Abstract
The Southern Andes Volcanic Zone (SVZ) represents one of the largest undeveloped geothermal provinces in the world. Development of the geothermal potential requires a detailed understanding of fluid transport properties of its main lithologies. The permeability of SVZ rocks is altered by the presence of fracture damage zones produced by the Liquiñe-Ofqui Fault System (LOFS) and the Andean Transverse Faults (ATF). We have therefore measured the permeability of four representative lithologies from the volcanic basement in this area: crystalline tuff, andesitic dike, altered andesite and granodiorite. For comparative purposes, we have also measured the permeability of samples of Seljadalur basalt, an Icelandic rock with widely studied and reported hydraulic properties. Specifically, we present the results of a systematic study of the effect of fractures and fracture offsets on permeability as a function of increasing effective pressure. Baseline measurements on intact samples of SVZ rocks show that the granodiorite has a permeability (10−18 m2), two orders of magnitude higher than that of the volcanic rocks (10−20 m2). The presence of throughgoing mated macro-fractures increases permeability by between four and six orders of magnitude, with the highest permeability recorded for the crystalline tuff. Increasing fracture offset to produce unmated fractures results in large increases in permeability up to some characteristic value of offset, beyond which permeability changes only marginally. The increase in permeability with offset appears to depend on fracture roughness and aperture, and these are different for each lithology. Overall, fractured SVZ rocks with finite offsets record permeability values consistent with those commonly found in geothermal reservoirs (>10−16 m2), which potentially allow convective/advective flow to develop. Hence, our results demonstrate that the fracture damage zones developed within the SVZ produce permeable regions, especially within the transtensional NE-striking fault zones, that have major importance for geothermal energy resource potential.
Highlights
The permeability of intact samples of all four lithologies from the Southern Andes Volcanic Zone (SVZ) are shown in Fig. 8, together with the change in permeability as a function of increasing effective pressure for samples of the same four rocks with mated macro-fractures
The largest increase is seen for the crystalline tuff, which had the lowest intact permeability, and the smallest increase is seen for the granodiorite, which had the highest intact permeability
As a first-order approximation, this could favour the existence of geothermal reservoirs in intrusive rocks, which is the main lithology making up the SVZ basement south of 39°S
Summary
Understanding the influence of fractures on the hydraulic properties of rocks is key to many fundamental and applied areas of geoscience, such as deformation and faulting (Barton et al, 1995; Evans et al, 1997), geophysical hazards (e.g. Scholz and Gupta, 2000), geothermal energy recovery (e.g. Meixner et al, 2016; Rowland and Sibson, 2004), genesis of ore deposits (e.g. Cox, 1999; Liotta et al, 2015; Micklethwaite et al, 2010) and subsurface storage of oils and gas (e.g. Gudmundsson et al, 2012, 2010). Fault zones accommodate most displacement in their cores, while simultaneously developing a variety of surrounding damage zone geometries defined by fractures at all scales, from micro-fractures to macrofractures (Kim et al, 2004; Mitchell and Faulkner, 2009). Permeability in such damage zones is enhanced by the hydraulic properties of the fault-fracture network
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