--> Abstract: Geochemical Characterization of the Hydrocarbons in the Petroleum System of the Southeastern Niger Delta, by M. A. Rooney, T. Matava, B. Nwankwo, H. M. Chung, and D. O. Lambert Aikhionbare; #90933 (1998).

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Abstract: Geochemical Characterization of the Hydrocarbons in the Petroleum System of the Southeastern Niger Delta

Rooney, M. A. and T Matava - Mobil Technology Company; B. Nwankwo - Mobil Producing Nigeria Ltd.; H. M. Chung - Mobil Technology Company; D. O. Lambert Aikhionbare - Mobil Producing Nigeria Ltd.

A study of hydrocarbons from producing fields in the Joint Venture (JV) Acreage of Mobil and the Nigerian National Petroleum Company (OML 67 and 70) has helped understand selected elements of the petroleum system in this region of the Niger Delta. The petroleum system present in the shallow, normally pressured sections is characterized by current charge, with dominantly vertical migration of hydrocarbons generated in the early-to mid-oil window. The major controls on reservoir gas/oil ratios and oil quality are source type, biodegradation, PT conditions, and seal competence. Results of this regional study are being used to help predict key elements of the petroleum system in deeper overpressured sections, leading to definition of new oil and gas opportunities.

Geochemical analyses of oils from fields distributed throughout the delta indicate that most of the hydrocarbons are generated from Tertiary marine deltaic source rocks containing Type II/III kerogen, varying in the amount of terrigeneous organic matter deposited and preserved across the delta. After source type, which is fairly invariant, oil quality is most affected by biodegradation and migration-induced compositional fractionation. Across Mobil's JV acreage, the geochemical differences attributable to source variation in oils occur as a mappable trend. Oils in the north/northwest part of the acreage, in fields such as Abang and Oso, are derived from a source with higher terrestrial organic carbon input than those oils found in the rest of the JV acreage. These oils are also found in stratigraphically older sediments, yet are similar in thermal maturity levels.

Regional gas geochemistry revealed that the majority of the samples analyzed showed similar source type and thermal maturity to the associated oils. The most thermally mature gases were found only in the northwestern portion of the Delta, whereas lower maturity gases were found in both the central and eastern parts of the Delta. In Mobil's JV acreage, the gases are likely generated from the same source as the oil, in the early to peak oil generation phase.

Physical analysis of the oils in the JV acreage revealed that the hydrocarbons are mostly bubble-point oils in which the bubble point is equal to the pore fluid pressure in the shallow section. Additionally, RFT pore pressure measurements indicate that the same pore pressure gradient occurs in both the hydrocarbon-bearing reservoirs and in the wet sands. Together with fault plane map analysis, these data show that in the shallow section of the JV acreage, many of the reservoirs are leaking traps, with the hydrocarbon accumulations a result of a charge rate that exceeds the leakage rate from the reservoir. These results, coupled with the geochemical data, allow the composition and physical properties of oil accumulations in the JV acreage to be predicted with depth.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil