17.5 Clastic Sedimentary Rocks—Sandstone/Shale
Like unconsolidated sediments, the porosity and permeability of sandstones is affected by sorting. grain size, and cementation. Well‑cemented sandstones can have very low permeabilities and hydraulic properties that are more like granite than sandstone. As a rule, older sedimentary rocks tend to be more cemented than younger, although this is not always the case.
Other factors control the suitability of sandstones as aquifers. The depositional system determines the grain size, grain distribution, and overall geometry of the deposits. A beach or wave‑dominated delta complex will usually result in continuous layers of well‑sorted sand, while a fluvial or submarine channel system will tend to have the sandy parts in channel deposits that may not be well‑connected. Some eolian sandstones can cover vast areas—like the Nubian Sandstone in Africa—and contain very productive aquifers. Cross bedding, clay drapes, and other sedimentary structures can create anisotropies that may influence and control flow directions.
Structures related to depositional and tectonic stresses can also affect flow in sandstones. Growth faults associated with prograding delta sediments can create linear trends of high‑sand (i.e., sands containing dense minerals) deposits that are not well connected to each other. Folds and faults from tectonic stresses can create anisotropies and affect the overall geometry of the aquifer. Fractures can play a major role in transmitting precipitation from the surface to the aquifer, which facilitates recharge. Fractures can greatly increase permeability in the subsurface; however, they can create barriers to flow when filled-in with post‑diagenetic minerals or where they juxtapose permeable sedimentary rocks against low‑permeability units.
Shales are generally very poor aquifers, and usually act as confining zones when interbedded with sandstones. If the shales are fractured and sufficiently thick, however, they can produce enough water to supply low‑yield water wells. In some places, like the Clare Valley of South Australia, the fractured shales are the most permeable units in the area and are the predominant source of water.