Deepwater Sequence Stratigraphy and Reservoir Prediction
By
Stephen S. Flint1, Peter Sixsmith1, Martin Grecula1, Graham Potts1, Stephen Johnson2
(1) University of Liverpool, Liverpool, United Kingdom (2) Statoil, Forus, Stavanger, Norway
The early Karoo basin of southwestern South Africa was segmented into the
Tanqua and Laingsburg sub-basins through the growth of antiform/synform pairs
oblique to the dominant shortening direction in the bounding Cape Fold Belt.
These structures grew episodically during deposition in the Laingsburg area but
did not affect the Tanqua sub-basin. Detailed sedimentological analysis and
regional 3-D mapping of basin floor and slope turbidite complexes from both
sub-basins has allowed development of a high resolution sequence stratigraphic
model with modifications to account for the influence of episodically growing
basin floor topography in the Laingsburg sub-basin. Individual basin floor fans
(50-300 m thick) can be divided into high frequency sequences that include sandy
growth phases (lowstand systems
tracts
) separated by fan-wide zones of reduced
sand deposition (transgressive and highstand
systems
tracts
). In both
sub-basins, these high frequency sequences form zones for reservoir modelling
and stack in progradational, aggradational and retrogradational styles, which
controls vertical and horizontal effective permeability. The main effect of
punctuated growth folding on the Laingsburg basin floor fans was to elongate fan
geometries parallel to the fold axes and to concentrate sand-rich flow fractions
in syntectonic lows, resulting in starvation on and beyond highs. Saddles in the
antiforms allowed cross-structure sand transport. The main effect of growth
folding on slope
systems
is episodic cycling (in space and time) between ponded
accommodation and bypass, resulting in complex facies patterns, net:gross trends
and connectivity.