Aqueous and petroleum fluid flow associated with sand injectites

Field, petrographic and fluid inclusion characteristics of sand injectites from five outcrop localities and from the subsurface of the Tertiary of the south Viking Graben are described. Although the case studies are from a wide variety of sedimentological, stratigraphic and tectonic settings, and hence their diagenetic evolutions differ significantly, it is possible and useful to assign diagenetic events to three distinct phases of fluid flow associated with sand injectites in sedimentary basins. Firstly, there is fluid flow associated with the injection of the fluid-sediment mix during shallow burial. Early diagenetic imprints in sand injectites reveal that basinal fluids, which may be released during movement along deeper-seated faults, can be associated with this process and thus the injection process may reveal information on the timing of basin-scale movement of fluids. Secondly, following the injection process, basinal fluids continue to migrate through uncemented injectites and mix with the ambient meteoric and/or marine pore fluids that invade injectites from the overlying and surrounding host sediments. Early, often pervasive, carbonate cementation is common within sand injectites and rapidly turns sand injectites into flow barriers during shallow (< 1 km) burial. If early carbonate cementation is not pervasive, fluid inclusions in late quartz cement (similar to > 2 km of burial) reveal additional information on fluid flow associated with sand injectites during deeper burial. The latest phase of fluid flow occurs when sand injectites are reactivated as preferential fluid conduits during phases of deformation, when well-cemented subvertical sand injectites become sites of focussed brittle deformation (fracturing). This study shows that sand injectites are a common and volumetrically important type of structural heterogeneity in sedimentary basins and that long-lived fluid flow associated with sand injectites in very different settings can be assessed and compared systematically using a combination of petrography and fluid inclusion studies.