INTERPRETING FLUID PRESSURE ANOMALIES IN SHALLOW ARGILLACEOUS FORMATIONS

Investigations in the last two decades reveal that pore fluid pressure anomalies – apparently isolated highs or lows in fluid potential - are common in shallow (< ~ 1 km depth) argillaceous formations, having been detected in about half of well-characterized sites worldwide. The nature and origin of the anomalies are of interest for understanding fluid transport in these materials generally and for applications that include isolation of nuclear waste. Low permeability and other conceptual and characterization complexities, however, make it challenging to evaluate models for the anomalies, which include separate-phase gas, osmosis and other coupled flows, non-Darcian flow, and measurement artifacts. Based on reported properties, formations with anomalies are distinguished by (1) relatively small ratios of hydraulic conductivity to formation thickness, and (2) relatively small hydraulic (or pressure) diffusivities. This suggests the anomalies are pressure transients in water-saturated systems; formations with anomalies are those in which such a model predicts pressure is most sensitive to forcing and in which anomalies, once generated, persist longest. This implies that even stable areas have been subjected to persistent pore strain at rates of 10^-17 to 10^-16 s^-1, to significant perturbing events in the past 10⁴ to 10⁶ a, or some combination of the two. In most instances, it is possible to identify processes, such as erosional downwasting, tectonic deformation, or glacial-mechanical loading, that may be responsible. Viewed in this light, the anomalies constrain formation-scale flow properties and behavior and also serve as records of forcing processes in the recent geologic past (< ~ 10⁶ a).