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AAPG Bulletin

Abstract

AAPG Bulletin, V. 95, No. 6 (June 2011), P. 1039-1065.

Copyright copy2011. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/12131010046

Hydrocarbon plumbing systems of salt minibasins offshore Angola revealed by three-dimensional seismic analysis

Katrine J. Andresen,1 Mads Huuse,2 Niels H. Schodt,3 Lene F. Clausen,4 Lars Seidler5

1Department of Earth Sciences, Aarhus University, Hoegh-Guldbergs Gade 2, 8000 Arhus C, Denmark; [email protected]
2School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Williamson Building, Oxford Road, Manchester, M13 9PL, England; [email protected]
3Exploration Department, Maersk Oil, Esplanaden 50, 1263 Copenhagen K, Denmark; [email protected]
4Exploration Department, Maersk Oil, Esplanaden 50, 1263 Copenhagen K, Denmark; [email protected]
5Maersk Oil Houston Inc., 2500 City West Boulevard, Suite 2500, Houston, Texas 77042; [email protected]

ABSTRACT

Analysis of three-dimensional seismic data from the lower Congo Basin, offshore Angola, reveals numerous fluid-flow features in the Miocene to Holocene succession and the potential for large, shielded traps underneath basinward overhanging salt structures. The fluid-flow evidence includes present-day sea floor pockmarks clustered above salt structures, Pliocene–Pleistocene stacked paleopockmarks and Miocene pockmark fields. Other fluid-flow features include high-amplitude cylindrical pipe structures 60 to 300 m (197–984 ft) wide and 25 to 300 m (82–984 ft) high within lower and middle Miocene strata, thick (lt150 m [492 ft]) high-reflectivity zones within the Pliocene succession associated with bottom-simulating reflections, and subvertical low-amplitude chimneys originating from the deeper section (gt1 km [0.6 mi] beneath the sea floor). The Miocene pockmark fields occur at a specific horizon, suggesting a regional fluid expulsion event at ca. 12 Ma, and the Miocene fluid-flow regime is interpreted to be dominated by thermogenic fluids supplied via carrier beds and leaking vertically above structural highs. The Pliocene–Pleistocene fluid-flow regime was dominated by short-distance vertical fluid migration and expulsion related to early stage diagenetic processes involving biogenic methane and pore water. The present-day fluid-flow regime is inferred to be dominated by thermogenic fluids primarily controlled by kilometer-scale salt-flank-controlled migration.

The study emphasizes the use of seismically imaged fluid-flow features in hydrocarbon systems analysis by documenting the evolution of an overburden plumbing system through time, involving several fluid types and flow regimes, depending on the spatiotemporal availability of thermogenic and diagenetic fluids and the tectonostratigraphic occurrence of aquifers, traps, and seals.

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