Porosity varies significantly in the Mt. Simon Sandstone, a regional aquifier in the Illinois Basin (basal unit in above cross section of the basin showing fluid flow.)

Left--Photograph of a rock thin section showing sandstone with low porosity; pore spaces between sand grains are completely filled with quartz cement.

Right--Photograph of a rock thin section showing sandstone with high porosity; empty pore spaces are readily seen upon injecting blue epoxy into specimen. Photograph on left is from a sample colleted 25 feet below sample in photograph on the right.

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