Hydrocarbon Charging Period of Cretaceous Reservoirs in AHDEB Oil Field: Evidence from Fluid Inclusion

The uncertainty of pool-forming rule of AHDEB oilfield in Iraq has influenced the oilfiled development. Combining with burial history, the study of fluid inclusion is effective to reveal the hydrocarbon charging period and time. The samples collected from Cretaceous reservoirs of AHDEB oil field in Iraq were used to study the characteristics of fluid inclusion. Based on the observation of thin sections and the fluorescence color, measurement of μFT-IR and homogenization temperature, combining with analysis of geologic setting, the hydrocarbon charging periods were researched. The results show that the fluorescence colors of oil inclusion were mainly green and yellow-green, while minor light yellow and yellowish-brown. It is found by the measurement of μFT-IR of oil inclusion that there are 2 types of maturity based on the value of CH_2a/CH_3a, X_inc and X_std. The data of burial history, thermal evolution history and the homogenization temperature indicate that there might be four terms of oil accumulation. The first term of oil charging occurred at 95.0-96.5Ma, equivalent to early Late-Cretaceous, and the second term occurred at 71.0-78.5Ma, equivalent to late Late-Cretaceous, which were consistent with the small scale of hydrocarbon generation by producing heavy oil that in turn was oxidized to asphalt. The third term of oil charging occurred at 14.0-15.0Ma, equivalently Middle Miocene, which was the main accumulation, being consistent with the peak of hydrocarbon generation. The forth term occurred at 10.0Ma, which was the charging event related to the third term.