Abstract
AbstractInjectites sourced from base‐of‐slope and basin‐floor parent sandbodies are rarely reported in comparison to submarine slope channel systems. This study utilizes the well‐constrained palaeogeographic and stratigraphic context of three outcrop examples exposed in the Karoo Basin, South Africa, to examine the relationship between abrupt stratigraphic pinchouts in basin‐floor lobe complexes, and the presence, controls, and character of injectite architecture. Injectites in this palaeogeographic setting occur where there is: (i) sealing mudstone both above and below the parent sand to create initial overpressure; (ii) an abrupt pinchout of a basin‐floor lobe complex through steep confinement to promote compaction drive; (iii) clean, proximal sand beds aiding fluidization; and (iv) a sharp contact between parent sand and host lithology generating a source point for hydraulic fracture and resultant injection of sand. In all outcrop cases, dykes are orientated perpendicular to palaeoslope, and the injected sand propagated laterally beneath the parent sand, paralleling the base to extend beyond its pinchout. Understanding the mechanisms that determine and drive injection is important in improving the prediction of the location and character of clastic injectites in the subsurface. Here, we highlight the close association of basin‐floor stratigraphic traps and sub‐seismic clastic injectites, and present a model to explain the presence and morphology of injectites in these locations.
Highlights
Improvements in subsurface imaging quality in recent years have led to increased recognition and understanding of the impact of injectites on the architecture and fluid flow of sedimentary basin-fills
The literature is dominated by examples of clastic injectites that are associated with primary deposits on a slope setting, such as deep marine channelfills (Hiscott, 1979; Dixon et al, 1995; Rowe et al, 2002; Parize & Fries, 2003; Duranti & Hurst, 2004; Huuse et al, 2005; Diggs, 2007; Duranti, 2007; Frey-Martınez et al, 2007; Hamberg et al, 2007; Jackson, 2007; Jonk et al, 2007; Surlyk et al, 2007; Vigorito et al, 2008; Kane, 2010; Svendsen et al, 2010; Szarawarska et al, 2010; Jackson et al, 2011; Løseth et al, 2013; Morton et al, 2014; Bain & Hubbard, 2016) and intraslope lobes (Monnier et al, 2014; Yang & Kim, 2014; Spychala et al, 2015)
We have presented three examples of basin-floor lobe complex pinchouts that have been subject to post-depositional fluidization of the parent sandbody and clastic injection into the surrounding mudstone
Summary
Improvements in subsurface imaging quality in recent years have led to increased recognition and understanding of the impact of injectites on the architecture and fluid flow of sedimentary basin-fills. Lithology is the principle control on basin wide fluid migration (Bjørlykke, 1993; Jonk et al, 2005a), and in the absence of clastic injectites fractures and faults form the most efficient conduits for fluid flow (Chapman, 1987; Knipe et al, 1998; Aydin, 2000). Net migration of fluids, including water and hydrocarbons, into an unconsolidated sandbody can provide the overpressure and trigger mechanism needed for sands to fluidize and inject (Vigorito & Hurst, 2010; Bureau et al, 2014). Post-injection, sandstone dykes and sills can act as fluid flow conduits for hydrocarbon leakage (Jonk, 2010) until cementation, at which point injectites become fluid flow baffles and barriers. Reactivation of clastic injectites as fluid flow conduits can occur
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