Comparative microbiostratigraphy of the Asmari Formation based on six biozonations (Barry anticline, Fars sub-basin, Zagros zone)

Authors

• Shiva Shakerjahromi ¹
• Reza Sadeghi ²
• Ali Rahmani ³

¹ M. Sc. in Stratigraphy and Paleontology, Department of Geology, Faculty of Sciences, University of Payame Noor, Tehran, Iran

² Assistant Professor, Department of Geology, Faculty of Sciences, University of Payame Noor, Tehran, Iran

³ Ph. D. in Stratigraphy and Paleontology, National Iranian Oil Company, Tehran, Iran

10.22067/sed.facies.2021.41986

Abstract

Introduction
Asmari Formation is a hard and thick carbonate succession in the Oligocene to Miocene series, which is the youngest and most important crude oil reservoir rock in southern Iran, Zagros zone (Motiei, 1993). Because the oil was discovered and extracted from this formation in the middle east for the first time, this Formation has a global reputation (Afsharharb, 2001). The Asmari Formation cropped out in the Zagros range and is present in many subsurface drilled wells, but its most complete sequence has been recorded in the Dezful embayment. The Asmari Formation, with three lithostratigraphic lower, middle and upper parts, is composed of limestone, marly, and dolomitic limestone (Motiei, 1993) and biochronostratigraphically has multichronous boundaries. This study aims to compare the age determination of the Asmari Formation sedimentary sequences in the Sahl-Abad section based on biozonations presented by Eames et al. (1962), Wynd (1965), Adams & Bourgeois (1967), Cahuzac & Poignant (1997), Ehrenberg et al. (2007), Laursen et al. (2009) and van Buchem et al. (2010). The mentioned section with geographical coordinates N 29° 19ʹ 3.07ʺ and E 52° 15ʹ 37.25ʺ is located in the northeastern flank of Barry anticline, within the Zagros simple folded zone, Fars sub-basin and interior Fars subzone.
Materials and Methods
Initially, visiting the area, preliminary stratigraphic studies, and identification and separation of possible lower and upper boundaries of the Formation were performed and the section was measured. In the following, field descriptions of lithology, stratification, color, biota, and photography of the outcrops, systematic sampling with a maximum distance of 2 meters, preparation of 169 thin microscopic sections of rock samples, and their study and photography by binocular microscope were achieved in natural light (PPL). To identify the microfossils, we used the proper literature such as Rahaghi (1980), Loeblich & Tappan (1988), Sirel (2003), Bassi et al. (2007), Hottinger (2007), Benedetti (2010), Sirel et al. (2013), Gedik (2014), Hottinger & Bassi (2014), Serra-Kiel et al. (2016), and Boudagher-Fedal (2018). Then, the biochronological zonation of the Asmari Formation was performed using all six biozonations. Finally, without prioritizing one of these six views over the other, the age conclusion based on the most recent views (Laursen et al., 2009 & van Buchem et al., 2010) is presented. Tables and figures were also drawn and prepared based on four software Google Earth, Microsoft Office Word, CorelDRAW, and Photoshop.
Discussion
According to paleontological studies on the Asmari Formation in the Sahl-Abad section, 35 genera and 64 species of benthic foraminifera were determined. Then, based on their standard range and distribution, three Assemblage biozones in the Asmari Formation were recognized and described. The relative age of this Formation was based on the views of van Buchem e al. (2010) and Laursen et al. (2009), Ehrenberg et al. (2007), Cahuzac & Poignant (1997), Adams & Bourgeois (1967), Wynd (1965) and Eames et al. (1962) respectively are as follows:
From 16 to 117 meters: Early Oligocene (Rupelian), Early Oligocene (Rupelian), Early-Late Oligocene (Early Rupelian-Early Chattian), Oligocene, Oligocene, and Middle Oligocene (Rupelian).
From 117 to 140 meters: Early-Late Oligocene (Rupelian-Chattian), Late Oligocene (Chattian), Early? - Late? Oligocene (Late Rupelian? - Early Chattian?) or Late Oligocene? (Middle? - Late? Chattian), Oligocene, Oligocene and Middle? - Late? Oligocene (Rupelian? - Chattian?).
From 140 to 307 meters: Late Oligocene (Chattian), Late Oligocene (Chattian), Early? - Late? Oligocene (Late Rupelian? - Early Chattian?) or Late Oligocene? (Middle? - Late? Chattian) and Late Oligocene (Middle-Late Chattian), Early Miocene (Early-Late? Aquitanian), Oligocene-Early Miocene (Aquitanian), and Late Oligocene (Chattian) or Early Miocene (Aquitanian).
Conclusion
Using the first view of Eames et al. (1962) and biozones of Cahuzac & Poignanat (1997), Ehrenberg et al. (2007), Laursen et al. (2009), and van Buchem et al. (2010), eventually, the same results are obtained. The age of Asmari is included in the Tang-e-Sahl-Abad section in the time range of the Early/Middle to Late Oligocene (Rupelian-Chattian), however this age, by comparison with the second view of Eames et al. (Middle Oligocene-Early Miocene: Rupelian-Aquitanian) and biozones of Wynd (Oligocene-Early Miocene: Aquitanian) and Adams & Bourgeois (Oligocene-Early Miocene: Early-Late? Aquitanian), is not only completely alike but also different from the first four views. All these six biochronostratigraphic patterns prove the Oligocene age for the Asmari Formation; of course, the four views of 1962, 1997, 2007, 2009, and 2010 only confirmed up to the Late Oligocene (Chattian), but the three views of 1962, 1965 and 1967 confirm up to the Early Miocene (Aquitanian).
References
Adams, T.D., & Bourgeois, E., 1967. Asmari Biostratigraphy (No. 1074). Iranian Oil Operating Companies Geological and Exploration Division, Tehran, 59 p.
Afsharharb, A., 2001. Geology of Iranian oil. Tehran Technical College, University of Tehran, 586 p. (In Persian)
Bassi, D., Hottinger, L., & Nebelsick, J., 2007. Larger foraminifera from the upper Oligocene of the Venetia area north-east Italy. Paleontology, 50: 845-868.
Benedetti, A., 2010. Biostratigraphy remarks on the Caltavuture Formation (Eocene-Oligocene) cropping out at Portella Colla (Madonie Mts., Sicily). Revue de Paleobiologie, 29: 197-216.
Boudagher-Fadel, M.K., 2018. Evolution and significance of larger benthic foraminifera, 2nd edition. University College London Press, London, 693 p.
Cahuzac, P.B., & Poignant, A., 1997. An attempt of biozonation of the European basin, by means of larger neritic foraminifera. Bulletin de la Société Géologique de France, 168: 155-169.
Eames, F.E., Banner, F.T., Blow, W.H., & Clark, W.J., 1962. Fundamental of Mid-Tertiary stratigraphical correlation. The Syndicate of the Cambridge University Press, America, 162 p.
Ehrenberg, S.N., Pickard, N.A.H., Laursen, G.V., Monibi, S., Mossadegh, Z.K., Svana, T.A., Aqrawi, A.A.M., Mc Arthur, J.M., & Thirlwall, M.F., 2007. Strontium isotope stratigraphy of the Asmari Formation (Oligocene–Lower Miocene), SW Iran. Journal of Petroleum Geology, 30: 107–128.
Gedik, F., 2014. Benthic foraminiferal fauna of Malatya Oligo-Miocene basin, (eastern Taurids, eastern Turkey). Bulletin of the Mineral Research and Exploration, 149: 93-136.
Hottinger, L., & Bassi, D., 2014. Paleogene larger rotaliid foraminifera from the western and central Neotethys. Springer Chem Heidelberg, New York, 196 p.
Hottinger, L., 2007. Revision of the foraminiferal genus Globoreticulina Rahaghi, 1978 and of its associated fauna of larger foraminifera from the late Middle Eocene of Iran. Carntis de Geologie / Notebooks on Geology, A: 1-51.
Laursen G.V., Monibi, S., Allan, T.L., Pickard, N.A.H., Hosseiney, A., Vincent, B., Hamon, Y., van Buchem, F.S.P., Moallemi, A., & Druillion, G., 2009. The Asmari Formation revisited: changed stratigraphic allocation and new biozonation. First International Petroleum Conference and Exhibition, Shiraz, 5 p.
Loeblich, A.R., & Tappan, H., 1988. Foraminifera genera and their classification. van Nostrand Reinhold Company, New York, 212 p., 847 plates.
Motiei, H., 1993. Stratigraphy of Zagros. Geological Survey & Mineral Explorations of Iran, Tehran, 536 p. (In Persian).
Rahaghi, A., 1980. Tertiary faunal assemblage of Qum-Kashan, Sabzewar and Jahrum areas. National Iranian Oil Company, Tehran, 64 p., 30 plates.
Serra-Kiel, J., Gallardo-Garcia, A., Razin, Ph., Robinet, J., Roger, J., Grelaud, C., Leroy, S., & Robin, C., 2016. Middle Eocene-Early Miocene larger foraminifera from Dhofar (Oman) and Socotra Island (Yemen). Arabian Journal of Geosciences, 9: 1-95.
Sirel, E., 2003. Foraminiferal description and biostratigraphy of the Bartonian, Priabonian and Oligocene shallow water sediments of the southern and western Turkey. Revue de Paleobiologie, 22: 269-339.
Sirel, E., Ozgen-Erdem, N., & Kangal, O., 2013. Systematics and biostratigraphy of Oligocene (Rupelian-Early Chattian) foraminifera from lagoonal-very shallow water limestone in the eastern Sivas basin (central Turkey). Geologia Croatica, 66: 83-109.
van Buchem, F.S.P., Allan, T., A., Laursen, G.V., Lotfpour, M., Moallemi, A., Monibi, S., Motiei, H., Pickard, N., Tahmasbi, A.R., Vedrenne, V., & Vincent, B., 2010. Regional stratigraphic architecture and reservoir types of the Oligo-Miocene deposits in the Dezful Embayment (Asmari and Pabdeh Formations) SW Iran. Geological Society of London Special Publication, 329: 219-263.
Wynd, T., 1965. Biofacies of the Iranian oil consortium agreement area (No. 1082). Iranian Oil Operating Companies Geological & Exploration Division, Tehran, Unpublished.

Keywords

• Folded Zagros
• Interior Fars
• Asmari Formation
• Six Biozonations
• Benthic Foraminifera

Main Subjects

• Systematic Paleontology & Biostratigraphy

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