Interpreting fluid pressure anomalies in shallow intraplate argillaceous formations

Abstract

Investigations have revealed several instances of apparently isolated highs or lows in pore fluid potential in shallow (< ~ 1 km depth) argillaceous formations in intraplate settings. 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 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 annum or by some combination of the two. Plausible causes include erosional downwasting, tectonic strain, and glaciation. In this conceptualization the anomalies provide constraints on formation‐scale flow properties, flow history, and local geological forcing in the last 106 annum and in particular indicate zones of low permeability (10−19–10−22 m2) that could be useful for isolation of nuclear waste.

Far‐field stress dependency of the failure mode of damage‐zone fractures in fault zones: Results from laboratory tests and field observations of siliceous mudstone

“…If dilatancy hardening occurred in the tests, it is also possible that local pore pressures dropped further in and around locally created open spaces, in contrast to observed pore pressures, because the propagation rate of pore pressure in a rock may be somewhat limited under a given strain rate and hydraulic conductivity of the rock. According to the model of Neuzil [, ], a significant pressure anomaly (i.e., transient Darcian flow) can develop when the following equation is satisfied: where |Γ| is the forcing rate, K is hydraulic conductivity, and l is the minimum flow distance required to dissipate the anomaly. Because the hydraulic conductivity of the matrix in the siliceous mudstone is about 10 −11 m/s [ Ishii et al ., ] and the height of the specimens is 0.06 m, the given strain rate (i.e., 0.02% min −1 ) during the undrained triaxial tests may have allowed the development of transient Darcian flow in the specimens, resulting in heterogeneous pore pressure.…”

Assessing Alternative Scenarios for the Cause of Underpressures in the Ordovician Sediments along the Eastern Flank of the Michigan Basin

“…Following exhumation, whereby an estimated maximum of 1000 m of Paleozoic sediment was eroded at the Bruce Site over an approximate 20-million-year period prior to the mid-Jurassic [4], Pleistocene glaciation imposed significant hydraulic and mechanical loads on the geosphere leading to the generation of both over-and underpressures, depending on the rock mechanical properties and the rate of glacial loading and unloading [20,29,37,40].…”

“…Due to harmonic averaging, relatively small, but contiguous, layers of reduced hydraulic conductivity can significantly lower effective vertical hydraulic conductivities and dominate the behaviour of vertical pore fluid migration. It should be noted that low vertical hydraulic conductivities (<10 −12 per Neuzil [20]) are necessary in order to generate and preserve underpressures [3,36]. Exhumation analyses were conducted whereby a helium tracer was simulated for each of the 20 calibrated paleohydrogeologic models over a period of 260 Ma.…”

“…The authors suggested that discrepancies between the model predicted results and observed underpressures could be the result of factors not evaluated, such as the presence of a gas phase, osmotic pressure, and/or erosion. Neuzil and Provost [40] and Neuzil [20] stated that glacial forcing was a possible cause of the underpressures based on a onedimensional analysis at the Bruce Site using a simplified lithology; however, other mechanisms could not be ruled out.…”

“…Numerical simulations by Al et al [10] suggest that the vertical effective diffusion coefficients of the Ordovician carbonates are up to an order-of-magnitude lower at the formation scale than laboratory estimates. A comparison of Bruce Site horizontal hydraulic conductivities and effective diffusion coefficients with international data reveals that both key solute transport parameters are among the lowest recorded anywhere [4,19,20].…”