1 Sedimentary Environment Diagenesis Sequence Stratigraphy and Reservoir Quality of the Ilam Formation in Dezful Embayment and Abadan Plain in

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Sedimentary Environment, Diagenesis, Sequence Stratigraphy, and Reservoir

Quality of the Ilam Formation in Dezful Embayment and Abadan Plain in

South-West Iran

Mahdiyeh Gholizadeh, Mohammad Hossein Adabia, Abbas Sadeghi, Mohammadfarid Ghasemi

and Maryam Moradi.

a Department of Sedimentary Basin and Petroleum, Faculty of Earth Sciences, Shahid Beheshti

University Tehran, Iran

Abstract

The Ilam Formation (Cenomanian–Santonian in age) is considered one of the main rock reservoirs

of the Bangestan Group in the southwest of Iran. This formation mostly consists of carbonate

rocks. To examine the sedimentary environment, diagenesis, sequence stratigraphy, and reservoir

quality of Ilam Formation in Dezful embayment and Abadan Plain, four subsurface sections in

wells No. A, B, C, and D, with a range of 106 to 146 meters of thickness, were studied. The

lithology of the Ilam Formation in the studied wells is limestone with interbedded shale and

argillaceous limestone. Considering the abundance of allochems and various fabrics in these

deposits, twelve microfacies and one shale petrofacies of the Ilam Formation were recognized in

these four wells. These microfacies were deposited in three facies belts, namely lagoon, shoal, and

open marine, in a homoclinal carbonate ramp setting. These deposits have been influenced by

meteoric, marine, and burial diageneses. Sequence stratigraphy of the Ilam Formation reveals that

the studied wells consist of a third-order sedimentary sequence. The sea-level fluctuations in this

area are the same as the global sea-level fluctuations. During the study of the Ilam Formation’s

reservoir quality based on the results of diagenesis, mainly porosity and permeability data in one

of studied wells from the depth of 2850.33 to 2911.13 meters, 6 flow units were identified. Flow

unit number 5 has the most potential reservoir quality, and flow unit number 6 has the undesirable

flow unit.

Keywords: Depositional models, Diagenesis, Ilam Formation, Reservoir rock clustering,

Hydraulic flow unit

1. Introduction

The Zagros fold and thrust belt (ZFTB) ,which its approximate length is more than 1500 km ,and

its width varies between 100 and 300 km, is located in the NE margin of the Arabian Plate

(Stöcklin,1968; Falcon, 1969; Berberian and King, 1981) and is a collisional belt between the

Iranian block (belonging to Eurasia) and the Arabian plate, whose convergence started at the

beginning of the Late Cretaceous (Berberian and King, 1981). This collision occurred after the

total consumption of the Neo-Tethys Ocean (Beydoun et al., 1992; Agard et al., 2005; Mouthereau

et al., 2007; Navidtalab et al., 2016; Navidtalab et al., 2019). In the past, Zagros was a part of the

Arabian Plate and was located in tropical latitudes, which caused the deposition of carbonate and

evaporite sediments in this basin (James and Wynd, 1965). Zagros is one of the most significant

basins in the Middle East because of the well-known oil productivity of this region (Afghah, and

Farhudi, 2012), and it is a part of the southern margin of Tethys (Stoneley. 1990). From the Lower

Cambrian to the Quaternary, a distorted sedimentary package is exposed by the ZFTB. (Stocklin,

1968; Falcon, 1969; Berberian and King, 1981; Sepehr and Cosgrove, 2005). This sedimentary

sequence has experienced folding and thrusting during the collision. (Molinaro et al., 2005;

Sherkati and Letouzey, 2004; Sherkati et al., 2005).

A sedimentary cycle from Albian to Campanian has been identified composed of Kazhdomi,

Sarvak, Surgah, and Ilam formations in Zagros, which are called as Bangestan Group, and name

of this group is taken from Bangestan mountain located in the northwest of Behbahan city (James

and Wynd, 1965). The most important interval of this group includes neritic carbonates of the

Sarvak and Ilam formations and their equivalent units (such as the Mishrif Formation of Iraq).

Cretaceous rocks of Zagros do not have the same rock facies and were not deposited in the same

sedimentary conditions. Accordingly, the Ilam Formation and its equivalents contain important

reservoir intervals in the south and southwest of Iran and throughout the Middle East (Aqrawi et

al. 1998; Adabi and Asadi-Mehmandosti 2008; Ghabeishavi et al. 2009). Throughout early Late

Cretaceous times, large parts of the Arabian Plate were covered by shallow subtropical seas

resulting in the deposition of thick limestone successions ,and these carbonates host a considerable

part of the world's total hydrocarbon reserves (Taghavi et al., 2006; Beiranvand et al., 2007).

Despite their outstanding economic importance, the stratigraphic assignment of the Cretaceous

neritic carbonates is notoriously difficult due to the absence of typical open-marine index fossils

(Omidvar et al., 2014).

Carbonate rocks are non-clastic sedimentary rocks that contain more than 50% of carbonate

minerals. Investigating the petrography of carbonate rocks, which includes texture and structure,

and identifying the main and secondary constituents of sedimentary facies are important in

investigating microfacies and changes in the sedimentary environment. Environmental factors

such as depth, temperature, salinity, biological substrate and disturbance, dispersion and spread of

organisms are among the controlling factors of carbonated environments (Tucker, 2001).

Carbonate rocks are sensitive to diagenesis changes and the replacement of crystals occurs over

time by temperature and pressure. In thin sections, this replacement of crystals can be seen with

changes in size, crystal shape, colors, and the creation of impurities compared to the original grains

(Wilson, 2012), but the carbonate facies sequences are the result of environmental changes over

time (Tucker and Wright, 1990). As a result, by identifying the microfacies of carbonate rocks, the

characteristics and conditions of their formation can be interpreted and a schematic model for the

paleo-sedimentary environment of these sediments can be presented.

The Cretaceous geological system is one of the most important systems in the history of geology,

especially in the geology of Iran in the Zagros fold region. This is due to the fact that various

sedimentary conditions led to the desirable setting for the formation of oil traps. Facies variations

of the Ilam Formation in horizontal and vertical directions as well as its diagenetic changes, led to

different reservoir qualities in the formation in various areas of Zagros. In this regard, the present

study aims to investigate the sedimentary environment, diagenetic processes, sequence

stratigraphy, and reservoir characteristics of this formation in subsurface sections (wells No. A, B,

C, D) located in the southwest of Iran.

2. Geological setting

The study area is located in the Dezful Embayment and Abadan Plain southwest of Iran (Fig. 1),

which is part of the Zagros fold-and-thrust belt (Alavi, 2004). The Dezful Embayment is located

southwest of the Zagros thrust (Alavi, 2007). Despite its relatively small area, the Dezful

Embayment produces a large portion of Iran’s oil reserves (Bordenave and Huc, 1995) because the

Gachsaran Formation has covered Asmari limestone as a cap rock in the oil system (Berberian,

1995). The Dezful Embayment is separated from other zones by the Mountain Front Fault (MFF),

Balarud Fault Zone (BFZ), Izeh Fault Zone (IFZ), Kazerun Fault Zone (KFZ), and Zagros Front

Fault (ZFF), Zagros Main Reverse Fault (ZMRF), High Zagros Fault (HZF), Zagros Deformation

Front (ZDF) (Fig. 2).

The Abadan Plain is located in southwest Iran and is surrounded by the Dezful Embayment, the

Persian Gulf, and the Iran-Iraq boundary (Fig. 2). It is a part of the Mesopotamian Basin, and its

structural characteristics differ from the Dezful Embayment (Zeinalzadeh et al., 2015).

Mesopotamian Basin is considered one of the richest petroleum systems in the world (Sadooni and

Aqrawi, 2000). The Mesopotamian Basin is a vital hydrocarbon province in Iraq and contains

several well-known oil fields. Also, Abadan Plain has some good oil fields that are considered for

future development and production. A thick Mesozoic succession and ranges dominate the

stratigraphic section in the Abadan Plain from Jurassic to Cretaceous. The Cretaceous rocks in the

basin are considered gas and oil reservoir rocks (Zeinalzadeh et al. 2015) and comprise fractured

and vuggy carbonates as well as clastic rocks. The Abadan Plain has been relatively less influenced

by the tectonic compression resulting from the closure of the Neo-Tethys Ocean (Abdollahie-Fard

et al., 2006; Aqrawi and Badics, 2015). Consequently, it is structurally characterized by gentle

folding compared to more eastward sub-basins in SW Iran (Aqrawi and Badics, 2015). In this

plain, similar to the other parts of the Mesopotamian Basin, due to the cover of recent alluvial

deposits and the absence of outcrops, the geological information is mainly limited to the

subsurface, including drilling boreholes and geophysical data (Sissakian, 2013).

Fig. 1. The location of the studied oil fields.

Fig. 2. (a) geological map of the Arabian Plate and Zagros Fold-Thrust-Belt. Tectonic features are

from reference (Alavi, 2007). Abbreviations: DE: Dezful Embayment (the region between the

Abadan Plain Basin and MFF), MFF: Mountain Front Flexure, MZT: Main Zagros Thrust, ZTZ:

Zagros Thrust Zone, ZDF: Zagros Deformation Front, ZS: Zagros structure. The Zagros front fault

bounds Abadan Plain Basin to the north and northeast. (b) cross-section across the Arabian Shield,

Arabian Platform, and the ZFTB along line A–A’ modified from reference (Abdollahie-Fard et al.,

2006), and all final changes are from reference (Atashbari et al., 2018).

3. Materials and methods

This study is based on the subsurface data (Fig. 1) and basic information, including thin sections

of cores, cuttings, and sonic and gamma-ray logs in the four studied wells. The petrographic

analysis includes examining and identifying carbonate and non-carbonate components, identifying

diagenetic processes, and recognizing different microfacies. In this study, 388 thin sections from

the Ilam Formation have been examined.

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1SedimentaryEnvironment,Diagenesis,SequenceStratigraphy,andReservoirQualityoftheIlamFormationinDezfulEmbaymentandAbadanPlaininSouth-WestIranMahdiyehGholizadeh,MohammadHosseinAdabia,AbbasSadeghi,MohammadfaridGhasemiandMaryamMoradi.aDepartmentofSedimentaryBasinandPetroleum,FacultyofEarthSciences,Shahi...

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