Porosity and permeability are important properties of sedimentary rocks that are controlled by grain size, shape, packing, and arrangement. Porosity is the ratio of pore space to total volume, and typically ranges from 5-25% in sediments. Permeability describes a rock's ability to allow fluid flow and is dependent on effective porosity, pore geometry, and fluid properties. These properties control fluid movement and diagenesis in rocks.

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SEDIMENTOLOGY (ES – 307)BB
CLASS – 7
POROSITY & PERMEABILITY
Dr. Biplab Bhattacharya
Dept. of Earth Sciences, IIT Roorkee
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Porosity & permeability
Porosity and permeability are important secondary, or
derived, properties of sedimentary rocks that are controlled
i t b th t t l tt ib t f i i hin part by the textural attributes of grain size, shape,
packing, and arrangement.
Because porosity and permeability are, in turn, controlling
parameters in the movement of fluids through rocks and
sediment, they are of particular interest to petroleum
geologists, petroleum engineers, and hydrologists
concerned with groundwater supplies and liquid waste
management.
Porosity and permeability also play an extremely important
role in the diagenesis of sediments by regulating the flow of
fluids through rocks that promote dissolution, cementation,
and authigenesis of minerals

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Porosity
The porosity of a rock is the ratio of its total pore space to
its total volume, that is, for a given sample:
Porosity = total volume – bulk volume
Ct- scan Image of a Carbonate Rock
Laboratory
Phi= 42% k=1050 mD
1st Computation
Phi= 24% k=1073 mD
Microporosity needs to be taken into account

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Porosity & permeability
Typically porosities in sediments range between 5 and 25%,
and porosities of 25-35 % are regarded as excellent if foundand porosities of 25 35 % are regarded as excellent if found
in an aquifer or oil reservoir.
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Primary and secondary Porosity

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1) Intergranular or interparticle
pore space that exists between or
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Primary (depositional) Porosity
pore space that exists between or
among framework grains, such as
siliciclastic particles and carbonate
grains (ooids, fossils, etc.)
2) Intragranular or intraparticle
pore space within particles, such
as cavities in fossils and open
space in clay minerals
(1) Solution porosity
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Secondary
(post-depositional) Porosity
caused by dissolution of
cements or metastable
framework grains (feldspars,
rock fragments) in siliciclastic
sedimentary rocks or
dissolution of cements,
fossils, framework crystals,, y ,
etc. in carbonate or other
chemically formed rocks
(Moldic, Vuggy porosity)

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(2) Intercrystalline
(cementation) porosity
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Secondary
Porosity
( ) p y
arising from pore space in
cements or among other
authigenic minerals
(Fenestral porosity, Bird’s eye)
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Secondary Porosity
(3) Fracture porosity
fracturing of any type of rock by
tectonic forces or other
processes such as compaction
and desiccation.

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Stromatactis
• Related to fenestral pore systems.
• Irregular patches of crystalline
calcite that are common on the flanks
of Paleozoic mud mounds around the
world (Monty et aL, 1995).
• Normally some 10 cm in length and
1-3 cm high. The base is flat, the
upper surface domed and irregular,
commonly dip radially from the center
of the mud moundof the mud mound.
• Stromatactis has been variously
attributed to a soft-bodied beast of
unknown origin, to bioturbation, to
algae, to micrite recrystallization, and
to gas bubbles, similar to the birds-
eye structure previously mentioned.
Geopetal fabric
Some stromatactis structures with a
partial infill of lime mud, the upper
surface of which is horizontal, while the
o erall str ct re dips do n the flank of
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overall structure dips down the flank of
the mound, indicates that stromatactis
developed as a penecontemporaneous
void just below the sea floor.
The rest of the void often shows two
phases of fill. There is a radiaxial fibrous
rim that formed prior to compaction at
shallow depth, and a later sparite cement
fill (B th t 1982)fill (Bathurst, 1982).
This structure is, therefore, a variety of
secondary porosity.
The geopetal fabric often seen in
stromatactis suggests the downslope
slumping of a lithified lime mud crust.

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Effective porosity is the amount of mutually interconnected
pore space present in a rock.
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Porosity & permeability
It is, of course, the effective porosity that is generally
economically important, and it is effective porosity that is
determined by many, but not all, methods of porosity
measurement.
The presence of effective porosity gives a rock the property of
permeability.
Example of thep f
injection of a CO2
saturated fluid
(pH=4) in two
bioclastic limestones
under constant flow
rate (Vialle, 2008)Estaillades limestone: bimodal
distribution of pore sizes due to the
presence of micrite.
St Maximin limestone: unimodal
distribution of pore sizes
Permeability is the ability of a fluid to flow through a
porous solid.
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Permeability
Permeability is controlled by many variables
- effective porosity of the rock,
- geometry of the pores, including their tortuosity, and
size of the throats between pores,
- capillary force between the rock and the invading fluid,
- its viscosity, and
pressure gradient- pressure gradient.
The permeability of rocks is highly variable, both depending on the
direction of measurement and vertically up or down sections.
Permeabilities ranging from 10 to 100 millidarcies are good, and
above that are considered exceptionally high.

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Permeability is conventionally determined from
Darcy's Law using the equation:
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Permeability
y g q
wherewhere,
Q is the rate of flow in cubic
centimeters per second,
Δ is the pressure gradient,
A is the cross-sectional area
 is the fluid viscosity in centipoises,
L is the length, and
K is the permeability.
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Porosity & permeability

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Porosity & permeability w.r.t. pore types
The pores themselves may be studied by a variety of methods –
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Measuring Porosity
i) examination of rough or polished rock surfaces by hand-lens
or stereoscopic microscope,
ii) through study of thin sections using a petrological
microscope,
iii) use of the scanning electron microscope.
iv) Another effective technique of studying pore fabric is to
impregnate the rock with a suitable plastic resin and then
dissolve the rock itself with an appropriate solvent.
Examination of the residue gives some indication, not only of
the size and shape of the pores themselves, but also of the
throat passages that connect pores (e.g., Wardlaw, 1976).

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BB Porosity & texture
BB Porosity & texture

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BB Porosity & texture
BB Porosity & structure

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Capillarity
Capillary pressure may be defined as the pressure difference across an
interface between two immiscible fluids. The height the liquid is drawn
up within a tube is related to the capillary pressure exerted at the
boundary between the two fluids, and to the diameter of the tube.
Considering sediment particles instead of tubes, there is a
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Capillarity
critical pore throat radius below which the capillary effect will
inhibit fluid flow for a given capillary pressure and a given
pressure differential.
An important petrophysical parameter of a sediment is its
capillary pressure curve. This is measured by plotting increasing
pressure against increasing saturation as one fluid is displaced
by anotherby another.
Because capillarity is a function of the radius of a pore throat, it
follows that the capillary pressure curve of a sediment will reflect
the size distribution of its constituent pore throat radii.