Fetter 4.2 & 4.3

Few naturally occurring sediments contain equidimensional spheres. If sediments are

well sorted and well rounded, their porosity will range between 25 - 50%. If they

are mixed, the porosity will be lowered, because the smaller particles fill the voids

between the larger ones. The wider the range in grain sizes, the lower the resulting

porosity. So, for our Wooster example we had little variation in grain sizes, resulting

in a high n_e.

Geological processes of running water, wind & glacial action create a wider range

of grain sizes, shapes and orientations.

process known as diagenesis. Diagenesis occurs when a sediment that is a product

of weathering or chemically precipitated material is buried. During burial, the weight

of the overlying materials causes compaction and movement of fluids which cement

the grains and reduce pore volume. Therefore, diagenetic processes tend to reduce

the porosity of the original sediments.

Ground water that is found between the grains is occupying the Primary Porosity

of the rock. Often rocks may become fractured. Fractures may represent very small

joints or large faults. Ground water stored in fractures is known as Secondary Porosity.

Ground water flowing through fractures may enlarge them by solution of material,

particularly in limestone, dolomites & chemical sedimentary rocks. These rocks are

composed of calcite CaCO₃ and dolomite CaMgCO₃, along with gypsum CaSO₄,

precipitated from solution and may easily re-enter the solution.

Some limestones, have dissolution cavities large enough to allow someone tall enough

to walk through them, for example Carlsbad Caverns or Mammoth Caves.

formed by applying heat and pressure to pre-existing rocks) typically have low porosity.

These rocks are not made by sedimentary processes, but are formed by sets of

interlocking grains having virtually no pores, or very little primary porosity. Often, if these

rocks are exposed at the earth’s surface, weathering an fracturing create secondary

porosity as large as 30 - 60 %.

similar in chemical composition to plutonic rocks. Both rocks cool from molten rock,

but volcanic rocks cool at the surface of the earth resulting in radically different porosities

from plutonic rocks.

Rapid cooling of volcanic rocks produces shrinkage cracks. If degassing occurs during

cooling, vesicles may form. Although these rocks may have many pores, most of them

are unconnected. Lava tubes may be produced as well.

i.e., Place a piece of pumice in water. It will float due to the trapped air.

Although the porosity of a rock controls the quantity of water that may be stored, the

effective porosity is the porosity available for fluid flow.

As water drains through pores, not all of the water will move. Specific yield (S_y) is

the ratio of the volume of water that drains from a saturated rock (due to gravity ) to

the total volume of the rock.

Water molecules cling to pore surfaces due to surface tension of the water. Gravity

exerts a force on the water film pulling some of it away from a grain and moves downward.

The remaining water film on the grain will be thinner, with a greater surface tension, so

that the force of gravity on the water particle will be equaled by the surface tension force,

stopping gravity, drainage.

The Specific Retention (S_r) of a rock or soil is the ratio of the volume of water a

rock can retain against gravity to the total volume of the rock.

Therefore, the total porosity is equal to the volume or water that a rock will yield

by gravity drainage (S_y) and the volume held by surface tension (S_r) or:

Specific retention is greatest with the smallest grain sizes. For instance, a clay may

have a porosity of 50% and a S_r (specific retention) of 48%. That means that if

you have 1 of clay, .5 will be water and only .02 of that water will drain by

gravity.

Maximum S_y occurs in sediments in the med. to coarse sand size range.

S_y may be determined in the lab. A sample of sediment of known volume is fully

saturated. This is usually done in a soil column which is slowly flooded from the

bottom, allowing the air to escape upward. The water is then allowed to gravity

drain from the column. The ratio of the volume of water drained to the volume

of the soil column is the specific yield.

Specific yield in the field is often estimated by a pumping test. We will return to

specific yield when we discuss aquifer analysis methods.