Oil and gas accumulation in the Umm Al-Anbar Field

The detailed investigation of the vertical variation of the hydrocarbon property in the Umm Al-Anbar (AR) field reveals that the top of the oil reservoir is sealed by a 30 feet thick anhydrite bed, but the volatile oil reservoir which is distributed below the black oil reservoir is clearly separated by an 8 feet thick anhydrite bed. The geochemical analyses of the samples from the studied area suggest that this oil and the volatile oil have not been generated from different source rocks but from the same source rock. Moreover, the paleostructure of the bitumen zone in the AR field shows that the oil was accumulated before Halul time. Also the saturation pressure of volatile oil reservoir calculated from the experimental data indicates that the gas migration was completed in Rus time. These facts will imply that the oil and gas migrated vertically and individually, which means that the oil migrated prior to the gas migration, and that the distrbution of the hydrocarbons was controlled by different seal potentials of anhydrite beds and clayey lime-mudstone at each migration time in proportion to the progress of the diagenesis. On the basis of the above assumption, the oil and gas migration in the structure is well-explained as follows: Firstly, oil migrated to the Arab reservoir from the Diyab Formation and accumulated in the crest before Halul time, when only more than 30 feet thick anhydrite bed could be affected as seal bed for the oil. Secondly, the gas migrated to the oil reservoir and changed this oil into the volatile oil. However, this gas migration was limited by the 8 feet thick anhydrite bed and the oil reservoir above the 8 feet thick anhydrite bed remained as oil reservoir itself.