Abstract:

Halite and other evaporite rocks are often considered to be viscous materials that never deform by brittle faulting. However, fractures and faults are observed very locally in some salt domes and glaciers, and are indicated by data or shocks from seismically active areas. Rock mechanics experiments show that halite begins to deform by faulting once strain rates are large enough. However, it is very rare that such high strain rates occur in salt. Faulting in salt is most likely to occur where effective confining stress is significantly decreased by fluid overpressure, so that the salt fails at much lower differential stress. Several examples are presented of dilational veins oriented in several directions, which strongly indicate the presence of overpressured fluids during fracturing. Hydrocarbon maturation in shales deposited within the salt interval or metamorphic reactions of evaporite minerals often produce fluid overpressure. Mining-induced fractures in the Boulby Mine, NE England produced 300 000 m3 of brine and 100 m3 of Carboniferous-derived oils through the Permian salt over a 1 year period. Evaporite beds are the most effective seal in a hydrocarbon system, but they are not a perfect seal and commercial volumes of hydrocarbons can occasionally migrate through salt over short time scales.

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