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

Definition: Porosity is the fraction of the bulk volume of a material (rock) that is occupied by pores (voids ).

Discussion Topics

• Origins and descriptions

• Factors that effect porosity

• Methods of determination

RESERVOIR POROSITY

ROCK MATRIX AND PORE SPACE

Rock matrix Pore spaceNote different use of “matrix”by geologists and engineers

Porosity: The fraction of the bulk volume of a rock that is occupied by pores

b

mab

b

p

VVV

VV

Porosity −==φ=

POROSITY DEFINITION

• Porosity is an intensive property describing the fluid storage capacity of rock

ROCK MATRIX AND PORE SPACE

Rock matrix Water Oil and/or gas

OBJECTIVESTo 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 ROCKSSEDIMENTARY

Roc

k-fo

rmin

gpr

oces

sSo

urce

of

mat

eria

l

IGNEOUS METAMORPHIC

Molten materials in deep crust andupper mantle

Crystallization(Solidification of melt)

Weathering anderosion of rocks

exposed at surface

Sedimentation, burial and lithification

Rocks under high temperatures

and pressures in deep crust

Recrystallization due toheat, pressure, or

chemically active fluids

SEDIMENTARY ROCKS

• Clastics

•Carbonates

•Evaporites

CLASTIC AND CARBONATE ROCKSClastic Rocks

Consist Primarily of Silicate Minerals

Are Classified on the Basis of:

- Grain Size- Mineral Composition

Carbonate RocksConsist Primarily of Carbonate Minerals(i.e. Minerals With a CO Anion Group)

Limestone - Predominately Calcite (Calcium Carbonate, CaCO3)Dolomite - Predominately Dolostone (Calcium Magnesium Carbonate, CaMg(CO3)2 )

3-2

Relative Abundances

Siltstoneand shale(clastic)

~75%

Sandstoneand conglomerate

(clastic)~11%

Limestone anddolomite

~14%

SEDIMENTARY ROCK TYPES,

SandGrains

ClayMatrix

ChemicalCement

QuartzFeldsparRock Fragments

QuartzCalcite

Hematite

IlliteKaoliniteSmectite

AverageSandstone

AverageMudrock(Shale)

AllochemicalGrains

ChemicalCement

MicrocrystallineMatrix

Calcite

FossilsPelloidsOolitesIntractlasts

Calcite

AverageSparry

LimestoneAverageMicritic

Limestone

Clastic Rocks Carbonate Rocks

Comparison of Compositions of Clasticand Carbonate Rocks

Grain-Size Classification for Clastic SedimentsName Millimeters Micrometers

BoulderCobblePebbleGranuleVery Coarse SandCoarse SandMedium SandFine SandVery Fine SandCoarse SiltMedium SiltFine SiltVery Fine SiltClay

4,096256644210.50.250.1250.0620.0310.0160.0080.004

500250125

623116

84

(modified from Blatt, 1982)

Average Detrital Mineral Compositionof Shale and Sandstone

Mineral Composition Shale Sandstone

Clay Minerals

Quartz

Feldspar

Rock Fragments

Carbonate

Organic Matter,Hematite, andOther Minerals

60 (%)

30

4

<5

3

<3

5 (%)

65

10-15

15

<1

<1

(modified from Blatt, 1982)

SANDSTONE CLASSIFICATIONQuartz + Chert

Feldspar

UnstableRock

Fragments

5 5

25 25

25 25

2525

50 50

5010 10

Quartzarenite

Subarkose Sublitharenite

LithicSubarkose

Arko

se

LithicArkose

FelspathicLitharenite

Litharenite

(modified from McBride, 1963)

Framework

Matrix

Cement

Pores

Sand (and Silt) Size Detrital Grains

Silt and Clay Size Detrital Material

Material Precipitated Post-Depositionally,During Burial. Cements Fill Pores andReplace Framework Grains

Voids Among the Above Components

FOUR MAJOR COMPONENTS OF SANDSTONE

FOUR COMPONENTS OF SANDSTONE

MATRIXFRAMEWORK

(QUARTZ)

FRAMEWORK(FELDSPAR)

CEMENT

PORE

Note different use of “matrix”by geologists and engineers

0.25 mm

1. Framework2. Matrix3. Cement4. Pores

Engineering“matrix”

Geologist’s Classification

ORIGINS OF POROSITY IN CLASTICS AND CARBONATES

(Genetic Classification)

• Primary (original)

• Secondary (induced)(Generally more complex thanprimary 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)

Dissolution

Micropores

Fractures

Interstitial Void Space BetweenFramework Grains

Partial or Complete Dissolution of

Framework Grains or CementSmall Pores Mainly Between Detrital

or Authigenic Grains (Can Also OccurWithin Grains

Breakage Due to Earth Stresses

FACTORS THAT AFFECT POROSITY

• Particle sphericity and angularity

• Packing

• Sorting (variable grain sizes)

• Cementing materials

• Overburden stress (compaction)

• Vugs, dissolution, and fractures

PRIMARY

SECONDARY (diagenetic)

ROUNDNESS AND SPHERICITYOF CLASTIC GRAINS

High

SPH

ERIC

ITY

Low

VeryAngular Angular Sub-

AngularSub-

Rounded Rounded Well-Rounded

ROUNDNESS

Porosity

Poro

sity

FACTORS THAT AFFECT POROSITY

• Particle sphericity and angularity

• Packing

• Sorting (variable grain sizes)

• Cementing materials

• Overburden stress (compaction)

• Vugs, dissolution, and fractures

PRIMARY

SECONDARY (DIAGENETIC)

Line of Traverse(using microscope)

Cement

Matrix(clays, etc.)

Tangential Contact

Sutured Contact

Long Contact

Concavo-ConvexContact

GRAIN PACKING IN SANDSTONE

(modified from Blatt, 1982)

This Example

Packing Proximity = 40%Packing Density = 0.8

4 Types of Grain Contacts

Packing Proximity

Packing Density

A measure of the extent towhich sedimentary particlesare in contact with their neighbors

A measure of the extent towhich sedimentary particles

occupy the rock volume

CUBIC PACKING OF SPHERESPorosity = 0.48

Porosity Calculations - Uniform Spheres

• Bulk volume = (2r)3 = 8r3

• Matrix volume =

• Pore volume = bulk volume - matrix volume

3r4 3π

( ) 476.032

18

3/483

33

=π

−=π−

=

−=

=

rrrVolumeBulk

VolumeMatrixVolumeBulkVolumeBulkVolumePorePorosity

RHOMBIC PACKING OF SPHERESPorosity = 0.27

FACTORS THAT AFFECT POROSITY

• Particle sphericity and angularity

• Packing

• Sorting (variable grain sizes)

• Cementing materials

• Overburden stress (compaction)

• Vugs, dissolution, and fractures

PRIMARY

SECONDARY (DIAGENETIC)

Packing of Two Sizes of SpheresPorosity = 0.14

Grain-Size Sorting in Sandstone

Very WellSorted

WellSorted

ModeratelySorted

PoorlySorted

Very PoorlySorted

SORTING

Change of Composition Change of Size

Change of Shape Change of Orientation

Change of Packing

Sand

Shale

Eolian

Fluvial

Slow CurrentFast Current

River

Beach

TYPES OF TEXTURAL CHANGES SENSEDBY THE NAKED EYE AS BEDDING

PROGRESSIVE DESTRUCTION OFBEDDING THROUGH BIOTURBATION

RegularLayers

IrregularLayers

Mottles(Distinct)

Mottles(Indistinct)

HomogeneousDeposits

(Whole Core)Bioturbated Sandstone

STS61A-42-0051 Mississippi River Delta, Louisiana, U.S.A. October 1985

STS084-721-029 Selenga River Delta, Lake Baykal, Russia May 1997

FACTORS THAT AFFECT POROSITY

• Particle sphericity and angularity

• Packing

• Sorting (variable grain sizes)

• Cementing materials

• Overburden stress (compaction)

• Vugs, dissolution, and fractures

PRIMARY

SECONDARY (DIAGENETIC)

DIAGENESIS

CarbonateCemented

OilStained

Diagenesis is the Post-Depositional Chemical andMechanical Changes thatOccur in Sedimentary Rocks

Some Diagenetic Effects Include

CompactionPrecipitation of CementDissolution of Framework

Grains and Cement

The Effects of Diagenesis MayEnhance or Degrade ReservoirQuality

Whole CoreMisoa Formation, Venezuela Photo by W. Ayers

DUAL POROSITY IN SANDSTONE

MATRIX

FRAMEWORK(QUARTZ)

FRAMEWORK(FELDSPAR)

CEMENT

PORE

Note different use of “matrix”by geologists and engineers

0.25 mm

Sandstone Comp.• Framework• Matrix• Cement• Pores

DISSOLUTIONPORE

FRACTURE

1. Primary and secondary “matrix” porosity system2. Fracture porosity system

SANDSTONE COMPOSITION,Framework Grains

Norphlet Sandstone, Offshore Alabama, USAGrains ~0.25 mm in Diameter/Length

PRF KF

P

KF = PotassiumFeldspar

PRF = Plutonic RockFragment

P = Pore

Potassium Feldspar isStained Yellow With aChemical Dye

Pores are Impregnated WithBlue-Dyed Epoxy

KF

Q

Q

Q = Quartz

Photo by R. Kugler

POROSITY IN SANDSTONE

QuartzGrain

Pore

Scanning Electron MicrographNorphlet Sandstone, Offshore Alabama, USA

Porosity in SandstoneTypically is Lower ThanThat of Idealized PackedSpheres Owing to:

Variation in Grain SizeVariation in Grain ShapeCementationMechanical and ChemicalCompaction

Photomicrograph by R.L. Kugler

POROSITY IN SANDSTONE

Scanning Electron MicrographTordillo Sandstone, Neuquen Basin, Argentina

Pore Throats inSandstone MayBe Lined WithA Variety ofCement MineralsThat AffectPetrophysicalProperties

Photomicrograph by R.L. Kugler

POROSITY IN SANDSTONE

Scanning Electron MicrographNorphlet Formation, Offshore Alabama, USA

Pores Provide theVolume to StoreHydrocarbons

Pore Throats RestrictFlow through pores

PoreThroat

Secondary Electron Micrograph

Clay Minerals in Sandstone Reservoirs,Authigenic Chlorite

Jurassic Norphlet SandstoneOffshore Alabama, USA (Photograph by R.L. Kugler)

Occurs as ThinCoats on DetritalGrain Surfaces

Occurs in SeveralDeeply BuriedSandstones WithHigh Reservoir Quality

Iron-Rich Varieties ReactWith Acid

~ 10 μm

Electron Photomicrograph

Clay Minerals in Sandstone Reservoirs,Fibrous Authigenic Illite

Jurassic Norphlet SandstoneHatters Pond Field, Alabama, USA (Photograph by R.L. Kugler)

Illite

SignificantPermeabilityReduction

Negligible PorosityReduction

Migration ofFines Problem

High IrreducibleWater Saturation

INTERGRANULAR PORE AND MICROPOROSITY

IntergranularPore

Microporosity

Kaolinite QuartzDetritalGrain

Intergranular PoresContain HydrocarbonFluids

Micropores ContainIrreducible Water

Backscattered Electron MicrographCarter Sandstone, Black Warrior Basin,Alabama, USA (Photograph by R.L. Kugler)

Clay Minerals in Sandstone Reservoirs,Authigenic Kaolinite

Secondary Electron Micrograph

Carter SandstoneNorth Blowhorn Creek Oil UnitBlack Warrior Basin, Alabama, USA

Significant PermeabilityReduction

High Irreducible WaterSaturation

Migration of FinesProblem

(Photograph by R.L. Kugler)

DISSOLUTION POROSITY

Thin Section Micrograph - Plane Polarized LightAvile Sandstone, Neuquen Basin, Argentina

Dissolution ofFramework Grains(Feldspar, for Example) and Cement may Enhance theInterconnected Pore System

This is SecondaryPorosity

Pore

Quartz DetritalGrain

PartiallyDissolvedFeldspar

Photo by R.L. Kugler

DISSOLUTION POROSITY

Scanning Electron MicrographTordillo Formation, Neuquen Basin, Argentina

PartiallyDissolvedFeldspar

Dissolution PoresMay be Isolated andnot Contribute to theEffective Pore System

Photo by R.L. Kugler

SandGrains

ClayMatrix

ChemicalCement

QuartzFeldsparRock Fragments

QuartzCalcite

Hematite

IlliteKaoliniteSmectite

AverageSandstone

AverageMudrock(Shale)

AllochemicalGrains

ChemicalCement

MicrocrystallineMatrix

Calcite

FossilsPelloidsOolitesIntractlasts

Calcite

AverageSparry

LimestoneAverageMicritic

Limestone

Clastic Rocks Carbonate Rocks

Comparison of Compositions of Clasticand Carbonate Rocks

Iles GambierTuamotu Archipelago

Maldive Islands

FOLK CARBONATE ROCK CLASSIFICATION

0-1% 1-10% 10-50% Over50%

SparseBiomicrite

Micrite &Dismicrite

Fossili-ferousMicrite

PackedBiomicrite

PoorlyWashed

Biosparite

UnsortedBiosparite

SortedBiosparite

RoundedBiosparite

Over 2/3 Lime Mud Matrix Over 2/3 Spar CementSubequalSpar &

Lime MudSortingPoor

SortingGood

Rounded,Abraded

Claystone SandyClaystone

Clayey orImmature Sandstone

Sub-mature SS

MatureSS

Super-mature SS

Depositional Texture Recognizable Depositional TextureNot Recognizable

D unham Carb onate Rock Class ificatio n

D epositional Texture Re cognizable Dep ositionalT extureNotR ecognizable

MudstoneW ackestone Packston eGrainst one Boundston eCry stalineCa rbonateGrainSupporte dLacks M ud,Grain -Suppo rted

Components Not Bound T ogether Duri ng Deposition

Mud Su pported Contains M ud(clay and silt size particles<10 %Grains >10 %Grains

Orig inal Compon entsB ound Togethe rDu ring Depositi on

DUNHAM CARBONATE ROCK CLASSIFICATION

Depositional Texture Recognizable DepositionalTexture

Not Recognizable

Mudstone Wackestone Packstone Grainstone BoundstoneCrystallineCarbonate

GrainSupported

Lacks Mud,Grain-

Supported

Components Not Bound Together During Deposition

Mud Supported

Contains Mud(clay and silt size particles

<10 %Grains

>10 %Grains

Original ComponentsBound Together

During Deposition

CARBONATES POROSITY TYPES

Interparticle

Intraparticle

Intercrystal

Moldic

Pores Between Particles or Grains

Pores Within Individual Particles or Grains

Pores Between Crystals

Pores Formed by Dissolution of anIndividual Grain or Crystal in the Rock

Fenestral

Fracture

Vug

Primary Pores Larger Than Grain-SupportedInterstices

Formed by a Planar Break in the Rock

Large Pores Formed by IndiscriminateDissolution of Cements and Grains

Interparticle Intraparticle Intercrystal Moldic

Fenestral Shelter Growth-Framework

FabricSelective

Fracture Channel Vug

Non-FabricSelective

Breccia Boring Burrow Shrinkage

Fabric Selective or Not Fabric Selective

Idealized Carbonate Porosity Types

(modified from Choquette and Pray, 1970)

CARBONATE POROSITY - EXAMPLE

Thin section micrograph - plane-polarized lightSmackover Formation, Alabama (Photograph by D.C. Kopaska-Merkel)

MoldicPores

• Due to dissolutionand collapse of ooids(allochemical particles)

• Isolated pores

• Low effective porosity

• Low permeability

Blue areas are pores.Calcite

Dolomite

MoldicPore

CARBONATE POROSITY - EXAMPLE

Thin section micrographSmackover Formation, AlabamaBlack areas are pores.

(Photograph by D.C. Kopaska-Merkel)

• Combination pore system

• Moldic pores formed throughdissolution of ooids (allochemicalparticles)

• Connected pores

• High effective porosity

• High permeability

MoldicPore

InterparticlePores

Moldic andInterparticle Pores

PORE SPACE CLASSIFICATION

(In Terms of Fluid Properties)

PORE-SPACE CLASSIFICATION

• Total porosity, φt =

• Effective porosity, φe =VolumeBulk

PoreVolumeTotal

VolumeBulkPore SpacectedInterconne

• Effective porosity – of great importance;

contains the mobile fluid

COMPARISON OF TOTAL AND EFFECTIVE POROSITIES

• Very clean sandstones : φe → φt

• Poorly to moderately well -cemented intergranular materials: φt ≈ φe

• Highly cemented materials and most carbonates: φe < φt

MEASUREMENT OF POROSITY

• Core samples (Laboratory)

• Openhole wireline logs

Quartz(Framework ) Sm allPor es IsolatedPoresLarge , Interconn ectedPoresClay Surfa ces& Interlay ersClayLayers

Hydration orBound W ater Hydrocarbo nPore Volum eStructu ral(OH -) W ater

Roc kMatr ix

Total Po rosity - Neu tron LogTotal Po rosity - Den sity LogAb solute or To tal PorosityOve n-Dried Co re Analysis PorosityHumid ity-DriedCore Analy sis Porosity

Cap illaryWa ter

VShale

Sa ndston e Por osity M easur edby Va rious T echn iques

Quartz(Framework ) Sm allPor es IsolatedPoresLarge , Interconn ectedPoresClay Surfa ces& Interlay ersClayLayers

Hydration orBound W ater Hydrocarbo nPore Volum eStructu ral(OH -) W ater

Roc kMatr ix

Total Po rosity - Neu tron LogTotal Po rosity - Den sity LogAb solute or To tal PorosityOve n-Dried Co re Analysis PorosityHumid ity-DriedCore Analy sis Porosity

Cap illaryWa ter

VShale

Sa ndston e Por osity M easur edby Va rious T echn iques

SANDSTONE POROSITY MEASUREDBY VARIOUS TECHNIQUES

Quartz(Framework)

SmallPores

IsolatedPores

Large, InterconnectedPores

Clay Surfaces& Interlayers

ClayLayers

Irreducible orImmobile Water

Hydration orBound Water

HydrocarbonPore Volume

Structural(OH -) Water

RockMatrix

Total Porosity - Neutron LogTotal Porosity - Density Log

Absolute or Total Porosity

Oven-Dried Core Analysis PorosityHumidity-Dried

Core Analysis Porosity

CapillaryWater

VShale

(modified from Eslinger and Pevear, 1988)

INFORMATION FROM CORES*

• Porosity

• Horizontal permeability to air

• Grain density

• Vertical permeability to air• Relative permeability

• Capillary pressure

• Cementation exponent (m) and saturation exponent (n)

Standard Analysis Special Core Analysis

*Allows calibration of wireline log results

PDC Cutters

Fluidvent

Drill collarconnection

Inner barrel

Outer barrel

Thrust bearing

Core retainingring

Core bit

CORING ASSEMBLYAND CORE BIT

COMING OUT OF HOLEWITH CORE BARREL

Whole Core Photograph,Misoa “C” Sandstone,

Venezuela

WHOLE CORE

Photo by W. Ayers

SIDEWALL SAMPLING GUN

Core bullets

Core sample

Formation rock

SIDEWALL CORING TOOL

Coring bit

Samples

WHOLE CORE ANALYSIS vs. PLUGS OR SIDEWALL CORES

WHOLE CORE

• Provides larger samples

• Better and more consistent representation of formation

• Better for heterogeneous rocks or for more complex lithologies

• Smaller samples

• Less representative of heterogeneous formations

• Within 1 to 2% of whole cores for medium-to high-porosity formation

• In low-porosity formations, φ from core plugs tends to be much greater than φ from whole cores

• Scalar effects in fractured reservoirs

WHOLE CORE ANALYSIS vs. PLUGS OR SIDEWALL CORES

PLUGS OR SIDEWALL CORES

Sparks and Ayers, unpublished

CORE PLUG

LABORATORY DETERMINATIONOF POROSITY

NEXT:

Student Questions / Answers• intraparticle porosity in carbonates (JC1):

– vugs and fractures• why are clays important (JC1):

– one major reason is that clays conduct electricity, this can effect water saturation calculations if not accounted for

• fines (ABW):– solid particles so small that they can flow with fluids

through pores - but they can also plug pore throats• tortuousity (ABW):

– the indirect curvy flow path through the pore system to get from point A to point B

• holocene:– referring to the Holocene Epoch (geology) or in general

meaning about the last 10,000 years.