Fracture sealing in limestones, a microstructural and mineralogical study

Abstract

Fractures significantly enhance the flow rate in rocks, if fracture density is high (Taylor 1999, Cox et al. 2001). This leads to rapid flux along a hydraulic gradient from high to low pressure reservoirs, and is represented in rocks as veins. Veins are precipitates from supersaturated fluid, and are formed by a change in pressure, temperature or geochemistry. The solubility of vein forming minerals such as quartz, calcite or halite is generally low and thus large (and sometimes unreasonable) fluid volumes are required to account for the precipitated mass. Rapid ascent of solution may explain the high supersaturation needed to seal fractures, either by fluid flow along deep reaching faults due to seismic ruptures, or mobile hydrofractures driven by pressure gradients in fluid filled fractured at deeper crustal sections (Bons 2001, Miller 2002). The vein microstructure is a unique tool to unravel the fracture sealing process. The most indicative microstructures are fractured minerals, which were sealed by a fluid of different composition. The repeated presence of fluid and solid host rock inclusions in fibrous, stretched crystal type veins (minerals which extend across the vein and into the host rock) also indicate repeated fracture-sealing processes (Ramsay 1980), although their presence is not a sufficient criteria (Hilgers 2005). In this study, we outline the different fault sealing processes associated in a still seismic zone. The faults are located in Carboniferous limestones, and thus present an analogue for fault sealing processes in hydrocarbon reservoirs and an in-depth study of seismogenic faults.