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New data (onshore, offshore) on the Upper Cretaceous Ilam Formation of southwestern Iran are provided concerning the common presence of dasycladalean green algae (families Triploporellaceae, Polyphysaceae) that have so far been treated mostly in open nomenclature. The assemblage consists of the five taxa Salpingoporella ubaiydhi Radoičić, 1979, Dissocladella ondulata (Raineri, 1922), Trinocladus tripolitanus Raineri, 1922, Clypeina cf. dusanbrstinai Radoičić, 1997, and gen. et sp. indet. The taxa typically prevail in wackestones-packstones but exhibit different distributional patterns. While S. ubaiydhi and T. tripolitanus, rarely associated with D. ondulata, form a characteristic assemblage in a distal infralittoral facies (more diverse assemblage; zone no. 31 of Wynd, 1965), C. cf. dusanbrstinai occurs in a proximal infralittoral facies associated with miliolids among also complex forms (rhapydioninids) (assemblage zone no. 30 of Wynd, 1965). It is concluded that the two assemblage zones refer to different palaeoenvironments, instead of different ages ('lower' and 'upper'). Based on benthic foraminifera, the main part of the Ilam Formation can be ascribed to the Santonian. For the lower part, a late Coniacian age is suggested implied indirectly by the comparably long vertical range of the rotaliid Orbitokathina vonderschmitti Hottinger, 1966 of the studied core. This would be in conformity with the K 150 Arabian Plate K 150 sequence boundary referring to a late Turonian?-early Coniacian hiatus. From a palaeobiogeographic perspective, T. tripolitanus is a cosmopolitan species, S. ubaiydhi is restricted to the area of the former Arabian Plate (subsurface Iraq, Saudi Arabia and Egypt), while C. dusanbrstinai has been recorded only from Serbia (type-locality). In addition to the algae, the stratigraphic discussion includes the occurrence of the benthic foraminifera Reticulinella? kaeveri Cherchi, Radoičić & Schroeder, 1989 and Orbitoides mid-orientalis (Eames & Smout, 1956) in the Ilam Formation. [ABSTRACT FROM AUTHOR]

The frequency of limestone has caused to be encountered in different projects of engineering. The Ilam Formation is one of the formations that is widely spread in the Zagros Mountains, and in the sample cutting, it contains 195 meters of cream-colored limestone. Due to the extent of this formation, many projects have been performed on this formation, and they also encounter this formation in oil drilling. Its lower and upper Boundaries are continuous with Surgah Formation and Gurpi Formations, respectively. The aim of this study is to investigate and introduce the engineering geological features of Ilam formation limestones in the type section. The Ilam Formation is a challenge in various projects like other formations with carbonate units. Especially, when the mentioned units are different in terms of lithology, physical, and mechanical. In order to identify the characteristics and engineering behavior of these rocks, 60 blocks were selected from different limestone horizons of the Ilam Formation and were transferred to the laboratory. Then, the physical and mechanical properties and lithology characteristics of limestones are determined. The samples have been used to test the physical properties, durability, point load index, uniaxial compressive strength, Brazilian tensile strength, and wave velocity. The results of the multivariate regression showed that the differences between petrological and physical properties have led to a variation in their mechanical properties. Statistical indices such as R², R²adjusted, RMSE have been evaluated by Minitab 16 software, and the relationship between physical and mechanical properties was determined. Based on the results, there is a higher reliability coefficient for estimating mechanical properties than other properties in the dry condition compared to the saturated condition. The results of this research showed that lithological and physical characteristics play an important role in the mechanical properties of Ilam limestones. [ABSTRACT FROM AUTHOR]

The late Cretaceous Ilam formation in the western part of the Zagros basin (W Iran) was inspected in two outcrops (namely Mehdiabad and Kuh-e Varzarin sections) which are located in the Ilam province (in the Zagros fold belt). The purpose of this research was to identify the lithofacies, microfacies and depositional environment of the Ilam formation. This formation consists of limestone and argillaceous limestone which comprise three microfacies (planktonic foraminifera wackestone/packstone, planktonic foraminifera-benthic bivalve wackestone/ packstone/ rudstone, benthic-planktonic foraminifera wackestone/ packstone/ rudstone that were deposited in two facies zones (c.f. slope and toe of slope). Evidence of the gravity flow including basal truncation (erosional) surface of beds, normal grading, weak amalgamation, abundant deep-water facies, coupled laminae, relatively thick gravite beds, mixed planktonic (deep water) and benthic (shallow water) biota, large scale slump folds and deformed structures, attest the influence gravity flows which transported the shallow water sediments into the deep setting. Poor quality of the outcrops and lack of well-preserved sedimentary structures makes it impossible to discriminate between various types of sediment gravity flow deposits (gravites). Gravites are observed only in the toe of slope facies zone. This facies zone in turn is classified into three parts including upper, middle and deep parts according to the texture and the argillaceous limestone beds thickness. Due to ack of platform top deposits, it was not possible to distinguish the carbonate platform type (ramp versus shelf type). However, the overwhelming resedimented carbonate facies points toward a carbonate shelf rather than a ramp. According to the occurences gravites in some part of the studied sucsessions (toe of slope facies zone), seemingly a carbonate platform-margin (shelf type?) with two distinct parts including middle (slope) and outer parts (toe of slope), was existed during the deposition of the Ilam formation in the study area. [ABSTRACT FROM AUTHOR]

This research is based on sedimentological study of 4 surface (Kabir kuh, Samand, Anjir and Anaran) and 8 subsurface wells in the Lurestan, North Dezful and Abadan plain with a total thickness of 1592 m. The Santonian – Lower Campanian Ilam Formation is a carbonate hydrocarbon reservoir in the Zagros fold – thrust belt, in SW Iran. Twenty microfacies were recognized based on microscopic study. Microfacies analysis led to the recognition of four facies belts including inner (shoal), mid, outer ramp, and basinal setting. Correlation of depositional environments together with lateral facies changes indicate deposition are distally steepend ramp. Deep marine and inner ramp facies are the most and least facies, respectively in the study area. As the distribution of inner ramp facies is very limited, it can only be recognized in wells from North Dezful and Abadan plain. The results obtained from facies together with analyses of wireline logs clearly shows a northward deepending of the depositional environment from Balarud Fault. Cementation, micritization, compaction, solution seams, neomorphism, disolution, fracturing, pyritization, phosphatization, gluconitization and bioturbation are the main diagentic processes. However, cementation is most dominant in shallow marine facies in the Abadan plain, whereas, pyritization, stylolite and fracture are the most prevailing processes in deeper marine facies of the Ilam Formation in Lurestan. There is a limited distribution of porosity, but the most dominant types are, interpartical, microporosity and fracture related porosity. The Ilam carbonates are mostly affected by shallow to deep burial diagenesis in Lurestan, whereas, in the Abadan plain and North Dezful, the marine and meteoric diagenesis are the most dominant diagenetic types. [ABSTRACT FROM AUTHOR]

The Upper Cretaceous Ilam Formation in southwest Iran was studied in order to provide a better understanding of the palaeoenvironments that lead to the deposition of heterogeneous carbonate rocks. This formation is an important hydrocarbon reservoir of the Bangestan Group in the Zagros Basin and the Abadan Plain. The Ilam Formation is comprised mainly of carbonate rocks and was studied from three subsurface sections (100, 74, and 100 m thick). Petrographic investigation reveals that the Ilam Formation consists of 12 microfacies and a shale petrofacies that were deposited in lagoonal, shoal, and open marine facies belts of a carbonate ramp. Two third–order depositional sequences are recognised. Comparison of Ni/Co reveals that the limestones of the Ilam Formation formed under oxic to anoxic conditions. The main diagenetic features of the Ilam Formation include compaction, dissolution, dolomitization and cementation that occurred in marine, meteoric and burial diagenetic environments. We document for the first time the relationship between facies and geochemistry in the carbonate rocks of the Ilam Formation. These findings provide a more comprehensive understanding of the palaeoenvironments that lead to the deposition of heterogeneous carbonate rocks and their relationship with geochemistry and reservoir characteristics. [ABSTRACT FROM AUTHOR]

Abstract To developing and explore hydrocarbon fields, it is necessary to know geological models and reservoir quality changes. Therefore, the Ilam Formation with approximate 145m thickness was examined in the Susangerd oilfield. The sedimentary model was considered carbonate ramp type due to the lack of slump and reef structures. Two complete and one incomplete third-order sedimentary sequences were identified. Close to the sequence boundary in the middle part of the Ilam Formation, reservoir conditions improve with the development of vug and mold dissolutions, especially in green algae by corrosive fluids passing through stylolites and fractures. But near the sequence boundary in the upper part of the Ilam Formation, however intergranular porosity develops between peloids and ooids but almost destroyed with cementation. The southeastern domain of the oilfield contains shallower microfacies in the lower sedimentary sequence and most part of transgressive system tract of the middle sequence in comparison with the northwestern domain. It seems that the southeastern part of the oilfield is located on paleo high and the lower values ​​of porosity log in the northwestern part are probably due to the distance from this paleo high. Therefore, the southeast of the oilfield is suggested for the development of the Susangerd oilfield and future explorations. Keywords: Sedimentary environment, Sedimentary sequence, Reservoir quality, Ilam Formation, Susangerd oilfield Introduction A large part of the Cretaceous carbonate reservoirs belongs to the Bangestan Group, including the Sarvak and Ilam formations in the Zagros and the Arabian platform. The Ilam Formation and its equivalents contain important reservoirs in the south and southwest of Iran (Dezful Embayment) and the Middle East. In this study, the sedimentary environment, sequence stratigraphy, heterogeneity and reservoir quality of the Ilam Formation in the Susangard oilfield in the southern part of the North Dezful Embayment have been investigated to develop and explore hydrocarbon reservoirs. The Susangard oilfield is located at 45 km in the west of Ahvaz oilfield and adjacent to the Juafair, Abteymour and Band-E-Karkheh oilfields. The predominant lithology of the Ilam Formation is carbonate and sometimes argillaceous limestones and shales in the studied area. The upper boundary of the Ilam Formation with the Gurpi Formation and its lower boundary with the Lafan Formation are transitional. In the past, many studies have been done on the sequence stratigraphy, sedimentary environment and diagenetic processes of the Ilam Formation in the southern part of the North Dezful Embayment (Adabi & Asadi Mehmandosti 2008; Hassani Giv & Abarghani 2009; Soleimani 2011; Mehrabi et al. 2013; Khanjani 2015; Omidi 2020; Khodaei 2020). Previous studies demonstrate that the sedimentation of the Ilam Formation is in a homoclinal ramp consisting of three inner, middle and outer ramp belts, while tidal flat facies has not been reported in most studies. Material & Methods The studied wells are named A, B and C from the northwest to the southeast of the oilfield, respectively. The thickness of the Ilam Formation is 140 m in well A, 150 m in well B and 141 m in well C. In this research, the 84 and 70 thin sections prepared from drilling cuttings of well A and well B, respectively and the 96 thin sections prepared from the core (from 3356 to 3438m) and the drilling cuttings of well C have been studied. The percentage and type of grains and matrix in these thin sections were examined and microfacies were named based on the classification of Dunham (962) for carbonate rocks. In this study, the lateral distribution of sedimentary facies has been reconstructed using standard Flugel (2010) facies models in and Burchette and Wright (1992) carbonate ramp sedimentary models. For investigation of the vertical distribution of microfacies, sequence stratigraphy of the Ilam Formation has been studied using petrography studies, petrophysical gamma and neutron logs and the sedimentary sequence stratigraphy model IV (Catuneanu 2006). Discussion of Results & Conclusions Based on petrography studies, one petrofacies (Anhydrite) and 15 microfacies were identified in the Ilam Formation. According to the results of this study, the Ilam Formation has been deposited in a carbonate ramp in three facies belts, including the inner ramp (tidal flat, lagoon and shoals), middle ramp and outer ramp. Two complete and one incomplete third-order sedimentary sequences were identified in the Ilam Formation. Reservoir conditions in the middle part of the Ilam Formation (the middle sequence) near the sequence boundary improve due to the development of vug and mold dissolutions, especially in green algae with aragonite mineralogy by corrosive fluids passing through stylolites and fractures. However, in the upper part of the Ilam Formation -- the upper sequence -- the presence of intergranular porosity in ooids and peloids is the main factor in increasing the reservoir quality and their cementation is a main factor in reducing the reservoir quality. In this oilfield, almost intergranular porosities of the upper part of the Ilam Formation were destroyed by cementation. The lower part of the Ilam Formation, the lower sequence, does not have a suitable reservoir quality due to the high content of mud in the matrix. Upon the onset of subduction of the oceanic crust of the Neo Tethys under the Central Iran Plate during the middle Cretaceous, the inactive and stable margins of the Neo Tethys became active margins and began the Alpine orogenic phase. At this time, due to the reactivation of basement faults and the movements of the Hormoz salt, several paleo highs have been formed in different parts of southwestern Iran and the Dezful Embayment. The presence of these paleo highs has caused heterogeneity and lateral facies changes and fluctuations. The Paleo highs in southwestern Iran are known in several oilfields, including the Gachsaran, Rag-E-Safied and Abteymour (Nazar Aghaei 1986; Afsharzadeh 1999; Van Buchem et al. 2001; Mehrabi 2013). The southeast of the Susangerd oilfield has a shallower sedimentary environment than the northwest of the oilfield at the time of deposition of the lower sedimentary sequence and almost transgressive system tract of the middle sequence which can indicate paleo high in the southeast of the oilfield. The lower values ​​of the porosity log in the northwest of the oilfield compared to the southeast are probably due to distance from the paleo high. Therefore, the southeast of the oilfield is suggested for the development of Susangerd oilfield and future explorations.

Permeability is a vital parameter in reservoir engineering that affects production directly. Since this parameter's significance is obvious, finding a way for accurate determination of permeability is essential as well. In this paper, the permeability of two notable carbonate reservoirs (Ilam and Sarvak) in the southwest of Iran was predicted by several different methods, and the level of accuracy in all models was compared. For this purpose, Multi-Layer Perceptron Neural Network (MLP), Radial Basis Function Neural Network (RBF), Support Vector Regression (SVR), decision tree (DT), and random forest (RF) methods were chosen. The full set of real well-logging data was investigated by random forest, and five of them were selected as the potent variables. Depth, Computed gamma-ray log (CGR), Spectral gamma-ray log (SGR), Neutron porosity log (NPHI), and density log (RHOB) were considered efficacious variables and used as input data, while permeability was considered output. It should be noted that permeability values are derived from core analysis. Statistical parameters like the coefficient of determination ( ), root mean square error (RMSE) and standard deviation (SD) were determined for the train, test, and total sets. Based on statistical and graphical results, the SVM and DT models perform more accurately than others. RMSE, SD and R2values of SVM and DT models are 0.38, 1.63, 0.97 and 0.44, 2.89, and 0.96 respectively. The results of the best-proposed models of this paper were then compared with the outcome of the empirical equation for permeability prediction. The comparison indicates that artificial intelligence methods perform more accurately than traditional methods for permeability estimation, such as proposed equations. Doi: 10.28991/HEF-2021-02-02-01 Full Text: PDF

Abstract Biostratigraphic studies of the Sarvak and Ilam formations in wells No. 1 and 14 of the Abteymour Oil Field led to the recognition of six species of foraminifers from the two mentioned formations for the first time. In this research, the species of Fischerina carinata and Spiroloculina cenomana in the Sarvak Formation and the species of Palaeosigmoilopsis apenninica and Sigmomassilina ottadunensis in the Sarvak and Ilam formations belonging to the suborder of MILIOLINA Delage and Herouard (1896) and the species Rotorbinella sp. nov. in the Sarvak and Ilam formation and Pararotalia boixae in the Sarvak and Ilam formations belonging to the suborder of ROTALIINA Delage and Herouard (1896) were identified and described. These species are not yet reported from the Sarvak and Ilam formations. Based on the identified fauna, the Sarvak Formation with a thickness of 812.98 and 238 meters in wells No. 1 and 14 respectively is Cenomanian–Turonian in age, and the Ilam Formation with a thickness of 151.7 and 136 meters in wells No. 1 and 14 respectively is Santonian–Campanian? in age. Keywords: Abeteymour Oil Field, Ilam Formation, Foraminifera, Rotaliidae, Sarvak Formation Introduction The Sarvak and Ilam formations, as one of the most important carbonate hydrocarbon reservoirs after the Asmari reservoir in the Zagros Basin, have special importance in petroleum studies. So far, several studies have been conducted on the two mentioned formations from different aspects especially biostratigraphic studies (James and Wynd 1965; Wynd 1965; Wood and Lacassagne 1956; Wells 1966, 1967, 1968; Stoneley et al. 1975; Speers 1967; Rahimpour-Bonab et al. 2012; Omidvar et al. 2014; Amiri Bakhtiar 1991; Khosroo Tehrani and Fonooni 1994; Allahbakhsh Ghiyasvand 2003; Teimourian 2004; Akbari Bas Kelayeh and Taheri 2005, 2006; Kamyabi Shadan 2005; Agh 2005; Sadeghi et al. 2006; Ghalavand 2009; Soleimani 2010; Ghobeyshavi et al. 2010; Sajjadi and Omidvar 2011; Omidvar 2011). However, these biostratigraphic studies of the Sarvak and Ilam formations in different areas of the Zagros Basin, introducing several biozones, some coexistence of foraminifera in these zones have been introduced . In the Dezful Embayment, due to the tectonic phases that occurred after Cenomanian and after Turonian (sub Hercynian tectonic phases), there is some absence of stratum or hiatus. The shallowing trend of the Late Albian to Cenomanian basin produced large amounts of shallow carbonate sediments of the Sarvak Formation in the Zagros Basin. However, during the Late Turonian, a regional uplift has taken place causing partial or complete erosion of the Turonian sediments, which create an erosional unconformitiy after Turonian, characterized by the development of iron-bearing sediments and sedimentary discontinuities in the Dezful Embayment and Fars areas. Omidi et al. 2018, in order to identify the fossil content with an emphasis on recognizing foraminifera of the Sarvak and Ilam formations in the south of Dezful embayment, studied two subsurface sections of wells 1 and 14 of the Abteymour oil field. They introduced four biozones based on the identification of 71 species belonging to 57 genera of foraminifera: one biozone in the Ilam Formation, called Biozone No. 1: Rotorbinella-Pararotalia Assemblage Zone, and three biozones in the Sarvak Formation called biozone No. 2: Nezzazatinella picardi-Dicyclina Schlumbergeri Assemblage Zone; biozone No. 3: Rotorbinella mesogeensis Total Range Zone and biozone No. 4: Nezzazata-alveolinid Assemblage Zone. Based on these identified biozones, the ages of the Sarvak and the Ilam formations are Cenomanian–Turonian and Santonian–Campanian, respectively. The identified benthic foraminifera in the Abteymour Oil Field is somehow similar to the other parts of the Zagros, but in addition to the benthic foraminifera reported in previous studies of these two formations; six other foraminifera species are identified and introduced from this oil field for the first time in this research. Since the Ilam and Sarvak formations face several high-pressure zones during drilling in the Abteymour oil field, identification of high-pressure oil and gas zones in strata is one of the issues that are of great importance. As a result, fossil studies, especially foraminifera can be helpful in finding the relative and sometimes exact location of high-pressure zones. The Ilam Formation in drilled wells in parts of the south of Dezful Embayment (Abteymour Oil Field) has low interest in terms of diversity of fossil genera and species (foraminifera); however, some foraminiferal members have been less studied, for example, the genus and species of the Rotaliidae family. In addition to the introduction of the Rotalia sp. 22-algae assemblage zone (due to the high diversity of Rotaliidae family members, sometimes up to 60 species, sp. 60) in the James and Wynd (1965) studies, and the high focus of systematic studies by Hottinger et al. (2014) on this family confirms the complexity and importance of the study in identifying and distinguishing the genus and species of this family; therefore, geologists on oil wells in Iran know the Ilam Formation as the "Rotalia Formation". Thus, the importance of the systematic study of the Rotaliidae family for separation of genera and species is more important. In this study, the foraminifera of the Ilam Formation have been systematically studied (to separate and distinguish several genera and species of Rotaliidae from each other), and this can be used to complete the chain of biostratigraphic studies in identifying high-pressure oil zones. In the Sarvak Formation, the study and knowledge of benthic foraminifera fossil assemblages could have great importance in the enrichment of fauna associations and recognizing biostratigraphic zones, determining the exact age of strata, knowing more local biozones, and identifying oil zones. Also, the rate of change in the diversity and abundance of foraminifera and their morphological characteristics increases in anoxic periods, for example, OAE1a (Leckie et al. 2002; Erba 2004). However, one of the biological characteristics of this incident is the presence of planktonic foraminifera with elongated chambers (Coccioni et al. 2006). However, here it is enough to describe the species that have been reported for the first time from the Sarvak and Ilam formations to identify more foraminifera and the fauna associations in the enrichment of the biozonations related to these two formation.. Material & Method In order to do biostratigraphy and identifying foraminifera in the Sarvak and Ilam formations in Abteymour Oil Field, the two wells of No.1 and 14, with the most suitable sample, were selected.A total of 1347 thin sections of the two wells (716 thin sections from well No. 1, mainly prepared from drilling cuttings and 631 thin sections from well No. 14, mostly prepared from drilling cores) were studied. These thin sections were studied microscopically to identify the microfossil contents, and after identification, they were photographed. Identification and description of the foraminifera were performed based on Loeblich and Tappan (1988), Boix (2007), Chiocchini et al. (2012), and Hottinger (2014). Discussion of Results & Conclusions The Sarvak and Ilam formations (Bangestan oil reservoir) are considered the second largest oil reservoir in Iran after the Asmari reservoir. Therefore, a careful study of the fossil content of these two oil formations is of particular importance in biostratigraphic studies. For this reason, in this study, a detailed study of the fossil content (especially foraminifera) of these two formations has been considered. The Ilam Formation in wells No. 1 and 14 has a thickness of 152 and 136 meters, respectively, and its lithology is mainly limestones and limestone with shale interbeds. The Sarvak Formation in well No. 1 is 813 meters thick, and in well No. 14, 238 meters of the Sarvak Formation has been drilled. The lithology of the Sarvak Formation is mainly limestones and dolomitic limestones. The biostratigraphy of the Sarvak and Ilam formations in wells No. 1 and 14 of Abteymour Oil Field, in addition to benthic foraminifera, which has been reported in previous studies of the Sarvak and Ilam formations, the following six benthic foraminifera are identified and introduced for the first time from these two formations:1- Fischerina carinata Peybernès, 1984 (Cenomanian, Sarvak Formation); 2- Spiroloculina cenomana Chiocchini, 2008 (Cenomanian, Sarvak Formation); 3- Sigmomassilina ottaduenesis Chiocchini, 2008 (Turonian, Sarvak Formation and Santonian–Campanian?, Ilam formations); 4- Paleosigmoilopsis apenninica Chiocchini, 2008 (Cenomanian–Turonian, Sarvak Formation); 5- Pararotalia boixae Piuz and Meister, 2013 (Cenomanian, Sarvak Formation and Santonian, Ilam Formation); 6- Rotorbinella sp. nov. (Turonian for Sarvak Formation and Santonian-Campanian? for Ilam Formation). These species have never been reported before from the Sarvak and Ilam formations.

The carbonate-dominated Upper Cretaceous Ilam Formation is representative of an important reservoir in the Zagros Basin. In this study, planktonic foraminiferal biostratigraphy of the Ilam Formation was investigated in the Kuh-e-Surgah section in western Iran. In the studied section, the 170 m thick Ilam Formation ismainly composed of limestoneswith shale intercalations. The formation conformably overlies and underlies the Surgah and Gurpi formations, respectively. Twelve genera and 24 species of planktonic foraminifera were identified. The following planktonic foraminiferal biozones were identified: the Dicarinella asymetrica Total Range Zone (latest Coniacian--late Santonian), the Globotruncanita elevata Partial Range Zone (Santonian to early Campanian), the Globotruncana ventricosa Interval Zone (middle to late Campanian.), the Radotruncana calcarata Total Range Zone (late Campanian.), the Globotruncanella havanensis Partial Range Zone (late Campanian) and the Globotruncana aegyptiaca Interval Zone (late to latest Campanian). In general, the biozones show that the Ilam Formation in the studied section is latest Coniacian--late Campanian in age. The Santonian--Campanian boundary in theKuh-e-Surgah section correlateswith the Globotruncanita elevata Partial Range Zone. The recognized biozones in the Ilam Formation are comparable with those recorded in other parts of the Tethyan Realm. The crinoid species Marsupites testudinarius is absent in the studied section. Generally, the last occurrence of this cosmopolitan species indicates the base of the Campanian stage. The Santonian/Campanian boundary was placed in the lower part of the Globotruncanita elevata Partial Range Zone at the base of the Ilam Formation in the studied section. [ABSTRACT FROM AUTHOR]

Abstract The Ilam Formation (Coniacian? –Santonian), is among the most important oil reservoirs of the Abadan Plain, SW Iran. Despite its reservoir significance, there is no comprehensive knowledge about the geological characteristics and factors controlling reservoir quality. In this study, the Ilam Formation is investigated using detailed core description, thin section study, and conventional petrophysical well log data to explain facies characteristics, sedimentary environment, and presenting the sequence stratigraphic framework. Accordingly, a total of 280 m of cores in four key wells from three oil fields as well as 620 thin sections were described and studied. The results of facies analysis lead to the recognition of 12 microfacies grouped in four facies belts including lagoon, shoal, shallow and deep open marine and two siliciclastic petrofacies (shales) related to brackish water and shallow lagoonal environment, which deposited in a carbonate ramp. Frequency analyses of facies associations and petrophysical well log signature indicate that the sedimentary basin was deepening to the east. Based on the identified sequence boundaries and maximum flooding surfaces, one third-order sequence in the studied interval was recognized and correlated by using the petrophysical well log data in all studied wells. The maximum flooding surfaces are discriminated by the development of deep-marine facies and also high gamma-ray responses on the well logs. Keywords: Ilam Formation, Abadan Plain, Facies, Sedimentary Environment, Sequence Stratigraphy Introduction Sedimentary facies, along with the distribution of diagenetic processes and fractures, are the three most important factors controlling the pores system and the reservoir characteristics of carbonates (Lucia 2007; Ahr 2008; Moore and Wade 2013). Despite the high impact of diagenetic processes on carbonate sequences, facies analysis and reconstruction of the ancient sedimentary environment is one of the most important and primary steps in the comprehensive geological studies of the reservoir (Ahr 2008). The sequence stratigraphic framework can provide an interconnection between the characteristics of the facies associations as components of a sequence and reservoir characteristics and also facilitate reservoir evaluation (Morad et al. 2012). To identify the key sequence boundaries, the integration of microfacies and core study results is applied (Tucker 1993; Taghavi et al. 2006). In other words, by using this framework, the pattern of facies distribution and the diagenetic processes trend related to facies could be examined (Mazzullo 1994). The focus of sequence stratigraphy is on the stacking pattern of sediments in space and time framework, which is formed by the contrast of the parameters of accommodation space and sediment supply (Catuneanu 2006; Catuneanu et al. 2012). Reservoir study in the sequence stratigraphy framework can lead to the identification, description, and study of the distribution of reservoir zones (Taghavi et al. 2006). To separate the sequences, it is important to identify the two key surfaces of maximum flooding surfaces and the sequence boundaries. The carbonate rocks of the Bangestan Group include the Sarvak and Ilam formations, where they are important reservoir rocks in many oil fields in the Zagros Basin and the Persian Gulf. The Ilam Formation in the Lorestan Basin is characterized by pelagic facies whereas in the Dezful Embayment and Fars appeared with neritic facies (James and Wynd 1965). In the type section in the northwest of Kabir-Kuh anticline, the lower boundary of this formation with Surgah Formation is conformable, while from the Lorestan Basin to the southeast, the Surgah Formation was disappeared and the Ilam Formation is placed on the Sarvak Formation disconformably. The purpose of this research is to identify and introduce microfacies and determine the geometry of the Ilam Formation platform along with determining the position of the studied wells in the sedimentary model. Also, we seek to identify the third-order depositional sequences and correlate them through petrophysical logs in other studied wells. Material & Methods In this study, four key wells in three oil fields of the Abadan Plain were used to identify microfacies, sedimentary environments and determine the depositional sequences of the Ilam Formation, by describing 280 meters of cores and studying 620 thin sections. Thin sections were stained by Alizarin Red S to differentiate calcite from dolomite (Dickson 1966). To identify microfacies and sedimentary environment, integration of core descriptions and thin section petrography studies have been used. In petrographical study of microscopic thin sections, mineralogy, texture, grain size, sorting, skeletal and non-skeletal components, and fossil content in each sample were identified and described quantitatively. The Dunham classification (Dunham 1962) has been used to describe and name microfacies. Standard facies models were used for facies analysis and nomenclature (Flügel 2010). Besides, based on the pattern of facies distribution, and also the frequency of facies, the location of the studied wells and palaeogeographical directions in the carbonate platform was determined. In order to determine the general time framework of the studied interval, based on identifying and investigating the distribution of fossil associations, especially planktonic and benthic foraminifera, biozones were compared with Wynd biozones (Wynd 1965) and to provide sequence stratigraphic framework, sequence boundaries, and maximum flooding surfaces were determined based on the pattern of facies changes as well as the determination of discontinuity surfaces (Hunt and Tucker 1992; Catuneanu 2006). Finally, using petrophysical logs (GR, RHOB, NPHI, and PHIE), the identified depositional sequences in studied wells are correlated. Discussion of Results & Conclusions Based on the integration of core description results, petrographic studies, and petrophysical logs, the Ilam Formation in some fields located in the northwestern part of Abadan plain were examined from several points of view such as facies features, sedimentary environment, platform geometry, biostratigraphy, and sequence stratigraphy framework. Facies studies led to the identification of two siliciclastic (shales) petrofacies PF1 and PF2, belonging to the estuary and shallow lagoon environment, respectively, and the 12 carbonate microfacies (MF1 to MF12) belonging to the four facies belts lagoon, shoal, shallow and deep open marine which are deposited in a carbonate ramp. An examination of the facies frequency showed that deep open marine facies, with the major abundance of planktonic foraminifera in a distinct trend from west to east, showed an increased thickness. Based on this, it seems that the west of the studied transect is the landward margin of the carbonate ramp, and by moving towards the high Zagros, in the east of transect, we approach the deep parts of this carbonate platform. Examination of the available cores and microfacies changes indicate that the only identified sequence boundaries are the lower contact (base of the Laffan Member) and the upper contact (the boundary between Ilam and Gurpi formations) in the studied succession, which both of them show evidence of subaerial exposure and brecciation (Sequence Boundary type 1); Therefore, the whole studied interval was identified as a third-order depositional sequence. The shaly Laffan Member was considered as Lowstand System Tract (LST) that mainly consisted of charophyte algal facies in the continental and estuary environment. The Maximum Flooding Surface (MFS) has corresponded to the deepest carbonate microfacies whereas Regressive Surface (RS) was considered by the continental fine-grained siliciclastic sediment supply and the increasing of lagoonal carbonate facies frequency. Transgressive Systems Tract (TST) is characterized by deep open marine microfacies, Highstand Systems Tract (HST) with shallow open marine microfacies, and Falling Stage Systems Tract (FSST) is characterized by the predominance of lagoonal microfacies with continental fine-grained siliciclastic sediments supply, and finally, correlation of sequence boundaries was performed in the studied wells by petrophysical logs. The thickness of HST and FSST facies show a reverse ratio in comparison with the whole thickness of the Ilam Formation. The maximum thickness of HST facies was observed in the east of the study area (Well D), whereas the maximum thickness of FSST was observed in the west of the studied transect (Well A). This confirms that the source of the continental siliciclastic sediments was closed to the west of the study area during the relative sea-level fall. The thickness changes observed for the Ilam Formation, along with the lateral facies change, could be attributed to tectonic activities and the beginning of collision and the closure of the Neo-Tethys.

Bangestan group in SW Iran consists of Sarvak and Ilam formations. The Sarvak and Ilam formations comprise a thick succession of carbonate rocks in Zagros basin of Southern Iran. These formations form one of the main hydrocarbon reservoirs in central Fars, Shiraz area. This study presents a facies analysis, sedimentary environments and sequence stratigraphy. To study petrography, microfacies, sedimentary environments and sequence stratigraphy of Sarvak and Ilam formations one stratigraphic section has been chosen and studied (MajAbad section). Facies analysis led to recognition of 13 microfacies and 2 lithofacies. Study of microfacies components such as allochems and ortochems, relative sea level change and sequence stratigraphy evidences show these facies were deposited in carbonate ramp sedimentary environment in 4 facies belt zones: open marine (A), Shoal (B), Lagoon (C) and Tidal flat (D) during Middle Creataceous. Petrography, microfacies analysis and sedimentary environment of Sarvak and Ilam formations show these formations in study area consist of 5 3 order depositional sequences. Sequence 1 to sequence 4 related to Sarvak Formation and sequence 5 covered of Ilam formation. These depositional sequences have been seperated by both type-1 and type-2 sequence boundaries. Sequence stratigraphy evidences show that the Sarvak and Ilam Formations constitute five separate depositional sequences which are separated by a thin palaeosol, representing a type-one and type -two sequence boundary which can be correlated with global curves of relative sea-level. [ABSTRACT FROM AUTHOR]

Microbiostratigraphy, microfacies and depositional environments of the Sarvak and Ilam formations have been studied in the Gachsaran Oilfield (well no. 335). The thickness of the Sarvak and Ilam formations are 95.5 and 66.5 meters respectively and they consist of limestone with minor shale. Biostratigraphic analysis of these formations has resulted in the recognition of two biozones in the Sarvak Formation: 1) Nezzazata-alveolinid assemblage zone, 2) Rudist debris; and one biozone in the Ilam Formation: the Rotalia skourensis-algae assemblage zone. The age of the Sarvak Formation is considered as late Cenomanian and the Ilam Formation as Santonian. The zones match with the previous biozonation scheme ofWynd (1965) in the Zagros area. The intervals are subdivided into 11 microfacies that were deposited in four major depositional environments. These include shelf lagoon, platform margin, slope, and basin depositional settings. A carbonate shelf without an effective marginal barrier is considered as a depositional model for both formations. [ABSTRACT FROM AUTHOR]

Petrophysical characteristics such as porosity, permeability, saturation, shale volume, and lithology are important in evaluating the petroleum potential of a reservoir. In this research, Ilam Formation (late Cretaceous age) in Ahvaz oil field, (SW Iran) has been evaluated using petrophysical parameters. Therefore, the available integrated digital petrophysical logs, and core analysis data consisted of input data of Geolog software (7.1). Petrographic thin sections studies were also made. The results indicated that limestone, dolomite and shale are main components. All documents revealed that the lithology is composed of clean porous limestone with a less quantity of shale (sporadically) and dolomite as well. Average effective porosity and water saturation of Ilam Formation are 15% and 27%, respectively. The shale volume, calculated using gamma ray logs as a shale indicator, is less than 10% indicating a clean formation. The estimated permeability is 8 mD, which is accommodated with the petrophysical logs and core analysis data. Pores are commonly including of intercrystal, cavity and fracture types. Reservoir fluids are distributed in a differentiated pattern (light oil in upper part and heavy oil in lower part) that water saturation degree and gravity derive mechanism (second migration) can be considered as the responsible factors. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Cretaceous carbonates host major hydrocarbon reserves in SW Iran and elsewhere in the Arabian Plate. Tectonic activity combined with eustatic sea-level changes resulted in periodic exposure of these carbonates which were subsequently modified by meteoric diagenesis under a warm and humid climate. Long-term exposure led to the formation of several disconformity surfaces within the middle Cretaceous succession which had important effects on the interval's reservoir characteristics. These disconformity surfaces in the Dezful Embayment were investigated using microfacies, diagenetic and geochemical studies at five subsurface sections. Facies differences across these boundaries, together with features such as karstification, palaeosol development and collapse-dissolution breccias, were used to identify emergent surfaces. Stable oxygen and carbon isotope ratios and trace element profiles indicate intense meteoric diagenesis. Disconformities were dated using biostratigraphic studies. The results indicate the presence of two major erosional disconformities: one is located at the Cenomanian - Turonian boundary separating the middle Sarvak Formation from its upper part; and the other is in the mid-Turonian at the boundary between the Sarvak and Ilam Formations. The latter disconformity is correlatable throughout the Arabian Plate. [ABSTRACT FROM AUTHOR]

Abstract: The relationship between the present-day stress field and natural fractures can have significant implications for subsurface fluid flow. In particular, fractures that are aligned in orientations favourable for reactivation by either shear or tensile failure in the in-situ stress field often exhibit higher hydraulic conductivities. The Ilam Formation of southwestern Iran is an important hydrocarbon reservoir containing numerous natural fractures. However, little is known about the state of stress in this region, or any of Iran''s petroleum provinces. We conducted analysis of the present-day maximum horizontal stress orientation and the density, orientation and hydraulic conductivity of natural fractures in the Ilam carbonates using high resolution Formation Micro Imager resistivity logs in two wells. A total of 51 breakouts with an overall length of 215m were observed in the two wells, indicating a maximum horizontal stress orientation of 68°N (±7.6°) in well A and 58°N (±6.3°) in well B. Furthermore, the wellbore-derived stress orientations determined herein are consistent with those inferred from nearby earthquake focal mechanism solutions, indicating that stresses in the sedimentary cover are linked to the resistance forces generated by Arabia–Eurasia collision. Furthermore, the correlation between stress orientations estimated from earthquake focal mechanism solutions and breakouts indicates that focal mechanism solution data, which is often considered to be unreliable for stress field analysis near transform margins, may provide reliable information on the stress orientation near continental collision zones. The image log data also reveals three sets of open, and presumably hydraulically conductive, fractures with strikes of (i) 160–170°N, (ii) 110–140°N and (iii) 070–080°N. Fracture set (iii) is consistent with being formed and open in the present-day stress field. However, fracture sets (i) and (ii) strike at a high angle to the present-day maximum horizontal stress, and are interpreted herein to be the result of either pre- or syn-folding related forces. The observation that different sets of open fractures in the field can be either sensitive or insensitive to the present-day stress is critical for improving hydrocarbon recovery. [Copyright &y& Elsevier]

Abstract: The Ilam Formation (Santonian–Campanian in age), part of the Bangestan Group, is disconformably overlain by the Sarvak Formation and underlain by the Gurpi Formation in the Tang-E Rashid, Peyon area, Izeh (Zagros), southwest of Iran. Facies analyses indicate that the Ilam carbonates formed in four microfacies belts: tidal flat, lagoon, shoal and open marine, in a platform ramp environment. Major and minor elements and carbon and oxygen isotope values were used to determine the original carbonate mineralogy of the Ilam Formation. Petrographic evidence and elemental and oxygen and carbon isotope values indicate that aragonite was the original carbonate mineralogy in the Ilam Formation. The elemental and isotopic compositions of the Ilam carbonates also illustrate that they have stabilized in the marine phreatic environment. Variations of Sr/Ca and δ18O values versus Mn suggest that diagenetic alteration occurred in a closed system. Temperature calculation based on the oxygen isotope value of the least-altered sample indicates that the very early shallow burial fluid temperature was around 28°C. Recognition of the exact boundary between the Ilam and Sarvak Formations is difficult, due to similar lithologies and the absence of the Surgah Formation in the study area. However, elemental and oxygen and carbon isotope analysis were used to determine the boundary between these formations. The δ18O and δ13C values, along with elemental results, clearly indicate a subaerial exposure surface, below which meteoric diagenesis affected the sediments. [Copyright &y& Elsevier]

Capillary pressure together with relative permeability curves are key parameters in carbonate reservoirs characterization. They serve as fundamental inputs in petrophysical and reservoir engineering studies such as rock typing, fluid contacts determination and saturation height function generation. In the current study, the NMR-T2 distribution diagrams were inverted to the synthetic capillary pressure and relative permeability curves. The results were compared to the laboratory-derived MICP and Kr curves and a satisfactory agreement was achieved between them. Afterwards, the reservoir rock types were determined from the integration of Conventional Core Analysis(CCAL) results with the NMR-derived Pc and Kr curves. Accordingly, the studied carbonate rocks were classified into four hydraulic flow units. Subsequently, a representative capillary pressure and relative permeability curve was derived for each hydraulic flow unit. The methodology is investigated through a case study of the upper Cretaceous Ilam Formation in the Abadan Plain. The results of this study demonstrated that NMR log contributes to a more precise carbonate reservoirs characterization and reducing uncertainty in reservoir rock typing. Through performing some additional computations and calibration with the core results, worthy information is extractable from NMR log. • Filling the information gaps in SCAL data by using NMR data. • Joint inversion of NMR T2 distribution to Pc and relative permeability. • Assigning representative PC and Kr curves to reservoir rock types. • Making a link between microfacies CCAL, SCAL and flow units. • Reducing uncertainty in reservoir dynamic simulation. [ABSTRACT FROM AUTHOR]