Pore pressure within dipping aquifers in overpressured basins
Abstract
We simulate the water flow associated with rapid burial of an aquifer by mudstone. A layer of mudstone 3000m thick is deposited over 1m.y. and it is then overlain by a sandstone aquifer 300m thick and 6000 meters long that is deposited over 0.3m.y.. Mudstone is then deposited at a rate of 6.0mm/yr on the left margin and 10.0mm/yr on the right margin for 1m.y.. The simulated aquifer pore pressure follows approximately the hydrostatic gradient whereas the pore pressure within the mudstone away from the aquifer follows approximately the lithostatic gradient: the lithostatic stress is higher on the left margin than on the right margin at equivalent depths because the sediment is more compacted. Near the tilted aquifer, flow is drawn into the base of the aquifer and expelled out the top of the aquifer. The overpressure of the aquifer is controlled by the permeability and compressibility of the bounding mudstone. At the crest of the aquifer, the effective stress of the bounding mudstone is lower and the permeability is higher than the adjacent mudstone at the same depth. However, at the bottom of the dipping structure, the surrounding mudstone has a higher effective stress and lower permeability than the adjacent mudstone at the equivalent depths. Along the top of the dipping aquifer, the effective stress in the bounding mudstone increases from 9.9PMa at the crest to 30.7MPa at the base. As effective stress increases, the rock compacts and the mudstone permeability decreases by 3 orders of magnitude. Previous models have assumed constant permeability within the mudstone and predicted that the pressure in the aquifer is equal to the mudstone pressure at the midpoint of the structure. In contrast, when the effect of stress-dependent permeability in the mudstone is included, the simulated aquifer pressure is much lower because flow into the base of the sandstone is inhibited by the lower permeability present. In this example, the aquifer pressure is found to equal the mudstone pressure at a height close to ¾ of the structure. The relationship between aquifer pressure and bounding mudstone pressure is a critical control on regional flow and can control whether natural hydro fracturing can occur. These types of simulations illustrate regional flow fields within sedimentary basins and the control of the large scale stratigraphy and material properties on subsurface pressures.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T11E..03G
- Keywords:
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- 1828 HYDROLOGY / Groundwater hydraulics;
- 1830 HYDROLOGY / Groundwater/surface water interaction;
- 8109 TECTONOPHYSICS / Continental tectonics: extensional;
- 8169 TECTONOPHYSICS / Sedimentary basin processes