An integrated model of clastic injectites and basin floor lobe complexes: implications for stratigraphic trap plays

Abstract

Injectites 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.

Quantification of Basin-Floor Fan Pinchouts: Examples From the Karoo Basin, South Africa

“…For this study, 39 sedimentological logs were collected around the Klein Hangklip and Gemsbok valley areas (Figure 2). The Gemsbok valley area logs compliment those of Prélat et al (2009), whereas the Klein Hangklip area logs for Fans 3 and 4 are new to this study, although some of the area overlaps with Cobain et al (2017) who focussed on injectites below Fan 3. Sedimentary logs from Fans 3 and 4 from five cored research boreholes (OR1, NS4, NB2, GBE01, NS2) (Hodgson, 2009;Kane et al, 2017) were utilized.…”

“…Individual lobes are difficult to distinguish in the oblique updip pinchout due to amalgamation and erosion by the sand-rich distributary channel-fills in the thickest part of the Fan 3 lobe complex. Near the area of most abrupt thinning a number of clastic injectites are observed below Fan 3, interpreted to originate from Fan 3 and follow the pinchout (Figures 8, 9) (Cobain et al, 2017).…”

“…Planar lamination is a product of traction (Best and Bridge, 1992). Structureless beds are formed by direct suspension fallout (Bouma, 1962) Lobe distal fringe environments Occurs when: (i) pore pressure in the parent sand body is higher than that within the mud-prone host strata, and (ii) the parent body is composed of clean, fine to very fine unconsolidated sand that is most susceptible to fluidization and grain transport (Richardson, 1971;Jolly and Lonergan, 2002;Cobain et al, 2017). These can be horizontal (sills) or (sub-) vertical (dike) injections.…”

“…It is recognized that submarine lobes exhibit diverse geometries, stacking patterns and facies distributions (Twichell et al, 1992;Bouma and Rozman, 2000;Hodgson et al, 2006;Deptuck et al, 2008;Prélat et al, 2009;Groenenberg et al, 2010), including the common occurrence of hybrid beds at lobe fringes (Haughton et al, 2009;Hodgson, 2009;Patacci et al, 2014;Porten et al, 2016;Kane et al, 2017;Pierce et al, 2018). The pinchout of basin-floor sandbodies can be used to better constrain the scale and orientation of seabed topography, and the paleogeographic configuration of basins at the time of deposition (Smith and Joseph, 2004;Bakke et al, 2013;Cobain et al, 2017;Spychala et al, 2017a,b). Interest in the architecture and facies distribution at basin-floor fan fringes, and the characteristics of fan pinchouts, have been driven by a need to improve prediction of pinchouts as stratigraphic trap targets for hydrocarbon reservoirs (Pickering, 1981;Bouma and Rozman, 2000;Etienne et al, 2012;Bakke et al, 2013;Marini et al, 2015;Nagatomo and Archer, 2015;Spychala et al, 2017b,c).…”

Disconnected submarine lobes as a record of stepped slope evolution over multiple sea-level cycles

“…Sand-rich pinch-outs are often associated with clastic injectite networks, up to several meters in thickness and kilometers in lateral extent (Cobain et al, 2017).…”