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

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

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

  • SEDIMENTARY
  • Rock-forming
  • process
  • Source of
  • material
  • IGNEOUS
  • METAMORPHIC
  • Molten materials in
  • deep crust and
  • upper mantle
  • Crystallization
  • (Solidification of melt)
  • Weathering and
  • erosion of rocks
  • exposed at surface
  • Sedimentation, burial
  • and lithification
  • Rocks under high
  • temperatures
  • and pressures in
  • deep crust
  • 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
  • Carbonate Rocks
  • Consist 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

SEDIMENTARY ROCK TYPES,

  • Relative Abundances
  • Siltstone
  • and shale
  • (clastic)
  • ~75%
  • Sandstone
  • and conglomerate
  • (clastic)
  • ~11%
  • Limestone and
  • dolomite
  • ~14%
  • Sand
  • Grains
  • Clay
  • Matrix
  • Chemical
  • Cement
  • Quartz
  • Feldspar
  • Rock Fragments
  • Quartz
  • Calcite
  • Hematite
  • Illite
  • Kaolinite
  • Smectite
  • Average
  • Sandstone
  • Average
  • Mudrock
  • (Shale)
  • Allochemical
  • Grains
  • Chemical
  • Cement
  • Microcrystalline
  • Matrix
  • Calcite
  • Fossils
  • Pelloids
  • Oolites
  • Intractlasts
  • Calcite
  • Average
  • Sparry
  • Limestone
  • Average
  • Micritic
  • Limestone
  • Clastic Rocks
  • Carbonate Rocks
  • Comparison of Compositions of Clastic
  • and Carbonate
  • Rocks
  • Grain-Size Classification for Clastic Sediments
  • Name
  • Millimeters
  • Micrometers
  • 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
  • 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
  • Quartz
  • Feldspar
  • Rock Fragments
  • Carbonate
  • Organic Matter,
  • Hematite, and
  • Other Minerals
  • 60 (%)
  • 30
  • 4
  • <5
  • 3
  • <3
  • 5 (%)
  • 65
  • 10-15
  • 15
  • <1
  • <1
  • (modified from Blatt, 1982)
  • SANDSTONE CLASSIFICATION
  • Quartz + Chert
  • Feldspar
  • Unstable
  • Rock
  • Fragments
  • 5
  • 5
  • 25
  • 25
  • 25
  • 25
  • 25
  • 25
  • 50
  • 50
  • 50
  • 10
  • 10
  • Quartzarenite
  • Subarkose
  • Sublitharenite
  • Lithic
  • Subarkose
  • Arkose
  • Lithic
  • Arkose
  • Felspathic
  • Litharenite
  • 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 and
  • Replace Framework Grains
  • Voids Among the Above Components
  • FOUR MAJOR COMPONENTS OF SANDSTONE
  • FOUR COMPONENTS OF SANDSTONE
  • MATRIX
  • FRAMEWORK
  • (QUARTZ)
  • FRAMEWORK
  • (FELDSPAR)
  • CEMENT
  • PORE
  • Note different use of “matrix”
  • by geologists and engineers
  • 0.25 mm
  • Framework
  • Matrix
  • Cement
  • Pores
  • Engineering
  • “matrix”
  • Geologist’s Classification

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)
  • Dissolution
  • Micropores
  • Fractures
  • Interstitial Void Space Between
  • Framework Grains
  • Partial or Complete Dissolution of
  • Framework Grains or Cement
  • Small Pores Mainly Between Detrital
  • or Authigenic Grains (Can Also Occur
  • Within 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 SPHERICITY
  • OF CLASTIC GRAINS
  • High
  • SPHERICITY
  • Low
  • Very
  • Angular
  • Angular
  • Sub-
  • Angular
  • Sub-
  • Rounded
  • Rounded
  • Well-
  • Rounded
  • ROUNDNESS
  • Porosity
  • Porosity

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-Convex
  • Contact
  • GRAIN PACKING IN SANDSTONE
  • (modified from Blatt, 1982)
  • This Example
  • Packing Proximity = 40%
  • Packing Density = 0.8
  • Packing Proximity
  • Packing Density
  • A measure of the extent to
  • which sedimentary particles
  • are in contact with their
  • neighbors
  • A measure of the extent to
  • which sedimentary particles
  • occupy the rock volume
  • CUBIC PACKING OF SPHERES
  • Porosity = 0.48

Porosity Calculations - Uniform Spheres

  • Bulk volume = (2r)3 = 8r3
  • Matrix volume =
  • Pore volume = bulk volume - matrix volume
  • RHOMBIC PACKING OF SPHERES
  • Porosity = 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 Spheres
  • Porosity = 0.14
  • Grain-Size Sorting in Sandstone
  • Very Well
  • Sorted
  • Well
  • Sorted
  • Moderately
  • Sorted
  • Poorly
  • Sorted
  • Very Poorly
  • Sorted
  • SORTING
  • Change of Composition
  • Change of Size
  • Change of Shape
  • Change of Orientation
  • Change of Packing
  • Sand
  • Shale
  • Eolian
  • Fluvial
  • Slow Current
  • Fast Current
  • River
  • Beach
  • TYPES OF TEXTURAL CHANGES SENSED
  • BY THE NAKED EYE AS BEDDING
  • PROGRESSIVE DESTRUCTION OF
  • BEDDING THROUGH BIOTURBATION
  • Regular
  • Layers
  • Irregular
  • Layers
  • Mottles
  • (Distinct)
  • Mottles
  • (Indistinct)
  • Homogeneous
  • Deposits
  • (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
  • Carbonate
  • Cemented
  • Oil
  • Stained
  • Diagenesis is the Post-
  • Depositional Chemical and
  • Mechanical Changes that
  • Occur in Sedimentary Rocks
  • Some Diagenetic Effects Include
  • Compaction
  • Precipitation of Cement
  • Dissolution of Framework
  • Grains and Cement
  • The Effects of Diagenesis May
  • Enhance or Degrade Reservoir
  • Quality
  • Whole Core
  • Misoa 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
  • DISSOLUTION
  • PORE
  • FRACTURE
  • Primary and secondary “matrix” porosity system
  • Fracture porosity system
  • SANDSTONE COMPOSITION,
  • Framework Grains
  • Norphlet Sandstone, Offshore Alabama, USA
  • Grains ~0.25 mm in Diameter/Length
  • PRF
  • KF
  • P
  • KF = Potassium
  • Feldspar
  • PRF = Plutonic Rock
  • Fragment
  • P = Pore
  • Potassium Feldspar is
  • Stained Yellow With a
  • Chemical Dye
  • Pores are Impregnated With
  • Blue-Dyed Epoxy
  • KF Q
  • Q
  • Q = Quartz
  • Photo by R. Kugler
  • POROSITY IN SANDSTONE
  • Quartz
  • Grain
  • Pore
  • Scanning Electron Micrograph
  • Norphlet Sandstone, Offshore Alabama, USA
  • Porosity in Sandstone
  • That of Idealized Packed
  • Spheres Owing to:
  • Variation in Grain Size
  • Variation in Grain Shape
  • Cementation
  • Mechanical and Chemical
  • Compaction
  • Photomicrograph by R.L. Kugler
  • POROSITY IN SANDSTONE
  • Scanning Electron Micrograph
  • Tordillo Sandstone, Neuquen Basin, Argentina
  • Pore Throats in
  • Sandstone May
  • Be Lined With
  • A Variety of
  • Cement Minerals
  • That Affect
  • Petrophysical
  • Properties
  • Photomicrograph by R.L. Kugler
  • POROSITY IN SANDSTONE
  • Scanning Electron Micrograph
  • Norphlet Formation, Offshore Alabama, USA
  • Pores Provide the
  • Volume to Store
  • Hydrocarbons
  • Pore Throats Restrict
  • Flow through pores
  • Pore
  • Throat
  • Secondary Electron Micrograph
  • Clay Minerals in Sandstone Reservoirs,
  • Authigenic Chlorite
  • Jurassic Norphlet Sandstone
  • Offshore Alabama, USA
  • (Photograph by R.L. Kugler)
  • Occurs as Thin
  • Coats on Detrital
  • Grain Surfaces
  • Occurs in Several
  • Deeply Buried
  • Sandstones With
  • High Reservoir
  • Quality
  • Iron-Rich
  • Varieties React
  • With Acid
  • ~ 10
  • m
  • Electron Photomicrograph
  • Clay Minerals in Sandstone Reservoirs,
  • Fibrous Authigenic Illite
  • Jurassic Norphlet Sandstone
  • (Photograph by R.L. Kugler)
  • Illite
  • Significant
  • Permeability
  • Reduction
  • Negligible
  • Porosity
  • Reduction
  • Migration of
  • Fines Problem
  • High Irreducible
  • Water Saturation
  • INTERGRANULAR PORE AND MICROPOROSITY
  • Intergranular
  • Pore
  • Microporosity
  • Kaolinite
  • Quartz
  • Detrital
  • Grain
  • Intergranular Pores
  • Contain Hydrocarbon
  • Fluids
  • Micropores Contain
  • Irreducible Water
  • Backscattered Electron Micrograph
  • Carter Sandstone, Black Warrior Basin,
  • Alabama, USA
  • (Photograph by R.L. Kugler)
  • Clay Minerals in Sandstone Reservoirs,
  • Authigenic Kaolinite
  • Secondary Electron Micrograph
  • Carter Sandstone
  • North Blowhorn Creek Oil Unit
  • Black Warrior Basin, Alabama, USA
  • Significant Permeability
  • Reduction
  • High Irreducible Water
  • Saturation
  • Migration of Fines
  • Problem
  • (Photograph by R.L. Kugler)
  • DISSOLUTION POROSITY
  • Thin Section Micrograph - Plane Polarized Light
  • Avile Sandstone, Neuquen Basin, Argentina
  • Dissolution of
  • Framework Grains
  • (Feldspar, for
  • Example) and
  • Cement may
  • Enhance the
  • Interconnected
  • Pore System
  • This is Secondary
  • Porosity
  • Pore
  • Quartz Detrital
  • Grain
  • Partially
  • Dissolved
  • Feldspar
  • Photo by R.L. Kugler
  • DISSOLUTION POROSITY
  • Scanning Electron Micrograph
  • Tordillo Formation, Neuquen Basin, Argentina
  • Partially
  • Dissolved
  • Feldspar
  • Dissolution Pores
  • May be Isolated and
  • not Contribute to the
  • Effective Pore System
  • Photo by R.L. Kugler
  • Sand
  • Grains
  • Clay
  • Matrix
  • Chemical
  • Cement
  • Quartz
  • Feldspar
  • Rock Fragments
  • Quartz
  • Calcite
  • Hematite
  • Illite
  • Kaolinite
  • Smectite
  • Average
  • Sandstone
  • Average
  • Mudrock
  • (Shale)
  • Allochemical
  • Grains
  • Chemical
  • Cement
  • Microcrystalline
  • Matrix
  • Calcite
  • Fossils
  • Pelloids
  • Oolites
  • Intractlasts
  • Calcite
  • Average
  • Sparry
  • Limestone
  • Average
  • Micritic
  • Limestone
  • Clastic Rocks
  • Carbonate Rocks
  • Comparison of Compositions of Clastic
  • and Carbonate
  • Rocks
  • Maldive Islands
  • FOLK CARBONATE ROCK CLASSIFICATION
  • 0-1%
  • 1-10%
  • 10-50%
  • Over
  • 50%
  • Sparse
  • Biomicrite
  • Micrite &
  • Dismicrite
  • Fossili-
  • ferous
  • Micrite
  • Packed
  • Biomicrite
  • Poorly
  • Washed
  • Biosparite
  • Unsorted
  • Biosparite
  • Sorted
  • Biosparite
  • Rounded
  • Biosparite
  • Over 2/3 Lime Mud Matrix
  • Over 2/3 Spar Cement
  • Subequal
  • Spar &
  • Lime Mud
  • Sorting
  • Poor
  • Sorting
  • Good
  • Rounded,
  • Abraded
  • Claystone
  • Sandy
  • Claystone
  • Clayey or
  • Immature Sandstone
  • Sub-
  • mature SS
  • Mature
  • SS
  • Super-
  • mature SS
  • Depositional Texture Recognizable
  • Depositional Texture
  • Not Recognizable
  • DUNHAM CARBONATE ROCK CLASSIFICATION
  • Depositional Texture Recognizable
  • Depositional
  • Texture
  • Not Recognizable
  • Mudstone
  • Wackestone
  • Packstone
  • Grainstone
  • Boundstone
  • Crystalline
  • Carbonate
  • Grain
  • Supported
  • Lacks Mud,
  • Grain-
  • Supported
  • Components Not Bound Together During Deposition
  • Mud Supported
  • Contains Mud
  • (clay and silt size particles
  • <10 %
  • Grains
  • >10 %
  • Grains
  • Original Components
  • Bound 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 an
  • Individual Grain or Crystal in the Rock
  • Fenestral
  • Fracture
  • Vug
  • Primary Pores Larger Than Grain-Supported
  • Interstices
  • Formed by a Planar Break in the Rock
  • Large Pores Formed by Indiscriminate
  • Dissolution of Cements and Grains
  • Interparticle
  • Intraparticle
  • Intercrystal
  • Moldic
  • Fenestral
  • Shelter
  • Growth-Framework
  • Fabric
  • Selective
  • Fracture
  • Channel
  • Vug
  • Non-Fabric
  • Selective
  • 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 light
  • Smackover Formation, Alabama
  • (Photograph by D.C. Kopaska-Merkel)
  • Moldic
  • Pores
  • Due to dissolution and collapse of ooids (allochemical particles)
  • Isolated pores
  • Low effective porosity
  • Low permeability
  • Blue areas are pores.
  • Calcite
  • Dolomite
  • Moldic
  • Pore
  • CARBONATE POROSITY - EXAMPLE
  • Thin section micrograph
  • Smackover Formation, Alabama
  • Black areas are pores.
  • (Photograph by D.C. Kopaska-Merkel)
  • Combination pore system
  • Moldic pores formed through dissolution of ooids (allochemical particles)
  • Connected pores
  • High effective porosity
  • High permeability
  • Moldic
  • Pore
  • Interparticle
  • Pores
  • Moldic and
  • Interparticle Pores

PORE SPACE CLASSIFICATION (In Terms of Fluid Properties)

PORE-SPACE CLASSIFICATION

  • Total porosity, t =
  • Effective porosity, e =
  • Effective porosity – of great importance;
  • contains the mobile fluid

COMPARISON OF TOTAL AND EFFECTIVE POROSITIES

  • Very clean sandstones : e  t
  • Poorly to moderately well -cemented intergranular materials: t  e
  • Highly cemented materials and most carbonates: e < t

MEASUREMENT OF POROSITY

  • Core samples (Laboratory)
  • Openhole wireline logs
  • SANDSTONE POROSITY MEASURED
  • BY VARIOUS TECHNIQUES
  • Quartz
  • (Framework)
  • Small
  • Pores
  • Isolated
  • Pores
  • Large, Interconnected
  • Pores
  • Clay Surfaces
  • & Interlayers
  • Clay
  • Layers
  • Irreducible or
  • Immobile Water
  • Hydration or
  • Bound Water
  • Hydrocarbon
  • Pore Volume
  • Structural
  • (OH
  • -
  • ) Water
  • Rock
  • Matrix
  • Total Porosity - Neutron Log
  • Total Porosity - Density Log
  • Absolute or Total Porosity
  • Oven-Dried Core Analysis Porosity
  • Humidity-Dried
  • Core Analysis Porosity
  • Capillary
  • Water
  • V
  • Shale
  • (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

CORING ASSEMBLY AND CORE BIT

  • PDC Cutters
  • Fluid
  • vent
  • Drill collar
  • connection
  • Inner barrel
  • Outer barrel
  • Thrust bearing
  • Core retaining
  • ring
  • Core bit

COMING OUT OF HOLE WITH CORE BARREL

WHOLE CORE

  • Whole Core Photograph,
  • Misoa “C” Sandstone,
  • Venezuela
  • 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

  • Smaller samples
  • Less representative of heterogeneous formations
  • Within 1 to 2% of whole cores for medium-to high-porosity formation
  • In low-porosity formations,  from core plugs tends to be much greater than  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 DETERMINATION OF 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.

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