Sedimentary Rocks Petrography

Role of Petrography on Reservoir Quality Assessment

Thin Section

Analysis

• Composition

• Texture

• Cement

• Porosity

• Fabric

• Sed. Structure

Microfacies

Lithofacies

ReservoirQuality

Dep. Env &Facies Group

Reservoir Distribution& Geometry

Reservoir Poregeometry

Reservoir Property(Ø, K, Sw)

Core & Log data

Seismic & Well dataCore, Paleontology, Log & Image data

Clast size indicates ancient relative current velocity

Detrital Sediments & Sedimentary Rocks

gravel&

conglomerate

sand&

sandstone

clay, silt&

mudstone

Sorting

Angularity Classification

Degree of roundness helps in knowing the distance of transportation

•Angular clasts- short distance transport from the source

•Rounded clasts- long distance transport

Sedimentary Fabric

Grain contacts

The diagram (after Taylor, 1950) shows the various types of grain contacts found in sandstones. You will often find more than one type in a single thin section, but are unlikely to find all four types together. Grain contacts start off as tangential

(point) contacts. As the sediment is buried and compacted,

grains are rotated and pushed closer together, and pore space is reduced: long contacts become common.

With further compaction, softer grains are compacted around harder ones, and grains start to dissolve in the pore waters at their contacts (grain contact dissolution*): concavo-convex contacts become important.

As grain-contact dissolution continues, grain boundaries become irregular or sutured, and original grain shapes are destroyed. Eventually all the contacts are sutured, and all the porosity is destroyed.

Time of formation Type Origin

Primary or depositionalIntergranular or interparticleIntragranular or intraparticle

Sedimentation

Secondary or postdepositional

IntercrystallineFenestral

Cementation

VuggyMoldic

Solution

Fracture

Tectonics,Compaction,Dehydration,

Diagenesis

Classification of the different types of porosity

Intergranular porosity

Intragranular porosity

Two Types of Primary Porosity

Sketch of a thin section showing moldic and

vuggy porosity in a limestone

Sketch of a thin section showing of crystalline

dolomite reservoir showing intercrystalline porosity

Type of Secondary Porosity

Silica, Calcite

Sketch of a slabbed core showing fracture porosity

Fracture porosity infilledBy cementation

Type of Secondary Porosity

Permeability

Poro

sity

Effect of grain size on porosity and permeability

Permeability

Poro

sity

Effect of sorting on porosity and permeability

Porosity & Permeability

Estimating Textural Maturity

• Immature– Poor sorting

– Angular grains

– High matrix content

• Mature– No matrix

– Moderate-good sorting

– Degree of rounding

• Supermature– No matrix

– Very good sorting

– High degree of roundness

Maturity

Maturity

Product composition

monocrystalline quartz (single crystal lattice for entire grain),  polycrystalline quartz (two or more crystals of quartz within the grain),  chert (cryptocrystalline quartz forming the grain). 

Sandstones

There are basically two reasons for this.  First, sandstones are easily studied because they contain sand sized grains that can easily be distinguished with a petrographic microscope.  Second, most of the world's oil and natural gas is found in sands or sandstones because of their generally high porosity.

ClassificationFor this course we will use a classification of sandstones that is partially based on Blatt and Tracey (p. 257) and partially based on Williams, Turner, and Gilbert (p. 326).  Sandstones that contain less than 10% clay matrix are called arenites. These can be subdivided based on the percentages of Quartz, Feldspar, and unstable lithic fragments (fragments of preexisting rock). 

A feldspar-rich sandstone is called an arkose.

If the rock has between 10 and 50% clay matrix, the rock is called a wacke. 

The term graywacke is seldom used today, but was originally used to describe a lithic-rich sandstone with between 10 and 50% mica, clay, or chlorite matrix.  Rocks with greater than 50% clay matrix are called sandy mudstones, and will be discussed in the lecture on mudrocks. 

Siliciclastic Sedimentary Rocks: Sandstones

Thin Section Analysis

HornblendeAmphiboles; Double-chain Silicates

Monoclinic (-,+)Relief – moderate, Colour - green or brown (oxy-hornblende) ,RI - 1.61-1.76 , Birefringence - 0.02 , Interference colours - 2nd-3rd order, masked by the body colour , Extinction - inclined, complete, Comments - simple twinning is common

 perfect cleavage in hornblendePPLFish Canyon Tuff

euhedral crystal of hornblende showing pleochroism as the crystal is roataed through 90°. This crystal also shows concentric zoning.

PPLFish Canyon Tuff

GlauconiteGlauconite is an iron-rich clay, restricted to marine environments. It forms grains composed of finely crystalline, bright green coloured clay-size particles and looks very similar in XPL to its appearance in PPL. The green colouration makes this distinctive in marine sediments.

British Cretaceous Greensand

BiotiteMica; Sheet Silicates

Monoclinic (-)Relief – moderate ;Colour - brown or green ;RI - 1.53-1.696 Birefringence - 0.028-0.08 ;Interference colours - 2nd-3rd order, masked by the body colour ;Extinction - straight, mottled ;Comments - alters to green chlorite, often contains inclusions of accessory phases

 c-axis parallel section (left) and subhedral basal section (right) of brown biotite. Note the presence of the one perfect cleavage on the c-axis parallel section and its absence on the basal section. The colourless inclusions in the basal section are apatite.PPL

PlagioclaseFramework Silicates

triclinic (+,-)Relief – low; Colour – colourless ; RI - 1.527-1.590; Birefringence - 0.012-0.013; Interference colours - 1st order; Extinction – inclined; Comments - show multiple 'albite' twinning; alters to montmorrillonite or kaolinite

anhedral crystal of plagioclase

XPL

Plagioclase with carlsbad-

albite twinsXPL

gabbro 

plagioclaseXPL

Fish Canyon Tuff

OrthoclaseFramework Silicates

triclinic (+,-)Relief – low; Colour – colourless; RI - 1.518-1.539; Birefringence - 0.006-0.012; Interference colours - 1st orderExtinction – inclined; Comments - Often shows simple 'Carlsbad' twinning; alters to the clay minerals illite, kaolinite and montmorrillonite.

Carlsbad twinned crystal of orthoclase

XPLFish Canyon Tuff

Fragments of orthoclase, showing concentric zoning

XPLFish Canyon Tuff

zoned crystal of orthoclase

XPLFish Canyon Tuff

MicroclineFeldspar Groupmonoclinic (-)Relief – low ; Colour – colourless; RI - 1.518-1.529; Birefringence - 0.006-0.010; Interference colours - low first order.; Comments - shows cross-hatched, 'tartan', multiple twins.

The typical appearance of microcline in a granite with 'Tartan Twins'

XPLGranite

Classification of

Carbonates

Calcite Carbonatetrigonal (-)Relief - variable; low to moderate; Colour – colourless; RI - 1.658-1.486; Birefringence - 0.172; Interference colours - 4th order; Extinction - inclined

calcitePPL

Loch Borrolan Carbonatite

calciteXPL

Loch Borrolan Carbonatite

DolomiteDolomite occurs frequently as a diagenetic alteration of calcite. Like the other carbonate minerals, dolomite has variable relief in PPL and extreme birefringence in XPL. Secondary dolomite often forms a mosaic of rhombic crystals, as shown below.

50x PPL 100x PPLDolomite and

Glauconite

25x XPLDolomite