Fault seal analysis: constraining fault seal risk using seismic velocities

Fault seal capacity is an important component in the conventional petroleum system. Assessing the capacity for a fault to seal or leak can be difficult, particularly where well constraint is lacking. In the frontier basin, in a marine setting, seismic velocities may be the only data available. However, useful constraints on a faults sealing capacity can be extracted from this data alone. This study investigates the robustness of a number of empirical relations that can assist in extracting useful constraints from seismic velocities and amplitudes. Reliable estimates on maximum and minimum stress tensors and pore pressures can be calculated and combined with basic fault architecture analysis, to place practical constraints on fault risk. In this study of an area on the Rankin Trend found good correlation between well-based and seismic velocity-based pore pressures and stress magnitudes allowing a coulomb failure function based only on stacking velocities to be calculated. Faults separating the Rankin 1 well block from the Dockrell/Keast Field, were shown to be within a stable stress regime. Calculated pore pressures match known RFT measurements and show that overpressure can be assessed using basic time-migrated velocity stacks. Furthermore, theoretical capillary pressure and hydrocarbon column calculations correlate with known values and highlight the capillary seal potential of sand-sand juxtaposition seals which can support 100 to 200 m column heights within the Triassic play. They suggest that many traps on the Rankin Trend within the Triassic play are limited by a combination of pore pressure and stress orientation. However, deeper intra-formational seals are likely to have increased seal capacity by virtue of overburden stress and reduced porosity exceeding any capacity for shear stress failure. Well data for this area confirms a Sh_max orientation of approximately 110 +/- 10^o and calculations show that faults striking within 20° of this direction may be at high risk of failure within the neotectonic setting, where pore pressures and dip predicate fault slip.