Effects of erosion on pore pressure and groundwater flow in the Western Canada Sedimentary Basin
Corbet, Thomas F., Jr.
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Permalink
https://hdl.handle.net/2142/23040
Description
Title
Effects of erosion on pore pressure and groundwater flow in the Western Canada Sedimentary Basin
Author(s)
Corbet, Thomas F., Jr.
Issue Date
1991
Doctoral Committee Chair(s)
Bethke, Craig M.
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Date of Ingest
2011-05-07T14:00:00Z
Keyword(s)
Geology
Hydrology
Language
eng
Abstract
Fluid pressures too low to be in equilibrium with the topography of the land surface occur in Cretaceous sediments over much of the western Canada sedimentary basin. I used a numerical model to study the origin of the low pressure in southern Alberta and its effects on patterns of regional groundwater flow over the past 5 m.y. The model accounts for changes in basin topography, conduction and advection of heat, cooling of the pore fluid, and rebound of pore volume during erosion.
Results of calculations of flow in idealized basins show that erosion at rates of a few tenths of millimeters per year can generate pressures several megapascals less than would exist if the flow patterns were in equilibrium with their land surfaces. Most of the underpressure forms because pore volume expands slightly as erosion reduces the load on the sediments. Flow patterns generated by erosion differ from equilibrium patterns in that hydraulic potential, in disequilibrium patterns, reaches its minimum at depth in the basin rather than at the point of lowest elevation on the land surface. Erosion generates potential gradients that drive groundwater along deep aquifers toward regions of lowest pressure in the aquifer's confining layers. Lateral flow generated by erosion is very slow (a few millimeters per year or less), but can be faster than flow that would occur if the system equilibrated.
Simulations of groundwater flow in southern Alberta show that the 3 MPa of underpressuring observed in this region could have formed because Plio-Pleistocene erosion removed some of the confining load from basin sediments. The simulations provide an estimate for the upper limit for the permeability of Cretaceous shales on a regional scale because results match observed pressures only if I assign vertical permeabilities less than 3 $\times$ 10$\sp{-16}$ cm$\sp2$ to these sediments. Pore fluid cooled and contracted as erosion reduced the burial depth of the sediments, but this effect could not have played a significant role in generating low pressure unless Cretaceous shales in southern Alberta are extremely stiff (pore compressibilities on the order of 6 $\times$ 10$\sp{-10}$ Pa$\sp{-1}$) and have regional permeabilities of about 10$\sp{-18}$ cm$\sp{-2}$. Erosion generated lateral flow in deep Cretaceous aquifers. However, groundwater moves too slowly to transport a significant amount of heat. The observed west to east increase in the geothermal gradient across the study area most likely occurs because sediments that were deeply buried in the west are more compacted and hence more thermally conductive than those to the east.
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