Biostratigraphy of the Asmari Formation in Gachraran oilfield, Southwestern Iran

رخساره‌های رسوبی

Article 6, Volume 13, Issue 2 - Serial Number 25, June 2021, Pages 239-251   

Document Type: مقالات پژوهشی

DOI: 10.22067/sed.facies.2021.46954

Authors

Narges Monjezi^{* 1}; Behzad Saeedi Razavi²

¹Assistant Professor, Department of Remote sensing, Dezful Branch, Islamic Azad University, Dezful, Iran

²Research Assistant Professor, Standard Research Institute, Technical and engineering faculty, Construction and mineral Department, Karaj, Iran

Abstract

Introduction
Asmari Formation is a thick succession of Oligocene-Miocene sedimentary rocks, the leading crude oil reservoir in the Zagros structural-sedimentary zone, and one of the largest oil reserves in the world (Alavi, 2004). Due to its economic significance, this formation has long been considered in recent decades. The presence of large benthic foraminifera as the most important fossil group in the Asmari Formation is essential for the biostratigraphic studies of this formation. This study aims to study the Asmari Formation in Gachsaran oil field in southwestern Iran, 200 km southeast of Ahvaz. In order to identify lithological changes and the biostratigraphic studies, three wells were selected from the Gachsaran oil field, including wells A, B, and C.
Materials and Methods
In the mentioned three wells of the Gachsaran oil field, the Asmari Formation has 590 m thickness in well A, 336 m in well B, and 382 m in well C. In this study, 3235 microscopic thin sections were prepared from the Asmari Formation, 1572 thin sections from well A, 809 from well B, and 854 from well C. Identification of microfossils have been done based on the Adams & Bourgeois (1967), Loeblish & Tappan (1988), and Hottinger (1997), and Biostratigraphic studies according to the Laursen et al. (2009) and Van Bochem et al. (2010).
Discussion and Conclusion
The Asmari Formation in the Gachsaran oil field consists of limestone, dolomitic limestone, and dolomite. Based on the distribution of benthic foraminifera in the studied wells, four faunal assemblages equivalent to those introduced by Laursen et al. (2009) and Van Bochem et al. (2010) have been recognized in the Asmari Formation. They are as follows:
1- Nummulites vascus - Nummulites fichteli Assemblage zone
2- Archaias asmaricus-Archaias hensoni - Miogypsinoides complanatus Assemblage zone
3- Miogypsina - Elphidium sp. 14- Peneroplis farsensis Assemblage Zone
4- Borelis melo curdica- Borelis melo melo Assemblage Zone
Assemblage zone 1 is equal to zone No. 2 of Laursen et al. (2009) and Van Bochem et al. (2010) (Nummulites vascus - Nummulites fichteli Assemblage zone) and contains index nummulitids such as Nummulites vascus, Nummulites fichteli, Nummulites intermedius with Rupelian in age. This biozone has been recorded in all three wells. Assemblage zone 2 is equivalent to zone No. 4 in Laursen et al. (2009) and Van Bochem et al. (2010) (Archaias asmaricus-Archaias hensoni - Miogypsinoides complanatus Assemblage zone). This assemblage contains index fossils such as Miogypsinoides complanatus, Archaias kirkukensis, Archaias hensoni. The appearance, abundance, and extinction of Archaias are occurred in this zone and represent the Chattian. This zone has also been recognized in all three wells. Assemblage zone 3 which is again present in all three wells, corresponds to zone No. 5 of Laursen et al. (2009) and Van Bochem et al. (2010) (Miogypsina - Elphidium sp. 14- Peneroplis farsensis Assemblage Zone). It has significant fossils such as Miogypsina, Elphidium sp. 14, and Peneroplis farsensis. Due to the presence of these index fossils as well as the disappearance of the genus Archaias at the end of the Chattian, even the absence of Borelis melo curdica (Burdigalian index), this zone belongs to Aquitanian. Assemblage zone 4 is comparable to biozone No. 7 in Laursen et al. (2009) and Van Bochem et al. (2010) (Borelis melo curdica- Borelis melo melo Assemblage Zone). Due to the index species Borelis melo curdica, this part of the sequence belongs to the Early Miocene (Burdigalian). This biozone has been identified only in wells A and C. Therefore, biostratigraphically, the most complete sequence of Oligocene - Early Miocene Asmari Formation in this study is recorded in wells A and C.
References
Adams, T.D., & Bourgeois, F., 1967. Asmari biostratigraphy. Iranian Oil Operating Companies. Geological and Exploration Division, Report 1074: 1-37. (Unpublished)
Alavi, M., 2004. Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution. American Journal of Science, 304: 1-20.
Hottinger, L., 1997. Shallow benthic foraminiferal assemblages as signals for depth of their deposition and their limitations. Bulleint de la Societ Geologique de france, 168: 491-505.
Laursen, G.V, Monibi, S., Allan, T.L., Pickard, N.A.H., Hosseiney, A., Vincent, B., Hamon, Y., Van Buchem, F.S.H., Moallemi, A., & Driullion, G., 2009. The Asmari Formation revisited: Changed stratigraphic allocation and new biozonation. First international petroleum conference & exhibition, Shiraz, Iran, 1-5.
Loeblich, A.R.Jr., & Tappan, E., 1988. Forminiferal Genera and their Classification. Van Nostrand Reinhold Campany, New York, 1-970.
Van Buchem, F.S.P., Allan, T.L., Laursen, G.V., Lotfpour, M., Moallemi, A., Monibi, S., Motiei, H., Pickard, N.A.H., 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, London, Special Publications, 329 (1): 219-263.

Keywords

Biostratigraphy; Asmari Formation; Oligocene; Miocene; Gachsaran oilfield

References

Adams, T.D., & Bourgeois, F., 1967. Asmari biostratigraphy. Iranian Oil Operating Companies. Geological and Exploration Division, Report 1074: 1-37. (Unpublished)

Alavi, M., 2004. Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution. American Journal of Science, 304: 1-20.

Amirshahkarami, M., Ghabishavi, A., & Rahmani, A., 2010. Biostratigraphy and paleoenvironment of the larger benthic foraminifera in wells sections of the Asmari Formation from the Rag-e-Safid oil field, Zagros Basin, south west Iran. Stratigraphy and Sedimentology Researches, 40: 63-84.

Berberian, M., & King, G.C.P., 1981. Towards a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Sciences, 18: 210-265.

Farzipour-Saein, A., Yassaghi, A., Sherkati, S., & Koyi, H., 2009. Basin evolution of the Lurestan region in the Zagros fold-and-thrust belt, Iran. Journal of Petroleum Geology, 32: 5-19.

Hakimzadeh, S., & Seyrafian, A., 2008. Late Oligocene-Early Miocene benthic foraminifera and biostratigraphy of the Asmari Formation, south Yasuj, north-central Zagros basin, Iran. Carbonate and Evaporites, 23: 1-10.

Heydari, E., 2008. Tectonics versus eustatic control on supersequences of the Zagros Mountains of Iran. Tectonophysics, 451: 56-70.

Heydari, E., Hassanzadeh, J., Wade, W.J., & Ghazi, A.M., 2003. Permian-Triassic boundary interval in the Abadeh section of Iran with implications for mass extiontion. Part 1. Sedimentology. Palaeogeography, Palaeoclimatology, Palaeoecology, 193: 405-423.

Hottinger, L., 1997. Shallow benthic foraminiferal assemblages as signals for depth of their deposition and their limitations. Bulleint de la Societ Geologique de france, 168: 491-505.

Laursen, G.V, Monibi, S., Allan, T.L., Pickard, N.A.H., Hosseiney, A., Vincent, B., Hamon, Y., Van Buchem, F.S.H., Moallemi, A., & Driullion, G., 2009. The Asmari Formation revisited: Changed stratigraphic allocation and new biozonation. First international petroleum conference & exhibition, Shiraz, Iran, 1-5.

Loeblich, A.R.Jr., & Tappan, E., 1988. Forminiferal genera & their classification. Van Nostrand Reinhold Campany, New York, 1-970.

Rahmani, A., Taheri, A., Vaziri-Moghaddam, H., & Ghabeishavi, A., 2012. Biostratigraphy of the Asmari Formation at Khaviz and Bangestan Anticlines, Zagros Basin, SW Iran. Neues Jahrbuch für Geologie und Paläontologie, 263: 1-16.

Rahmani, A., Vaziri-Moghaddam, H., Taheri, A., & Ghabeishavi, A., 2009. A model for the paleoenvironmental distribution of larger foraminifera of Oligocene-Miocene carbonate rocks at Khaviz Anticline, Zagros Basin, SW Iran. Historical Biology, 21: 215-227.

Roozpeykar, A., & Moghaddam, I.M., 2015. Biostratigraphy, facies analysis and paleoecology of the Asmari Formation in the northwest of Behbahan, south-western Iran. Carbonates and Evaporites, 30: 387-400.

Roozpeykar, A., & Moghaddam, I.M., 2016. Benthic foraminifera as biostratigraphical and paleoecological indicators: An example from Oligocene-Miocene deposits in the SW of the Zagros basin, Iran. Geoscience Frontiers, 7: 125-140.

Sadeghi, R., Vaziri-Moghaddam, H., & Taheri, A., 2009. Biostratigraphy and palaeoecology of the Oligo-Miocene succession in Fars and Khuzestan areas (Zagros Basin, SW Iran). Historical Biology, 21: 17-31.

Seyrafian, A., Vaziri-Moghaddam, H., Arzani, N., & Taheri, A., 2011. Facies analysis of the Asmari Formation in central and north-central Zagros basin, southwest Iran: Biostratigraphy, paleoecology and diagenesis. Revista Mexicana de Ciencias Geológicas, 28: 439-458.

Sooltanian, N., Seyrafian. A., & Vaziri-Moghaddam, H., 2011. Biostratigraphy and paleo ecological implications in microfacies of the Asmari Formation (Oligocene), Naura anticline (Interior Fars of the Zagros Basin), Iran. Carbonates and Evaporites, 26: 167-180.

Thomas, A.N., 1948. The Asmari limestone of southwest Iran. Anglo-Iranian Oil Company Report, 706 p., Unpublished.

Van Buchem, F.S.P., Allan, T.L., Laursen, G.V., Lotfpour, M., Moallemi, A., Monibi, S., Motiei, H., Pickard, N.A.H., 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, London, Special Publications, 329 (1): 219-263.

Wynd, J.G., 1965. Biofacies of the Iranian Oil Consortium Agreement area. Iranian Oil Operating Companies, Geological and Exploration Division, Report 1082. (Unpublished).