Fluid Pressure Anomalies in Shallow Intraplate Argillaceous Formations

Abstract:

Fluid transport in shales and other argillaceous formations is difficult to study because these materials often have extremely low permeability. However, recent investigations have revealed a number of instances of apparently isolated highs or lows in pore fluid potential in shallow (< ~ 1 km depth) argillaceous formations in intraplate settings. The presence (or absence) of such pressure anomalies may provide clues to fluid flow. Formations with the pressure anomalies are distinguished by (1) smaller ratios of hydraulic conductivity to formation thickness and (2) smaller hydraulic (or pressure) diffusivities than those without anomalies. This is consistent with water-saturated transient Darcian flow caused by strain at rates of ~ 10^-17 to 10^-16 s^-1, by significant perturbing events in the past 10⁴ to 10⁶ years or by some combination of the two. Plausible causes include erosional downwasting, tectonic strain, and glaciation. In this conceptualization the anomalies constrain formation-scale flow properties, flow history, and local geological forcing in the last 10⁶ years and in particular indicate zones of low permeability (10^-19 – 10^-22 m²) that could be useful for isolation of nuclear waste.