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AAPG Bulletin

Abstract

AAPG Bulletin, V. 87, No. 7 (July 2003),

P. 1193-1206.

Copyright copy2003. The American Association of Petroleum Geologists. All rights reserved.

Migration effects on the composition of hydrocarbon accumulations in the OML 67–70 areas of the Niger Delta

Tim Matava,1 Melodye A. Rooney,2 H. Moses Chung,3 Basil C. Nwankwo,4 George I. Unomah5

119707 Ivory Brook, Houston, Texas; email: [email protected]
2Buenos Aires, Argentina
3Mobil Technology Company, Dallas, Texas
4P.O. Box 864302, Plano, Texas
5Mobil Producing Nigeria, Lagos, Nigeria

AUTHORS

Tim Matava is a geophysicist in Houston, Texas, working on research and applications in petroleum systems. He has a Ph.D. from the University of Alaska, Fairbanks in geophysics and an M.B.A. degree finance.

Melodye Rooney has a B.S. degree in chemistry from Indiana University (1983) and a Ph.D. in chemistry from the University of Wisconsin (1988). Melodye's experience as a research geochemist at Mobil EampP included modeling natural gas generation, thermochemical sulfate reduction, and predicting carbon dioxide levels for field development optimization. She currently is a consulting geochemist living in Buenos Aires, Argentina.

Moses Chung retired from Mobil as a geochemist with more than 20 years of industry experience. Moses is currently a hospital chaplain in Dallas and a deacon in the Roman Catholic Church.

Basil Nwankwo retired from Mobil Producing Nigeria, where he worked for more than 23 years. He was chief geologist and participated in many of the large hydrocarbon discoveries in Nigeria during his career. Basil received an M.Sc. degree in petroleum geology from University of London and is currently consulting on several international projects.

George Unomah is a geochemist in Lagos, Nigeria, where he works on exploration and producing geochemical problems. George has a strong background in field sampling and integrating data sets.

ACKNOWLEDGMENTS

K. Peters contributed his time to edit the text and provided suggestions that significantly improved the quality of this manuscript. K. Shields also read an early version of this manuscript and made valuable suggestions. J. Bond and S. Joiner kindly helped with portions of the paper pertaining to the stratigraphic and depositional models. D. Helber helped prepare many of the figures.

ABSTRACT

Systematic changes in gas-oil ratios in the OML 67–70 area (joint venture acreage) of the Niger Delta (Nigeria) illustrate the effects of hydrocarbon migration on the compositional changes observed in the accumulations over large depth intervals. One interpretation is that the reservoirs are currently both filling and leaking hydrocarbons, indicating that trap styles affect the migration pathways in this region of the Niger Delta. The accumulations are more liquid prone with depth, based on the bubble point of the fluids in the reservoir. Applying these results to regional exploration opportunities indicates that fluid contacts in the normally pressured section are controlled by leaking seals (class III traps). The potential exists for pressure-controlled filled-to-spill traps (class II traps) in the overpressured section, where a high-quality seal impedes the vertical flux of hydrocarbon fluids. Although these conclusions apply to a specific portion of the Niger Delta, combining the concept of trap style with compositional changes resulting from migration processes is useful in identifying new play concepts and opportunities in other regions of the world.

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