Abstract
AbstractGypsum veins and faults were studied in red mudstones of the Upper Triassic Mercia Mudstone Group, in the Bristol Channel Basin, exposed in E–W-trending cliffs at Watchet on the Somerset Coast (SW England). In nodular gypsum horizons, individual gypsum nodules are connected by subhorizontal gypsum veins. In evaporite-free mudstone layers, however, dense anastomosing networks of gypsum veins occur. In a 300 m long profile dissected by 28 (mostly) normal faults with small displacements, 24 faults have veins following them, indicating palaeofluid transport along the fault planes. Ninety-seven cross-cutting relationships and mostly perpendicular vein fibres indicate that the veins are primarily extension fractures. The thickest veins in the vein network are subhorizontal (160 measurements), indicating a vertical orientation of the minimum principal compressive stress (horizontal basin compression). Such a stress state may have existed during basin inversion associated with Alpine compression (late Cretaceous to early Tertiary). I propose that the gypsum veins are the result of hydrofracturing. In the gypsum nodules, then presumably consisting of anhydrite, overpressure was generated related to the hydration of anhydrite to gypsum. Stress concentration around the nodules led to rupturing and injection of thin subhorizontal hydrofractures. Some of the calcium-sulphate saturated fluids were then transported upwards along the faults and gained access to evaporite-free mudstone layers where dense anastomosing vein networks developed. Most veins were arrested during their propagation by layers with contrasting mechanical properties (stress barriers). Some veins, however, propagated through the barriers along faults to shallower levels. The dense networks of mineral veins observed in Watchet indicate that hydrofractures can generate a very high temporary permeability in fluid reservoirs.
Highlights
The permeability of fractured reservoirs, such as those used for production of groundwater, geothermal water and petroleum, depends on fluid pressure and transport in fractures
The fluid overpressure of hydrofractures is commonly defined as the fluid pressure exceeding the stress normal to the fracture, which for extension fractures is the minimum principal compressive stress σ 3
The mineral veins occur in different geometrical arrangements, including anastomosing vein systems in thick mudstone layers as well as short subhorizontal veins that connect individual gypsum nodules in the nodular layers
Summary
The permeability of fractured reservoirs, such as those used for production of groundwater, geothermal water and petroleum, depends on fluid pressure and transport in fractures I present a hydrofracture-based model for the formation of gypsum veins in red mudstones of the Upper Triassic Mercia Mudstone Group near the village of Watchet on the Somerset Coast of SW England. I propose that the gypsum veins formed as hydrofractures related to fluid transport along faults into the host rocks and the stress concentration associated with the transformation of nodular anhydrite to gypsum. Coastal cliffs near the village of Watchet on the Somerset Coast, the south coast of the Bristol Channel Basin (Fig. 1), are largely comprised of the Upper Triassic ‘Mercia Mudstone Group’ In the upper part of the red mudstones there are laterally impersistent (non-continuous) evaporite-rich horizons, mainly composed of white nodular gypsum Many of these horizons are associated with grey-green beds (Whittaker & Green, 1983). During the Late Cretaceous and Early Tertiary N–S contraction associated with Alpine tectonics, the basin was inverted and many older structures were reactivated (Dart, McClay & Hollings, 1995; Nemcok, Gayer & Miliorozos, 1995)
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