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 RESERVOIR POROSITY

ROCK MATRIX AND PORE SPACE Rock matrix Pore space Note different use of “matrix” by geologists and engineers

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

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 - Mineral Composition Carbonate Rocks Consist Primarily of Carbonate Minerals (i.e. Minerals With a CO Anion Group) Limestone - Predominately Calcite (Calcium Carbonate, CaCO 3 ) Dolomite - Predominately Dolostone (Calcium Magnesium Carbonate, CaMg(CO 3 ) 2 ) 3 -2

Relative Abundances Siltstone and shale (clastic) ~75% Sandstone and conglomerate (clastic) ~11% Limestone and dolomite ~14% SEDIMENTARY ROCK TYPES,

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 CarbonateRocks

Grain-Size Classification for Clastic Sediments NameMillimetersMicrometers 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, (modified from Blatt, 1982)

Average Detrital Mineral Composition of Shale and Sandstone Mineral Composition ShaleSandstone Clay Minerals Quartz Feldspar Rock Fragments Carbonate Organic Matter, Hematite, and Other Minerals 60 (%) 30 4 <5 3 <3 5 (%) <1 (modified from Blatt, 1982)

SANDSTONE CLASSIFICATION Quartz + Chert Feldspar Unstable Rock Fragments Quartzarenite Subarkose Sublitharenite Lithic Subarkose Arkose Lithic Arkose Felspathic 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 1.Framework 2.Matrix 3.Cement 4.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 Sub- Angular Sub- Rounded Well- Rounded ROUNDNESS 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)

CUBIC PACKING OF SPHERES Porosity = 48%

Porosity Calculations - Uniform Spheres Bulk volume = (2r) 3 = 8r 3 Matrix volume = Pore volume = bulk volume - matrix volume

RHOMBIC PACKING OF SPHERES Porosity = 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 = 14%

Grain-Size Sorting in Sandstone

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

STS 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 1.Primary and secondary “matrix” porosity system 2.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 Typically is Lower Than 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 Hatters Pond Field, Alabama, USA (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

FOLK CARBONATE ROCK CLASSIFICATION

DUNHAM CARBONATE ROCK CLASSIFICATION

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

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

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 AnalysisSpecial Core Analysis *Allows calibration of wireline log results

PDC Cutters Fluid vent Drill collar connection Inner barrel Outer barrel Thrust bearing Core retaining ring Core bit CORING ASSEMBLY AND CORE BIT

COMING OUT OF HOLE WITH 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,  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