Assessing Alternative Processes for the Cause of Under-pressures in the Ordovician Sediments at the Eastern Flank of the Michigan Basin

Abstract:

This study examines the occurrence of anomalous hydraulic under-pressures observed within low hydraulic conductivity (<10^-14 m/s) Paleozoic age sediments on the eastern flank of the Michigan Basin. The primary goal of the study was to explore hypotheses regarding the nature of long-term phenomena responsible for the generation and preservation of formation under-pressures. This approach has provided a unique basis to develop an understanding of up-scaled aquiclude properties, and groundwater system evolution and stability as influenced by external perturbations on geologic time scales. The analysis is based on evidence gathered during detailed field investigations of an 840 m thick sedimentary sequence at the location of a proposed deep geologic repository for low and intermediate level radioactive waste in Ontario, Canada. At a nominal depth of 460 to 820 m below ground surface, hydraulic formation pressures of 300 m below hydrostatic were measured within Ordovician age carbonates and shales. Three hypotheses for under-pressure formation are examined by means of 1-dimensional numerical analyses: 1) glacial ice-sheet loading; 2) sedimentary erosion and exhumation; and 3) the presence of an immiscible gas phase. Calibration of the numerical models to observed conditions was achieved through manipulation of formation hydraulic diffusivity, loading efficiencies, and hydraulic boundary conditions. In all cases, the calibrated numerical models were able to replicate the observed hydraulic pressure profiles. Simulations of glacial ice-sheet advance-retreat required three 120 ka cycles to yield observed head conditions. The exhumation model suggests that the erosion of approximately 1-2 km of overlying rock can lead to the formation of under-pressures only if the scaled hydraulic diffusivities are 3-4 orders of magnitude less than observed. The immiscible gas analysis, using TOUGH2-MP, provides a favourable match between the simulated and the measured under-pressures if the effective permeabilities are reduced by 2-3 orders of magnitude from observed values. Formation scale hydraulic properties must be exceedingly small to allow under-pressure development and preservation and reflects the value of self-analogues in developing an integrated understanding of deep-seated groundwater systems.