Mechanisms and controls on the formation of sand intrusions

Sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep-water settings. They also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments. A variety of mechanisms have been cited as triggers for clastic intrusions. These include seismicity induced liquefaction, application of tectonic stresses, excess pore fluid pressures generated by deposition-related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body. The formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures. When the seal on an unconsolidated, overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize. The fluidized slurry can then inject along pre-existing or new fractures to form elastic intrusions. The scale and the geometry of an intrusive complex is governed by the stress state, depth and pre-existing joints or faults within the sedimentary succession, as well as the nature of the host sediments. For the simplest tectonic setting, where the maximum stress in a basin is vertical (gravitational loading), small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface, whereas at greater depth dykes and sills forming elastic intrusion networks are more typical. A simple relationship is derived to calculate the maximum burial depth at which a dyke-sill complex forms as a function of the source-bed to sill height, the bulk density of the surrounding sediments, and the ratio of the vertical to horizontal effective stresses, K-0. When applied to three examples of large-scale dyke-sill complexes developed within Paleocene and Eocene deep-water reservoir sand bodies of the North Sea, maximum burial depths in a range of 375 to c. 500 m, 450-700 m and 550-850 m are estimated for intrusion of each of the three complexes.