With integrated data from core，well logging and laboratory analysis，the geological characteristics and main controlling factors of a giant marine delta of the Upper Kirkuk reservoir in the Halfaya Oilfield were studied. The results demonstrate that the Upper Kirkuk reservoir was developed under the background of giant marine delta，which underwent a complete cycle of sea level rising first and then declining from bottom to top. Near stretched distributary channels evolved into banded ones during the period of the maximum flooding surface（MFS），and finally into a stretch of distributary channels with width of 5-15 km as the sea level declined. Besides，the lithology of distributary channels is dominated by pure sandstone，which forms the best reservoir in the Upper Kirkuk reservoir. The overbank and sand sheets are thin，with limited lateral continuity，and particles are filled with debris and clay，indicating a poor storage capacity. The mudstone in the front delta and the silty mudstone in the interdistributary bay are mostly non-reservoirs，serving as the main barriers. Moreover，the distributary channels controlled by sedimentation are the main reservoirs distributing continuously in the Upper Kirkuk reservoir，and a stable mudstone barrier was developed during the MFS. The top and bottom are prone to dolomitization due to the contact with the near carbonate rock formation，leading to the stable barriers in the reservoir. Sedimentation and diagenesis jointly determine the layered structure in the Upper Kirkuk reservoir.