RESERVOIR POROSITY

POROSITY

Many slides contain more detailed notes that may be shown using the “Notes Page View”


Acknowledgments
• Dr. Walt Ayers, PETE 311, Fall 2001
• NExT PERF Short Course Notes, 1999
– Note that many of the NExT slides appears to have been
obtained from other primary sources that are not cited


RESERVOIR POROSITY
Definition: Porosity is the fraction of a rock that
is occupied by voids (pores).
Discussion Topics
• Origins and descriptions
• Factors that effect porosity
• Methods of determination


ROCK MATRIX AND PORE SPACE

Rock matrix

Pore space
Note different use of “matrix”
by geologists and engineers


POROSITY DEFINITION

Porosity: The fraction of a rock that is
occupied by pores
• Porosity is an intensive property describing the
fluid storage capacity of rock

Vp

Vb − Vma
Porosity = φ =
=
Vb
Vb


ROCK MATRIX AND PORE SPACE

Rock matrix

Water

Oil and/or gas


OBJECTIVES
To provide an understanding of
• The concepts of rock matrix and porosity
• The difference between original (primary) and
induced (secondary) porosity
• The difference between total and effective
porosity

• Laboratory methods of porosity determination
• Determination of porosity from well logs


CLASSIFICATION OF ROCKS

Rock-forming Source of
process
material

IGNEOUS

SEDIMENTARY

METAMORPHIC

Molten materials in
deep crust and
upper mantle

Weathering and
erosion of rocks
exposed at surface

Rocks under high
temperatures
and pressures in
deep crust

Crystallization

(Solidification of melt)

Sedimentation, burial
and lithification

Recrystallization due to
heat, pressure, or
chemically active fluids


SEDIMENTARY ROCKS
• Clastics
•Carbonates
•Evaporites


CLASTIC AND CARBONATE ROCKS
Clastic Rocks
Consist Primarily of Silicate Minerals
Are Classified on the Basis of:
- Grain Size
- Mineral Composition

Carbonate Rocks
Consist Primarily of Carbonate Minerals
(i.e. Minerals With a CO3-2 Anion Group)
Limestone - Predominately Calcite (Calcium
Carbonate, CaCO3)
Dolomite - Predominately Dolostone (Calcium
Magnesium Carbonate, CaMg(CO3)2 )

SEDIMENTARY ROCK TYPES,
Relative Abundances
Sandstone
and conglomerate
(clastic)
~11%

Siltstone
and shale
(clastic)
~75%

Limestone and
dolomite
~14%


Comparison of Compositions of Clastic
and Carbonate Rocks
Clastic Rocks
Sand
Grains

Carbonate Rocks
Fossils
Allochemical Pelloids
Oolites
Grains

Intractlasts

Quartz
Feldspar
Rock Fragments
Average
Sandstone

Average
Mudrock
(Shale)

Average
Sparry
Limestone
Average
Micritic
Limestone

Clay
Matrix

Chemical
Cement

Illite
Kaolinite
Smectite

Quartz

Calcite
Hematite

Microcrystalline
Matrix

Chemical
Cement

Calcite

Calcite


Grain-Size Classification for Clastic Sediments
Name
Boulder
Cobble
Pebble
Granule
Very Coarse Sand
Coarse Sand
Medium Sand
Fine Sand
Very Fine Sand
Coarse Silt
Medium Silt
Fine Silt
Very Fine Silt
Clay

Millimeters

Micrometers

4,096

256
64
4
2
1
0.5
0.25
0.125
0.062
0.031
0.016
0.008
0.004

500
250
125
62
31
16
8
4
(modified from Blatt, 1982)

Average Detrital Mineral Composition
of Shale and Sandstone
Mineral Composition

Shale

Sandstone

Clay Minerals

60 (%)

Quartz

30

65

4

10-15

<5

15

3

<1

<3

<1

Feldspar
Rock Fragments
Carbonate
Organic Matter,
Hematite, and
Other Minerals

5 (%)

(modified from Blatt, 1982)


SANDSTONE CLASSIFICATION
Quartz + Chert
5

5

Quartzarenite

Subarkose

Sublitharenite

25

25

Lithic
Subarkose
50

50

Ar
ko

se

ren
ha
Lit

Feldspar

10

Lithic
Arkose

25

(modified from McBride, 1963)

Felspathic
Litharenite

50

25

i te

25

25

10

Unstable
Rock
Fragments


FOUR MAJOR COMPONENTS OF
SANDSTONE
Framework
Sand (and Silt) Size Detrital Grains

Matrix
Silt and Clay Size Detrital Material

Cement
Material Precipitated Post-Depositionally,

During Burial. Cements Fill Pores and
Replace Framework Grains

Pores
Voids Among the Above Components


FOUR COMPONENTS OF SANDSTONE
Geologist’s Classification

1.
2.
3.
4.

Framework
Matrix
Cement
Pores

Note different use of “matrix”
by geologists and engineers

Engineering
“matrix”

PORE
FRAMEWORK

CEMENT

(QUARTZ)

MATRIX

FRAMEWORK
(FELDSPAR)

0.25 mm


ORIGINS OF POROSITY IN
CLASTICS AND CARBONATES
(Genetic Classification)

• Primary (original)
• Secondary (induced)
(Generally more complex than
primary porosity)


PRIMARY (ORIGINAL) POROSITY
• Developed at deposition
• Typified by
– Intergranular pores of clastics or
carbonates
– Intercrystalline and fenestral pores of carbonates

• Usually more uniform than induced porosity

SECONDARY (INDUCED) POROSITY
•

Developed by geologic processes after
deposition (diagenetic processes)

•

Examples
–
–
–

Grain dissolution in sandstones or carbonates
Vugs and solution cavities in carbonates
Fracture development in some sandstones, shales,
and carbonates


SANDSTONES POROSITY TYPES
Intergranular (Primary) Interstitial Void Space Between
Framework Grains
Micropores

Small Pores Mainly Between Detrital
Framework Grains or Cement

Dissolution

Partial or Complete Dissolution of
or Authigenic Grains (Can Also Occur
Within Grains

Fractures

Breakage Due to Earth Stresses


FACTORS THAT AFFECT POROSITY
PRIMARY
• Particle sphericity and angularity
• Packing
• Sorting (variable grain sizes)

SECONDARY (diagenetic)
• Cementing materials
• Overburden stress (compaction)
• Vugs, dissolution, and fractures


SPHERICITY

Porosity

ROUNDNESS AND SPHERICITY
OF CLASTIC GRAINS

High

Low
Very
Angular
Angular

SubSubWellRounded
Rounded
Angular Rounded

ROUNDNESS

Porosity


FACTORS THAT AFFECT POROSITY
PRIMARY
• Particle sphericity and angularity
• Packing
• Sorting (variable grain sizes)

SECONDARY (DIAGENETIC)
• Cementing materials
• Overburden stress (compaction)
• Vugs, dissolution, and fractures


GRAIN PACKING IN SANDSTONE
Line of Traverse
(using microscope)

4 Types of Grain Contacts
Packing Proximity

Sutured Contact

A measure of the extent to
which sedimentary particles
are in contact with their
neighbors

Long Contact

Packing Density

Tangential Contact

Cement

Matrix
(clays, etc.)

A measure of the extent to
which sedimentary particles
occupy the rock volume
Concavo-Convex
Contact

This Example
Packing Proximity = 40%
Packing Density = 0.8

(modified from Blatt, 1982)