Sand-rich injectites in the context of short-lived and long-lived fluid flow: Sand-rich injectites

Sand-rich injectites are a common attribute of clastic sedimentary successions, and they have received increased attention in the last decade by geoscientists and engineers dealing with shallow subsurface flow and retention of aqueous and hydrocarbon fluids. Injectites form due to fluidized flow of sediment-entraining fluids through high-permeability strata. Given that sediment entrainment is confined to high-permeability sediments with low horizontal effective stress and negligible cementation, the formation of sand-rich injectites is restricted to the first kilometre of burial, where sand-rich sediments are prone to fluidization and clay-rich sediments are generally more cohesive and may hydraulically fracture to allow the creation of injectites. When reviewing conditions that may lead to aqueous and hydrocarbon flow within this shallow section at velocities that may cause sand fluidization and injection, we can rule out geologic time-scale processes (disequilibrium compaction, hydrocarbon migration, and lateral pressure transfer) as plausible causes. This emphasizes the need for high-strain-rate processes to cause rapid fluid overpressuring and flow associated with sand injection. Earthquake-induced shaking, instantaneous loading and release of overpressured fluids along moving fault planes are the most likely causes of sand remobilization and injection and these processes may enforce each other during tectonic stress-release events. An additional mechanism that may trigger sand-rich injections is the mechanical failure of shallow oil accumulations, particularly as the fluidization velocity of sand entrained in oil can be several orders of magnitude lower than for sand entrained in aqueous fluids, particularly if the oil is biodegraded (and thus has a high dynamic viscosity). Flow of hydrocarbon gas is unlikely to cause sand injection, although gas dissolved in upward-flowing aqueous fluids may evolve at or near the surface as pressures in the aqueous phase drop.