Regionally extensive emplacement of sandstone intrusions: a brief review

Large sandstone intrusion provinces (LSIPs) are described from three basins with contrasting tectonic settings, the North Sea Basin, the Faeroe-Shetland Basin and the San Joaquin Basin. Hundreds of reservoir-scale sandstone intrusions have been identified in each of these basins over areal extents of > 500 km2, and with vertical extents of > 1 km from their underlying source units. It is likely that these large intrusion networks were emplaced over geologically short time periods i.e. as a single hydrodynamic event, because of their highly interconnected geometry. Large sills are extensively intruded into the host claystones in each basin, and the presence and distribution of these intrusions demonstrates that pore fluid pressures in the source units may have been in excess of lithostatic at the time of remobilization and intrusion. Mechanisms that could bring source sand bodies of large volume and areal extent to a condition of supralithostatic pore fluid pressure are not well understood in the context of sandstone intrusion. Of the possible candidate mechanisms, shear-induced liquefaction resulting from a large earthquake or bolide impact and geologically rapid pressure transfer from higher pressure cells are the most likely. The relative rarity of LSIPs worldwide is either a result of undersampling at outcrop or perhaps signifies that a rare combination of factors is required to form them. Once formed, they disrupt the sealing properties of the sequences they intrude, and this has major implications for fluid migration in these areas.