LECTURE 7

Lecture 7

Porosity

Fetter 4.1 and 4.2

As was mentioned in the last lecture, almost no geologic materials are totally solid. Most rocks

and sediments contain voids, or open spaces. If you remember from your Physical Geology

Course, a ROCK is an aggregate of one or more minerals. If these minerals are unconsolidated,

they are known loosely as sediments. SEDIMENTS are materials which may be transported

by the water, wind, ice or gravity. The voids that occur in rocks and sediments are where

ground water is stored and transported.

The POROSITY of geologic materials is defined as:

	The percentage of the rock or soil that is void of material.
	A mathematical expression is the following:

Where:
	N = the porosity, expressed as a %.
	V_V = volume of void space in the geologic material.
	V_T = total volume of material.

In the lab, porosity is determined by taking a sample of known volume (V_T). The

sample is then dried in an oven at 105^OC until it reaches a constant weight. This

drives out all of the water in the pores. The dried sample is then submerged in a

known volume of water and sealed until the sample is re-saturated. The volume

or voids (V_V) is equal to the original water volume less the volume in the chamber

after the sample is removed. This method excludes pores that are not large enough

to contain water molecules and pores which are not connected.

Porosity of Sediments (Unconsolidated ROCKS)

Let us begin examining porosity by considering sediments that are solid spheres with an equal diameter.

One type of packing is Cubic Packing, where each sphere sits directly on the crest of another sphere. Porosity = 48%

Cubic Packing

In Rhombohedral Packing - the spheres lie in the hollows formed by four adjacent

spheres at the underlying layer. Porosity = 26%

Rhombohedral Packing

As you can imagine very few naturally occurring rocks or sediments contain

equidimensional spheres.

Porosity varies for different types of geologic materials, whether they be sediments

(Unconsolidated material) or rock (Consolidated). As you remember from your

physical geology course, there are three types of rocks, igneous, sedimentary and

metamorphic. We will examine some examples of porosity for sediments and then

move on to the different rock types.

The porosity of sediments will be dependent on the fundamental properties for grains:

	1) Grain size (or texture).
	2) Grain shape.
	3) Grain orientation (fabric).
	4) Grain packing.

Grain size - is used to classify sediments.

This overhead shows the different classifications used for grain sizes.

Udden-Wentworth - Standard class scale.
(grain size in mm)	à measure of diameter of grain.

	à grain size in base 2

Where: d_O = 1mm à standard grain

d_(mm) = diameter of grain

Grain size distribution chart

Measure Sediment diameter by:
	1) Direct/Visual - for sand or greater size grains.

	2) Sieving - a series of screens that retain grains larger than each mesh size.

Various Size Grade Scales in Common Use

Udden-Wentworth	f values	German scale † (after Atterberg)	USDA and Soil Sci. Soc. Amer.	U.S. Corps Eng., Dept. Army and Bur. Reclamation‡
Cobbles		(Blockwerk)
		--- 200 mm ---	Cobbles	Boulders
			--- 80 mm ---	--- 10 in. ---
--- 64 mm ---	-6			Cobbles
Pebbles		Gravel		--- 3 in. ---
		(Kies)		Gravel
--- 4 mm ---	-2		Gravel
Granules				--- 4 mesh ---
				Coarse sand
--- 2 mm ---	-1	--- 2 mm ---	--- 2 mm ---	--- 10 mesh ---
Very coarse sand			Very coarse sand
--- 1 mm ---	0		--- 1 mm ---
Coarse sand		Sand	Coarse sand	Medium sand
--- 0.5 mm ---	1		--- 0.5 mm ---
Medium sand			Medium sand	--- 40 mesh ---
--- 0.25 mm ---	2		--- 0.25 mm ---
Fine sand			Fine sand	Fine sand
--- 0.125 mm ---	3		--- 0.10 mm ---
Very fine sand			Very fine sand	--- 200 mesh ---
--- 0.0625 mm ---	4	--- 0.0625 mm ---
			--- 0.05 mm ---
Silt		Silt
			Silt	Fines
--- 0.0039 mm ---	8
		--- 0.002 mm ---	--- 0.002 mm ---
Clay		Clay	Clay
		(Ton)

† Subdivisions of sand sizes omitted.

‡ Mesh numbers are for U.S. Standard sieves: 4 mesh = 4.76 mm, 10 mesh = 2.00 mm,

40 mesh = 0.42 mm, 200 mesh = 0.074 mm

Blatt, Middleton, and Murry, 1980. p. 57.

3) Settling - pipette analysis - used to separate fine sediments by letting them settle in a column of water.

	Use Stokes Law.

4) Electronic - pass grains between a current.

The distribution of grain sizes in a sample can be plotted on semi-log paper. Grain size is plotted on the log scale and the cumulative percent weight finer or retain is plotted on the arithmetic scale.

	Wooster Example
	- production well S2
	- well is 120 ft deep
	- sample interval 100 - 105 ft

		We see that these sediments are predominately sand and gravel. There is a very small percent that are finer than sand(Pan fraction)

		Well sorted - little variation in grain size of this sample, very little clay, has a porosity of around 25 to 30%.

Grain-size distribution curve flow Wooster. Ohio well 52 at a depth of 100 to 105 feet.

U.S. Sieve	Sieve Opening		Cumulative
NO.	INCHES	MM.	WEIGHT	%
	.187		245	30.82
6	.1320	3.35	346	43.53
8	.0937	2.36	442	55.60
12	.0661	1.70	525	66.04
16	.0469	1.18	606	76.23
20	.0331	.085	677	85.17
30	.0234	.060	730	91.83
35	.0197	0.50	757	95.23
40	.0165	.425	768	96.61
50	.0117	.300	783	98.50
70	.0083	.212	790	99.38
100	.0059	.150	--
140	.0041	.106	--
	PAN		795	100.01

Grain-sized distribution curve for the Coconino Sandstone at a depth

of 500 feet from a well near Flagstaff, Arizona.


	Very well sorted sediments.
	Wind blown sand.

Grain-sized distribution curve for the Supai Formation at a

depth of 1,400 feet from a well near Flagstaff, Arizona.



	- Not as well sorted as the Coconino.

Grain Shape - measures

	1) Surface texture of grains.
	2) Roundness - of edges or corners (results from transport of sediments).
	3) Sphericity - how closely does grain approximate sphere?

Example

Orientation - How are the grains arranged around each other.
	i.e., Imbrication - oblong grains stacked by current flow

Grain Packing - the properties of groups of grains.
		i.e., Cubic vs. Rhombohedral

ENV 302 - Lectures