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

Abstract

Geoscientific investigations for a proposed deep geologic repository at the Bruce Site, located on the eastern flank of the Michigan Basin, have identified unique and significant underpressured conditions. Along with the measurement of environmental tracer profiles (e.g., helium), this study aims to explore, through a series of numerical simulations, the nature of long-term phenomena responsible for the generation and preservation of formation underpressures. Three families of inverse numerical experiments for underpressure formation were examined by means of one-dimensional hydromechanically coupled models through the vertical hydrostratigraphic column: (i) uncertainty in glaciation scenarios; (ii) uncertainty in initial heads prior to glaciation; and (iii) uncertainty in the degree of hydraulic connectivity between the more permeable Guelph Formation at the Bruce Site and the applied glacial loading, for a total of 20 scenarios, assuming fully saturated conditions. Underpressured initial heads for the paleohydrogeologic simulations lead to lower calibrated vertical hydraulic conductivities. The robustness and resilience of the groundwater system to external perturbations are greater for the state where underpressured conditions predate the onset of glaciation and are better able to preserve the present day helium tracer profile in 260 Ma exhumation analyses.

Possible Effects of Multiphase Methane Evolution During a Glacial Cycle on Underpressure Development in Sedimentary Basins: An Analysis With Application to the Northeast Michigan Basin

“…Due to the large capillary pressures in the very fine-grained media in this system, this caused the liquid phase to be significantly underpressured initially. Similar initially underpressured scenarios have been tested by Plampin and Neuzil (2018) and Normani et al (2017). Simulation 29 tests whether pre-existing underpressures could persist through a glacial loading and unloading cycle in a multiphase system.…”

“…Permeabilities used in a previous modeling study by Normani et al. (2017) are shown as black lines. The methodology for estimating these parameters is discussed in Section 3.2.…”

“…In Figure 1a, vertical permeability (k) values for the five hydrostratigraphic units as estimated in this study by means of inverse analysis using an updated paleoclimate reconstruction are shown as red lines. Permeabilities used in a previous modeling study by Normani et al (2017) are shown as black lines. The methodology for estimating these parameters is discussed in Section 3.2.…”

“…Boundary conditions were applied in a groundwater flow model published by Neuzil and Provost (2014), and also in the models described in this paper, at depths of hydraulically conductive aquifers, which exist approximately at the locations denoted by the gray horizontal dashed lines in Figure 1 (0.164, 0.327, and 0.840 kmBGS). The middle aquifer, the A1 Carbonate, was the most important drain for Normani et al (2017). Labels in (b) represent five hydrostratigraphic units.…”

Glacially Induced Hydromechanical Coupling in Shale May Have Caused Underpressured Water in the Eastern Michigan Basin Despite the Possible Presence of Gas Phase Methane

“…Likewise, while multiple lines of evidence suggest the possibility of in situ gas phase methane within the low‐permeability strata at the proposed DGR site (Sterling et al, 2011), the effects of gas phase on hydromechanical processes are still rather uncertain. A recent study (Normani et al, 2017) suggested that the underpressures may have predated glaciation, but this leaves open the question of their cause.…”