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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