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AAPG Bulletin
Abstract
AAPG Bulletin, V.
Evolution of deep-water Tertiary sinuous channels offshore Angola (west Africa) and implications for reservoir architecture
V. Kolla,1 Ph. Bourges,2 J.-M. Urruty,3 P. Safa4
16907 LaPuente Drive, Houston, Texas, 77083; email: [email protected]
2Total Fina Elf Exploration and Production, Luanda, Angola
3Total Fina Elf Exploration and Production, CSTJF, Pau, 64000, France
4Total Fina Elf Exploration and Production, CSTJF, Pau, 64000, France
AUTHORS
V. Kolla is now a consultant in Houston. He received a D.Sc. degree from Andhra University, India, and worked as a research associate, scientist, and senior scientist at the University of Washington (1968-1970) and Lamont-Doherty Earth Observatory (1970-1981). He also worked as a stratigrapher at Superior Oil Company (1982-1985), senior staff geologist and seismic-sequence stratigrapher at Elf Exploration, USA (1985-1991), and at Elf Exploration and Production Technology Center, France (1992-1997), and seismic-sequence stratigraphy advisor at Elf Exploration Angola, Angola (1998-1999). He has extensive experience in exploration-production geology in a variety of sedimentary basins--Gulf of Mexico, North Sea, offshore west and east Africa, Indian continental margins, and others. He has published extensively on sequence stratigraphy and sedimentology of especially deep-water depositional systems.
Philippe Bourges received a Ph.D. in sedimentology from the University of Toulouse (France). After working in uranium prospecting for a short time, he joined Elf, where he has been working for the past 12 years in hydrocarbon exploration. At Elf he was involved in sedimentology and seismic-stratigraphy specialty projects, mainly on clastic reservoirs, for five years. He later worked for four years in exploration in several French sedimentary basins. He joined Elf Exploration Angola in 1997 and is currently involved in Block 17/Block 33 exploration as area manager in Luanda.
Jean-Marc Urruty is currently a seismic stratigrapher at the Total Fina Elf Technology Center in France. He worked for Elf Exploration Angola for five years on the exploration of turbidite deposits off Angola. Before this, he worked on several seismic stratigraphy projects at the Elf Technology Center, providing technical support for Elf's subsidiaries.
Philippe Safa holds a Ph.D. from the University of Paris, France. He worked as a mining geologist for Elf during 1980-1986. Since 1987, he has worked in oil exploration as a geologist, senior geologist, team leader, and seismic stratigrapher at Elf's subsidiaries and at Elf Technology Center in France. During this time, he worked on the Tertiary deposits of the North Sea (United Kingdom, Norway) and off west Africa (off Gabon, Congo, and Angola).
ACKNOWLEDGMENTS
We thank Gary Parker for many discussions on hydrodynamic concepts relating to evolution of sinuous channels. Ray Martin, Carlos Pirmez, John Damuth, Patrice Imbert, the AAPG reviewers, Paul Weimer, Jory Pacht, and AAPG Editor Neil Hurley offered many useful and constructive comments that helped to improve the manuscript. Rob Merritt helped to improve the quality of figures electronically. We thank Elf Exploration Angola, Esso, BP-Amoco, Statoil, Norsk Hydro, and Total Fina for permission to publish this article.
ABSTRACT
Simple to complex, highly sinuous deep-water channels are common in the subsurface Tertiary off Angola, west Africa, and have been important exploration targets there in recent years. We discuss in this article three examples of these sinuous channels that are generally characterized by high-amplitude seismic reflection events, resulting from reservoir-prone lithologies. The planform characteristics of these channels--sinuosities and sinuous loop wavelengths--are similar to those of the fluvial channels; however, the planform combined with sectional seismic characteristics of the deep-water channels suggest that high sinuosities generally evolved through repeated channel aggradation and lateral migration. In detail, the influence of lateral migration vs. vertical aggradation in the evolution of the deep-water channel sinuosities varies from one channel to another and even along the length of a single channel or across a single sinuous loop. The lateral migrations may be continuous or discrete, separated in distance, and resolvable in seismic. Along some parts of the channels, very high initial sinuosities, not significantly affected by lateral migration, suggest that the initial sea-floor topography and gradients were a major factor in their development. In such cases, the channels essentially aggraded vertically. Thus, the modes of deep-water channel migration and sinuosity evolution are complex and different from those of fluvial channels that exhibit largely lateral (without much aggradational component) shifts within a meander loop. These differences are the result of unique hydrodynamic characteristics of the currents in these systems. The gross reservoir shapes in both the fluvial and deep-water channels are obviously dictated by their high sinuosities. The degree of lateral migration vs. vertical aggradation, however, determines the details of the lateral extent vs. vertical stacking of reservoir lithologies and their connectivities in deep-water sinuous channels. A fluvial, (Begin page 1374) point-bar-type continuity may be present in a part of a deep-water sinuous channel loop but is not expected to extend all across it.
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