Gypsum veins as hydrofractures in layered and faulted mudstones: implications for reservoir permeability

Abstract

Mineral veins form when water solutions passing through fluid-transporting fractures gradually seal the fractures as minerals precipitate. Many mineral veins are hydrofractures, that is, fractures generated at least partly by an internal fluid pressure. For most mineral veins, the fluid generating the hydrofracture is geothermal water. Other hydrofractures include fractures generated by magma (dykes, sills, inclined sheets), oil, gas and groundwater (many joints), as well as manmade hydraulic fractures in petroleum engineering. Hydrofractures are primarily extension fractures (Gudmundsson et al. 2002). The formation of hydrofractures is one of the two basic mechanisms for the generation and maintenance of permeability, particularly in fluid-filled heterogeneous reservoirs such as those commonly associated with petroleum, groundwater, volcanic and geothermal fields. The other, and better-known, mechanism for permeability development is the formation of shear fractures, that is, faults. The permeability development in fractured reservoirs, such as those for groundwater, geothermal water and petroleum, depends on fluid overpressure and transport in hydrofractures (Aguilera 1995). It has been proposed that a high fluid pressure in a reservoir can create high temporary permeability through hydrofracturing (Aguilera 1995; Gudmundsson et al. 2002). This hydrofracturing may result in mineral vein networks. Such palaeohydrofractures give information about past fluid flow and flow networks. Studying mineral veins is thus important for understanding fluid and mineral transport in rocks and reservoirs...