Physical characteristics of sand injectites

Abstract

Almost two hundred years of research is reviewed that focuses on the physical characteristics of sandstone intrusions. It is concerned with mechanisms of sand injection, particularly with fluid-grain transport and sedimentation processes during the remobilization, injection and extrusion of sand. Outcrop and subsurface studies in combination with laboratory experimental data are drawn on to present the state-of-the-art of sand injection. The text covers 1) geometry, internal structure, and microtexture of deformed parent units, injected and extruded sandstones, 2) host-strata and their seal characteristics that contribute to basin-wide overpressure generation, 3) common trigger mechanisms for sand injection such as high magnitude seismicity and the rapid injection of large volumes of fluids, 4) fluid types that drive sand into fractures, 5) hydrofracture mechanisms that induce regional-scale seal failure, 6) liquefaction and fluidization processes that transport sand into fractures, 7) sedimentation processes in fractures, 8) the flow regime of fluidized sand during injection, 9) post-sand-injection fluid flow and diagenesis, 10) porosity and permeability characteristics of injected sandstones and 11) post-sand-injection fluid-flow over geological timescales. Processes of sand remobilization, injection, and extrusion are complex and depend on many interrelated factors including: fluid(s) properties (e.g. pressure, volume, composition), parent unit and host-strata characteristics (e.g. depositional architecture, grain size and distribution, clay-size fraction, thickness, permeability) and burial depth at the time of injection. Many studies report erosional contacts between host strata and injected sands and these record high-velocity, erosive flow during injection. The flow regime is poorly constrained and similar features are interpreted as records of laminar and turbulent flow, or both, during injection. Internal structures are common in sandstone intrusions and can be accounted for by a variety of processes. The interpretational limits largely result from a lack of laboratory experiments that focus on developing analogues for sand injection. The relationship between grain fabric developed during injection and its control on permeability in sandstone intrusions is poorly understood and failure to advance this field of research will hinder the quantitative characterization of sandstone intrusions as fluid-flow conduits during basin evolution. We conclude that future research should focus on: 1) quantification of sediment transport modes under different flow conditions in different fracture dimensions with laboratory data relevant to sand injection; 2) estimation of the effect of injection on the bulk permeability of otherwise low-permeability seals (host strata) so that their effect on fluid flow can be assessed at all scales; and 3) incorporation of sand injection into quantitative basin models. Although an enormous amount of data have arisen from existing studies there remains a need to advance many fields of research related to sand injection so that the significance of these important structures can be fully appreciated in the geological record.