New findings in biostratigraphy of the Sarvak and Ilam formations of Abteymour Oil Field (Dezful Embayment)

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.