1. Sequence Stratigraphic Characterisation of Petroleum Reservoirs in the Deepwater Southern Outeniqua basin- South Africa
Emmanuel Nformi1, Paul Carey2
1&2University of the Western Cape, South Africa
Corresponding
8th Inkaba yeAfrica Workshop: 28 August - 2 September Geosynthesis 2011 (Cape Town)
Introduction
Deepwater depositional systems are the one type of reservoir system that cannot be easily reached, observed, and studied in the modern environment, in contrast to other systems. It requires many remote observation systems (Weimer and Slatt, 2004). Their study then requires multiple data types which include detailed outcrop studies, 2D and 3D seismic-reflection data, cores, log suites, and biostratigraphy.
If deepwater production was viewed as its own “country,” it would exceed that of every
other country except Saudi Arabia, Russia, and the United States (IHS CERA, 2010).
7th Inkaba Workshop, Potsdam (Germany):1-5th Nov 2010
New seismic data (of the SOB) indicates deepwater drilling might be the answer to South Africa's unquenchable thirst for oil (Roux et al, 2004).
Model of submarine-fan deposition, relating facies, fan morphology, and depositional environment. D.B indicates disorganized-bed conglomerates (After, Walker 1978)
Methodology
Identification of facies in time and space in the basin margin Pletmos Basin) to PREDICT their location in the deep SOB.
Brown et al., 2004 proposed a hypothesis which maintains that third-order, lowstand, growth-faulted subbasins develop seaward of the shelf break, where coarser grained sediments accumulate on unstable slope.
Facies were fitted into a model of submarine fan deposition (Walker, 1978) to predict their basin location.
Results
Ga-V1 displayed basin margin facies deposits mainly while Ga-S1 displayed basin plain facies.
Progradational sequences very much observed on the deepwater seismic lines- typical lowstand deposits.
Conclusion
Seismic analyses and interpretation as well as structural, core, log and biostratigraphic analyses and interpretation confirm that depositional rate especially along the slope was moderate to very fast suggesting great sediment-bypass into the deepwater Southern Outeniqua basin.
It also suggests to high degrees that the Southern Outeniqua Basin is a sand-rich system. Such systems tend to have broad, low-sinuosity channels that do not have well developed levees.
The SOB is seen here as an untested world class frontier basin having a viable and unexplored petroleum system.
Well Ga- S1
Sand mound shape gamma ray log signature at depths of between 2925 m to 3050m.Over 80m thick of clean sand layer. Sharp termination at the base and top.
Bell and crescent shaped gamma ray log signature observed at depths of between 2500m and
2750m- about 150m thick. Indicates fining upward sequence as channel is abandoned.
Stratigraphic surfaces showed a huge difference in depth, occurring at greater depth in Ga-V1 and Ga-S1. Tectonic subsidence or uplift most probably was the cause of this gap. Tectonic subsidence created a relative sea level change orchestrating the deposition of lowstand prograding sequences that were observed in Ga-V1 and Ga-S1 wells.
Acknowledgements to Inkaba YeAfrica for funding this research.