Hydrocarbon migration in the Niger Delta: Evidence from natural gas occurrence

摘要

The Niger delta is a major hydrocarbon province with estimated reserves of about 25 billion barrels of oil and 124 trillion cubic feet of gas. Although a lot has been published about the source rocks, the migration history of the petroleum is still largely unknown. In the present study we have investigated hydrocarbon migration by studying the stable carbon isotopic composition of eighteen natural gas samples from depths ranging from 1640-4153m in 10 fields located across four depositional belts (depobelts). From the results, the maturity of gas was estimated to range from 0.68 to 1.45 % R_o, indicating that they were largely formed within the catagenesis zone, that is, in association with oil. The sediments adjacent to the respective gas reservoirs were found to be mostly thermally immature (R_o%, 0.34-0.53) hence ruling them out as the gas source rocks. The estimated depths of the presumed source beds averaged 5625 m inferring average vertical migration distances of nearly 3km. Such long distance migration is likely to have been via the growth faults.