The development of secondary porosity and permeability in fractured rock

Abstract

Systems of faults and joints are observed to span many scales of length and displacement. Spacing and orientation are rarely constant within any area of interest to groundwater engineers. It is suggested that the methods used by engineering surface topographists should be applicable to geological fracture surfaces. Each fissure is regarded as resulting from the displacement of two, initially matched, rough surfaces. It is argued that a fractured aquifer is best modelled as a pipe network. This would be a similar system to the pore network in a small unfractured subvolume of that aquifer However, the manipulation of models having an infinite number of elements and many scales of length is not in an advanced state. Accordingly, the fissure system is treated as a pipe network, and the joint blocks are regarded as being subject to one dimensional diffusion. The concept of Pore Span Distribution can be developed from the extensive literature on petroleum reservoir engineering Experimentally determined values of porosity and permeability should be functions of lithology, and also of specimen size and shape. It is a measurable lithological property which describes the degree of interconnection displayed by a pore network. A falling head gas method is developed and used to determine the low permeabilities found in weathered limestone joint block margins. Unusually high seismic anisotropy is observed in the Corallian Limestone due to layering of weathered and unweathered rock, parallel to a dominant joint set. A new interpretation of the deformational phase affecting the Scourie dykes near Loch Torridon, Ross-shire is included. It is related to the problem of describing fault and joint sets adequately.