Understanding how clastic injectites form is important, as they are increasingly being recognised as significant components of sedimentary basin-fills, but are not predicted by standard sedimentary facies models. This study focuses on exhumed examples of injectites from the Karoo Basin, South Africa, and utilises a multidisciplinary approach to investigate clastic injectites across a variety of scales.
Small-scale analysis of injectites allows a classification of fracture patterns preserved on sill and dyke margins. These are used to interpret propagation direction through brittle, fine grained sediments under a laminar flow regime at depth in closed fracture networks. In contrast, shallow injectites, where they do not extrude, are identified by; fewer dykes, less stratigraphy crosscut, lower volume of injected material, and in some cases burrows on injectite margins—suggesting exploitation of injectite networks close to the surface.
These insights are applied to larger-scale (100s m to km) analysis, where extensive outcrop and well constrained paleogeography permits the injectite geometry to be related to parent sandstone facies and architecture. The influence of fluid flow pre-, syn- and post-injection is investigated across multiple scales. A model for the predictive distribution of injectites is proposed, which highlights the close association of basin-floor stratigraphic traps and sub-seismic clastic injectites.The outcrop data permits construction of forward seismic models demonstrate injectite architecture is scale invariant, which supports the use of outcrop-scale data in seismic-scale interpretations.
The integration of outcrop panels, well log data, forward seismic models and subsurface seismic sections has aided the identification of injectites in the subsurface and therefore the ability to discriminate between clastic injectites and parent sandbodies. The increased predictability in the location and character of injectites allows subsurface uncertainty in the impact of clastic injectites on hydrocarbon reservoirs to be reduced.
