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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