Abstract - Faults have the potential to be both barriers to lateral flow and conduits for vertical movement of fluids. We examine where and why faults locally enhance the flow of fluids using mainly outcrops, tunnel excavations and seismic reflection lines. Outcrop studies of normal faults within the Mount Messenger Formation reveal that the thickness of fault zones (containing clay-rich fault rock and fractures) generally increases with displacement and varies by several orders of magnitude on individual faults. Elevated densities of small-scale faults, and the greatest fault zone widths, typically occur at irregularities on fault surfaces (e.g., relays, bends and fault intersections). Data from ground water flow in tunnels and gas chimneys imaged in seismic reflection lines show that for low permeability rocks (e.g., seals) fluid flow is primarily achieved via fault zones, that flow rates are greatest on larger faults and that migration of fluids is locally enhanced along fault irregularities (i.e. the sites where the highest densities of small-scale faults occur). Geomechanical analysis of faults does not appear to be a reliable predictor of gas chimney locations and migration pathways on individual faults, perhaps because stress conditions near faults can depart significantly from the regional stress field used in these analyses.
Abstract of talk given to:
Advantage Geotechnical Petroleum Forum, Wellington, New Zealand, April 2014.