The location or extent of hydrostatic and geopressured zones is commonly interpreted from porosity assessments using downhole (wireline) geophysical tools such as sonic, neutron, density and resistivity (e.g. Magara 1978;Fertl & Chilingarian 1989;Aminzadeh 2002). The boundary between the two regions is determined by analysing where the decrease in porosity with depth stabilises and becomes significantly reduced or constant, usually at around 5-10% porosity. In the Sydney Basin, actual porosity measurements (compiled by Blevin et al. 2007) allow the accurate identification of this horizon at approximately 1000-1200 m depth ( Figure 4). Borecore (horizontal) permeability **data** (Blevin et al. 2007) also indicate that a significant flow barrier region exists at approximately 800-1000 m depth; and in a large area such as the Sydney Basin, it is reasonable to expect the horizon to vary between these depth ranges in different areas. The significance of this is that it is possible for methanogen-bearing meteoric recharge to penetrate 