Lithostratigraphy and biostratigraphy of Gurpi Formation in one of the borehole of Marun Oilfield, southwest of Iran (Zagros)

In order to conduct stratigraphic and paleoenvironmental studies in the Gurpi Formation, the sediments of this formation in a borehole in the Marun Oilfield located in southwestern Iran based on calcareous nannofossils were studied. In this study, Gurpi Formation consists of 10 rock units consisting of marl, marly limestone, limestone and shaly limestone. Also, the study of biostratigraphy based on calcareous nannofossils led to the identification of 22 genera and 43 species. Based on distribution of the index species have been recognized biozones, including Calculites obscurus Zone (CC17/ UC13), Aspidolithus parcus Zone (CC18/ UC14), Calculites ovalis Zone (CC19/ UC15aTP),Ceratolithoides aculeus (CC20/ UC15bTP), Quadrum sissinghii (CC21/ UC15cTP), Quadrum trifidum (CC22/ UC15dTP - UC15eTP), Tranolithus phacelosus (CC23/ ‌UC16-UC17), Reinhardtites levis (CC24/ UC18), Arkhangelsiella cymbiformis (CC25/ UC19- UC20aTP- UC20bTP), Nephrolithus frequens (CC26/ UC20c, dTP)‌, that corresponding to UC13- UC20dTP. According to the identified bio-zones, the age of the Gurpi Formation in this section is late Santonian/early Campanian to latest Maastrichtian. The paleoenvironmental study indicates that the basin depth has increased for the deposition of the Gurpi Formation in the Late Santonian. Following this, the deposition of the Gurpi Formation continued until the Late Maastrichtian, so that the last bio-event related to the Gurpi Formation is the first occurrence of Micula prinsii in this formation. Then, at the end of the Last Maastrichtian, with the drop of sea level, the deposition of this formation in this oil field was completed, which followed by the Pabdeh Formation with a sedimentary gap on the Gurpi Formation.