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Range of Stratigraphic Trap Architecture in the Basin-floor Fans, Karoo Basin, South Africa

Hodgson, David1 and Flint, Stephen
1[email protected]

Stratigraphic traps occur due to changes in rock character at the pinchouts of sandbodies. Detailed mapping of basin-floor fans in the SW Karoo Basin has documented a range of stratal termination styles, including up-dip, down-dip, and lateral sandstone pinchouts. These are analogous to stratal pinchouts identified on seismic data, but the sub-seismic analysis of the sedimentology and depositional architecture indicates that not all configurations are suitable stratigraphic trap targets.

Up-dip terminations occur where the physical passage from the erosional submarine slope to the basin-floor results in a complicated stratigraphic record of process change from confined channels to unconfined lobes. Predicting the degree of up-dip sand connectivity is critical when assessing stratigraphic trap potential in this transition zone. In a sand-detached system, such as Units E and F in the Laingsburg depocentre, a widespread area dominated by erosional processes and coarse sediment bypass can lead to the formation of an up-dip stratigraphic trap. The facies associations preserved in these sand-poor areas are thin -bedded, but commonly, their top surface includes megaflutes, rip-up clast lags, and soft-sediment deformation, indicating a high energy erosional bypass environment. In reflection seismic datasets a physical connection might be imaged, although the degree of reservoir connectivity, and the stratigraphic trap potential, will be less clear. Oblique up-dip pinchouts from axial to off-axis settings, such as Fan 3 in the Tanqua depocentre, fine- and thin- abruptly, and include minor soft-sediment deformation features. Lateral and down-dip stratal terminations are commonly sand-rich, particularly in mature systems. However, these settings commonly contain large numbers of argillaceous hybrid beds and low porosity sandstones with complicated geometries that increase the heterogeneity of pinchout.

Although up-dip and proximal pinchouts are the best targets as stratigraphic traps in terms of sandstone quality, there will be uncertainty in terms of their 3D connectivity with sandstones, and these bodies could suffer from fluid leakage. Furthermore, stratal pinchouts in basin-floor settings without hybrid beds commonly suffer from widespread post-depositional clastic injection. This study indicates that sub-seismic analyses of a range of basin-floor pinchout styles are needed to help in the prediction of stratigraphic trap potential in the subsurface.

 

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013