Paleozoic-aged brine and authigenic helium preserved in an Ordovician shale aquiclude

Abstract

Consideration of the geosphere for isolation of nuclear waste has generated substantial interest in the origin, age, and movement of fl uids and gases in low-permeability rock formations. Here, we present profi les of isotopes, solutes, and helium in porewaters recovered from 860 m of Cambrian to Devonian strata on the eastern fl ank of the Michigan Basin. Of particular interest is a 240-m-thick, halite-mineralized, Ordovician shale and carbonate aquiclude, which hosts Br --enriched, post-dolomitic brine (5.8 molal Cl) originating as evaporated Silurian seawater. Authigenic helium that has been accumulating in the aquiclude for more than 260 m.y. is found to be isolated from underlying allochthonous, 3 He-enriched helium that originated from the rifted base of the Michigan Basin and the Canadian Shield. The Paleozoic age and immobility of the pore fl uids in this Ordovician aquiclude considerably strengthen the safety case for deep geological repositories, but also provide new insights into the origin of deep crustal brines and opportunities for research on other components of a preserved Paleozoic porewater system.

Generic Argillite/Shale Disposal Reference Case

“…In the Eastern Interior Province, the characteristics of the Upper Ordovician Black River Group and Middle Ordovician Trenton Group of the Michigan Basin were considered (Clark et al 2013;Beauheim et al 2014) . In the Great Plains Province, most of the data analyzed refers to the Pierre Shale (Neuzil 1986;1993;Bredehoeft et al 1983;Olgaard et al 1995;Smith et al 2013).…”

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

“…At the Bruce Site, the Ordovician shale and carbonate formations have preserved brine compositions with minimal evidence of transport since the Silurian, with this being attributed to permeability reduction resulting from secondary halite mineralization [9]. Clark et al [9] determined helium isotope ratios with depth at the Bruce Site (see Figure 4) which show a distinct increase at the base of the Cobourg formation and further suggest that the Ordovician behaves as an aquiclude with limited transport capability over Paleozoic time. Xiang et al [18] have measured effective diffusion coefficients at the Bruce Site on the order of 10 −12 to 10 −14 m 2 /s, indicative of extremely low solute transport.…”

“…Helium in groundwater originates from the natural radioactive decay of uranium and thorium and from degassing of the crust and mantle [16,17]. At the Bruce Site, the Ordovician shale and carbonate formations have preserved brine compositions with minimal evidence of transport since the Silurian, with this being attributed to permeability reduction resulting from secondary halite mineralization [9]. Clark et al [9] determined helium isotope ratios with depth at the Bruce Site (see Figure 4) which show a distinct increase at the base of the Cobourg formation and further suggest that the Ordovician behaves as an aquiclude with limited transport capability over Paleozoic time.…”

“…These tracers represent natural analogues that provide a means to determine mass transport characteristics of solutes (e.g., radionuclides) over geologic time scales [6][7][8][9][10]. In various studies, helium isotopes have been used to quantify fluxes in low permeability media and to determine media properties [6,[11][12][13][14][15].…”

An Evaluation of the Long-Term Stagnancy of Porewater in the Neogene Sedimentary Rocks in Northern Japan

“…However, there have been only a few studies that investigated the porewater age in formation with low hydraulic conductivity [1,[13][14][15]. The evaluation of porewater age in formations with low hydraulic conductivity is much more difficult compared to that for flowing groundwater because of the following two reasons.…”