Facies distribution, paleoenvironment and sequence stratigraphy model of the OligoMiocene Asmari Formation (Fars Province, south of Iran)

Abstract

The Oligocene (Rupelian-Chattian) to Miocene (Aquitanian)-aged Asmari Formation is considered one of the most significantpetroliferous reservoirs in the Middle East. The Oligocene–Miocene Asmari Formation represents diverse facies patterns in differentparts of Zagros, so analysis of its facies in different provinces of the Zagros Basin is necessary. In this study, the sedimentary depositionalsequences of Asmari Formation in the Firuzabad (south of Shiraz) and Bastak (west of Bandar-Abbas) sections in the Zagros Basin (FrasPlatform) have been analyzed for distinguishing facies characteristics, interpreting the paleoenvironment and determining dominantdepositional sequences. Based on field investigations and laboratory studies, 13 microfacies have been determined; these are organizedin 3 facies associations containing outer ramp (open marine), mid ramp, and inner ramp (shoal, lagoon, and tidal flat). Sedimentarycharacteristics, arrangement, and comparison of horizontal and vertical facies indicated that the successions of the Asmari Formationsediments were deposited in a homoclinal ramp-like platform with a gentle slope and the deeper part of the basin located in theFiruzabad section. Analysis of detailed facies frequency showed that the outer ramp is the most frequent facies association in theFiruzabad and the inner ramp (lagoon subenvironment) is the most frequent facies association in the Bastak. Sea level variations duringOligo-Miocene time resulted in creation of two third-order depositional sequences in both sections. The depositional sequences consistof transgressive systems tract and highstand systems tract. In all the identified sequences, transgressive systems tract (TST) demonstrateretrogradational stacking pattern with deepening-upward trend while highstand systems tract (HST) indicate progradational stackingpattern with shallowing-upward trend.