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Rock properties: Porosity and density


Go to: TABLE of porosity and density measurements | Understanding density |Details on sample preparation and density measurement | Introduction to porosity and density


UNDERSTANDING POROSITY

What is porosity?

Porosity is the percentage of void space in a rock. It is defined as the ratio of the volume of the voids or pore space divided by the total volume. It is written as either a decimal fraction between 0 and 1 or as a percentage. For most rocks, porosity varies from less than 1% to 40%.
porosity equation where "n" equals pore space volume divided by total volume
The porosity of a rock depends on many factors, including the rock type and how the grains of a rock are arranged. For example, crystalline rock such as granite has a very low porosity (<1%) since the only pore spaces are the tiny, long, thin cracks between the individual mineral grains. Sandstones, typically, have much higher porosities (10–35%) because the individual sand or mineral grains don’t fit together closely, allowing larger pore spaces.

Go to: Details on sample preparation and porosity measurement

Sandstone—porosity

diagram showing relatively large pore spaces between mineral grains of sandstone


Crystalline rock—porosity

diagram showing very small pore spaces between mineral grains of crystalline rock

Porosity measurements of Wisconsin rocks

The porosities of the rocks measured vary from 2% to more than 30%. Much of this variation is due to lithology (rock type). Table 1 lists the porosities of the tested samples and the figure at right shows the range and distribution of porosities by lithology. The dolomites have the lowest porosities (2–6%), the shales have the widest range of porosities (8–29%, although most are less than 15%), and the sandstones have the highest porosity (11–32%).

Figure 1. Distribution of porosities for dolomite, shale, and sandstone.

range of porosities for sandstone, shale, and dolomite

 


UNDERSTANDING DENSITY

Density measurements of Wisconsin rocks

Density is defined as the mass per volume. In rocks, it is a function of the densities of the individual grains, the porosity, and the fluid filling the porosity. There are three types of density in rocks: dry density, wet density, and grain density.

View table 1 lists the dry, wet, and grain densities of the samples. Additional wet densities for Wisconsin rocks can be found in Dutch and others, 1995.

Reference:
Dutch, S.I., R.C. Boyle, S.K. Jones-Hoffbeck, and S.M. Vandenbush, 1995, Density and Magnetic Susceptibility of Wisconsin Rock, Geoscience Wisconsin, Vol. 15, pp. 53-70.


Dry density measurements and distribution

Dry density is measured on rocks without any water or fluid in their pores.
Formula: Dry density porosity equals solid mass divided by total volume

See figure 2 for dry density distribution for dolomite, shale, and sandstone.

Figure 2. Distribution of dry density for dolomite, shale, and sandstone.

Dry density distributions for dolomite, shale, and sandstone


Wet density measurements and distribution

Wet density is measured on fully saturated cores.
Formula: Wet density porosity equals sum of solid mass and pore fluid mass divided by total volume

See figure 3 for wet density distribution for dolomite, shale, and sandstone.

Figure 3. Distribution of wet density for dolomite, shale, and sandstone.

Dry density distributions for dolomite, shale, and sandstone


Grain density measurements and distribution

Grain density describes the density of solid or mineral grains of the rock.
Formula: Grain density porosity equals solid mass divided by solid volume

Grain density can give an indication of the mineralogy of the rock:
•  Dolomite, &rho = 2.8–3.1 g/cm3

•  Shales, &rho = 2.65–2.8 g/cm3
Shales are composed of several minerals that have different densities in different relative amounts. The minerals may include clays such as illite (&rho = 2.6–2.9 g/cm3) and kaolinite (&rho = 2.6 g/cm3) mixed, for example, with dolomite (&rho = 2.8–3.1 g/cm3) and calcite (&rho = 2.71 g/cm3).

•  Sandstones, &rho = 2.65–2.80
Nearly half of the sandstones have grain densities close to 2.65 g/cm3, the density of quartz, suggesting that those sandstones are composed of quartz grains and cement. The remaining sandstones have slightly larger grain densities, most likely due to mixing of quartz with more dense minerals like calcite (&rho = 2.71 g/cm3) or dolomite (&rho = 2.8–3.1 g/cm3).

See figure 4 for grain density distribution for dolomite, shale, and sandstone.

Figure 4. Distribution of grain density for dolomite, shale, and sandstone.

Dry density distributions for dolomite, shale, and sandstone

 

Go to: TABLE of porosity and density measurements | Understanding porosity |Details on sample preparation and density measurement | Introduction to porosity and density

 

 

 

 


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