Fault Void Fills: pervasive and persistent fluid flow pathways in fractured crystalline and carbonate reservoirs

Abstract

Fracture-hosted dilational cavities, or fault voids, and their infilling geological materials, are widely observed cutting crystalline and carbonate basement terranes worldwide. To date, however there has been a paucity of detailed descriptions and comparative studies of fault voids and their fills formed at different depths and geological settings. Here we examine four such examples of naturally-formed fault void fills in exhumed basement, each of which formed at different palaeodepths throughout the brittle upper crust ranging from the surface to 15km. We describe their geological characteristics and evolution, and how their relationships with fluid flow changes through geological time. Through the description and comparison of different field analogues, we gain a better understanding of the general features of fault void fills at given depths and make predictions regarding the architecture and connectivity of fault void networks for subsurface fractured reservoirs.
Here we define three depth domains, each with independent fault void fills and architectures. These are the “shallow zone” (0-2km), the “intermediate zone” (2-8km), and the “deep zone” (8-15km). The shallow zone features typically comprise voluminous and interconnected dilatant fissure networks which connect to the palaeosurface and form complex and irregular cavity shapes and architectures. They are, inevitably, associated with major regional unconformities and are best developed on the upfaulted flanks of active rift basins or basement highs. These shallow fissures are variously filled by sediments and detrital fossiliferous material, hydrothermal mineralisation, and dilational wall rock collapse breccias. The intermediate zone is a hybrid or transitional zone between the textures and architectures observed in the shallow and deep zones. The faults here comprise frequently bifurcating and branching networks of faults and fractures which have irregular fault traces, forming dilational and contractional jogs with a network of interconnected void spaces. These fault voids are typically filled by both attritional and collapse breccias, in addition to extensive and chemically complex mineral deposits. The deep zone faults typically comprise high-strain zones of deformation, where multiple parallel highly planar faults form shear zones or fault corridors. Between parallel slip surfaces localised stresses develop and can form fault linkages and even dilatant fractures with complex shapes, architectures, and fills. Deep zone fault voids are commonly filled by attritional breccias, cataclasite, hydrothermal mineralisation, and frictional melts; however locally dilatant breccias have also been shown to form in association with ladder fracture arrays.
In addition to the contrasting fabrics, processes, and structures which define the different depth zones, we have observed a number of key similarities which link the evolution and preservation of fault voids fills throughout the brittle upper crust. All the outcrops studied show faults and fractures which extensively reactivate inherited fabrics, such as vein margins, pre-existing faults, or metamorphic foliations. In addition, all field areas show strong evidence for fluid flux in void networks through the deposition of vein deposits, alteration of wall rocks, and redistribution of sediments. This illustrates that fault voids and their fills represent important fluid conduits, both during their development, and through deep geological time. Finally we have observed that the widespread development of fault voids and their fills are closely associated with wider tectonic and seismogenic events such as rifting, strike-slip faulting, and orogenic uplift.