Depositional environment and sequence stratigraphy of the Upper ...

Carbonates Evaporites DOI 10.1007/s13146-013-0168-z

ORIGINAL ARTICLE

Depositional environment and sequence stratigraphy of the Upper Cretaceous Ilam Formation in central and southern parts of the Dezful Embayment, SW Iran Hamzeh Mehrabi • Hossain Rahimpour-Bonab • Amir Hossain Enayati-Bidgoli • Amin Navidtalab

Accepted: 3 July 2013 Ó 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 studies 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 conceptual 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 stratigraphic framework. Keywords Depositional sequences Sedimentary environment Ilam Formation Santonian Dezful Embayment

H. Mehrabi (&) H. Rahimpour-Bonab A. H. Enayati-Bidgoli A. Navidtalab Department of Geology, College of Science, University of Tehran, Tehran, Iran e-mail: [email protected]; [email protected]

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 microscale) 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 Formation (Fig. 2), these successions host up to one-third of the total Iranian oil reserves (Motiei 1993). Considering its reservoir 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

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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 Mesopotamian basin) the Cretaceous successions constitute thick sedimentary packages, which host numerous economically important hydrocarbon reserves (Setudehnia 1978; Al-sharhan 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 depositional 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 situated 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 differentiated 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

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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)

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 shallowwater limestones that unconformably overlies the carbonate 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 sections (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, compositional 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.

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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 associations 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, Hedbergella 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 lamination, 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 Hedbergella in a mud dominated matrix. Other components are debris of bivalves, echinoderms, calcareous sponge

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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 peloids), 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 environment (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 spicules 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 association of echinoderms debris, planktic/benthic foraminifera, 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,


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/wackestone, F4 Peloid-Oligosteginid wackestone, F5 microbioclastic

wackestone, F6 foraminiferal mudstone/wackestone, F7 bioclasticforaminiferal wackestone to packstone, F8 bioclastic-intraclastic wackestone, F9 rudist debris floatstone to wackestone

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).

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 taluses that were extended to the middle parts of mid-ramp and mixed with mid-ramp bioclasts (Flugel 2010; Jez et al. 2011).

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).

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 peloidal-bioclastic shoals and re-deposited in the low energy

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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 bioclastic 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 foraminiferal wackestone to mudstone

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

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 conditions. Considering the variations in size and type of bioclasts, 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).

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.

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 intraclasts are composed of some bioclasts such as echinoderms debris, green algae, gastropods and benthic

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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

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 bioclasticpeloidal 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 bioclasts 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

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Carbonates Evaporites 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

Nonskeletal

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

Bioclasticforaminiferal wackestone/ packstone

Lime, brown– gray, wackestone/ packstone


Small benthic foraminifera, rare planktonic foraminifera, debris of echinoderms, bivalves, rudists and green algae

Peloids

MF 6, MF 8, MF 9


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


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


MF 10

Fine bioclastspeloidal packstone/ wackestone

Lime, cream light brown, packstone/ wackestone


Diverse fine bioclasts include, echinoderms, rudists, bivalves, small benthic foraminifera debris and rare planktic foraminifera

Peloids

MF 8, MF 9


MF 11

Diverse size bioclastic wackestone

Lime, dark cream-gray, wackestone


Coarse and fine debris of rudists, bivalves, echinoderms, large and small benthic foraminifera

Peloids

MF 12

Channel

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Carbonates Evaporites Table 1 continued MF code

Microfacies name

Lithology, color and texture

Grain size and sorting

Components

Facies association

Interpretation (environment)

Skeletal

Nonskeletal

MF 12

Rudist debrisintraclastic 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 foramgreen 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


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


Large benthic foraminifera echinoderms and green algae debris, calcareous sponge spicules and gastropods

Peloids

MF 16, MF 17

Restricted lagoon

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).

lagoon) to the proximal mid-ramp and leeward shoal (Bauer et al. 2002; Bucur and Saˇsaˇran 2005; Flugel 2010).

Benthic foraminifera-green algae debris grainstone (F16) This facies is composed of abundant green (dasycladaleans) 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 Saˇsaˇran 2005). The association of green algae, benthic foraminifera and graindominated 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

Green algae debris wackestone/mudstone (F17) Green algae debris, bivalves, echinoderms, large benthic foraminifera 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 foraminifera and a mud-dominated fabric indicates that this facies deposited in a lagoon environment near to an open-marine setting (Bucur and Saˇsaˇran 2005; Schulze et al. 2005; Flugel 2010).

Open-marine and restricted lagoon facies

Bioclast-large benthic foraminifera wackestone/mudstone (F18) Large benthic foraminifera such as Rotalia and

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Fig. 6 Frequency diagrams for the six studied wells based on analyses of various facies associations of the Ilam Formation

Miliolidae, echinoderms, green algae debris, calcareous sponge spicules, gastropods and peloids are the principal components of this facies (Figs. 4, 5; Table 1). The presence 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-sharhan 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

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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 generalized positions are determined by considering the frequency 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. 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

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 indicates 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).

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 Bangestan 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 subsurface sections of the Dezful Embayment formed in a ramplike 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

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 comprehensive 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

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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

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 ramplike. 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;

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Rahimpour-Bonab et al. 2012a; Mehrabi and RahimpourBonab 2013) it could be concluded that depositional environment of the Ilam Formation was considerably different 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. 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)

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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 assemblage’s deposition (in response to the sea-level fluctuations), 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 maximum 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:

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).

Sequence 1

1.

This sequence is recorded in all of the studied wells and its thickness varies from 80 to 25 m. The transgressive systems tract (TST) of sequence 1 is mainly composed of basin and outer ramp facies (pelagic foraminifer wackestone/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 considerable 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

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Conclusions

2.

3.

4.

In the subsurface sections of the Dezful Embayment the Ilam Formation consists of 18 representative microfacies 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 depositional environments. This study indicates that the Ilam Formation was formed on a ramp-like carbonate platform under warm and humid (tropical) climatic condition. Lateral distribution of various sub-environments was also determined in this ramp-like carbonate platform. 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. 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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