Biostratigraphy of the Shahbazan and Asmari formations in well no.3 of Qaleh Nar oil field, Lurestan

The Paleogene deposits in the Zagros have formed during two sedimentary cycles. The Shahbazan and Asmari formations formed during the Jahrum (Paleocene to Eocene) and Asmari cycles (Paleocene to Early Miocene), respectively (Aghanabati, 2006; Motiei, 1993). The type section of the Shahbazan Formation is located in the Shahbazan village (near Andimeshk-Dourod railroad) and consists of white and brown-weathering, thick beds of porous, dolomite and dolomitic limestone (Aghanabati, 2006; Motiei, 1993). The type section of the Asmari Formation is located in south of Masjed Soleyman and Haftkel (Aghanabati, 2006). Anhydrite deposits (Kalhur Member) and lithic and limy sandstones (Ahwaz Member) also occur within the Asmari Formation (Motiei, 1993). The Qaleh Nar oil field with geographical coordinate of N: 32° 43' 54" and E: 48° 17' 44" is located about 4 km east Andimeshk-Bidrobeh (Seraj, 2005; Bakhtiari, 2013).The thickness of the Shahbazan Formation in the studied well is 228 meters. This formation consists mainly of limestone and dolomite. The lower boundary of the Shahbazan Formation with the Pabdeh Formation is conformable and its upper boundary with Asmari Formation is marked by an unconformity. The Asmari Formation with thickness of 157 meters consists mainly of limestone and dolomite. It is located between the Shahbazan Formation below and the Gachsaran Formation above. In order to study the biostratigraphy of the Asmari and Shahbazan formations in the studied well, 707 thin sections were prepared and examined, from which 564 thin sections belong to the Shahbazan Formation and 143 thin sections belong to the Asmari Formation. Paleontological studies documented a high diversity of shallow-water benthic foraminifera and rare planktonic foraminifera at this section. The distributions of foraminifera in the study area is used to determine the age of the Shahbazan and Asmari formations. Three assemblage Zones were recognized in the Shahbazan Formation. Turborotalia cf. Boweri (Lutetian) and Guembelitrioides (Globigerina) cf. lozanoi (early Yapresian to early Lutetain) are present in assemblage Zone 1, so that this assemblage Zone is indicative of early Lutetian age (BouDagher-Fadel, 2012). Turborotalia cf. Increbecens (late Bartonian to middle Ruplian) and Morozovella are identified in assemblage Zone 2. The last appearance of Morozovella was recorded in late Bartonian (BouDagher-Fadel, 2012; Berggern and Pearson, 2005). Therefore, this assemblage suggests the Bartonian age. Assemblage Zone 3 consists of Nummulites fabianiiand Chapmaninasp. This assemblage represents Priabonian age. This assemblage Zone is time equivalent to Chapmanina-Pellatispira-Baculogypsinoides assemblage Zone of Wynd (1965) and two assemblage Zones SBZ 19 and SBZ 20 of Serra-kiel et al. (1998). Two assemblage Zones have been recognized in the Asmari Formation. Assemblage Zone 1 consists of Archaias cf. kirkukensis and Archaias cf. hensoni (middle Ruplian to late Chattian). All species of Nummulites genus were extinct in Ruplian-Chattian boundary (Laursen et al., 2009; van Buchem et al., 2010) so that this assemblage indicates a Rupplian-Chattian age. This assemblage Zone is equivalent to Archaias asmaricus-Archaias hensoni-Miogypsinoides complanatus assemblage Zone of Laursen et al. (2009) and van Bouchem et al. (2010). Assemblage Zone 2 of the Asmari Formation due to lack of index fossils corresponds to Indeterminate Zone of Laursen et al. (2009) and van Bouchem et al,( 2010) and indicates an Aqitanianage. Finally the Shahbazan and Asmari formations in the studied well are considered Middle Eocene (early Lutetian) to Late Eocene (Priabonian) and Middle Ruplian to Aqitanian age respectively