نشریه رخساره های رسوبی
سال نهم شماره 2 (پیاپی 17، پاییز و زمستان 1395)

Intoduction: In the geological record of the Zagros fold-thrust belt and Dezful Embayment, Cretaceous successions consist of a thick sedimentary packages, which host numerous economically important hydrocarbon reservoirs such as Sarvak Formation (Ziegler, 2001; Ghabeishavi et al, 2010; Rahimpour-Bonab et al. , 2012, Mehrabi et al, 2014). Sarvak Formation (Albian-Turonian) is one of the main reservoirs in Iran and is the second major hydrocarbon reservoir in southwest Iran. (James and Wynd, 1965; Alavi, 2007). Ahwaz Oilfield is one of the largest oilfield in Iran that is located in northern Dezful Embayment (Motiei, 1995). In this research, petrographic studies and facies identification, for interpretation of sedimentary environment, have been carried out in order to understand the reservoir rocks of the Sarvak Formation. Furthermore, the influence of diagenetic processes on carbonate rocks of this formation in well No. 6 in the Ahwaz oil field, has been investigated which could be important in understanding of this formation for further development in this field.

Materials and Method: 205 thin sections, prepared from drilled cores, from Sarvak Formation in well No. 6 Ahwaz oil field have been studied in order to identify facies and interpret depositional environment as well as to investigate the diagenetic processes affected the Sarvak Formation. The frequency of the components which constitute the studied carbonates have been estimated by Flügel chart (Flügel, 2010). Different researchers study, for example Boudagher-Fadel (2008), Ghabeishavi et al. (2010) and Rahimpour-Bonab et al. (2012), have been taken into consideration for identification of foraminifera which are important component in the studied carbonates. For classification of carbonate facies followed the nomenclature of Dunham (1962) and Embry and Klovan (1971). The thickness of the Sarvak Formation in well No. 6 in the Ahwaz oilfield is 345 m that consists mainly of limestone. Since Sarvak Formation in the studied oilfield has not been completely drilled and with regard to the available data, the depth of 3310 to 3545 of this well which contains core has only been studied.

Discussion and conclusion: Petrographic studies of 205 thin sections from Sarvak Formation (235m thickness) in the studied well has led to identification of 7 carbonate facies including miliolid mudstone, packstone with highly diverse benthic foraminifera, bioclastic peloidal packstone, foraminiferal peloidal grainstone, rudist boundstone, rudist floatstone-rudstone, and planktonic foraminiferal wackestone. Based on identified of facies as well as vertical facies distribution and lack of slumping and turbidity deposits (Tucker and wright, 1990; Watts and Blome, 1990), gradual changes of facies, lack of aggregate grains, pisoids and oncoids (Burchette and Wright, 1992), the carbonate deposits of studied formation have been deposited in three environments including inner, middle and outer ramp, where sedimentation took place in a homoclinal ramp. Carbonates of the Sarvak Formation in the studied well have been affected by various diagenetic processes such as micritization, compaction, cementation, neomorphism, dissolution, fracturing and replacement (dolomitization, hematitization and pyritization) that operated in diagenetic environments such as marine, meteoric, burial and uplifting. According to Petrographic studies, in carbonates of the Sarvak Formation, dissolution and cementation processes are more important than the other. In these carbonates, the dissolution process increased and the cementation decreased the reservoir quality

Introduction: Garau Formation has a good distribution and outcrops at the Zagros sedimentary basin in Iran. This formation has been studied from different aspects of sedimentology and paleontology (e.g., Vincent et al. 2010; Jamalian et al., 2011; Ezampanah et al., 2012). In the present study, 510 m from the upper part of the Garau Formation at the south west of the Kabir-Kuh anticline has been investigated. The Kabir-Kuh anticline is located at the south east of Ilam and north east of Qaleh-Darreh village. The studied interval consists mainly of green to gray marls, marly limestone, limestone, shale and marly shale. This formation overlies the Gotnia Formation and is overlain by Sarvak Formation disconformably at the studied interval. The studied interval is investigated with regard to the calcareous nannofossils for determining the exact age the studied interval.

Materials and Methods: A total of 226 samples with a sampling resolution of 1 to 4 m were collected from the upper part of the Garau Formation and were processed using the simple smear slide technique described in Perch-Nielsen (1985). The prepared slides were examined using an Olympus BH2 transmitting light microscope at x1000 magnification.

Discussion: In the present study, the standard calcareous nannofossil zonation in low-latitude sections of Sissingh (1977) emended by Perch-Nielsen (1979, 1985) and Applegate and Bergen (1988) and Roth (1978) emended by Bralower (1987) and Bralower et al. (1993, 1995) were applied. The studied interval spans from CC5 to CC7 biozone of Sissingh (1977) and from NC5C to NC7B biozone of Roth (1978). Regarding to calcareous nannofossils, ninety species from 43 genus and 15 families have been recognized at the studied interval. Six important calcareous nannofossil bioevents were identified within the studied interval. Calcicalathina oblongata is recorded from the first part of the studied interval and its last occurrence (LO) is the lowermost nannofossil datum observed at 28 m of the studied interval, which defines the top of CC5 and NC5C. The first occurrence (FO) of H. irregularis (75 m) is recorded after the FO of Braarudosphaera africana (48 m) and Flabellites oblongus (64 m) at the studied interval and marks the top of CC6 zone of Sissingh (1977) and NC5 zone of Roth (1978) emended by Bralower et al. (1993, 1995). At low latitudes, the FO of R. irregularis and H. irregularis are the most reliable bio-events for defining the Barremian/Aptian boundary (e.g., Applegate and Bergen, 1988). The next important bio-event at the studied interval is the FO of E. floralis (122 m), defining the top of NC6 and CC7a (Early Aptian). The other important bio-event at the studied interval is the last occurrence of Micrantholithus hoschulzii (370 m) defines the top of NC7A biozone of Roth (1978) emended by Bralower et al. (1993, 1995)

Introduction: In order to investigation the Carboniferous micro-fauna of northwest Iran, the Khemes section in the Ardebil Province has been studied systematically. According to structural divisions of Iran proposed by Nabavi (1976), the Khemes section is part of Alborz-Azerbayjan zone in Iran. On the basis of foraminifers, Shabanian et al., (2014) stated that the Carboniferous sequence in this section is Early Nisean-the beginning of Early Bashkirian. These sediments are comparable to partly or completely of Dozdehband, Qezelgaleh, Bagherabad, Khashachal and Sardar formations. The purpose of this study is to investigate the Carboniferous micro-biostratigraphy in the Khemes section and it compare with other regions of Iran, according to foraminiferal stratigraphic distribution.

Materials and Methods: The study of the Khemes stratigraphic section of Ardebil Province, with 163 meter thickness and 7 lithological units, led to identification of 30 genera and 15 speices belonge to 8 families of benthic foraminifera. Systematical investigation of these foraminifera led to definition of two biozones: 1) Endothyra-Lysella Assemblage Biozone This biozones starts from the beginning of this section up to 23 meter. This assemblage was recognized from the Early Carboniferous sediments of Mobarak Formation from Central-Eastern Alborz (Bozorgnia, 1973) and Shishtoo Formatiom from Central Iran (Ruttneret al., 1968). Also these foraminiferal assemblages are comparable to the Donetz area of Russia with the age of Early Visean (Reitlinger, 1950), biozone No. 10 of North America with the age of Middle Visean-Late Visean (Mamet & Skipp, 1970) and biozone No. 30 of Illanlu section (Shabanian & Ghanizadeh, 2012) with the age of Late Visean. As a result, the age of this Biozone is Late Visean. 2) Eostaffella- Millerella Assemblage Biozone According to Kulagina et al., (2001-2006), association of foraminiferas such as Eostaffella and Millerella are marker of the Syuranian substage from Early Bashkirian of Europe. Also these mentioned species were reported from the Early Bashkirian sediments of Anarak and Zaladu sections of Central Iran (Leven & Gorgig, 2010., Gaetani et al., 2009), lower part of Qezelgaleh Formation (Jenny, 1977), Dozdehband Formation (Bozorgnia, 1973) and Tauride of Turkey (Ozermir-Altiner-Yilmaz, 2011). Based on the age of this assemblage and their correlations with other regiogns, we can state that the exactly age of this biozone is Serpukhovian to Syuranian substages from Early Bashkirian.

Discussion and Conclusion: According to Leven & Gorgij (2011), Bashkirian Fusulinida assemblages of Alborz and Central Iran are similar to each other and also similar to Ural and Eastern Europe foraminiferal assemblages. Comparison of the Iranian Fusulinid assemblages with the Eastern Europe indicate that during Carboniferous, Iranian and the Eastern Europe seas were connected and diversity in their fauna arising from local changes. According to the statements above, we can conclude that foraminifera assemblage of the Khemes stratigraphy section are the same as the Alborz and Central Iran. Also it is comparable to Ural and Eastern Europe and local variations were the reason for differences

Introduction The Dash-Aghel area is located about 20 km east of Bukan town, south of West-Azarbaidjan Province, NW Iran. Shales of the Shemshak Formation (Lower Jaurassic) form cap rock of layers and lenses of the iron-rich laterite ores in this area. The propose of the present study is to indentify the source rock characteristics as well as the source-area weathering, tectonic setting, and paleo-oxidation conditions by using major, trace, and rare earth elements geochemistry of shales of the Shemshak Formation exposed in the Dash-Aghel area.

Materials and Methods For the identification of mineralogical phases in the shales, four samples from a selective section were chosen for X-ray diffraction (XRD) analyses. XRD analyses were carried out using diffractometer model D5000 SIEMENS in Geological Survey of Iran (Tehran). The chemical compositions of the shales (#10) were determined at the ACME Analytical Laboratories Ltd. , Vancouver, Canada. The values of major and trace elements were determined by inductively coupled plasma emission spectrometry (ICP-ES). Rare earth element (REEs) contents were determined by inductively coupled plasma mass spectrometry (ICP-MS). Loss on ignition (LOI) was determined by weight loss of 1 g sample after heating at 950 °C for 90 min. The REE contents were normalized relative to chondrite and PAAS (Taylor & McLennan, 1985). Additionally, the geochemical data were evaluated by correlation coefficient analysis, elemental ratios, and plot of binary and ternary diagrams.

Discussion The result of XRD analyses revealed that quartz, kaolinite, illite, montmorillonite, calcite, albite, plagioclase, and goethite are the main mineral constituents of the studied shlaes. The positive correlations of K2O, TiO2, and Na2O with Al2O3 indicate that these elements are associated entirely with detrital phases in the samples. The Dash-Aghel shales show lower TiO2 values than the PAAS, which suggests more evolved (felsic) material in the source rocks. The depletion of P2O5 may be explained by the lesser amount of accessory phases such as apatite and monazite compared to PAAS in the studied samples. The high Sr content in shales indicates that Sr may be associated with calcite minerals. Correlation coefficients of positive and strong between K with elements such as Cr, U, Th, Ba, Hf, Nb, Rb, V, Sc, Co, and Ni suggest that illite have played the important role in distribution and concentration of these trace elements in the studied shales. The distribution pattern of REEs normalized to chondrite shows differentiation and strong enrichment of LREEs relative to HREEs and Eu and Ce negative anomalies in the studied shales. The geochemical signatures of clastic sediments have been used to find out the provenance characteristics (Armstrong-Altrin et al. , 2004). Al2O3/TiO2 ratios of most clastic rocks are essentially used to infer the source rock compositions, because the Al2O3/TiO2 ratio increases from 3 to 8 for mafic igneous rocks, from 8 to 21 for intermediate rocks, and from 21 to 70 for felsic igneous rocks (Hayashi et al. , 1997). In the studied shales, the Al2O3/TiO2 ratio ranges from 23. 57 to 44. 62, which suggests felsic rocks must be the probable source rocks for the shales of the Shemshak Formation. This interpretation is further supported by the TiO2 vs. Ni and Al2O3 vs. TiO2 bivariate plots, which also indicate that these shales were mainly derived from felsic source rocks. Ratios such as La/Sc, Th/Sc, Th/Cr, and Cr/Th are significantly different in felsic and basic rocks and may allow constraints on the average provenance composition. La/Sc, Th/Sc, Th/Cr, and Cr/Th ratios of shales from this study are compared with those of sediments derived from felsic and basic rocks (fine fraction) as
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well as to upper continental crust (UCC) and PAAS values. This comparison suggests that these ratios are within the range of felsic rocks. The REEs pattern and europium anomaly in the sedimentary rocks will provide important clues regarding the source rock characteristics. Higher LREEs/HREEs ratios and negative Eu anomalies are generally found in felsic igneous rocks, whereas the mafic igneous rocks exhibit lower LREEs/HREEs ratios and no or small Eu anomalies (Cullers, 1994). The Dash-Aghel shales show enrichment of LREEs relative to HREEs pattern with strong negative Eu anomalies. This pattern suggests that these sedimentary rocks were mainly derived from the felsic source rocks. Geochemical parameters such as U (0. 1-2. 6), authigenic U (0. 00-0. 28), Mn* (0. 41-1. 11), Ni/Co (3. 04-4. 92), Cu/Zn (0. 12-0. 47), U/Th (0. 33-0. 43), and Ce/Ce* (0. 44-0. 86) suggest that Dash-Aghel shlaes were deposited under oxic environmental conditions (Rimer, 2004). Alteration of rocks during weathering results in depletion of alkalis and alkaline earth elements and preferential enrichment of Al2O3. Therefore, weathering effects can be evaluated in terms of the molecular percentage of the oxide components by using the geochemical indices such as chemical index of alteration (CIA= [Al2O3/(Al2O3 + CaO* + Na2O +K2O] × 100) (Nesbitt & Young, 1982), chemical index of weathering (CIW = [Al2O3/( Al2O3 + CaO* + Na2O] × 100) (Harnois, 1988) and plagioclase index of alteration (PIA = (Al2O3–K2O) /[ (Al2O3–K2O) + CaO* +Na2O)] ×100) (Fedo et al. , 1995). However, samples having highly variable CaO contents due to variation in calcite abundance (such as those included in this study), may produce misleading conclusions if the CIA, PIA, and CIW are used to infer the degree of weathering (Cullers, 2000). Therefore, in this study we used a modified chemical index of weathering (CIW´ = molecular [Al2O3/ (Al2O3 + Na2O] × 100, in which CaO is left out of the CIW; Cullers, 2000). The CIW´ value of the Dash-Aghel shale samples vary from 96. 18-97. 07 %, which refer to intense weathering of source material of the shales. Based upon geochemical data such as ternary diagrams of (K2O+Na2O) (TiO2+Fe2O3+Al2O3) (Kreonberg, 1994) and Th-Sc-Zr/10 (Bhatia & Crook, 1986), the potential tectonic setting for Dash-Aghel shales is the passive continental margin environment.

Introduction: Oil-bearing fluid inclusions within oil reservoirs are usually trapped in the course of migration and during the formation of diagenetic neomorph mineral phases and associated cements. Their size often varies from 2mµ to 50µm (England et al. , 1987). They preserve the physic-chemical data such as temperature and composition of the fluids at the time of entrapment (Roedder, 1986; Barker & Goldstein, 1990; Sisson et al. , 1993). Since the oil reservoirs are commonly filled by oils of many origins with different maturity at various stages, investigation of the oil inclusions can help researchers determine the history of accumulation of oil within the reservoirs (England et al. , 1987). Microthermometric studies have been done in order to determine the maximum burial temperature, paleogeothermal gradient and maximum depth of burial (Burruss, 1987; Lowenstein & Brown, 1998; Ceriani et al. , 2002; Ceriani et al. , 2006; Ceriani et al. , 2011) The main objective of this research is to use the petrographic and fluorescence spectroscopy along with oil and aqueous inclusions microthermometry in cements of Asmari and Bangestan sedimentary formations of the Kupal oil field. For recognition of oil inclusions flourescenc microscopy was applied. The phenomenon of fluresence in organic matter consists of the emission of photons by chemical entities. Flurophores are able to absorb part of the incident energy, rising from a fundmental energy state of higher energy. The return to the ground state may then produce a luminescence phenomenon such as fluorescence (Hercules, 1965; Mousseron et al. , 1969). Essentially the fluorescence of organic molcules is related to the energy tranistion phenomena in the c=c bands. The conjugated system reguires a lower excitation energy therefore, aromatic and polyaromatic compounds are mainly responsible for the flurescence properites of sedimentary organic matters (Wehry, 1967). Oil inclusions are Classified with respect to changes in color of fluorescence: Orange colors with maturity (API): 15-20, Yellow colors with maturity (API): 20-30, blue colors with maturity (API): 30-40, white colors with maturity (API): 40-50 and White with maturity (API) > 50 (Lang & Gelfand, 1985).

Geologic setting: Kupal oil field is one of the main oil fields which is located about 60 km northeast of Ahwaz, Khuzestan Province. The area is located in central part of northern Dezful embayment. This oil field includes Asmari and Bangestan reservoirs both of which are currently producing oil.

Methods: In this study, samples were collected from sedimentary rocks and core logs of the Asmari reservoir (No. 40 and 44) and Sarvak reservoir (No. 20 and 4). To carry out this study, wafers of doubly polished sections (150 -200 µm) were prepared for fluorescence studies and discremination of inclusions (aqueous and petroleum) was done by using Olympus microscope equipped with UV light (with 340 nm filter). The microthermometric measurements were performed using a Linkam THMS600 heating - freezing stage.

Discussion: Fluid inclusions petrograohy: Based on petrographic studies, The majority of the studied fluid inclusions are hosted by sparry calcite cements and calcites filling the solution casts and cavities. The sparry calcite cements exhibit a variety of primary and secondary fluid inclusions. Most of them are hydrocarbon bearing fluid inclusions. Fluid inclusions micro termomerty: The obtained results showed that the highest frequency of homogenization temperatures in oil inclusions are 60-70 ºC and 70-80 ºC in Asmari Formation and 65-85 ºC and 100-120 ºC in Sarvak Formation. Also the salinity of fluid inclusions in the studied samples vary from 5 to 15 wt% NaCl eq. Fluid inclusions fluorescence: The fluorescence studies on oil reservoirs of Asmari and Bangestan (Sarvak) Formations illustrate two types of fluorescent colors, yellow and blue. Combination of the results obtained from micro-thermometry and fluorescence studies indicate that Asmari and Bangestan (Sarvak) reservoirs were fed by two different sources or were filled through several stages of diagenetic processes. The blue fluorescent color indicates the higher degree of maturity (API: 50-40) while yellow fluorescent color reflects relatively immature and heavy (API: 30-20) oil. Aqueous fluids are non-fluorescent.

Conclusion: The sparry calcite cements exhibit a variety of primary and secondary fluid inclusions. Most of them are hydrocarbon bearing fluid inclusions. Highest frequency of homogenization temperatures in oil inclusions are 6070 ºC and 70-80 ºC in Asmari Formation and 65-85 ºC and 100-120 ºC in Sarvak Formation. The results of fluorescence stuies showed that the oils from Asmari and Bangestane (Sarvak) reservoirs have two types of fluorescent colors (yellow and blue) that indicating the reservoirs were fed by two different sources. The blue fluorescent color indicates the higher degree of maturity (API: 50-40) while yellow fluorescent color reflects relatively immature and heavy (API: 30-20) oil. Combination of fluorescent, geochemical, and microthermometric data in Asmari and Bangestan Formations revealed that there were various generations of hydrocarbons filling the reservoirs.

Acknowledgments: The authors would like to thank the National South Oil Company for providing samples and funds for this research project. Our appreciation also extend to University of Tabriz, Research Institute of Petroleum Industries, and Iran Mineral Processing Research Center for furnishing laoratory facililities for this research

Introduction The Zagros orogeny belt as a part of the Alpine Himalayas orogeny belt extended about 2000km in a northwest-southeast trend from the Anatolian fault in the southeast of Turkey to the Makran zone in southern Iran. Bisotun limestone was deposited on the southeastern border of the Neotetis Sea, where it was separated from the Arabic platform by the narrow and radiolarite basin of Kermanshah. A 2500 m thick succession Bisotoun carbonate rocks of the Late Triassic to Late Cretaceous in age were exposed at the east of Kermanshah as a part of the High Zagros zone. The studied section consists of dark gray color, medium to thick bed and massive limestone. Little studies have been done on the microfacies analysis, sedimentary environments and diagenesis history of the Bisotoun limestones. Since these limestones are deposited along with the radiolarite deposits, sedimentary studies and facies analysis help to reconstruct the paleogeography, their relation with the Kermanshah radiolarite basin, as well as investigate the diagenic history of the Bisotoun carbonate successions.

Study method 105 thin sections were prepared for petrography. To recognize the calcite from dolomite under microscope, all thin sections were stained with Alizarin red solution (by Dickson 1965 method). The Cathodoluminescence (CL) microscopy and scanning electron microscope (SEM) have been conducted to study the dolomites and cements. Classification of dolomites was based on Sibley & Gregg (1987). The blue epoxy has been used to estimate the porosity in studied samples.

Results and discussion Petrographic study of 105 thin sections from this succession led to identification of 8 carbonate facies that are deposited in four depositional environments including open marine, shoal, lagoon and tidal flat. Due to the absence of turbidity facies, cortoid, ankoid, pizoid and aggregate allochem, which indicate carbonate shelf and rarely found in a carbonate ramp, and also because of gradual changes in vertical sequence, the depositional environment of Bisitoun carbonate succession is a homoclinal ramp (consisting of middle and inner ramp). Facies are classified according to Dunham (1962) and Embry & Klovan (1971) classifications. The facies analysis is based on Schlager (2002) and Flugel (2010). The main diagenetic processes that affected the studied succession are micritization, bioturbation, replacement (dolomitization, silicification, pyritization and dedolomitization), compaction, stylolitization, cementation (blocky, syntaxial overgrowth, poikilotopic, isopachous and drusy equant), fracturing and vein filling, neomorphism, dissolution and hematitizationt. These processes were conducted in marine, burial (shallow and deep burial) and uplifted (meteoric) stages of diagenetic environments. The marine diagenetic process include micritization (that destroyed the internal components of the allochems), syntaxial overgrowth calcite cement, bioturbation (that is mainly found in rudist containing facies). The burial (shallow and deep burial) diagenetic process include physical (compaction in mudsupported facies) and chemical (stylolitization) phenomena. These process affected on most carbonate facies and played important role in the studied succession. Most of the allochems are cut off by stylolitization in which the presence of dolomite and iron oxide is common. Channel porosity and stylolitization porosity are significant in studied carbonate rocks, which some of them are filled by spary calcite cement. Dolomite dissolution, dedolomitization, channel and moldic porosity were created in uplift and meteoric diagenetic environments

Conclusion Based on field observation and petrography studies it is concluded that the Bisotoun carbonate rocks have been deposited in middle and inner parts of a homoclinal ramp. The most important diagenetic processes observed in Bisotoun carbonate rocks are micritization, bioturbation, replacement, compaction, stylolitization, cementation, fracturing and vein filling, neomorphism, dissolution and hematitizationt, which are mainly occurred in marine, burial and uplift stages of diagenetic environments. Channel porosity and stylolitization porosity are significant in studied carbonate rocks

Introduction The Tirgan Formation, as a cliff forming carbonate stratigraphic unit, consists of medium to thick bedded gray limestone with some thin intercalations of shales, considered as a probable hydrocarbons reservoir rock in the Kopet-Dagh basin (Robert et al. , 2014). Throughout the basin, this formation overlaid the red siliciclastic succession of the Shurijeh Formation and overlain by the gray shales of the Sarcheshmeh Formation. Relative age of the Tirgan Formation, especially on the western Kopet-Dagh sections, have been discussed in a lot of publications (e. g. , Kalantari, 1969; Rivandi et al. , 2010; Khodadadi & Hadavi, 2012; Carevic et al. , 2013; Bucur et al. , 2013; Taherpour et al. , 2013). However the provided results are not practical for many of the sections particularly in the eastern parts of the basin. The outcomes of different biostratigraphical tools such as orbitolinids and calcareous nannofossils are dissimilar and it is not possible to find them in all paleoenvironments, especially in the shallower settings. Embryonic chambers of orbitolinids as the most important characters of their taxonomy (Schroeder et al. , 2010) are absent in most of specimens, not possible to find them even in the serial sections. The previously introduced biozones based on the foraminifera are problematic and the name of some biozone maker fossils had been chosen in a wrong way (for example Dictyoconus arabicus in Rivandi et al. , 2010). In addition, pelagic macrofauna like ammonites as the fundamental biostratigraphic tool for dating of the Cretaceous marine environments are invisible in the eastern outcrops of the Tirgan Formation. Due to these limitations, we try to use a new tool for biostratigraphy and provide a suitable explanation for relative age of the sequences at least in the TaherAbad and Eshlir anticline sections, east of Kopeh-Dagh. These two stratigraphic sections are respectively located at the distance of 75 and 95 kilometers northeast of Mashhad City. Totally, more than 150 thin sections belong to 100 limestone samples have been prepared. Furthermore, 25 shale samples for palynological investigations were taken and studied.

Results and Discussion Micropaleontological studies on the Tirgan Formation is led to identification of 17 genera and 21 species of the benthonic foraminifera and 16 genera and 20 species of the calcareous algae in the Taher-Abad section. They are as follow: Andersenolina alpina, Balkhania balkhanica, Charentia cuvillieri, Comaliamma sp. , Dictyoconus sp. , Haplophragmoides joukowskyi, Haplophragmoides sp. , Istriloculina eliptica, Istriloculina sp. , Lenticulina sp. , Mayncina bulgarica, Melathrokerion valserinensis, Nautiloculina oolithica, Neotrocholina aptiensis, Orbitolinids, Palorbitolina lenticularis, Quinqueloculina sp. , Rumanoloculina robusta, Rumanoloculina cf. pseudominima, Rumano-loculina sp. , Torinosuella peneropliformis, Vercorsella scarsellai, Acicularia sp. , Arabicodium sp. , Boueina sp. , Boueina hochstetteri, Cayeuxia sp. , Carpathoporella sp. , Coptocompylodon sp. , Deloffrella sp. , Deloffrella quercifoliipora, Girvanella sp. , Halimedaceae, Holosporella sp. , Kopetdagaria sphaerica, Marinella lugeoni, Montiella? elitzae, Neomeris cretacea, Permocaculus sp. , Russoella radoicicae, Terquemella sp. . In addition, 17 genera and 19 species of benthonic foraminifera and 12 genera and 16 species of calcareous algae are recognized in the Eshlir section. They comprise Andersenolina alpina, Balkhania balkhanica, Charentia cuvillieri, Dictyoconus sp. , Haplophragmoides joukowskyi, Istriloculina eliptica, Istriloculina sp. , Lenticulina sp. , Mayncina bulgarica, Melathrokerion valserinensis, Nautiloculina oolithica, Neotrocholina aptiensis, Novalesia cf. producta, Orbitolinids, Palorbitolina lenticularis, Quinqueloculina sp. , Rumanoloculina robusta, Torinosuella peneropliformis, Vercosella arenata, Vercorsella scarsellai, Acicularia sp. , Boueina sp. , Boueina hochstetteri, Coptocampylodon sp. , Deloffrella sp. , Deloffrella quercifoliipora, Halimedaceae, Holosporella sp. , Kopetdagaria sphaerica, Marinella lugeoni, Montiella? elitzae, Neomeris cretacea, Permocaculus sp. , Russoella radoicicae, Terquemella sp. The mentioned microfossils characterized three biozones in the sections: Palorbitolina lenticularis Range zone, Kopetdagaria sphaerica local zone and Balkhania balkhanica local zone. Palorbitolina lenticularis Range zone contains the first to last occurrences of the nominal species, demonstrates the Late Barremian – Early Aptain ages (Bachman & Hirsch, 2006) for the Taher-Abad section. This biozone is equivalent to two shorter local zones of Balkhania balkhanica and Kopetdagaria sphaerica. The first zone includes the entire existence of the Balkhania balkhanica, has a regional biostratigraphic value for the Late Barremian. The thickness of Balkhania balkhanica zone in the Taher-Abad and Eshlir sections is respectively 20. 65 and 37 meters, includes the carbonate units of the lower part of the Tirgan Formation. The second zone comprises the whole range of Kopetdagaria sphaerica in the studied sections, confirms the Late Barremian to Earlier Aptian (Bedoulian). Because of the incompleteness of Palorbitolina lenticularis Range zone in the Eshlir section, it is not possible to simply use of that. Instead, the later defined shorter Balkhania balkhanica and Kopetdagaria sphaerica biozones are appropriately useful in the Eshlir section. This biozone has a thickness of 43. 35 and 39. 65 meters in Taher-Abad and Eshlir, occupies a major upper part of the carbonate rocks of these two sections.

Conclusion In the absence of proper orbitolinid fauna as the index benthonic fossils of the Early Cretaceous (Late Barremian – Early Aptain), the Balkhania balkhanica and Kopetdagaria sphaerica biozones have a worthy performance in the age dating of the Tirgan Formation, particularly in the eastern Kopet-Dagh. These ages have been examined by palynological contents of the shaly intervals. Some species such as Cribroperidinium cf. tenuiceras, Muderongia staurota, Odontochitina operculata and Pseudoceratium pelliferum which formerly reported from the Late Barremian – Early Aptian strata of Australia and Egypt (e. g. , Oosting et al. , 2006; Deaf et al. , 2015) confirm the proposed age by foraminifera and calcareous algae

Introdaction Generally, sedimentary facies is a primary factor controlling framework and geometry of pores in reservoir rocks. However, in carbonate reservoirs, many of primary properties are changed due to the effect of diagenesis (Mazzullo 1994; Lucia, 2007; Ahr, 2008; Moore and Wade 2013). Microfacies and sedimentary environment of the Sarvak Formation have been vastly studied in different parts of the Zagros and Persian Gulf (Taghavi et al. 2006; Razin et al., 2010; Ghabeishavi et al. 2010; Mehmandosti et al. 2013; RahimpourBonab et al. 2012;). Nevertheless, there is less information about facies architecture and paleo-environment of the formation in the Abadan Plain, as one of the important exploration regions of SW Iran. The main parts of this paper is related to Facies description and depositional environment interpretation of the Sarvak Formation in two key wells of one of the giant oil field in the Abadan Plain region.

Materials and Methods The data used in this study includes a collection of information derived from cores and thin sections belonging to two key wells of one of the giant oil field in the Abadan Plain (430 m core and 1116 thin sections). The results from core description and thin section study were used for facies analysis. In this regard, some sedimentary properties such as lithology, texture, sedimentary structure and types of skeletal and non-skeletal grains were taken into account. Also, to reach a better understanding of sedimentary characteristics in thin sections, all of them were scanned using a CREO-IQSMART3 scanner at the Research Institute of Petroleum Industry (RIPI). The apparatus is a powerful scanner, which can be used for achieving high resolution images up to 10000 DPI.

Result and discussion Based on the results from the core descriptions and petrographic studies, the sedimentary texture, grain size, type and frequency of allochems, fossil content and other sedimentary parameters for the Sarvak Formation in the key wells were identified. Then, using this information and comparison with previous studies (Razin et al., 2010; Hollis, 2011; Mehmandosti et al., 2013; Rahimpour-Bonab et al., 2012, 2013) and standard models (Wilson 1975; Flugel 2010), 15 facies were recognized. The defined facies are representative of five facies belts including distal open marine, proximal open marine, slope, rudist biostrome-shoal and lagoon. The lagoon facies belt based on diversity of components and texture can be subdivided into open and restricted lagoon. On the basis of such evidences as facies association, rudist distribution as well as tectonic setting a shelf carbonate platform for deposition of reservoir part of the Sarvak Formation in an oil field in the Abadan plain (equivalent of Mishrif Formation in Mesopotamian Basin) has been proposed. With regard to reservoir potential of the area, the findings from facies analysis is thought to be valuable for better reservoir characterization and development