John M. Guthrie¹, Melodye A. Rooney², H. Moses Chung², George Unomah³
(1) Mobil Exploration and Producing Technical Center, Dallas, TX
(2) Mobil Exploration and Producing Technical Center
(3) Mobil Producing Nigeria, Lagos, Nigeria

Abstract: The Effects of Biodegradation versus Recharging on the Composition of Oils, Offshore Nigeria

Oils from eastern offshore Niger Delta were studied in detail to determine whether variations in composition can be attributed to recharging (separation/migration processes) of oil with gas condensate or to biodegradation. The ability to distinguish the effects of separation/migration versus biodegradation on the composition of oils trapped in shallow reservoirs could be useful for assessing the extent of vertical migration from source to trap and for predicting the accumulation of oil versus gas in deeper traps.

Oil composition ranges from unaltered (full range of n-alkanes from n-C₅ to n-C₃₀ present) to bimodal (loss of n-alkanes in the range of n-C₈ to n-C₁₅) to biodegraded (removal of n-alkanes). The recognition of the effects of biodegradation versus recharging on the composition of the oils can be deciphered by correlating the ¹³C of n-C₅ (pentane) and n-C₆ (hexane) with several established biodegradation parameters. Oils that have been biodegraded show carbon isotopic fractionations of 3 to 4‰ for n-C₆ and n-C₅, respectively.

In the Ekpe WW field, geochemical results have been integrated with geology and engineering data to demonstrate that light hydrocarbons have migrated from deep to shallow reservoirs along major faults. Oils in the shallow reservoirs with the highest water-driven pressures show the greatest degree of biodegradation, presumably due to a greater influx of meteoric waters.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana