oa Sedimentary environments and sequence stratigraphy of the Asmari Formation, northwest of Dezful Embayment, SW Iran

The Oligocene-Miocene Asmari Formation is the most productive reservoir in the folded belt of the Zagros Basin, in southwestern Iran. In the study area, it is of Early Miocene age, and composed mainly of limestone, dolomite, sandstone, anhydrite, siltstone and shale. Its lower contact with the Pabdeh Formation (Paleocene- Oligocene) is conformable and its upper contact with the Gachsaran Formation (Middle Miocene) is unconformable. Detailed petrographic and well-log analysis of the formation led to the recognition of five facies belts, which were deposited in a distally steepend ramp platform. Two third-order depositional sequences were recognized. The sequence boundaries of the older AS1 sequence are type one (SB1). The maximum flooding surface (MFS) of AS1 is near the top of Pabdeh Formation and its highstand systems tract (HST) occurs in the Asmari Formation. The younger AS2 sequence has a lowstand systems tract (LST) that can be recognized by deep basinal evaporites of the Kalhur Member and sandstones of Ahwaz Member. The TST is thin, whereas the HST is thick with permeable facies. The sequence boundaries of AS2 are type one (SB1). The lowstand systems tract of the AS2 sequence formed during the fall of sea-level in Early Miocene. In the deep narrow paleo-seaway of the study area, deep basinal evaporites of the Kalhur Member were deposited. Interfingering of pelagic facies with the evaporites and the presence of thick salt is the best criteria for deep-water evaporites. The carbonates, situated updip of these evaporites, may be the most productive units in the Asmari Formation. Their productivity may be due to the preferential distribution of reservoir facies along the break of ramp and the spatial relationship of dolomitization with evaporites. Accordingly stratigraphic traps could be developed with the best reservoir units situated in the HST.