Lesson 4: Porosity and Permeability Flashcards
what is the key difference between porosity and permeability
porosity refers to the pore spaces between sediment grains (the ability for sediments to hold fluids), while permeability refers to the interconnectedness between those pore spaces (the permissiveness for fluid to flow)
defined as the ratio of the pore spaces vs the total volume of the sedimentary rocks
porosity
pore spaces that are interconnected
effective porosity
in terms of assessing resources such as groundwater and petroleum, what is more important, total porosity or effective porosity?
effective porosity
formula for % porosity
% porosity = (Vp/Vb)100
Vp - pore volume
Vp - bulk volume of rock
formula for % effective porosity
% effective porosity = (IVp/Vb)100
IVp - interconnected pore volume
Vp - bulk volume of rock
2 types of porosity
primary (depositional)
secondary (post-depositional)
pore spaces formed during deposition/lithification
primary (depositional)
three subtypes under primary porosity
intergranular/interparticle
intragranular/intraparticle
intercrystalline
pore spaces in between grains
intergranular/interparticle
pore spaces with in the grains themselves (rock or mineral grain)
intragranular/intraparticle
pore spaces mostly referring to chemical/biochemical sedimentary rocks, between crystals
intercrystalline
rock example with intercrystalline porosity
tavertine
pore spaces formed after deposition/diagenesis
secondary (post-deposition)
three subtypes under secondary porosity
solution porosity
intercrystalline
fracture
porosity when cementing materials are chemically eroded away from or between primary sediments, the pore spaces can then be filled with fluid
solution porosity
examples of rocks that have solution porosity
sandstone with calcite/feldspar/clay minerals, those three can be chemically weathered away from the primary sediments
porosity that mostly happens because of authigenic sediments.
In pore spaces that have chemical matter, authigenic sediments can grow and the spaces that remain after they grow refer to this porosity
intercrystalline
porosity that results from tectonic forces
fracture
the ultimate porosity of a rock depends on what two things, and which kinds of porosities specifically?
The ultimate porosity of a rock thus depends upon:
- initial depositional conditions: all of the primary porosities
- diagenetic history of the rock: the first 2 of secondary porosities
3 ways to measure porosity
Most of these techniques determine the effective porosity of a specimen by first measuring the pore volume
- Pore volume is calculated by determining the volume of fluid or gas that can be forced into the rock to completely fill the pores.
- Porosity can be examined thru thin sections by point counting: u count pore spaces v bulk volume using the equation
- alternative technique: determine pore volume indirectly by:
- determining the grain volume of a specimen: by dividing the dry weight of the specimen by the rock grain density
- assuming that the composition is uniform, the pore volume is the grain volume subtracted from the bulk volume
- done with out cutting
can indirect pore volume measurement be used for all types of sedimentary rocks?
it’s best for homogeneous rocks, but it can be used for heterogeneous rocks too, you just have to weigh the dry and wet weight very carefully
2 examples of homogeneous rocks
quartz sandstone and crystalline limestone
commonly defined as the ability of a medium to transmit a fluid
permeability
thought of more simply as the property of a rock that permits the passage of a fluid through the interconnected pores of the rock
rock permeability
rock permeability formula (flow rate equation)
Q = K (P1-P2) (A/L)
Q - flow rate
K - permeability constant
P1-P2 - pressure drop along the medium
A - cross section of the medium
L - length of the medium
what are the 6 factors that influence porosity and permeability
the effect of:
- grain shape
- grain size
- sorting
- fabric
- secondary/tectonic porosity and permeability
- diagenesis
how does grain shape affect effective porosity and permeability
sphericity -not porous/permeable if spherical esp if tight packing
roundedness -more porous if more rounded (brick walls, not rounded, no space, not porous)
how does grain size affect effective porosity and permeability
greatly affects, the larger, the more Eporous and permeable, i.e. sand and larger
how does sorting affect effective porosity and permeability
the more poorly sorted, the less Eporous and permeable because the smaller particles can get deposited in the voids
how does fabric affect effective porosity and permeability
fabric refers to the orientation of individual particles among each other.
- parallel packing: less space, poor porosity, especially for elongated (oblate) sediments
- not parallel: more space
how does secondary/tectonic porosity and permeability affect effective porosity and permeability
less porous is there are no fractures, esp if tightly packed
how does diagenesis affect effective porosity and permeability
- early (shallow burial stage): cementation, early lithification, primary porosities
- middle (deep burial): sediments are maturing, removal of incompatible minerals
- dissolution
- induration: in very mature clastics, induration fills primary sediments in voids, that’s why they consider age when looking for groundwater or petroleum - late (uplift from deep burial): if either exposed to the surface, tilted, deformed. depends on the geologic history from deep burial to uplift
the oldest age of rocks with petroleum/groundwater
uplifted carboniferous
what should be considered in late diagenesis effects in porosity and permeability
discontinuities between rocks, joints, faults, bedding planes, contacts (planes of weaknesses)
