Melting of the Laurentide Ice Sheet profoundly altered the groundwater and surface hydrology of North America over the last 2 million years. Subglacial meltwaters infiltrated into saline aquifers along the margins of low-lying Paleozoic basins, reversing groundwater flow directions and suppressing the freshwater-saline water mixing zones to over 1 kilometer depth. Economic accumulations of microbial methane in fractured organic-rich shales are located in areas of freshwater recharge along the basin margins. We will assess the impact of Pleistocene glacial loading and permafrost development on recharge history and circulation of meteoric waters in the Illinois Basin, focusing on two organic-rich zones that contain microbial methane (Late Devonian New Albany Shale, Pennsylvanian coals), and adjacent regional aquifer systems. Results of this study will greatly expand our understanding of ice sheet-aquifer interactions, age distribution of confined groundwaters, and hydrogeochemical controls on microbial activity in the deep subsurface.
Formation waters, gas, and microbial samples will be collected from ~50 producing oil/gas and household wells in the New Albany Shale, Pennsylvanian Coalbed Methane Play, and adjacent regional aquifers, along a transect from the central Illinois Basin to the northeastern basin margin. We will analyze the elemental and stable isotope (O,H,C) chemistry, 14C, and noble gases of formation waters and gas. In addition, microbial populations will be determined by DNA analyses, and archived shale and coal materials will be analyzed for biodegradation of hydrocarbons. We have recently developed a new hydrologic model, which solves equations for fluid motion, heat, multi-species (salinity, stable/radiometric isotope, and noble gas) solute transport, permafrost, and lithosphere flexure/mantle flow. This model will be compared to observed salinity and isotope tracers to test the hypothesis that ice sheet dynamics has substantially modified the plumbing of the Illinois Basin during the Late Pleistocene.
Despite numerous studies of the paleohydrology of the Illinois Basin the maintenance of high salinity fluids of marine-origin in deep basin aquifers is still enigmatic. This study will help to constrain the residence times of brines and freshwater resources in sedimentary basins, and hydrodynamic forces required to drive continental-scale fluid and solute transport against strong salinity gradients. Results from this study will also help to constrain the hydrologic and geochemical conditions necessary for economic gas accumulation in unconventional reservoirs, such as fractured black shales and coalbeds. Importantly, this multi-disciplinary project will be an integral part of a University of Arizona Ph.D. student's thesis, and involve several undergraduate researchers.
