Porosity varies from 0% to 70% in natural sediments but exceeds 70% for freshly deposited mud.
Several factors control porosity.
a) Packing Density
Packing density: the arrangement of the particles in the deposit.
The more densely packed the particles the lower the porosity.
e.g., perfect spheres of uniform size.
Porosity varies from 0% to 70% in natural sediments but exceeds 70% for freshly deposited mud.
Several factors control porosity.
a) Packing Density
Packing density: the arrangement of the particles in the deposit.
The more densely packed the particles the lower the porosity.
e.g., perfect spheres of uniform size.
Porosity can vary from
48% to 26%.
Shape has an important effect on packing.
Tabular rectangular particles can vary from 0% to just under 50%:
Natural particles such as shells can have very high porosity:
In general, the greater the angularity of the particles the more open the framework (more open
fabric) and the greater the possible porosity.
b) Grain Size
On its own, grain size has no influence on porosity!
Consider a cube of sediment of perfect
spheres with cubic packing.
VT VG
P 100
VT
d = sphere diameter; n = number of grains along a side (5 in this example).
VT VG
P 100
VT
Length of a side of the cube = d ´ n = dn
Volume of the cube (VT):
VT dn dn dn d 3n3
Total number of grains: n ´ n ´ n = n3
Volume of a single grain:
V d3
6
Total volume of grains (VG):
VG n d n d 3
3 3 3
6 6
VT VG 3 3
P 100 Where: VT d n and VG n 3
d3
VT 6
3 3 3
d n n d3
Therefore: P 6 100
3 3
d n
3 3
d n 1
Rearranging: 6
P 3 3
100
d n
Therefore:
P 1 100 48%
6
d (grain size) does not affect the porosity so that porosity is independent of grains size.
No matter how large or small the spherical grains in cubic packing have a porosity is 48%.
There are some indirect relationships between size and porosity.
i) Large grains have higher settling velocities than small grains.
When grains settle through a fluid the large grains will impact the substrate with larger
momentum, possibly jostling the grains into tighter packing (therefore with lower porosity).
ii) A shape effect.
Unconsolidated sands tend to
decrease in porosity with increasing
grain size.
Consolidated sands tend to increase in
porosity with increasing grain size.
Generally, unconsolidated sands undergo little burial and less compaction than consolidated
sands.
Fine sand has slightly higher porosity.
Fine sand tends to be more angular than coarse sand.
Therefore fine sand will support a more open framework (higher porosity) than better rounded,
more spherical, coarse sand.
Consolidated sand (deep burial, well compacted) has undergone exposure to the pressure of
burial (experiences the weight of overlying sediment).
Fine sand is angular, with sharp edges, and the edges will break under the load pressure and
become more compacted (more tightly packed with lower porosity).
Coarse sand is better rounded and less prone to breakage under load; therefore the porosity is
higher than that of fine sand.
c) Sorting
In general, the better sorted the sediment the greater the porosity.
In well sorted sands fine grains are not available to fill the pore spaces.
This figure shows the relationship between sorting and porosity for clay-free sands.
Overall porosity decreases with increasing sorting coefficient (poorer sorting).
For clay-free sands the reduction in porosity with increasing sorting coefficient is greater for
coarse sand than for fine sand.
The difference is unlikely if clay was also available to fill the pores.
For clay-free sands the silt and fine sand particles are available to fill the pore space between
large grains and reduce porosity.
Because clay is absent less relatively fine
material is not available to fill the pores of
fine sand.
Therefore the pores of fine sand will be less
well-filled (and have porosity higher).
d) Post burial changes in porosity.
Includes processes that reduce and increase porosity.
Porosity that develops at the time of deposition is termed primary porosity.
Porosity that develops after deposition is termed secondary porosity.
Overall, with increasing burial depth
the porosity of sediment decreases.
25% reduction in porosity with burial to
6 km depth due to a variety of
processes.
i) Compaction
Particles are forced into closer packing by the weight of overlying deposits, reducing porosity.
May include breakage of grains.
Most effective if clay minerals are present (e.g., shale).
Freshly deposited mud may have 70% porosity but burial under a kilometre of sediment
reduces porosity to 5 or 10%.
http://www.engr.usask.ca/~mjr347/prog/geoe118/geoe118.022.html
ii) Cementation
Precipitation of new minerals from pore waters causes cementation of the grains and acts to fill
the pore spaces, reducing porosity.
Most common cements are calcite and quartz.
iii) Clay formation
Clays may form by the chemical alteration of pre-existing minerals after burial.
Feldspars are particularly common clay-forming minerals.
Clay minerals are very fine-grained and may accumulate in the pore spaces, reducing porosity.
iv) Solution
If pore waters are undersaturated with respect to the minerals making up a sediment then some
volume of mineral material is lost to solution.
Calcite, that makes up limestone, is relatively soluble and void spaces that are produced by
solution range from the size of individual grains to caverns.
Quartz is relatively soluble when pore waters have a low Ph.
Solution of grains reduces VG, increasing porosity.
Solution is the most effective means of creating secondary porosity.
v) Pressure solution
The solubility of mineral grains increases under an applied stress (such as burial load) and the
process of solution under stress is termed Pressure Solution.
The solution takes place at the grain contacts where the applied stress is greatest.
Pressure solution results in a reduction in porosity in two different ways:
1. It shortens the pore spaces as the grains are dissolved.
2. Insoluble material within the grains accumulates in the pore spaces as the grains are dissolve.
v) Fracturing
Fracturing of existing rocks creates a small increase in porosity.
Fracturing is particularly important in producing porosity in rocks with low primary porosity.
