TSK 11 Göttingen 2006 Philipp & Gudmundsson Gypsum veins as hydrofrac- tures in layered and faulted mudstones: implications for reservoir permeability Poster Sonja L. Philipp1 Agust Gudmundsson1 Mineral veins and reservoir perme- ability Mineral veins form when water solu- tions passing through fluid-transporting fractures gradually seal the fractures as minerals precipitate. Many mineral veins are hydrofractures, that is, frac- tures generated at least partly by an internal fluid pressure. For most min- eral veins, the fluid generating the hy- drofracture is geothermal water. Other hydrofractures include fractures gen- erated by magma (dykes, sills, in- clined sheets), oil, gas and ground- water (many joints), as well as man- made hydraulic fractures in petroleum engineering. Hydrofractures are pri- marily extension fractures (Gudmunds- son et al. 2002). The formation of hydrofractures is one of the two ba- sic mechanisms for the generation and maintenance of permeability, particu- larly in fluid-filled heterogeneous reser- voirs such as those commonly associ- ated with petroleum, groundwater, vol- canic and geothermal fields. The other, and better-known, mechanism for per- meability development is the formation of shear fractures, that is, faults. The permeability development in frac- tured reservoirs, such as those for groundwater, geothermal water and petroleum, depends on fluid overpres- 1 Geowissenschaftliches Zentrum der Georg- AugustUniversität Göttingen, Abteilung Strukturgeologie und Geodynamik, Gold- schmidtstr. 3, 37077 Göttingen Figure 1: Reverse fault in the profile at Watchet.There is a network of white gyp- sum veins on either side of the fault plane, which stops abruptly at a grey siltstone layer and is absent in the overlying mud- stones. A gypsum vein follows the fault plane to a higher level than the vein net- work in the adjacent layers. View south- east; the person provides a scale. sure and transport in hydrofractures (Aguilera 1995). It has been proposed that a high fluid pressure in a reservoir can create high temporary permeabil- ity through hydrofracturing (Aguilera 1995; Gudmundsson et al. 2002). This hydrofracturing may result in mineral vein networks. Such palaeohydrofrac- tures give information about past fluid flow and flow networks. Studying min- eral veins is thus important for under- standing fluid and mineral transport in rocks and reservoirs. Faults and gypsum veins in mud- stones Here we present field measurements of mineral veins in coastal sections near the village of Watchet on the Somerset Coast of Southwest England (Fig. 1). The cliffs provide excellent outcrops of subhorizontal to gently dipping red mudstone beds with horizons of nodu- 1 Philipp & Gudmundsson TSK 11 Göttingen 2006 Figure 2: Aperture /dip relationship of 160 gypsum veins measured at Watchet. The thick veins are subhorizontal indicat- ing that the minimum principal compres- sive stress was oriented vertically at the time of vein formation, a situation occur- ring during basin inversion. lar gypsum and of siltstone of the Upper Triassic Mercia Mudstone Group. In the upper part of the red mud- stones there are laterally impersistent evaporite-rich horizons, mainly of white nodular gypsum. In part of the stud- ied section west of Watchet Harbour, there are many thin beds of grey-green carbonatic siltstones, as well as nodu- lar gypsum horizons. The beds are dis- sected by many faults and numerous gypsum veins. In some beds, there are dense anastomosing networks of fibrous gypsum (satin spar) veins which may have formed during transient hydraulic fracturing (Cosgrove 2001). The exact mechanism for the formation of gyp- sum veins, however, is still a matter of debate. Proposed mechanisms include mineral precipitation in open fractures, formation due to crystallisation pres- sure, and hydraulic overpressure (Shear- man et al. 1972; Gustavson et al. 1994). Based on 160 measurements, the veins in a dense vein network do not show any preferred orientation. The thick- est veins, however, are subhorizontal (Fig. 2), indicating a horizontal orienta- tion of the maximum principal compres- sive stress during their formation. Thus, the vein networks were partly devel- oped during horizontal basin compres- sion, a stress state which may have ex- isted during basin inversion associated with Alpine Tectonics in the late Cre- taceous and early Tertiary. 97 cross- cutting relationships, as well as mostly perpendicular vein fibres, indicate that the veins are primarily extension frac- tures; that is, they show hardly any evi- dence of shear displacement. In a 300m- long profile dissected by (mostly) nor- mal faults with small displacements, 24 faults (out of 28) have veins following them, indicating palaeofluid transport along the fault planes. Formation of gypsum veins as hy- drofractures Mineral veins form through several re- lated processes: fracturing, mineral dis- solution, transport and precipitation. These processes may occur simultane- ously and be repeated many times to form a single mineral vein. For a vein to form there must thus be a fluid, a ma- terial source, a fracture providing space, and suitable pressure-temperature con- ditions for material precipitation. We propose that for the gypsum veins at Watchet water was transported from deeper levels in the sedimentary basin along faults into the mudstones where it got access to nodular anhydrite. The water then dissolved the anhydrite and formed gypsum. The volume increase due to this reaction and the low perme- ability of the mudstones lead to build- up of high fluid pressure in the nod- ules that, eventually, created hydrofrac- tures at the ends of their long axes. The resulting hydrofractures connected 2 TSK 11 Göttingen 2006 Philipp & Gudmundsson the gypsum nodules. Calcium-sulphate saturated fluids, transported along the faults, got access to evaporite-free mud- stone layers where dense anastomos- ing vein networks developed. Most veins were arrested during their prop- agation by layers with contrasting me- chanical properties (generating stress barriers). Some veins, however, prop- agated through the barriers along faults to shallower levels. Permeability of heterogeneous reservoirs Our results have important implica- tions for fluid transport in reservoirs and the formation of hydrofractures. The gypsum veins at Watchet indi- cate hydrofracturing rather late in basin history during inversion and exhuma- tion (Cosgrove 2001). The veins show that fluids from deeper levels in the sedimentary basin can be transported along faults into rather impermeable host rocks. When injected into the host rocks, the overpressured fluids induce new hydrofractures. Provided the host rock has a low permeability and is seal- ing the nodules, the fluid overpressure is then partly related to the volume change at the hydration of nodular anhydrite to gypsum. The hydrofractures propa- gate until they become arrested at layers with contrasting mechanical properties. Individual layers or ‘compartments’ in a fluid reservoir can be connected ver- tically through faults in which case the reservoir may develop a high temporary permeability. The gypsum veins fol- lowing many fault planes indicate the faults transported water through the mudstones. This transport, presum- ably, occurred either during fault slip or through the formation of hydrofrac- tures along the fault planes. Hydrofrac- tures can transport fluids through low- permeability rocks. When the fluids are supersaturated with respect to cer- tain minerals, or when there exists a lo- cal material source, mineral veins may form. At Watchet, nodular anhydrite acted as local material source. The present results suggest that high tempo- rary permeabilities can develop in reser- voirs not only during early burial but also during basin inversion — and that these permeabilities are primarily due to the formation of hydrofractures. Acknowledgements We thank Sta- toil for a PhD-Grant (to AG) for Sonja Philipp, née Brenner. References Aguilera R (1995) Naturally Fractured Reser- voirs. PennWell Publishing Company, Tulsa, Oklahoma Cosgrove JW (2001) Hydraulic fracturing dur- ing the formation and deformation of a basin: A factor in the dewatering of low- permeability sediments. Am Assoc Petrol Geol Bull 85, 737–748 Gudmundsson A, Fjeldskaar I & Brenner SL (2002) Propagation pathways and fluid transport of hydrofractures in jointed and layered rocks in geothermal fields. J Volc Geotherm Res 116, 257–278 Gustavson TC, Hovorka SD & Dutton AR (1994) Origin of satin spar veins in evaporite basins. J Sed Res A64, 88–94 Shearman DJ, Mossop G, Dunsmore H & Mar- tin M (1972) Origin of gypsum veins by hy- draulic fracture. Institution of Mining and Metallurgy Transaction, Section B: Applied Earth Science 81 149–155 3