Turkish Journal of Earth Sciences. Research Article

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1 Turkish Journal of Earth Sciences Research Article Turkish J Earth Sci (2013) 22: TÜBİTAK doi: /yer Microfacies correlation analysis of the Oligocene-Miocene Asmari Formation, in the central part of the Rag-e-Safid anticlinal oil field, Zagros Basin, south-west Iran Mahnaz AMIRSHAHKARAMI* Geology Department, Payame Noor University, Iran Received: Accepted: Published Online: Printed: Abstract: The Oligocene-Miocene Asmari Formation was deposited in a carbonate ramp setting at the margin of the Zagros Basin in south-western Iran. The subsurface sedimentary successions of the Asmari Formation have been studied using cores from the Rag-e- Safid oil field, in order to determinate their microfacies and sedimentary palaeoenvironments. Based on texture analysis and faunal assemblages, 10 microfacies types have been recognised and interpreted. They indicate different depositional settings: open marine, oolitic and bioclastic shoal, lagoon, tidal flat and beach. The microfacies have been interpreted as indicative of the inner and middle ramp. In accordance with the temporal and spatial correlation model for the Asmari Formation across the south-western part of the Zagros basin, deposition of the Asmari Formation in the south-west had started in a deeper environment and continued in a shallower high energy environment. Key Words: Asmari Formation, Oligocene-Miocene, microfacies correlation, carbonate platform ramp, larger benthic foraminifera, Zagros Basin, Iran. 1. Introduction The Oligocene-Miocene shallow marine carbonates of the Asmari Formation are one of the most important oil reservoirs at the margin of the Zagros Basin in southwestern Iran. This formation has been studied on both outcrop and subsurface layers. The type section of the Asmari Formation, measured in the Tang-e-Gele Torsh outcrop in Khuzestan province by Richardson (1924), consists of 314 m of limestones, dolomitic limestones, and argillaceous limestones (Motiei 1993). Generally, the Asmari Formation conformably overlies the deeper microfacies of the Palaeocene Oligocene Pabdeh Formation. The Gachsaran Formation unconformably overlies the Asmari Formation in most places. The Asmari Formation was originally named after the Kuh-e-Asmari outcrop in Khuzestan province by Busk and Mayo (1918) as a sequence of Cretaceous-Eocene age. The first publications on the Asmari Formation (Richardson 1924, Boeckh et al. 1929) were revised by Lees (1933), who considered the Asmari Formation to be Oligocene-Miocene in age and Thomas (1948), who dated it as Oligocene Burdigalian. Biostratigraphic data on the Asmari Formation were established by Wynd (1965) and reviewed by Adams and Bourgeois (1967) in unpublished reports. Ehrenberg et al. (2007) and Laursen et al. (2009) * Correspondence: [email protected] 204 have applied the method of strontium stratigraphy to date the Asmari Formation biozones (Table 1). More recent studies about palaeoecology, microfacies and sequence stratigraphy of the Asmari Formation were carried out by Seyrafian and Hamedani (1998, 2003); Seyrafian (2000); Vaziri-Moghaddam et al. (2006); Amirshahkarami et al. (2007a, 2007b); Rahmani et al. (2009); Vaziri-Moghaddam et al. (2010) and Seyrafian et al. (2011). The Asmari Formation is widespread, with very varied characteristics of lithostratigraphy, biozones and microfacies in different locations in the Zagros Basin (Motiei 1993). Most researches of the Asmari Formation involve outcrop sections. Also, there has been no correlation between subsurface and outcrop sections of the Asmari Foramation in previous works. Therefore the Asmari formation needs more detailed microfacies analysis for its correlation model and palaeoenvironment reconstruction. Also, studies of the subsurface sedimentary successions of the Asmari Formation are necessary for exploration of the oil reservoirs. The Rag e Safid oil field is one of the most important oil fields in Iran, and it needs comprehensive research about microfacies analyses and palaeoenvironment reconstruction. This paper has two objectives: (1) the interpretation of the depositional settings of the Asmari Formation in

2 Table 1. Biozonation of the Late Oligocene Early Miocene using the distribution of larger benthic foraminifera (Laursen et al. 2009). Standard Chronostratigraphy Age (Ma) Epoch Stage Biozonation of the Asmari Formation Burdigalian Borelis melo curdica-borelis melo melo 20 Miocene Aquitanian Miogypsina Elphidium sp. 14, Peneroplis farsensis Indeterminate 25 Oligocene Chattian Archaias asmaricus Archaias hensoni Miogypsinoides complanatus Lepidocyclina Operculina Ditrupa 30 Rupelian Nummilites vascus Nummulites fichtelii Globigerina- Turborotalia cerroazulensis-hantkenina subsurface sections at the Rag e Safid oil field, using the microfacies analysis of cores and (2) the microfacies correlation of subsurface sections of the Asmari Formation in the Rag-e-Safid oil field with some of the previously studied outcrop sections. The Asmari Formation contains numerous species of larger benthic foraminifera that thrived in the photic zone of tropical to subtropical seas and these provide additional useful tools for the reconstruction of the sedimentary palaeoenvironments (Vaziri-Moghaddam et al. 2006; Amirshahkarami et al. 2007a, 2007b; Rahmani et al. 2009). The field investigations of the Asmari Reservoir in the Rag-e-Safid oil field were carried out by Shirmohammadi et al. (1974), Wiley and Habibi (1978) and Zahrabzadeh (2007). The biostratigraphy of the Asmari Formation in the Rag-e-Safid oil field has been studied by Amirshakarami et al. (2010), who recognised four assemblage zones based on the distribution of the larger benthic foraminifera (Table 2). However, the present work is the first full study of the microfacies analysis and palaeoenvironment of the Asmari Formation in the Rag-e-Safid oil field. 2. Study area and geological setting Iran and some adjacent countries were detached from the Arabian Plate in the Permian (Berberian & King 1981). From the Middle Eocene to Early Miocene, the Arabian Plate began to impact the southern Asian Plate border and the Zagros belt orogeny began. The Zagros Basin extends from Turkey, north-eastern Syria and north-eastern Iraq through north-western Iran and continues into southeastern Iran (Figure 1). The Zagros Mountains of Iran are divided into three principal tectonic units (Stocklin 1968; De Jong 1982) namely the Zagros fold thrust zone, the imbricated zone and the Urumieh Dokhtar magmatic zone (Alavi 2004). The study area is in the fold thrust zone of the Zagros Basin (ZFTB in Figure 1) and is located in the Rag-e-Safid oil field, about 150 km south-east of Ahvaz in southwestern Iran (Figure 2a). The Rag-e-Safid oil field extends from N, 49 4 E to N, E at the surface, and is an asymmetric anticline, so the most comprehensive and deepest wells have been drilled almost in the centre of the oil field. This study involves wells numbers 13 and 21 at the Rag-e-Safid oil field (Figure 2b), but cores from the boundary between the Asmari Formation and the underlying Pabdeh Formation (Palaeocene Oligocene) are not available because the contact lies much deeper. The Asmari Formation is overlain by the Miocene Gachsaran Formation. 3. Materials and methods Two well sections of the Asmari Formation have been studied in the Rag-e-Safid oil field. Well number 13 is 2576 m deep and well number 21 is 2702 m deep (Figure 2b). The samples include both cores and cuttings but the majority of thin sections have been prepared from the cores (Table 3). The microfacies characteristics were described in more than 1100 thin section of the cores and 170 thin sections of the cuttings samples. The classification of carbonate rocks followed the nomenclature of Dunham (1962) and Embry and Klovan (1971). 4. Microfacies description Based on the study of the textures, allochems and skeletal components in thin sections of the cores, ten microfacies 205

3 Table 2. Biozonation of the Asmari Formation using distribution of larger benthic foraminifera in the Rag-e-Safid oil field (Amirshahkarami et al. 2010). Stage Assemblage biozone Burdigalian IV Borelis melo, Borelis curdica Aquitanian Chattian Rupelian III II I Barren Zone Peneroplis thomasi, Peneroplis evolutus, Peneroplis sp., Miogypsinoides deharti Archaias kirkukensis, Archaias hensoni, Archaias operculiniformis, Archaias asmaricus, Borelis pygmaea, Miogypsinoides complanatus, Austrotrillina asmariensis, Austrotrillina howchini, Austrotrillina striata Nummulites vascus, Nummulites fichteli, Nephrolepidina sp., Eulepidina sp., Eulepidina dilitata * This table was mistakenly omitted from the print version. were identified (FR or Microfacies of the Rag-e-Safid oil field, Figures 3-4). Microfacies FR1 Bioclastic Nummulitidae-Lepidocyclinidae packstone-grainstone (Figure 5/a-b) This microfacies type is a grain-supported texture (packstone-grainstone) with densely packed, flat larger benthic foraminifera. The foraminiferal assemblage comprises numerous perforated larger foraminifera such as Lepidocyclinidae and Nummulitidae. The Nummulitidae are represented by Nummulites, Operculina, Heterostegina and Spiroclypeous. Other skeletal grains include bryozoans, corallinaceas, Ostrea, gastropoda, echinids, ostracods and small benthic foraminifera. This facies occurs in well number 13 in the lower part of the Asmari Formation and is Rupelian in age (Figure 3). TURKEY ARMENIA AZERBAIJAN Caspian Sea TURKMENISTAN N 38 I R A N AFGHANISTAN Zagros Orogen 34 IRAQ Ahvaz ZFTB UDMA ZIZ 30 KUWAIT PAKISTAN 206 ARABIA Study area 300 km Persian Gulf ZDF Figure 1. Subdivisions of the Zagros orogenic belt and geological setting of the study area: OL, Oman line; UDMA, Urumieh-Dokhtar magmatic arc; ZDF, Zagros deformational front; ZFTB, Zagros fold-thrust belt; ZIZ, Zagros imbricate zone; ZS, Zagros suture (After Alavi 2004). OL 26

4 49 10ʹ 49 20ʹ 49 30ʹ To Ahvaz 49 40ʹ 49 50ʹ 50 00ʹ 50 10ʹ 50 20ʹ 50 30ʹ 50 40ʹ 50 40ʹ 14Km N 31 10ʹ 31 00ʹ 30 50ʹ Omidiyeh 30 40ʹ Mahshahr Behbahab 30 30ʹ Hendijan Rage Safid Oil Field 30 20ʹ 30 10ʹ (a) Persian Gulf Bandar Daylam 30 00ʹ 49 40ʹ 49 50ʹ 50 00ʹ 50 10ʹ N 30 30ʹ 1800 m Rag-e-Safid Field Asmari Reservoir Core Compendium Data Top Asmari Formation 2450 m m 100% Roc m 2400 m T.D m 30 20ʹ 3000m Top Asmari Formation 2203 m U D 3000 m 1800 m , m 86% Roc. T.D m 4 km U D 30 10ʹ (b) Figure 2. (a) Location of the study area at the Rag-e-Safid oil field in south-western Iran. (b) Location of wells numbers 13 and 21 from the Rag-e-Safid oil field (After Amirshahkarami et al. 2010). Microfacies FR1 was deposited in a medium-high energy open marine environment. This interpretation is supported by the abundance of typical open marine skeletal fauna including large and flat Nummulitidae, Lepidocyclinidae, bryozoans, and echinoids (Romero et al. 2002). The presence of those fauna, in comparison with analogues in modern platforms (Hottinger 1983; Reiss & Hottinger 1984; Leutenegger 1984; Hohenegger 1996; Hottinger 1997; Hohenegger et al. 1999), suggests that this microfacies type has been deposited in the lower photic zone. This microfacies has also been reported from the lower parts of the Asmari Formation in other sections, such as Chaman-Bolbol and Tang-e-Gurgdan (Amirshahkarami et al. 2007a, 2007b). Microfacies FR2 Bioclastic bryozoans-coral floatstonerudstone (Figure 5/c-d) This microfacies is characterised by abundant and densely packed skeletal grains. The texture is floatstone-rudstone with coarse-grained fragments of coral colonies and bryozoans. Other bioclasts are small benthic foraminifera, miliolids, fragments of mollusca and corallinacean algae. 207

5 Table 3. The sample data from wells numbers 13 and 21 at the Rag-e-Safid oil field, Zagros Basin, south-west of Iran. Well no. 13 Total Depth: m Top of the Asmari Formation: 2203 m Well no. 21 Total Depth: m Top of the Asmari Formation: 2450 m Sample no. Depth (m) Sample no. Depth (m) (Driller depth in feet) (below sea level) to to to to to to to to Cuttings Cores (Driller depth in feet) (below sea level) 8039 to to to to to to to to to to to to to to to to % Cores This facies is interpreted as an open marine facies that formed between the fair weather wave base and the storm wave base (Wilson 1975; Flügel, 2004). Coarse-grained debris of corals and bryozoans and the floatstone-rudstone texture of Facies FR2 suggest the absence of an effective barrier. A similar microfacies has been also reported by Amirshahkarami et al. (2007a) and Vaziri-Moghaddam et al. (2010) from outcrop sections of the Asmari Formation. Microfacies FR3 Bioclastic grainstone (Figure 5/e-f) This microfacies is characterised by clean calcareous rounded and coated skeletal grains in a depositional texture of grainstone. Common biota are large bivalves, gastropods, echinoids, dasycladacean algae fragments and small benthic foraminifera and abraded biota such as corallinacean algae. Dasycladacean algae indicate shallow marine conditions within the euphotic zone and the grainstone texture suggests sufficient energy to winnow away the fines in this microfacies. In accordance with the standard microfacies types described by Wilson (1975) and Flügel (2004), this microfacies suggests a bioclastic sandy shoal. A similar microfacies has been also reported by Amirshahkarami et al. (2007a) from the Chaman-Bolbol section. Microfacies FR4 Bioclastic-ooidal packstone-grainstone (Figure 6/a) This microfacies is characterised by rounded ooids in a packstone-grainstone depositional texture. Other grains are miliolids, micritised skeletal grains and bioclasts. Minor elements are foraminifera such as Miogypsinoidae and Dendritina. The ooids are well sorted, small with multiple concentric laminates and exhibit distinct tangential structures. Some of them are micritised and a few are dissolved. The rounded ooids of this facies suggests an ooid shoal, with a depositional environment located in the high energy shoals of the outer platform margin (Flügel 2004). A similar microfacies has been reported from the Asmari Formation in other sections, such as the outcrop Khaviz section (Kimiagari 2006) and well number 30 from the Aghajari oil field (Yazdani 2006). Microfacies FR5 Bioclastic perforate foraminifera miliolid wackestone-packstone (Figure 6/b-c) The major components of this microfacies are benthic foraminifera and micritised bioclasts with a wackestone packstone texture. The larger benthic foraminifera include both perforate and imperforate forms. Common foraminifera with perforate walls are small-medium sized Nummulitidae, Miogypsinidae, Neorotalia and Amphistegina. Imperforate forms are miliolids, Borelis and Austrotrillina. Minor components are small benthic foraminifera, Dendritina, fragments of molluscs, echinoids and corallinacean algae. The coexistence of perforate benthic foraminifera (Nummulitidae, Miogypsinidae and Amphistegina, Neorotalia) and imperforate foraminifera (miliolid, Borelis and Austrotrillina) of microfacies FR5 indicate that deposition took place in an open shelf lagoon. The small Nummulitidae were reported from open marine conditions by Romero et al. (2002). Miogypsinoids lived in shallow waters of normal salinity (Geel 2000) and 208

6 Series M I O C E N E O L I G O C E N E Stage? Rupelian C h a t t i a n Aquitanian Formation Depth (m) (Below sea level) Gachsaran 2200 A s m a r i Sample no. (Driller depth in feet) Lithology No thin IBiozone Barren Zone section III II Open marine Fr1 Microfacies and sedimentary paleoenvironment Fr2 Shoal Fr3 Fr4 Lagoon Fr5 Fr6 Tidal flat Fr8 Fr9 Beach Fr Pabdeh Total Depth: Figure 3. Vertical distribution of the microfacies of the Asmari Formation at the Rag-e-Safid oil field, well no. 13, (Zagros Basin, SW Iran). For Biozones see Table 2. Fr: Microfacies of the Rag-e-Safid oil field. (For lithology symbols see Figure 4). 209

7 Microfacies and sedimentary paleoenvironment Series Stage Formation Depth (m) (Below sea level) Sample no. (Driller depth in feet) Lithology Biozone Open marine Shoal Fr2 Fr3 Fr4 Lagoon Fr5 Fr6 Fr7 Tidal flat Fr8 Fr9 Beach Fr10 M i o c e n e O l i g o c e n e C h a t t i a n A q u i t a n i a n Burdigalian A s m a r i Gachsaran IV Barren Zone III II Pabdeh Total Depth: Bioclastic limestone with diagenetic recrystallized skeletal and dolomitization Limestone with abundant larger benthic foraminifera Anhydrite Mudstone with anhydrite Quartzarenite and Quartz mudstone Figure 4. Vertical distribution of the microfacies of the Asmari Formation in the Rag-e-Safid oil field, well no. 21, (Zagros Basin, SW Iran). For Biozones see Table 2. Fr: Microfacies of the Rag-e-Safid oil field. 210

8 AMIRSHAHKARAMI / Turkish J Earth Sci b a c d e f Figure 5. (a-b) Microfacies FR1: (a) Bioclastic Lepidocyclinidae packstone grainstone; Rag-e-Safid oil field; Well no. 13 (Sample No ). (b) Bioclastic Neorotalia Nummulitidae packstone grainstone; Rag-e-Safid oil field; Well no. 13 (Sample no ). (c-d) Microfacies FR2: (c) Bioclastic coral floatstone; Rag-e-Safid oil field; Well no. 21 (Sample no ). (d) Bioclast bryozoan floatstone; Rag-e-Safid oil field; Well no. 21 (Sample no ). (e-f) Microfacies FR3: (e) Corallinacea Dasycladacea bioclastic grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8531). (f) Bioclastic packstone grainstone. Rag-e-Safid oil field; Well no. 21 (Sample no. 8227). 211

9 recent Amphistegina and Neorotalia live in the photic zone of shallow water (Romero et al. 2002). The occurrence of the imperforate foraminifera such as miliolids, Borelis and Austrotrillina was reported from restricted lagoon conditions by Hallock and Glenn (1986); Geel (2000) and Romero et al. (2002). The open lagoon depositional setting is characterised by microfacies types that include mixed open marine bioclasts and protected environment bioclasts (Vaziri-Moghaddam et al. 2010). Microfacies FR5 has also been reported in other sections of the Asmari Formation by Vaziri-Moghaddam et al. (2006); Yazdani (2006) and Amirshahkarami et al. (2007a, 2007b). A similar microfacies with abundant Neorotalia was also identified in well sections of the Asmari Formation from the Aghajari oil field by Yazdani (2006). Microfacies FR6 Bioclastic peloid imperforate foraminifera packstone-grainstone (Figure 6/d-f, Figure 7/a) The main components of this microfacies are larger benthic foraminifera with imperforate walls, such as Archaias, Peneroplis, Dendritina, Meandropsina, Borelis, Austrotrillina and miliolids. Other bioclasts are corallinacean algae, corals, bryozoans, molluscs and shell fragments. Peloids are also present. The minor components are small benthic foraminifera, Ostracode, peloids and micritic skeletal grains. The poorly medium sorted grains of this facies are fine medium in size and subangular to round in shape. The depositional texture is represented by packstone grainstone. The existence of abundant larger benthic foraminifera with imperforate walls in Microfacies FR6 indicates deposition in a restricted shelf lagoon. Imperforate foraminifera such as miliolids, peneroplids, alveolinids and soritids lived in a restricted shelf lagoon (Hallock & Glenn, 1986). The restricted conditions are suggested by rare to absent normal marine biota and abundant restricted biota (imperforate foraminifera) (Geel 2000; Romero et al. 2002). This microfacies has also been recognised in other sections of the Asmari Formation (Yazdani 2006; Amirshahkarami et al. 2007a, 2007b; Rahmani et al. 2008; Vaziri-Moghaddam et al. 2006, 2010). Microfacies FR7 Intraclastic bioclastic miliolid packstonegrainstone (Figure 7/b) The main skeletal grains consist of miliolids, echinoids and intraclasts. Minor skeletal grains are corallinaceans, Neorotalia; Miogypsinoides and small benthic foraminifera such as Discorbis. The depositional texture is a poorly sorted packstone grainstone. Some grains have been partially micritised. Because of the coexistence of biota, including miliolids and peneroplids with intraclasts, the depositional setting of Microfacies FR7 is recognised as being at the lagoonal end of the platform margin. Miliolids and peneroplids lived in restricted lagoon conditions (Geel 2000; Hallock & Glenn 1986). This microfacies has also been identified in other sections of the Asmari Formation, such as the Chaman Bolbol outcrop section by Amirshahkarami et al. (2007a). Microfacies FR8 Mudstone (Figure 7/c) This microfacies consists of homogeneous micrite with a low diversity of foraminifera and very rare carbonate and non-carbonate grains. These sediments are mainly composed of 90% to 100% of lime mud. In some samples, subordinate detrital quartz grains are also present. There is no evidence of subaerial exposure (such as a vesicular fabric, microcodium, birdseye and fenestral fabric) in lime mudstone in microfacies FR8. However, those unfossiliferous homogeneous micritic limes are interbedded with the lagoonal facies. Therefore lime mudstone with a paucity of fauna in microfacies FR8 was deposited in a protected lagoon (Tucker 1985; Flügel 2004). Microfacies FR9 Quartz mudstone (Figure 7/d-e) This microfacies is lime mudstone with grains of detrital quartz. Fenestrate structures and evaporite materials such as anhydrite can be found in some thin sections (Plate 7/e). These sediments are mixed siliciclastic-carbonate rocks or alternating layers of sandy limestone, lime sandstone and carbonates. Mixed siliciclastic-carbonate rocks can be common in near-coast environments (Flügel 2004). The input of terrigenous materials into the carbonate environment can take place by erosion of the underlying sediments in a tidal zone (Flügel 2004). The microfacies characteristics, including its fine grained nature, lack of fauna and the presence of fenestrate fabric, are common in tidal flat sediments (Vaziri-Moghaddam et al. 2010). Microfacies FR10 Quartzarenite (Figure 7/f) This quartzarenite microfacies is mature sandstone composed primarily of subangular to angular grains of quartz containing more than 95% detrital quartz grains in a clay matrix. Minor components are bioclastic grains such as mollusc shell fragments. The quartzarenite facies has been recognised in the Ahvaz Member sandstones from the Asmari Formation and is interbedded with the Chattian-Aquitanian carbonate layers (Motiei 1993). The occurrence of detrital quartz with skeletal grains suggests a beach facies on a coastal environment. Mixed siliciclastic-carbonate rocks can be common on the shoreline (Flügel 2004). A similar microfacies has also been reported from the Asmari Formation in the subsurface sediments from wells 30 and 66 in the Aghajari oil field (Yazdani 2006). 212

10 AMIRSHAHKARAMI / Turkish J Earth Sci 200 µm a c b d e f Figure 6. (a) Microfacies FR4: Bioclastic ooid packstone grainstone. Rag-e-Safid oil field; Well no. 21 (Sample no ). (b-c) Microfacies FR5: (b) Bioclastic miogypsinidae miliolids packstone; Rag-e-Safid oil field; Well no. 21 (Sample no ). (c) Bioclastic miliolids Neorotalia packstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8229). (d-f) Microfacies FR6 (d) Bioclastic miliolid corallinacean packstone grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8733). (e) Bioclastic miliolid Peneroplidae packstone grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8727). (f) Miliolid peloid bioclastic packstone-grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no ). 213

11 AMIRSHAHKARAMI / Turkish J Earth Sci a c e b d f Figure 7. (a) Microfacies FR6: Miliolid peloid bioclastic grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8224). (b) Microfacies FR7: Intraclastic bioclastic Peneroplidae miliolid grainstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8730). (c) Microfacies FR8: Mudstone; Rag-e-Safid oil field; Well no. 13 (Sample no. 7975). (d-e) Microfacies FR9: (d) Quartz mudstone; Rag-e-Safid oil field; Well no. 21 (Sample no. 8670). (e) Quartz mudstone with anhydrite; Rag-e-Safid oil field; Well no. 21 (Sample no. 8382). (f) Microfacies FR10: Quartzarenite; Rag-e-Safid oil field; Well no. 21 (Sample no. 8128). 214

12 XRF analysis and the CIW Index in the Ahvaz Member sandstones from the Asmari Formation in the Ahvaz oil field indicate a marginal marine or coastal environment depositional setting (Hosseinibarzi et al. 2008). 5. Sedimentary model Microfacies analyses have allowed the interpretation of several carbonate marine system environments including open marine, oolitic and bioclastic shoal, open lagoon, restricted lagoon, tidal flat and shoreline or beach in the Asmari Formation in the Rag-e-Safid oil field. The palaeoenvironments of the Asmari Formation can be reconstructed by means of the arrangement of the facies belts and the distribution of the larger benthic foraminiferal assemblages. By comparing the textures, lithofacies and biofacies with those of modern carbonate depositional settings, such as the Persian Gulf, a very low gradient homoclinal carbonate ramp model is suggested for the Asmari Formation in the Rag-e-Safid oil field (Figure 8). It seems that the Asmari Formation depositional environment was similar to the modern homoclinal carbonate ramp of the Persian Gulf (Read 1985; Jones & Desrochers 1992). A carbonate ramp is separated into inner ramp, middle ramp and outer ramp in classical facies models (Burchette & Wright 1992). Larger benthic foraminifera are important to recognition of the palaeoecology of the Cenozoic carbonate platforms and ramps. The palaeoenvironmental distribution of foraminiferal assemblages depends on intrabasinal conditions, including nutrient, temperature, salinity, depth, light, substrate and water energy (Geel 2000; Hottinger 1983). The gradient of the light is the most important factor in the distribution of species because it is effective on symbioses and nutrient (Hottinger 1983). The size, degree of flatness and the wall of the larger foraminifera tests, can also provide environmental information (Hallock & Gleen 1986; Geel 2000). Larger and flatter individuals become more common at the lower limit of the euphotic zone (Geel 2000). For example, lepidocyclinidae and large and flat nummlitidae occur in the lower photic zone of an open marine but small to medium-sized and lens-shaped nummulites lived together with alveolina in the upper part of photic zone in an interior platform (Hallock & Gleen 1986; Geel 2000). The occurrence of abundant imperforate foraminifera (e.g. miliolids in Microfacies FR5-FR7) is generally taken as evidence for a relatively nutrientrich restricted lagoon with a slightly hypersaline habitat (Hallock & Gleen 1986; Geel 2000). Perforate foraminifera with symbiotic algae (e.g. lepidocyclinidae, large and flat nummlitidae in Microfacies FR1) occur in open marine shallow water conditions (Hallock & Gleen 1986). Key to symbols Bioclasts Bryozoan fragments Coralline algae detritus Corals fragments Echinodermata fragments and spines Quartz mudstone with anhydrite Imperforate foraminifera Beach Inner ramp Tidal flat Lagoon Proximal Oolitic and bioclastic shoal Middle ramp Distal Skeletal grains and flat Nummulitidae Outer ramp? Nummulitidae and Lepidocyclinidae-rich grainstone and packstone Ooid Peloid Rotalia Sandstone FR10 FR9 FR8? FR7 FR6 FR5 FR4 Figure 8. Depositional model for the platform carbonates ramp of the Asmari Formation in the central part of the Rag-e-Safid oil field, Zagros Basin, SW Iran. FR3 FR2 FR1 215

13 Planktonic foraminifera are individuals indicative of open marine conditions with slope and basin facies (Hallock & Gleen 1986; Geel 2000). In the Asmari Formation sediments in the Rage-Safid oil field, the middle ramp is recognised by open marine, oolitic and bioclastic shoal environments and the inner ramp is recognised by lagoon, tidal flat and shoreline or beach environments. The shoreline and beach facies is characterised by mixed siliciclastic-carbonate rocks and detrital sediments affected by a high energy environment (Microfacies FR10). The tidal flat setting is identified by fenestral lime mudstone and quartz mudstone (FR9). The most common microfacies of the inner ramp is medium-coarse-grained imperforate foraminifer bioclastic wackestone-packestone (Microfacies FR5- FR7). The restricted shallow sub-tidal environment or restricted lagoon is commonly dominated by imperforate foraminifera (Geel 2000; Romero et al. 2002) such as Archais, Meandropsina, Peneroplis, Borelis and miliolids. The mudstone microfacies is interbedded with these sediments (Microfacies FR10). The coexistence of imperforate and perforate foraminifera indicates that the depositional setting was a semi-restricted lagoon (Microfacies FR5). The middle ramp can be divided into a proximal and distal middle ramp. The proximal middle ramp is characterised by oolitic and bioclastic shoals in a skeletal packstone grainstone texture (Microfacies FR3 and FR4). Skeletal grains are dominated by echinoids, mollusca, bryozoan, corallinacean and benthic foraminifera. This evidence suggests a shallow water depositional setting near the fair weather wave base. The distal middle ramp is identified by the foraminiferal assemblage with perforate walls such as Nummulitidae and Lepidocyclinidae and the textures that correspond to an increase in the water depth (Microfacies FR1 and FR2). These faunal associations adapt to a symbiontbearing strategy (Hottinger 1983; Hallock & Glenn 1986; Leutenegger 1984; Hottinger 1997). The most common facies of the outer ramp environment in the Asmari Formation is bioclastic planktonic foraminifera wackestone (Vaziri-Moghaddam et al. 2006, 2010; Amirshahkarami et al. 2007a; Rahmani et al. 2009). In accordance with the studies of the palaeoenvironment and sequence stratigraphy by Vaziri-Moghaddam et al. (2010) an outer ramp environment can mostly be recognised in the lower part of the Asmari Formation. Because of the undrilled strata in the lower part of the Asmari Formation in the Rag-e-Safid oil field, no cores of the Asmari Formation reach the underlying Palaeocene Oligocene Pabdeh Formation. Therefore, the existence of an outer ramp environment cannot be discussed. 6. Correlation of depositional environments Following correlation of the Palaeocene Oligocene Asmari Formation depositional environments across the study area and with other sections such as the Chaman- Bolbol and Tange-Gurgdan sections (Amirshahkarami et al. 2007a, 2007b), tidal flat and beach environments are more extensive in the Rage-Safid oil field (Figure 9). The sandstones of the Ahvaz Member of the Asmari formation have been recognised in the Aquitanian stage in the Rage- Safid oil field too. Patch reef, identified in the Chattian stage in the Tange-Gurgdan section, comprises coral boundstone. Correlation diagrams (Figure 9), in the Chaman- Bolbol section identify a bioclastic shoal without any ooids in the Chattian stage but in the Rag-e-Safid oil field, the bioclastic ooids shoal facies is widespread in the Chattian and Aquitanian stages (Figure 9). Sandstone of the Ahvaz Member of the Asmari Formation in the Aquitanian stages has been recognised with a beach facies in the subsurface layers at the Rag-e-Safid oil field. Based on the chronostratigraphic scheme (Figure 9), outer ramp platform microfacies with a planktonic foraminiferal fauna has been recognised in the Rupelian stage of the lower part of the Asmari Formation in the Chaman-Bolbol section. Therefore, in accordance with temporal and spatial correlation model for the Asmari Formation across the south-western part of the Zagros basin (Figure 9), deposition of the Asmari Formation in the south-west (Chaman-Bolbol section) was in a deeper environment and continued in a shallower higher-energy environment (Rage-Safid oil field). 7. Conclusions This investigation shows that the Asmari Formation was deposited with varied microfacies on a homoclinal carbonate ramp, so that the Asmari Formation formed in a deeper environment with lower energy in the southwestern parts of the Zagros Basin (e.g. Chaman-Bolbol section). Ten major microfacies are recognised in the subsurface sediments of the Asmari Formation in well numbers 13 and 21 of the Rag-e-Safid oil field. These are grouped into five depositional settings including open marine, ooids and bioclastic shoal, lagoon (open lagoon and restricted lagoon), tidal flat and beach. These sedimentary settings correspond to the inner and middle ramp. The open marine environment is separated from the lagoonal setting by ooids-bioclastic shoals. An ooids shoal is widespread in the Aquitanian stage of the Asmari Formation at the Rag-e-Safid oil field and indicates a high energy shallow environment. Beach facies sandstones of the Ahvaz Member have been recognised in the Asmari 216

14 Rag-e-Safid oil field Well no. 21 IV III? 20 m 40 m Well no. 13? III 50 m IIb IIb 140 m 170 m Tang-e-Gurgdan section Amirshahkarami et al. 2007b IV III IIb IIa 30 m 70 m 120 m 80 m section Chaman-Bolbol (Amirshahkarami et al. 2007a IV III IIb 40 m 50 m 59 m Amirshahkarami and Taheri 2010) Stage Burdigalian Aquitanian 44 Caspian Sea I R A N Rage Safid oil field Persian km 0 IV III IIb IIa I Pa? Tang-e-Gurgdan Chaman Bolbol Gulf Biozone-Age N U. Asmari (Burdigalian) M. Asmari (Aquitanian) L. Asmari (Chattian) L. Asmari (Chattian) L. Asmari (Rupelian) Pabdeh Formation (Oligocene) No information Correlation sections 30 m 0 I? 80 m I Pa 60 m IIa I Pa 65 m 15 m Chattian Rupelian Disconformity Depositional environment Outer ramp Open marine [ Inner ramp [ Middle ramp ] Bioclastic Shoal Ooids Shoal Patch reef Lagoon Tidal flat Shore line Quartzarenite Well no. 21 Well no. 13 Rag-e-Safid oil field Tang-e-Gurgdan section Chaman-Bolbol section Figure 9. Temporal and spatial correlation of depositional environments and biozones of the Asmari Formation across the central part of the Rag-e-Safid oil field (study area), Tange-Gurgdan (Amirshahkarami et al., 2007b) and Chaman-Bolbol sections (Amirshahkarami et al., 2007a; Amirshahkarami and Taheri, 2010) Zagros Basin, SW Iran. Formation subsurface at the Rag-e-Safid oil field. Therefore, microfacies evidence from the Asmari Formation indicates a shallower palaeoenvironment, with more high energy at the Rage-Safid oil field. Acknowledgements The author is grateful to Payame Noor University of Isfahan for providing financial support and would also like to thank the reviewers for their helpful comments. The author thanks the National Iranian South Oil Company (NISCO) for providing the thin sections of cores. References Adams, C.G. & Bourgeois, E Asmari biostratigraphy. Iranian Oil Offshore Company, Geological and Exploration Division, Report 1074 [Unpublished]. Alavi, M Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution. American Journal of Science 304, Amirshahkarami, M., Vaziri-Moghaddam, H. & Taheri, A. 2007a. Sedimentary facies and sequence stratigraphy of the Asmari Formation at Chaman-Bolbol, Zagros Basin, Iran. Journal of Asian Earth Science 29,

15 Amirshahkarami, M., Vaziri Moghaddam, H. & Taheri, A. 2007b. Paleoenvironmental model and sequence stratigraphy of the Asmari Formation in southwest Iran. Historical Biology 19(2), Amirshahkarami, M., Ghabishavi, A. & Rahmani, A Biostratigraphy and paleoenvironment of the larger benthic foraminifera in wells sections of the Asmari Formation from the Rag-e-Safid oil field, Zagros Basin, southwest Iran. Stratigraphy and Sedimentology Research 40(3), Berberian, M. & King, G.C.P Towards a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Science 18, Boeckh, H. de, Lees, G.M. & Richardson, F.D.S Contribution to the stratigraphy and tectonics of the Iranian ranges. In: Gregory JW (ed), The Structure of Asia, Burchette, T.P. & Wright, V.P Carbonate ramp depositional systems. Sedimentary Geology 79, Busk, H.G. & Mayo, H.T Some notes on the geology of the Persian oilfields. Journal Institute Petroleum Technology 5, De Jong K.A Tectonics of the Persian Gulf, Gulf of Oman and southern Pakistan region. In: Narin, A.E.M. & Stehli, F.G. (eds), The ocean basins and margins: The Indian Ocean. Elsevier, Amesterdam 6, Dunham, R.J Classification of carbonate rocks according to their depositional texture. In: Ham, W.E. (ed), Classification of Carbonate Rocks. A Symposium: American Association of Petrology Geologists Bulletin, Ehrenberg S.N., Pickard, N.A.H., Laursen, G.V., Monibi, S., Mossadegh Z.K., Svånå, T.A., Aqrawi, A.A.M., McArthur J.M. & Thirlwall M.F Strontium isotope stratigraphy of the Asmari Formation (Oligocene Lower Miocene), SW Iran. Journal of Petrolium Geology 30(2), Embry A.F. & Klovan, J.E A Late Devonian reef tract on Northeastern Banks Island, NWT. Canadian Petroleum Geology Bulletin 19, Flügel, E Microfacies of carbonat rocks. Springer, Berlin- Heidelberg, New York, p Geel, T Recognition of stratigraphic sequences in carbonate platform and slope deposits: empirical models based on microfacies analysis of paleogene deposits in southeastern Spain. Palaeogeography, Palaeoclimatology, Palaeoecology 155, Hallock, P. & Glenn, E.C Larger foraminifera: A Tool for Paleoenvironmental Analysis of Cenozoic carbonates depositional facies. Palaios 1, Hohenegger, J Remarks on the distribution of larger foraminifera (Protozoa) from Palau (western Carolines). In: Aoyama, T. (ed), The Progress Report of the 1995 Survey of the Research Project, Man and the environment in Micronesia. Kagoshima University Research Center for the Pacific Islands, Occasional Papers 32, Hohenegger, J., Yordanova, E. & Tatzreiter, Y Habitats of larger foraminifera on the upper reef slope of Sesko Island, Okinawa. Marine Micropaleontology 36, Hosseinibarzi, M., Jafarzadeh, M. & Adabi, M.H Geochemistry of the Ahvaz member sandstone from the Asmari Formation in the Ahvaz oil Field: A tools for recognization of the tectonically setting and primary weathering of parent rock. Science Journal of Shahid Chamran University of Ahvaz 19, (In Persian). Hottinger, L Processes determining the distribution of larger foraminifera in space and time. Utrecht Micropaleont Bulletin 30, Hottinger, L Shallow benthic foraminiferal assemblages as signals for depth of their deposition and their limitations. Bulletin of Society of Geology of France 168(4), James, G.A. & Wynd, J.G Stratigraphic nomenclature of Iranian oil consortium agreement area. American Association Petroleum Geology Bulletin 49, Jones, B. & Desrochers, A Shallow platform carbonates. In: Walker, R.G. & James N.P. (eds), Facies models response to sea level changes. Geological Association, Canada, St. Jones, Newfoundland, Kimiagari, M Biostratigraphy, microfacies and sequence stratigraphy of the Asmari Formation in the Gurpi Anticline (Lali area) to Khaviz Mountain (Behbahan area. PhD Thesis, Isfahan University, p. 220, [In Persian with English abstract]. Laursen, G.V., Monibi, S., Allan, T.L., Pickard, N.A.H., Hosseiney, A., Vincent, B., Hamon, Y., Buchem, F.S.P. van, Moallemi, A. & Druillion, G The Asmari Formation Revisited: Changed Stratigraphic Allocation and New Biozonation. First International Petroleum Conference and Exhibition Shiraz, Iran B29. Lees, G.M Reservoir rocks of Persian oil Wells. American Association Petroleum Geology Bulletin 17, Leutenegger, S Symbiosis in benthic foraminifera, specificity and host adaptations. Journal of Foraminifera Research 14, Motiei, H Stratigraphy of Zagros. Geological Survey of Iran, p Rahmani, A., Vaziri-Moghaddam, H., Taheri, A. & Ghabeishavi, A A model for the paleoenvironmental distribution of larger foraminifera of Oligocene Miocene carbonates rocks at Khaviz Anticline, Zagros Basin, SW Iran. Historical Biology 21, Read, J Carbonate platform facies models. American Association Petroleum Geology Bulletin 69(1), Reiss, Z. & Hottinger, L The Gulf of Aqaba: Ecological Micropaleontology. Berlin, Springer, p Richardson, R.K The geology and oil measures of southwest Persia. Journal Institute Petroleum Technology 10, Romero, J., Caus, E. & Rossel, J A model for the palaeoenvironmental distribution of larger foraminifera based on Late Middle Eocene deposits on the margin of the south Pyrenean basin. Palaeogeography, Palaeoclimatology, Palaeoecology 179, Seyrafian, A Microfacies and depositional environments of the Asmari Formation, at Dehdes area (a correlation across Central Zagros Basin). Carbonates and Evaporites 15,

16 Seyrafian, A. & Hamedani, A Microfacies and depositional environment of the Upper Asmari Formation (Burdigalian), north-central Zagros Basin, Iran. Neues Jahrbuch für Geologie und Paläontology, Abhandlungen 21, Seyrafian, A. & Hamedani, A Microfacies and palaeoenvironmental interpretation of the lower Asmari Formation (Oligocene), North-Central Zagros Basin, Iran. Neues Jahrbuch für Geologie und Paläontology, Montashefte 3, Seyrafian, A., Vaziri-Moghaddam, H., Arzani, N. & Taheri, A Facies Analysis of the Asmari Formation in central and north-central Zagros basin, southwest of Iran: Biostratigraphy, paleoecology and diagenesis. Revista Mexicana de Ciencias Geolόgicas 28(3), Shirmohammadi, N., Verstfelt, P. & Wiley, J Geology study of Asmari Reservoir in Rage-Safid oil field. Report p- 2451, National Iranian South Oil Company (Nisoc), Ahwaz p. 90. Stöcklin, J Structural history and tectonics of Iran: A review. American Association Petroleum Geologists Bulletin 52, Thomas, A.N The Asmari Limestone of southwest Iran. National Iranian Oil Company, Report 706 [unpublished]. Tucker, M.E Shallow marine carbonate facies and facies models, in: Brenchley P.J. & Williams B.P.J. (eds), Sedimentology, recent development and applied aspects. Geological Society of London, Special Publication 18, Vaziri-Moghaddam, H., Kimiagari, M. & Taheri, A Depositional environment and sequence stratigraphy of the Oligocene-Miocene Asmari Formation in SW Iran, Lali Area. Facies 52(1), Vaziri-Moghaddam, H., Seyrafian A., Taheri, A. & Motiei, H Oligocene-Miocene ramp system (Asmari Formation) in the NW of the Zagros basin, Iran: Microfacies, paleoenvironmental and depositional sequence. Revista Mexicana de Ciencias Geolόgicas 27(1), Wells, A.J Lithofacies and geological History of Lower Tertiary sediments in southwest Iran. Iranian Oil Offshore Company Report 1108 [Unpublished]. Wiley, J. & Habibi, F Geology study of Asmari Reservoir in Rage-Safid field. National Iranian South Oil Company (NISCO), Report P-3543 [Unpublished]. Wilson, J.L Carbonate facies in geological history. Springer, Berlin Heidelberg, New York p Wynd, J Biofacies of Iranian Oil Consortium Agreement Area. Iranian Oil Offshore Company, Report 1082 [Unpublished]. Yazdani, R Subsurface stratigraphy of the Asmari Formation in the Aghajari oil field, southeast Ahvaz. M.Sc. Thesis, Isfahan University, p. 153 [In Persian with English abstract]. Zahrabzadeh, M Geology studying of the Asmari Reservoir in the Rag-e-Safid oil field. National Iranian South Oil Company (Nisco), Ahvaz, Report 5954, [Unpublished, In Persian]. 219