FACIES ANALYSIS AND STRATIGRAPHIC DEVELOPMENT OF THE NAHRUMR FORMATION IN LUHAIS OIL FIELD, SOUTHERN IRAQ

Abstract

The Clastic succession of NahrUamr Formations is deposited during the Albian stage within the Early Cretaceous epoch. The present study includes the facies analysis and stratigraphic development for the NahrUmr Formation in three boreholes (Lu -1, Lu-3, and Lu-12) within the Luhais oil field. Two types of successions are observed within the NahrUmr Formation; the first is the upper part which characterized by shale dominated rocks and the second (lower part) is characterized by sand dominated rocks. Five major lithofacies are recognized in this succession as follow:-well sorted quartz arenite lithofacies; poorly sorted quartz arenite lithofacies; poorly sorted graywacke lithofacies; sandy shale lithofacies and shale lithofacies. Four facies associations (depositional environments) are distinguished in the NahrUmr Formation, they are: fluvial channel, shoreface, delta plain, and distributary channels. The microfacies analysis and reconstructed the paleo environments for the Albian basin in the studied area has contributed to determine two stages of the deposition: -

The first stage is showed clear when the facies development from the fluvial channel to the shore face association facies within the sand dominated unit of NahrUmr Formation (lower unit). This succession is representing transgression system tracts (TST), which was ended with appeared the shale lithofacies within the well sorted quartz arenite facies to mark the maximum flooding surface (mfs). The deposition was continued during still stand sea level to develop at last to delta plain association and start the second stage.

The second stage is started with the delta plain in Lu-3 and Lu-12 but the distributary channel in the Lu-1 which represent high-stand system tract (HST) within the upper unit. This pattern of deposition (HST) continued until a change in depositional environment from delta plan to a distributary channel in all studied wells to became TST. The clear change in the lithofacies from the shale dominated (delta plain association) to sand dominated lithofacies to became high maturity fine grained sandstone (distributary channel), that onset to sea level rise. Therefore, the upper part of the upper unit is representing the TST, which it was continue to deposition of the Mauddud Formation during the sea progress.