaman-Bolbol
section). Ten major microfacies are recognised in the
subsurface sediments of the Asmari Formation in well
numbers 13 and 21 of the Rag-e-Safid oil field. These
are grouped into five depositional settings including
open marine, ooids and bioclastic shoal, lagoon (open
lagoon and restricted lagoon), tidal flat and beach. These
sedimentary settings correspond to the inner and middle
ramp.
The open marine environment is separated from the
lagoonal setting by ooids-bioclastic shoals. An ooids
shoal is widespread in the Aquitanian stage of the Asmari
Formation at the Rag-e-Safid oil field and indicates a high
energy shallow environment. Beach facies sandstones of
the Ahvaz Member have been recognised in the Asmari


AMIRSHAHKARAMI / Turkish J Earth Sci

Tang-e-Gurgdan section
Amirshahkarami et al. 2007b

Well no. 13

?

III

III

I

R

120 m

140 m

59 m

IIb

50 m

170 m

80 m

IIb

40 m

IIa
?

I

Pa

15 m

0
?

Chattian

I
Pa

48

50

28

26

52

54 56

58

60

62

Correlation sections

Biozone-Age
IV
III
IIb
IIa
I
Pa

U. Asmari (Burdigalian)
M. Asmari (Aquitanian)
L. Asmari (Chattian)
L. Asmari (Chattian)
L. Asmari (Rupelian)
Pabdeh Formation (Oligocene)
No information

Disconformity

Depositional environment
Rupelian

Middle ramp

Shore line
Tang-e-Gurgdan section

30

Chaman Bolbol

500km

Inner ramp

Well no. 13
Well no. 21
Rag-e-Safid oil field

32

Gulf

Outer ramp

60 m

80 m

I

30 m

46

?

65 m

IIa

44

Burdigalian

Aquitanian

34

Tang-e-Gurgdan

0

III

36

N

ian

IV

38

Rage Safid oil field

Stage

IIb

A

rs
Pe

IIb

Chaman-Bolbol section
(Amirshahkarami et al.
2007a
Amirshahkarami and Taheri
2010)

40 m

III

IV
70 m

IV

50 m

20 m

Well no. 21

30 m

Rag-e-Safid oil field

N

Caspian
Sea

[

[

]Open marine

Bioclastic Shoal
Ooids Shoal
Patch reef
Lagoon
Tidal flat
Quartzarenite

Chaman-Bolbol section

Figure 9. Temporal and spatial correlation of depositional environments and biozones of the Asmari Formation across the
central part of the Rag-e-Safid oil field (study area), Tange-Gurgdan (Amirshahkarami et al., 2007b) and Chaman-Bolbol sections
(Amirshahkarami et al., 2007a; Amirshahkarami and Taheri, 2010) Zagros Basin, SW Iran.

Formation subsurface at the Rag-e-Safid oil field. Therefore,
microfacies evidence from the Asmari Formation indicates
a shallower palaeoenvironment, with more high energy at
the Rage-Safid oil field.

Acknowledgements
The author is grateful to Payame Noor University of
Isfahan for providing financial support and would also like
to thank the reviewers for their helpful comments. The
author thanks the National Iranian South Oil Company
(NISCO) for providing the thin sections of cores.

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