--> Abstract: Reservoir Development and Distribution of a Dolomitized Lacustrine Carbonate: The Toca Formation, Offshore Cabinda, Angola, by Phillip Bassant, Peter S. Brown, Rachel Preece, Martinho Fernandes, and Anthony J. Lomando; #90039 (2005)

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Reservoir Development and Distribution of a Dolomitized Lacustrine Carbonate: The Toca Formation, Offshore Cabinda, Angola

Phillip Bassant1, Peter S. Brown2, Rachel Preece1, Martinho Fernandes3, and Anthony J. Lomando4
1 ChevronTexaco Energy Technology Company, San Ramon, CA
2 ChevronTexaco, Houston, TX
3 ChevronTexaco, Luanda, Angola
4 ChevronTexaco, San Ramon, Kuwait

The pre-salt, Barremian age Toca carbonate reservoir is a vuggy, often extensively dolomitized, lacustrine carbonate. It is a prolific oil producer in Angola and in Brazil (where the equivalent is the Lagoa Feia Formation). Predicting reservoir quality and distribution has always been problematic due to rapid lateral thickness and facies variations, complex diagenesis and poor seismic imaging. Exploration for this elusive reservoir in the relatively mature Cabinda Basin has been challenging. Core data, non-marine ostracode assemblages, facies analysis, petrophysical analysis, well-log correlations, and seismic interpretation were integrated in this study to develop a sequence stratigraphic framework and diagenetic model which was then used to predict where the best reservoirs should occur.

Barremian lacustrine basin morphology consists of deep, NW-SE trending, elongate rift basins separated by tilted, fault-block highs. Three lacustrine carbonate-basin shale sequences (T1, T2, T3) are identified. Each sequence formed during a phase of minor fault-block adjustment followed by thermal subsidence. Algal-dominated carbonates composed of coated-grains (oncoids, some ooids) and peloids dominate the lower T1 and T2 sequences. Molluscan shell debris typically dominates the uppermost T3 carbonate sequence. Frequent subaerial exposure during deposition and at each sequence boundary created vuggy porosity in vadose zones. Subsequent dolomitization by hydrothermal fluids can be extensive and has a net corrosive effect close to the major faults. We conclude that the best reservoirs should occur in relatively narrow, NW-SE trending structural highs where T3 molluscan carbonates with good primary interparticle porosity have been subaerially exposed, leached, and later aggressively dolomitized in close proximity to major faults.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005