Depositional environment, seismic stratigraphy, and Sr-isotope geochronology, Bangestan reservoir, Ahwaz oilfield, SW Iran

Abstract

The Bangestan reservoir, which occurs in the Ahwaz oilfield, consists of the middle Cretaceous limestone Ilam and Sarvak Formations that were deposited in the Zagros Basin. The reservoir is divided into ten Zones (A to J) formed in the upper Albian-Santonian and contains considerable hydrocarbon accumulations. The limestones were deposited on an extensive shallow carbonate platform on a passive margin and are dominated by rudist biostrome and grainstone facies. Paleogeographical changes mean that identification of the facies is complex. Seismic stratigraphy and isotopic data are used to better understand the structural and geological setting and develop an understanding of the sedimentary environment. The results show that the rudist biostrome facies extends to a shallow interior shelf as a patch reef, indicating a regressive trend. Petrographic studies on 3174 thin sections identified twelve microfacies in the Bangestan reservoir. In the Sarvak Formation, the dominant microfacies consist of rudist biostrome, shoal bar, and lagoon facies, with a high sedimentation rate in the basin, whereas the Ilam Formation involves open and deep-marine facies. Seismic tectono-stratigraphy shows that there are seven sedimentary sequences with mostly regressive cycles and six maximum flooding surfaces from the upper Albian to the Santonian. In addition, the Ahwaz oilfield had experienced pre-tectonic extension before the Early Cretaceous and post-tectonic extension after the Late Cretaceous. The isotopic data from the Bangestan reservoir demonstrate two important disconformities (Cenomanian-Turonian and post-Turonian) that had an important influence on the evolution of the area evolution. Sr geochronology confirms long-term exposure to two main disconformities within the Ahwaz Bangestan reservoir. • C/O geochemical data show that Ilam Formation experienced fewer diagenesis event. • Ilam Formation represents open- and deep-marine facies. • Sarvak consists of rudist biostrome, shoal, back shoal, lagoon and tidal flat facies. • 7 sedimentary sequences with regressive cycles and 6 MFS from Albian to Santonin. • Bangestan reservoir involves two important disconformities of Cenomanian-Turonian.