Biostratigraphy of the Asmari Formation in Gachraran oilfield, Southwestern Iran

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.

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).

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.