Since the onset of production in 1993, extraction and injection in Strathspey has focused on the central region of the field, around the ‘Central Fault’. Analysis of RFT measurements from intermittent, but regular drilling on either side of this fault has allowed reconstruction of footwall/hanging wall pore-pressure fields for particular time instances. Comparison of these similar age pressure fields shows the Central Fault capable of maintaining up to 1300 psi pressure differentials. The results of the pressure analysis were used to calibrate a fault seal attribute, termed Shale Gouge Ratio (SGR) which is an estimate of clay concentration within fault gouge, mapped across the surface of the Central Fault. The calibration showed that as SGR increases, so does the measured pressure differential across the fault. This positive relationship between SGR and pressure differential suggests SGR is a guide to potential fluid-flow resistance exerted by faults. We therefore suggest that SGR can potentially be used as a guide to defining differences in permeability within and between faults in a given field. This ‘scaleability’ of SGR as an indicator of fault permeability within a field could provide hitherto unachievable flexibility in the systematic modelling of the hydraulic behaviour of faults during fluid flow simulations.

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This is a technical report prepared for the Federal Emergency Management Agency at the request of the County of Santa Cruz to determine the geologic hazards that may have resulted from the Loma Prieta earthquake. Neither the Federal Emergency Management Agency nor any of the institutions or individuals participating in the Technical Advisory Group have any control on the use of this report by the County of Santa Cruz or other parties.