Post on 06-Feb-2023
ORIGINAL ARTICLE
Depositional environment and sequence stratigraphy of the UpperCretaceous Ilam Formation in central and southern partsof the Dezful Embayment, SW Iran
Hamzeh Mehrabi • Hossain Rahimpour-Bonab •
Amir Hossain Enayati-Bidgoli • Amin Navidtalab
Accepted: 3 July 2013
� Springer-Verlag Berlin Heidelberg 2013
Abstract This study is focused on the sedimentary
environments, facies distribution and sequence stratigraphy
of the Santonian intervals (Ilam Formation of Bangestan
Group) that host enormous hydrocarbon reserves in five
giant and supergiant oilfields in the central and southern
parts of the Dezful Embayment (SW Iran). This reservoir
formation is investigated using detailed petrographic
analysis assisted by microscopic image analyses to explain
its depositional facies and sedimentary environment in the
subsurface sections of this embayment. Petrographic stud-
ies led to the recognition of 18 microfacies that formed in
four facies belts: inner ramp (including shoal facies and
open to restricted lagoons), mid-ramp (including channels
and patch reef talus facies), outer ramp and basin. To locate
the approximate position of the studied wells in the con-
ceptual depositional model, frequency analyses for facies
associations are carried out. The studied intervals consist of
two, thick shallowing-upward 3rd-order sequences. Facies
variations of the Ilam Formation investigated throughout
the studied oilfields using correlation in a sequence strati-
graphic framework.
Keywords Depositional sequences � Sedimentary
environment � Ilam Formation � Santonian � Dezful
Embayment
Introduction
Carbonate reservoir quality and architecture depend on
several factors including spatial distribution of depositional
facies, secondary alterations (diagenetic events) and
depositional cycles (high frequency cycles and depositional
sequences; Lucia 2007; Ahr 2008). Generally, in carbonate
reservoirs, the sedimentary facies (microfacies) control the
primary porosity and permeability distribution (Schlager
2005). In the absence of intensive diagenetic alterations the
reservoir characteristics are mostly controlled by the
depositional facies features and distributions (in micro-
scale) and sedimentary environments (in macro-scale).
The Albian-Campanian aged Bangestan Group hosts some
of the most prolific reservoirs of the Arabian Platform and
Zagros fold-thrust belt hydrocarbon provinces. The most
important interval of this group includes neritic carbonates of
the Sarvak and Ilam Formations and their equivalent units
(such as Mishrif Formation of Iraq). Accordingly, the Ilam
Formation and its equivalents contain important reservoir
intervals in south and southwest Iran (including Dezful
Embayment) and throughout the Middle East (Motiei 1993;
Aqrawi et al. 1998; Adabi and Asadi-Mehmandosti 2008;
Ghabeishavi et al. 2009; Rahimpour-Bonab et al. 2012a, b). In
the Dezful Embayment, this formation provides the reservoir
for many giant and supergiant oilfields such as Ahwaz,
Gachsaran, Marun, Rag-e-Safid and Abteymour (Fig. 1).
Along with the Cenomanian-middle Turonian Sarvak For-
mation (Fig. 2), these successions host up to one-third of the
total Iranian oil reserves (Motiei 1993). Considering its res-
ervoir quality, this formation represents a heterogeneous unit
previously described as shallow-water carbonates, with beds
of algal and rudist-bearing limestones, capped by deep water
marls and shales (Motiei 1993) (Fig. 2). The aim of this
research is to introduce microfacies and depositional
H. Mehrabi (&) � H. Rahimpour-Bonab �A. H. Enayati-Bidgoli � A. Navidtalab
Department of Geology, College of Science,
University of Tehran, Tehran, Iran
e-mail: hamze.mehrabi@khayam.ut.ac.ir;
hamze.mehrabi@yahoo.com
123
Carbonates Evaporites
DOI 10.1007/s13146-013-0168-z
environment of the Ilam Formation in six boreholes from five
giant and supergiant oilfields in SW Iran (Dezful Embayment).
By this approach, a conceptual depositional model for this unit
is prepared, basin-scale facies distributions and variations are
presented, and the cyclic nature of different microfacies and
their spatial distributions are illustrated in the sequence
stratigraphic framework. By frequency analysis of microfacies
in each studied wells, relative abundance of various facies
associations are assessed, and their relative position in the
proposed conceptual depositional model is illustrated.
Geological setting and stratigraphy
In the geological record of the Arabian Platform and Zagros
fold-thrust belt (including Dezful Embayment and Mesopo-
tamian basin) the Cretaceous successions constitute thick
sedimentary packages, which host numerous economically
important hydrocarbon reserves (Setudehnia 1978; Al-shar-
han and Nairn 1993; Ghabeishavi et al. 2009; Hollis 2011).
These successions, host a considerable part of the world’s total
hydrocarbon reserves (Scott et al. 1993) and huge amount of
oil reserves of the Middle East region. In their general
paleogeographical studies of the Cretaceous, Murris (1980)
and Koop and Stoneley (1982) proposed a ramp-type depo-
sitional regime associated with shelf carbonates, established
and gradually surrounded most parts of the Middle East region
in response to the eustatic sea-level rise. During this period,
the Arabian plate moved toward the tropical and subtropical
latitudes (Murris 1980; Beydoun 1991; Beydoun et al. 1992;
Sharland et al. 2001; Alavi 2004, 2007; Heydari 2008). At this
time, local salt diapirisms or movement of the basement
blocks triggered sporadic regional uplifts and subaerial
exposure of carbonate platforms (Sepehr and Cosgrove 2005;
van Buchem et al. 2011; Hollis 2011; Casini et al. 2011;
Mehrabi and Rahimpour-Bonab 2013). The study area is sit-
uated on the northeastern domain of this moving plate (Fig. 1).
During the Late Cretaceous, the study area was near the
equator, in the northern hemisphere (Sharland et al. 2001;
Heydari 2008; van Buchem et al. 2011). At this time, the
general basin configuration evolved from a passive differen-
tiated margin, including shallow shelves and intrashelf basins
(that developed during Jurassic), into an active margin ramp
system with low relief (e.g., Setudehnia 1978; Murris 1980;
Motiei 1993; Ziegler 2001; Sharland et al. 2001; Piryaei et al.
2010; Hollis 2011).
Fig. 1 Location map of the studied oilfields in the Dezful Embayment of southwest Iran. The main geological and structural subdivisions of SW
part of Iran are also shown and the location of the Dezful Embayment in this framework is marked
Carbonates Evaporites
123
The type section of the Ilam Formation is situated in the
Kabirkoh area, Lurestan province. At its type section this
formation is overlain by the Surgah Formation and
underlain by the Gurpi Formation (Fig. 2). However, in
most parts of the SW Iran (including Dezful Embayment),
the Ilam Formation is generally represented by the shallow-
water limestones that unconformably overlies the carbon-
ate sediments of the Sarvak Formation and is conformably
overlain by shale and marls of the Gurpi Formation
(Fig. 2).
Materials and methods
To reconstruct the depositional environment of the Ilam
Formation, six subsurface sections have been selected from
exploration wells in five giant and supergiant oilfields located
in central and southern parts of the Dezful Embayment (SW
Iran). These include Ahwaz, Abteymour, Marun, Gachsaran
and Rag-e-Safid oilfields (Fig. 1). More than 750 thin sec-
tions (mostly from core samples) were described using the
modified Dunham (1962) textural classification of Embry and
Klovan (1971). Facies types and depositional setting were
interpreted on the base of matrix and grains content, com-
positional and textural fabrics, fossil content, energy index
and sedimentary data and in comparison with modern and
ancient environments (e.g., Wilson 1975; Tucker and Wright
1990; Wright and Burchette 1996; Flugel 2010). Several
factors were considered to differentiate sedimentary facies
including abundance of large benthic foraminifera, green
algae, sponge spicules, molluscs and echinoderms, as well as
non-skeletal grains (e.g., ooids, intraclasts, peloids, and
aggregate grains). These interpretations were verified using
microscopic image analyses.
Fig. 2 Generalized stratigraphy of the Cretaceous successions in the Dezful Embayment (a) and in different parts of the Zagros fold-thrust belt,
including the Ilam Formation of the Bangestan Group and its lateral facies and thickness variations (b)
Carbonates Evaporites
123
Facies analysis
The Ilam Formation in the Dezful Embayment contains
shallow to relatively deep water carbonates composed of a
large variety of skeletal and non-skeletal grains, micrite,
calcite cements and late diagenetic dolomites (Figs. 3, 4, 5;
Table 1). The dominant skeletal grains are dasycladacean
algae, echinoids, rudists, benthic and planktic foraminifera,
gastropods, sponge spicules and bivalves, respectively.
Non-skeletal grains are abundant and mainly include ooids,
intraclasts, peloids, and aggregate grains (Fig. 5; Table 1).
Based on the lithology, sedimentary features, textures, and
fossil contents, 18 microfacies are distinguished for the
Ilam Formation in the studied area (Figs. 3, 4, 5; Table 1).
These microfacies are grouped in four main facies associ-
ations that include basin, outer ramp, mid-ramp (including
channel and patch reef talus facies) and inner ramp
(including shoal facies and open/restricted lagoon)
(Table 1). These facies are briefly described below.
Basin facies association
Mudstone (F1)
It is the deepest microfacies which is characterized by
abundant planktonic foraminifera (e.g. Heterohelix, Hed-
bergella and Globigerinoides), oligosteginids, fine grain
size, infrequent laminations, anoxic minerals and organic
matters (Fig. 3; Table 1). Because of its high organic (OM)
content and pyrite it displays brownish color. This facies
likely was deposited below the normal wave base in very
low energy conditions (Flugel 2010; Al-Dabbas et al. 2009;
Ghabeishavi et al. 2010) (Figs. 3, 4, 5; Table 1).
Microbioclastic wackestone (F2)
The main components of this microfacies are silt-sized
bioclasts and planktic bivalve debris, echinoderms and
planktic foraminifera such as Heterohelix and Hedbergella.
High OM content and anoxic conditions are indicated by its
brownish color (Figs. 3, 4, 5; Table 1). The preservation of
laminations is weaker than the facies discussed below. The
mud-supported fabric, silt-sized particles, crude lamina-
tion, OM preservation and planktic fauna (foraminifera and
bivalves) all are indications of deposition under the calm
and deep conditions (Wilson 1975; Flugel 2010).
Planktic foraminifera mudstone/wackestone (F3)
The main distinctive feature of this facies is the abundance
of planktic foraminifera such as Globigerina and Hedber-
gella in a mud dominated matrix. Other components are
debris of bivalves, echinoderms, calcareous sponge
spicules and small-sized peloids (mostly less than 0.1 mm;
Figs. 3, 5; Table 1). The silt-sized allochems of this facies
are poorly sorted. The presence of recognizable bioclasts is
indicative of increased relative energy and thus, shallower
water. Considering the abundance of planktic foraminifera
in the mud-supported fabric it could be concluded that this
facies is mainly deposited from basin to the outer ramp
setting (Bauer et al. 2002; Schulze et al. 2005; Al-Dabbas
et al. 2009; Ghabeishavi et al. 2009; Flugel 2010).
Outer ramp facies association
Peloid-Oligosteginid wackestone (F4)
The main constituents of this facies are oligosteginids,
planktic and benthic foraminifera, fine peloids (micro pe-
loids), as well as echinoderms debris, bivalves and scarce
rudist debris. The abundance of oligosteginids along with
planktic foraminifera, fine bioclasts and mud-supported
fabric all indicate a low energy and deep water environ-
ment (Figs. 3, 5; Table 1). This evidence as well as low
OM content are suggestive of deposition near the storm
wave base.
Bioclast wackestone (F5)
This facies is mainly composed of various bioclasts
including echinoderms, bivalves, calcareous sponge spic-
ules as well as planktic and some benthic foraminifera
(Rotalia) and fine peloids. The frequency of echinoderm
debris and their fine size is the evidence for the proximity
of mid-ramp environment (Figs. 3, 5; Table 1). The asso-
ciation of echinoderms debris, planktic/benthic foraminif-
era, fine bioclasts and mud supported fabric indicate the
proximal parts of an outer ramp environment (Flugel
2010).
Mid-ramp facies association
Middle to distal mid-ramp facies
Foraminiferal mudstone/wackestone (F6) Small benthic
foraminifera, some planktic foraminifera and bioclasts
(including debris of echinoderms, bivalves and green
algae) are the main components of this facies (Figs. 3, 5;
Table 1). The presence of small benthic and planktic
foraminifera, fine bioclasts and mud-dominated fabric all
indicate that the depositional environment of this facies is
distal parts of the mid-ramp setting.
Bioclastic-foraminiferal wackestone/packstone (F7) The
main constituents of this facies are small benthic and, more
rarely, planktic foraminifera, debris of echinoderms,
Carbonates Evaporites
123
bivalves, rudists, green algae as well as peloids. The small
benthic foraminifera are partly micritized. As compared to
F6 this facies is formed under higher energy condition of
the middle parts of the mid-ramp, as indicated by the larger
volume of coarse bioclasts (millimeter to centimeter in
size) and lesser mud content (Figs. 3, 5; Table 1).
Bioclastic–intraclastic wackestone (F8) The frequency
of intraclasts is an important attribute of this facies. The
intraclasts are composed of echinoderms debris, small
benthic foraminifera and peloids in a mud-dominated
matrix. In this facies, small benthic and in the some cases
planktic foraminifera are present. Its bioclasts include
echinoderm debris and bivalves (Figs. 3, 5; Table 1).
Seemingly, the abundant intraclasts are originated from
storm wave base zone (Flugel 2010).
Rudist debris floatstone/wackestone (F9) This facies is
mainly composed of rudist debris of variable sizes, peloids,
small benthic foraminifera and echinoderms fragments. A
significant characteristic of this facies is floating rudist
debris in a muddy matrix (Figs. 3, 5; Table 1). This facies
constitutes the terminal parts of the rudists patch reefs ta-
luses that were extended to the middle parts of mid-ramp
and mixed with mid-ramp bioclasts (Flugel 2010; Jez et al.
2011).
Fine bioclastic-peloidal packstone/wackestone (F10)
The main constituents of this facies are peloids, diverse
fine bioclasts and small benthic foraminifera. The peloids
are mainly accompanied with micritized bioclasts and
small benthic foraminifera that originated from the peloi-
dal-bioclastic shoals and re-deposited in the low energy
Fig. 3 Photomicrographs of microfacies (F1 to F9) from the Ilam
Formation in the studied oilfields (all photos in ppl). F1 mudstone, F2
microbioclastic calcisiltite, F3 planktic foraminifera mudstone/wa-
ckestone, F4 Peloid-Oligosteginid wackestone, F5 microbioclastic
wackestone, F6 foraminiferal mudstone/wackestone, F7 bioclastic-
foraminiferal wackestone to packstone, F8 bioclastic-intraclastic
wackestone, F9 rudist debris floatstone to wackestone
Carbonates Evaporites
123
setting of this facies. The bioclasts include echinoderm
debris, rudists, bivalves, and small planktic and benthic
foraminifera (Figs. 4, 5; Table 1). This facies is interpreted
to have been deposited in the middle to terminal parts of
the mid-ramp as a continuation of peloidal-bioclastic
shoals.
Channel facies
Diverse-sized bioclastic wackestone/packstone (F11)
The main characteristics of this facies are poorly sorted
bioclasts of various sizes. The bioclasts include coarse
(mostly more than 1 cm) and fine (less than 2 mm) rudist
debris, bivalves, echinoderms, gastropods, green algae,
calcareous sponge spicules and large to small benthic
foraminifera that are distributed in a mud-supported matrix.
Peloids are also present as non-skeletal grains (Figs. 4, 5;
Table 1). The fabric, textural characteristics and fossil
content of this facies are indications of turbulent condi-
tions. Considering the variations in size and type of bio-
clasts, textural inversion, admixture of the planktic and
benthic fauna and turbulent fabric, this facies could be
ascribe to the channels developed from proximal parts of
the mid-ramp to outer-ramp settings (Flugel 2010).
Rudist debris-intraclastic wackestone (F12) This facies
is composed of intraclasts and rudist debris floating in a
mud rich matrix and also shows some textural inversion
(i.e. high energy allochems such as crushed and coarse
bioclasts in a low energy/mud-dominated matrix). The in-
traclasts are composed of some bioclasts such as echino-
derms debris, green algae, gastropods and benthic
Fig. 4 Photomicrographs of microfacies (F10 to F18) from the Ilam
Formation in the studied oilfields (all photos in ppl). F10 fine
bioclastic-peloidal packstone to wackestone, F11 diverse-sized bio-
clastic wackestone to packstone, F12 rudist debris-intraclastic
wackestone, F13 Peloid-small benthic foraminiferal grainstone, F14
bioclastic-peloidal grainstone/packstone, F15 ooid grainstone, F16
benthic foraminifera-green algae debris grainstone, F17 green algae
debris wackestone/mudstone, F18 Bioclast-large benthic foraminif-
eral wackestone to mudstone
Carbonates Evaporites
123
foraminifera, that indicate the shallower environments such
as lagoon and proximal mid-ramp. The rudists debris
originated from the destruction of rudists patch reefs
located in the inner-ramp environment (Figs. 4, 5;
Table 1). Depositional setting of this facies could be
attributed to channels located in the proximal to middle
parts of the mid-ramp setting near a high energy surface
fair weather wave base (FWWB)(Flugel 2010).
Inner ramp facies association
Shoal facies
Peloid-small benthic foraminifera grainstone (F13) The
abundance of small benthic foraminifera and presence of
peloids, are the main characteristics of this facies. The
peloids are usually accompanied with micritized benthic
foraminifera. The mud free and cemented fabrics with good
sorting indicate a high energy setting (Figs. 4, 5; Table 1).
This facies formed in the seaward part of the bioclastic-
peloidal shoal in the distal parts of the inner-ramp to the
proximal mid-ramp environment (Korbar et al. 2001;
Blomeier et al. 2009).
Bioclast-peloid grainstone/packstone (F14) In this facies,
peloids and bioclasts such as echinoderm debris, green
algae, rudists and small benthic foraminifera are present.
The peloids are mainly accompanied by micritized bio-
clasts and benthic foraminifera (Figs. 4, 5; Table 1). The
grain-supported fabric and a relatively good sorting of
grains are related to higher energy and turbulence during
deposition. This facies formed in the central part of a shoal
(Adabi and Asadi-Mehmandosti 2008; Ghabeishavi et al.
2009; Flugel 2010).
Ooid grainstone (F15) This facies is composed of
grainstone with higher frequency of ooids comparing with
the other shoal-builder constituents. The ooids display
relatively clear concentric fabrics and likely formed under
high energy conditions (Kahle 1974; Davies et al. 1978).
The cortoids are also present as subordinate allochems.
These are mostly bioclasts with a micritic coating
Fig. 5 Schematic cross section of the proposed ramp-like carbonate platform model for the Ilam Formation in the Dezful Embayment. Lateral
distribution of microfacies in the sedimentary model, petrographic characteristics, and main carbonate particles of various facies are shown
Carbonates Evaporites
123
Table 1 Determined microfacies and facies associations (facies belts) of Ilam carbonates in the studied wells. The main components, grain
properties, and mineralogy of the microfacies are also shown
MF
code
Microfacies
name
Lithology,
color and
texture
Grain size
and sorting
Components Facies
association
Interpretation
(environment)Skeletal Non-
skeletal
MF 1 Mudstone Lime–
Dolomite,
brownish,
mudstone
Calcilutite,
moderately
sorted
– – MF 2, MF 3 Basin
MF 2 Microbioclastic
calcisiltite
Lime–
Dolomite,
brownish,
wackestone
Calcilutite,
poorly
sorted
Silt-sized bioclasts. Debris
of planktonic bivalves,
echinoderms and
planktonic foraminifera
– MF 1, MF 3 Basin
MF 3 Planktonic
foraminifera
mudstone/
wackestone
Lime–
Dolomite,
light brown,
mud/
wackestone
Calcilutite,
poorly
sorted
Planktonic foraminifera
(globigerina and
hedbergella), debris of
bivalves (mainly
planktonic), echinoderms
and calcareous sponge
spicules
Fine
peloids
MF 1, MF 2, MF 4 Basin
MF 4 Peloid,
oligosteginid
wackestone
Lime–rare
Dolomite,
light brown,
wackestone
Calcilutite,
moderately
sorted
Oligosteginids, planktonic
and benthic foraminifera
and debris of
echinoderms, bivalves
and rudists
Fine
peloids
MF 3, MF 5 Outer ramp
MF 5 Microbioclast
wackestone
Lime, light
brown–
yellow,
wackestone
Calcilutite –
calcarenite,
poorly
sorted
Debris of echinoderms,
bivalve; calcareous
sponge spicules;
Planktonic foraminifera
Fine
peloids
MF 4, MF 6 Outer ramp
MF 6 Planktonic–
benthic foram
mudstone/
wackestone
Lime, light
brown–
cream,
mud/
wackestone
Calcilutite–
calcarenite,
poorly
sorted
Small benthic foraminifera,
low planktonic
foraminifera, debris of
echinoderms, bivalves
and green algae
– MF 5, MF 7, MF 8 Middle-distal
mid-ramp
MF 7 Bioclastic-
foraminiferal
wackestone/
packstone
Lime, brown–
gray,
wackestone/
packstone
Calcilutite–
calcarenite,
poorly
sorted
Small benthic foraminifera,
rare planktonic
foraminifera, debris of
echinoderms, bivalves,
rudists and green algae
Peloids MF 6, MF 8, MF 9 Middle-distal
mid-ramp
MF 8 Bioclast–
intraclastic
wackestone
Lime, light
brown,
wackestone
Calcarenite–
calcilutite,
poorly
sorted
Small benthic foraminifera,
rare planktonic
foraminifera, debris of
echinoderms, bivalves
Intraclasts MF 7, MF 9, MF 10 Middle-distal
mid-ramp
MF 9 Rudist debris
floatstone/
wackestone
Lime, light
brown,
floatstone/
wackestone
Calcirudite–
calcilutite–
calcarenite,
poorly
sorted
Rudists debris with
different sizes, small
benthic foraminifera and
echinoderms fragments
Peloids MF 8, MF 10 Middle-distal
mid-ramp
MF 10 Fine bioclasts-
peloidal
packstone/
wackestone
Lime, cream
light brown,
packstone/
wackestone
Calcilutite–
calcarenite,
poorly
sorted
Diverse fine bioclasts
include, echinoderms,
rudists, bivalves, small
benthic foraminifera
debris and rare planktic
foraminifera
Peloids MF 8, MF 9 Middle-distal
mid-ramp
MF 11 Diverse size
bioclastic
wackestone
Lime, dark
cream-gray,
wackestone
Calcarenite–
calcilutite,
poorly
sorted
Coarse and fine debris of
rudists, bivalves,
echinoderms, large and
small benthic
foraminifera
Peloids MF 12 Channel
Carbonates Evaporites
123
composed of irregular to regular laminations (Figs. 4, 5;
Table 1). Good sorting and roundness indicate that the
depositional setting of this facies was a high energy central
shoal (Bauer et al. 2002; Schulze et al. 2005; Blomeier
et al. 2009; Jamalian et al. 2011).
Benthic foraminifera-green algae debris grainstone
(F16) This facies is composed of abundant green (da-
sycladaleans) algae fragments, echinoderm debris, small
to large (few millimeters to centimeter in size) benthic
foraminifera and cortoids (Figs. 4, 5; Table 1). The
presence of green algae is an indicator for the inner
ramp environment (Zhicheng et al. 1997) and a shallow
marine setting (Bucur and Sasaran 2005). The associa-
tion of green algae, benthic foraminifera and grain-
dominated fabric indicate a high energy environment
close to the inner ramp and lagoon. Seemingly, the green
algae fragments are reworked from an inner ramp (open
lagoon) to the proximal mid-ramp and leeward shoal
(Bauer et al. 2002; Bucur and Sasaran 2005; Flugel
2010).
Open-marine and restricted lagoon facies
Green algae debris wackestone/mudstone (F17) Green
algae debris, bivalves, echinoderms, large benthic forami-
nifera such as Miliolidae and Rotalia along with peloids are
the main constituents of this facies (Figs. 4, 5; Table 1).
The bioclasts diversity, presence of large benthic forami-
nifera and a mud-dominated fabric indicates that this facies
deposited in a lagoon environment near to an open-marine
setting (Bucur and Sasaran 2005; Schulze et al. 2005;
Flugel 2010).
Bioclast-large benthic foraminifera wackestone/mudstone
(F18) Large benthic foraminifera such as Rotalia and
Table 1 continued
MF
code
Microfacies
name
Lithology,
color and
texture
Grain size
and sorting
Components Facies
association
Interpretation
(environment)Skeletal Non-
skeletal
MF 12 Rudist debris-
intraclastic
wackestone
Lime, very
light
brown–
gray,
wackestone
Calcirudite–
calcilutite–
calcarenite,
poorly
sorted
Rudists debris Intraclasts MF 11 Channel
MF 13 Peloid-small
benthic
foraminiferal
grainstone
Lime, cream–
yellow,
grainstone
Calcarenite,
well sorted
Small benthic foraminifera Peloids MF 14, MF 15 Shoal
MF 14 Bioclast-peloid
grainstone/
packstone
Lime, cream–
dark
yellow,
grain/
packstone
Calcarenite,
well/
moderately
sorted
Echinoderms, green algae,
rudists debris and small
benthic foraminifera
Peloids MF 13, MF 15 Shoal
MF 15 Ooid grainstone Lime, cream–
light,
grainstone
Calcarenite,
very well
sorted
– Ooids
cortoids
MF 14, MF 16 Shoal
MF 16 Benthic foram-
green algae
debris
grainstone
Lime, light
cream,
grainstone
Calcarenite,
well/
moderately
sorted
Green algae
(dasycladaleans)
fragments, debris of
echinoderms; and small/
large benthic foraminifera
Cortoids MF 13, MF 14, MF 15 Shoal
MF 17 Green algae
debris
wackestone/
mudstone
Lime, brown–
cream,
wackestone/
mudstone
Calcarenite–
calcilutite,
poorly
sorted
Green algae, bivalves and
echinoderms debris, large
benthic foraminifera such
as Miliolidae and Rotalia
Peloids MF 16, MF 18 Open lagoon
MF 18 Bioclast-large
benthic
foraminifera
wackestone/
mudstone
Lime, brown,
wackestone/
mudstone
Calcarenite–
calcilutite,
poorly
sorted
Large benthic foraminifera
echinoderms and green
algae debris, calcareous
sponge spicules and
gastropods
Peloids MF 16, MF 17 Restricted
lagoon
Carbonates Evaporites
123
Miliolidae, echinoderms, green algae debris, calcareous
sponge spicules, gastropods and peloids are the principal
components of this facies (Figs. 4, 5; Table 1). The pres-
ence of large benthic foraminifera (few millimeters to
centimeter in size) and gastropods are typical for a more
restricted environment (comparing with F17). This facies is
ascribed to the semi-restricted lagoon environment on the
basis of these characteristics (Flugel 1982, 2010; Al-shar-
han and Nairn 2003; Schulze et al. 2005; Cross et al. 2010;
Jez et al. 2011).
Frequency analysis of facies associations
In this study, frequencies of different facies associations
(subenvironments) in each studied well were examined to
estimate the approximate position for each well in the
proposed sedimentary model. Frequency diagrams are
depicted for the facies associations of six studied wells to
give a better understanding about distribution patterns of
various sub-environments in the proposed sedimentary
model (Figs. 6, 7). It is necessary to note that these gen-
eralized positions are determined by considering the fre-
quency analyses of shallow to deep facies associations and
their relations in the vertical sequence (separately in each
well) as well as their comparison with the other studied
wells (Fig. 5). As shown (Fig. 6), the mid-ramp and outer
ramp facies associations are the most frequent facies in
AT-1, AZ-1, GS-1 and GS-2 wells. In MN-1 and RS-1
wells, the mid-ramp and shoal facies show their maximum
frequencies. The channel facies commonly display low
frequencies of about 8 percent. They reach their maximum
frequency in GS-2 well (nearly 15 percent). These channels
are considered to have been developed between shoals and
Fig. 6 Frequency diagrams for the six studied wells based on analyses of various facies associations of the Ilam Formation
Carbonates Evaporites
123
to a lesser extent between patch reefs in proximal to distal
parts of the mid-ramp settings. In most cases, lagoonal
facies have relatively low frequencies across all of the
studied wells that range from 5 to 10 percent. This indi-
cates the absence of important barriers on this carbonate
platform. In addition, traces of the rudist patch reefs are
present as rudist bioclasts in the studied intervals. They
mostly occur as the rudist debris floatstone to wackestone
of patch reefs taluses. On the whole, the frequency of these
facies is lower than in the Sarvak Formation (the other
member of Bangestan group), (Rahimpour-Bonab et al.
2012a, b; Mehrabi and Rahimpour-Bonab 2013).
Conceptual depositional model
Microfacies analysis and depositional environment of the
Ilam Formation were the subject of several studies in various
parts of the south and southwest Iran (including Dezful
Embayment). Van Buchem et al. (2006) in their compre-
hensive studies on the Cretaceous sedimentary record of the
Middle East specified that the shallow-water carbonates of
the Ilam Formation were deposited on low angle ramps of
tens of kilometers in size during a transgression phase. Adabi
and Asadi-Mehmandosti (2008) investigated the microfacies
and geochemistry of the Ilam Formation in the Tang-E
Rashid area, Izeh (Zagros) and presented four microfacies
belts: tidal flat, lagoon, shoal and open marine formed in a
ramp platform. Ghabeishavi et al. (2009) distinguished nine
microfacies types formed in continental lacustrine to very
shallow and relatively deep-water (hemipelagic to pelagic)
marine environments for the Ilam succession in the Bange-
stan anticline (Zagros). The influence of the Late Cretaceous
tectonic events on the sedimentation patterns along the
northeastern Arabian plate margin (Fars Province, SW Iran)
was studied by Piryaei et al. (2010). According to this study,
the Ilam Formation in the Fars Province formed on a distally
steepened ramp in an incipient foreland basin configuration.
In our study, four major depositional subenvironments were
identified in the Santonian successions of the Dezful
Embayment on the basis of faunal elements distribution and
vertical facies relationships (Table 1). These include basin,
outer ramp, mid-ramp (including channels and patch reef
talus facies) and inner ramp (including shoal facies and open/
restricted lagoons). These four depositional settings are
represented by 18 microfacies types (Figs. 3, 4; Table 1).
According to this study the Ilam Formation in subsur-
face sections of the Dezful Embayment formed in a ramp-
like depositional platform (Figs. 5, 7). During the Late
Cretaceous, intensive tectonic activities in the NE margin
of the Arabian Plate resulted in severe evolution in the
depositional environment of carbonate platforms (both in
Fig. 7 Proposed carbonate ramp model for the Ilam Formation in the
studied oilfields. The location of the microfacies determined the main
energy surfaces (fair weather wave base and storm wave base) and
schematic microfacies illustrations for the main facies belts are
shown. Approximate positions for studied wells were proposed based
on the results of frequency analyses
Carbonates Evaporites
123
regional and local scales). As stated by many authors (e.g.,
Murris 1980; Koop and Stoneley 1982; Alsharhan and
Nairn 1988, 2003) the Cretaceous carbonate platforms of
the Middle East (and nearly all over the world) were ramp-
like. In some cases, tectonic activities resulted in evolution
of these platforms to form distally steepened ramps. In
other cases, they were rimmed by the ooid-bioclastic sand
shoals associated with relatively permanent shallow
lagoons formed behind these barriers. However, in the
studied interval, lagoonal facies are scarce.
Considering depositional setting and facies association
of the Sarvak Formation (Ghabeishavi et al. 2010;
Rahimpour-Bonab et al. 2012a; Mehrabi and Rahimpour-
Bonab 2013) it could be concluded that depositional
environment of the Ilam Formation was considerably dif-
ferent from other units. Some of the major differences are
elaborated on in our discussion of frequency analysis.
Depositional sequences
Lateral distribution of depositional facies depends on
depositional environments while their vertical stacking is
dictated by the sea-level fluctuations and is reflected in
Fig. 8 Correlation of facies associations and main facies characteristics (textures and energy levels) in the framework of 3rd-order sequences
determined in AT-1 well. Gamma-ray log and lithological variations are also included
Carbonates Evaporites
123
Fig. 9 Sequence stratigraphic correlation of 3rd-order sequences in studied wells. The main sequence surfaces (maximum flooding surfaces and
sequence boundaries) determined are based on facies (microfacies) properties and log data (GR logs)
Carbonates Evaporites
123
their sequence stratigraphic framework (Schlager 2005;
Roger 2006). Therefore, for prediction of 3D distribution of
facies patterns, clear understanding about the relative
timing of facies deposition is essential. In other words, to
completely appreciate the relative time of facies assem-
blage’s deposition (in response to the sea-level fluctua-
tions), sequence stratigraphic analysis is required (Schlager
2005). In this study, a sequence stratigraphic framework is
established as a basis for future reservoir modeling. For
construction of this framework and determination of the
main sequence surfaces (sequence boundaries and maxi-
mum flooding surfaces), various data including results of
facies analysis, log data (especially gamma-ray logs),
energy index classification and paleontological data (such
as frequency of pelagic and benthic foraminifera) are
considered. Two 3rd-order depositional sequences were
recognized in the Ilam Formation (Figs. 8, 9) that includes:
Sequence 1
This sequence is recorded in all of the studied wells and its
thickness varies from 80 to 25 m. The transgressive sys-
tems tract (TST) of sequence 1 is mainly composed of
basin and outer ramp facies (pelagic foraminifer wacke-
stone/mudstone). In the upper part [highstand systems tract
(HST)], a gradual shift from deep-water facies toward the
middle ramp facies can be observed. Above the middle
ramp facies, mudstone facies of lagoon and wackestone/
packstone of channel facies succeed, which indicate a
shallower environment. The lower part is interpreted as a
TST (because of the deepening trend). This system tract
includes mud-dominated (mudstone to wackestone) facies
that contains pelagic components (such as planktic
foraminifera). Gamma-ray log response display consider-
able increasing upward pattern in this system tract and
reaches to its maximum values towards the upper boundary
of this TST [maximum flooding surface (MFS)], especially
in AT-1, AZ-1 and MN-1 wells (Fig. 9). This TST is
overlain by a HST with varying thicknesses (from 10 to
30 m). The lower boundary of this sequence corresponds to
a highly weathered and karstified surface (type-I sequence
boundary) and its upper boundary corresponds to a weakly
weathered surface that shows minor meteoric dissolutions.
Variations in energy levels correspond to these changes, as
the higher energy levels parallel the HST and lower levels
of energy correspond to the TST part (Figs. 8, 9).
Sequence 2
The complete interval of this sequence is only recorded in
the AT-1 well. In the other studied wells, only the TST is
recorded. As in sequence 1, the lower part of sequence 2
(TST) consists of basinal and outer ramp facies (pelagic
foraminifer’s wackestone/mudstone). The upper part of the
TST (MFS) is marked by the maximum deepening of the
facies and lowest energy level. In AT-1 well, the upper part
of the sequence 2 (HST) is composed of middle to inner
ramp facies associations that include lagoon (restricted and
open-marine lagoon), shoal and channel facies (Fig. 8).
The lower boundary of sequence 2 is characterized by a
weakly weathered surface and its upper sequence boundary
is not recorded in most of the studied wells. In the AT-1
well the upper sequence boundary corresponds to the
maximum shallowing of facies that reaches a somewhat
highly weathered and karstified surface (probably a type-I
sequence boundary; Fig. 8). The thickness of this sequence
is about 50 m in AT-1 well (Figs. 8, 9).
Conclusions
1. In the subsurface sections of the Dezful Embayment
the Ilam Formation consists of 18 representative mi-
crofacies that are grouped into four facies associations
from distal to proximal part of the platform. These are
basin, outer ramp, mid-ramp and inner ramp deposi-
tional environments.
2. This study indicates that the Ilam Formation was
formed on a ramp-like carbonate platform under warm
and humid (tropical) climatic condition. Lateral dis-
tribution of various sub-environments was also deter-
mined in this ramp-like carbonate platform.
3. Based on the frequency analyses of facies associations,
the approximate position of each studied section is
illustrated in the conceptual depositional model of the
studied successions and the frequencies of various
facies associations were investigated.
4. Sequence stratigraphic analyses resulted in recognition
of two 3rd-order sequences in the studied intervals of
the Ilam Formation and facies variations of this unit
throughout the studied wells were investigated using
correlation in the sequence stratigraphic framework.
Acknowledgments We are grateful to the University of Tehran for
the provision of facilities for this research and to the National Iranian
South Oil Company (NISOC) for support and data preparation. We
thank A. R. Ashrafzadeh and M. Omidvar for their useful suggestions.
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