Facies architecture and stratigraphic sequence of Zubair Formation in Majnoon and Suba oil fields, Southern Iraq

Abstract

The Zubair Formation was deposited during the Barremain stage within the Early Cretaceous epoch. This study includes the lithofacies analysis and stratigraphic development of the Zubair Formation in four boreholes (Mj-2, Mj-4, Su-2 and Su-7) within the Majnoon and Suba oil fields. This succession is divided into three lithofacies units using well logs such as GR, SP, DT and RT. The upper member is composed mostly of shale layers, the middle member consists of thick layers of sandstone and the lower member consists mainly of layers of shale with less sandy layers. Five major lithofacies are recognized in this succession as follows: quartz arenite well-sorted subrounded lithofacies, quartz arenite poorly sorted sub-angular lithofacies, graywacke poorly sorted lithofacies, sandy claystone and shale lithofacies. Three facies associations (depositional environments) are distinguished in Zubair Formation: delta plain, delta front and backshore. The microfacies analysis and reconstructed paleoenvironments for the Barremain basin in the studied area have contributed to determine two stages of the deposition: the first stage showed clear when the facies was developed from the delta plain to the delta front association facies within the sand-dominated unit of Zubair Formation (lower and middle units). This succession is represented by three depositional cycles of transgression system tracts, which was ended with appeared the shale lithofacies within the well-sorted quartz arenite lithofacies to mark the maximum flooding surface (mfs) in the Suba oil field. While in the Majnoon oil field, these three cycles were represented by alternating delta plain and delta front association with dominated shale lithofacies. The deposition of this part of Zubair Formation was continued during still stand of the sea level to develop at the last time of this stage to delta front environment to mark a start a second stage. The second stage started with the appearance of the backshore association facies as a high-stand system tract (HST) within the upper unit. This pattern of deposition (HST) continued until a change in facies associations from delta front to a backshore in all studied wells. The clear change in the lithofacies occurred from the sand-dominated (delta front association) with high maturity fine-grained sandstone to shale-dominated lithofacies (back-shore) that onsets to still sea level. Therefore, the upper part of the upper unit is represents the HST, and then the Mauddud Formation was formed.