Significance of large-scale sand injectites as long-term fluid conduits: evidence from seismic data

Sand injectites and related features that are interpreted to have formed by large-scale, often sudden, fluid escape in the shallow (typically <500 m) crust are readily imaged on modern seismic data. Many of the features have geometrical similarity to igneous dykes and sills and cross-cut the depositional stratigraphy. Sand injectites may be multiphase and form connected, high-permeability networks that transect kilometre-scale intervals of otherwise fine-grained, low-permeability strata. North Sea examples often form significant hydrocarbon reservoirs and typically contain degraded, low-gravity crude oil. Fluid inclusion and stable isotope data from cements in sand injectites record a mixing of aqueous fluids of deep and shallow origin.