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Clastic Injection at Deep-Water Stratigraphic Traps


The abrupt pinchout of a well-connected deep-water sandbody with reservoir quality sand that is sealed by mud can form an ideal stratigraphic trap. However, this situation is also susceptible to becoming overpressured and a prime location for clastic injection prior to hydrocarbon migration. Here, the relationship of stratigraphic traps and the presence and character of injectites in terminal lobe complexes and base-of-slope fan settings is assessed through analysis of several examples exposed in the Karoo Basin, South Africa. The palaeogeographic and stratigraphic context of the injectite complexes, which can be studied in three dimensions and over large distances, is very well constrained permitting their relationship to different styles of sand-prone pinchouts to be assessed. Three detailed studies demonstrate that the particular style and geometry of clastic injection is similar where there is an abrupt pinchout of relatively clean sands in a lobe complex. Several near vertical dykes extend from the erosive base of the parent body and feed a main sheet or sill, which steps upwards and outwards, following the thinning direction of the parent sand, with the overall propagation direction being horizontal rather than vertical as is often assumed with vertical dyke emplacement. In cross-section the geometries observed are comparable to those imaged on seismic sections of known injected units in the subsurface, including cone-shaped sheet injections and large scale steps. The extent and detail afforded by this outcrop study means that links between injectite and parent sand architecture and context across core, outcrop and seismic scales can be made, and that the presence and character of injectites at deep-water stratigraphic traps can become more predictive.