rock properties in sedimentary basins: diagenesis ... · diagenesis and sedimentary rocks ......
TRANSCRIPT
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Rock Properties in Sedimentary Basins: Rock Properties in Sedimentary Basins: Diagenesis, Lithology, Depth, AgeDiagenesis, Lithology, Depth, Age……
Maurice Dusseault
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Rock Properties (in generalRock Properties (in general……))
� Sediments become stiffer & stronger with age
� Also, they are stiffer and stronger with depth
� And, with the degree of chemical diagenesis
� Porosity is also reduced, and permeability is usually degraded with age, depth & diagenesis
� There are also effects of lithology (sands, shales, carbonates, evaporites, basalt flows…)
� …and mineralogy (NaCl, quartz, clays…)…
� …with correlations to geophysical log values and seismic properties as well…
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The Basic Rock PropertiesThe Basic Rock Properties……
� Porosity
� Permeability
� Stiffness (rigidity, compressibility, elastic modulus, Young’s modulus…)
� Strength
� vP, vS, Q…
� c,κ, βT (thermal properties…)
Stiffness – E - GPa
Depth - z
0 50 100
5 km - ~16,000′
Example:E = ƒ(z)
What caused this zone of anomalously low E?
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Age of the RockAge of the Rock……
Canadian Oil Sands
Orinoco Heavy Oils
Maracaibo Heavy Oils
St Peter Sandstone
Sherwood Sandstone
Rotliegend Sandstone
Frigg Sandstone
Brent, Statfjord Sss
Old Red Ss (Buchan)
Tui Field (offshore NZ)
Frio Sands (GoM)
Murdoch Gas Field
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Diagenesis and Sedimentary RocksDiagenesis and Sedimentary Rocks
� Mechanical diagenesis (compaction)�Burial → σ′↑ → φ↓ → strength↑ & stiffness↑�This can be impeded by high pore pressures
�Very important in shales (higher po common)
� Chemical diagenesis�Grain contact dissolution & re-precipitation
�Mineral alterations (eg: smectite → illite + SiO2)
�Cementation at grain contacts (e.g. CaCO3)
�Massive pressure solution (limestones)
� Geologists provide valuable diagenetic info.
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Effect of Burial on Rock PropertiesEffect of Burial on Rock Properties
� Rocks that have been buried at greater depths� Have lower porosities
� Are much stiffer
� Are stronger
� Are less likely to crush
� Are less permeable
� Have more cementation
� And so on…
� And, stresses can be changed (σ′hmin/σ′v ratio alterations)
Line
of σ v
= σ h
yiel
d,φ= 3
0°
σ’v
σ’hbu
rial
chemical diagenesis
eros
ion
Current state
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Mechanical Compaction & PorosityMechanical Compaction & Porositypo
rosi
ty -
φ
Stress - log(σ′v)
1 MPa 10 MPa 100 MPa
50
40
30
20
10
0
Some comments:Sands compress less than clays with ∆σ′Porosity recovery with -∆σ′ is modestPre-compacted strata are far stiffer∆φ immediate in sands, slow in shales
Quartzose sand Mixed-clay shale
More grain crushing
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Mechanical Compaction Effects Mechanical Compaction Effects
� Purely mechanical diagenesis effects in sands:�Not too important: loss of φ from 35% to 28-30%
for SiO2 sands and ∆σ′ of 10-15 MPa
�Drop in k by a factor of perhaps 2 to 3..
�More important in litharentites, arkoses…
� Purely mechanical effects in shales:�Extremely important: loss of φ from 60% to 15%
for shales and ∆σ′ of 10-15 MPa
�Permeability becomes negligible
� For sands and shales, persistence of high overpressures retard mechanical compaction
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Chemical DiagenesisChemical Diagenesis
timetemperaturechemistry
cementation,25-32%
pressuresolution,25-32%
initial state: 35-38% porosity after
sedimentation, shallow burial (25 m)
porosity reduction
stresses
Both solution and cementation reduce φ, increase E, vP, etc.
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Sandstone Stiffness & DiagenesisSandstone Stiffness & Diagenesis
∆l = ∆φ
T, t, σ′, p
chemistry
σij
p
high σ, small A
low σ, large A
DIAGENESIS!
p
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An Unconsolidated SandstoneAn Unconsolidated Sandstone
� St Peter Snds, (source of Ottawa Sand), φ = 26%
� Ordovician age, max Z perhaps 800-1000 m
� 99.5 SiO2
� Highly rounded grains –aeolian/beach sand
� Indentations evidence of contact pressure solution
� No cement whatsoever
� High friction angles because of interlocking
This rock has lost 30% of its original porosity, (3 5 - 26)/35
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Extreme Diagenesis CaseExtreme Diagenesis Case
SiO2 grains
Highly soluble grain
Crystal overgrowths
Interpenetrating fabric
This rock has lost 90% of its original φφφφ, expelling fluids
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Diagenesis and PropertiesDiagenesis and Properties
� Cohesion (c′) is greatly increased� Tensile strength (To) somewhat increased
� Friction angle (φ′) increased somewhat� Stiffness (E) is increased greatly� Shale → anisotropy in all properties is
generated by compaction diagenesis� Shale Poisson’s ratio decreased (0.4 to 0.3)� Weak anisotropy generated in sandstones� Permeability drops (pore throat dia., fractures)� Thermal properties are altered slightly…
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Sandstone Diagenesis EvidenceSandstone Diagenesis Evidence
� Dense grain packing
� Many long contacts
� Concavo-convex grain contacts
� SiO2 precipitated in interstitial regions
� Only 1% solution at contacts = 8% loss in volume
� -A stable interpenetrative fabric develops with high stiffness and strength
Fine-grained unconsolidated sandstone
- Alberta Oil Sands
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erosion
Precompaction Effect by ErosionPrecompaction Effect by Erosion
porosity
log(σ′v)
present state
“virgin”compression curve
apparent threshold ∆σ′
stiff reloadresponse
Burial compaction (diagenesis) is largely irreversi ble…
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Stiffness, Strength, DiagenesisStiffness, Strength, Diagenesis
� A larger contact area means the sandstone is now much stiffer
� Also, the porosity is lower, giving a higher frictional strength
� If mineral cements are added, strength and stiffness are higher yet
� Also, the 3-D interlock that arises from the diagenesis increases strength
Contact force distribution
Egrain, νgrain
Egrain, νgrain
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Diagenesis and StrengthDiagenesis and Strength
shear stress
normalstress
chemical cementation
densification(more interlock)
originalsedimentdiagenetic
strengthincrease
σ′1σ′3
cohesion
diagenesiseffects on the
Mohr-Coulomb strength envelope
σa
σrσr = σ′3
σa = σ′1 τmax planesslip
planes
TriaxialTest
Stresses
c′
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Porosity vs Depth & OverpressurePorosity vs Depth & Overpressure
0 0.25 0.50 0.75 1.0
clay & shale,“normal” line
sands &sandstones
effect ofoverpressures
on porositydepth
porosity
4-8 km
Specific details of these relationships are related to basin
age, diagenesis, heat flux, geochemistry ...
mud
clay
mud-stone
shale
slate (deep)
+T
Different basins → ~different φ = ƒ(z) for sandstones, shales
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Permeability and DepthPermeability and Depth
� Muds and shales have low k, < 0.00001 D, and as low as 10-10 D
� Exception: in zones of deep fractured shale, k can approach 0.1-1 D
� For sands: k↓ with z� Exception, high φ sands
in overpressured zones can have high k
� Anhydrite, salt = 0 D� Carbonates, depends…
0 1 2 3 4 5
5
10
15
20
25
Fractured shales at depth may have high fracture permeability
Permeability – k – Darcies
Dep
th –
z –
1000
’s ft
High porosity OP sands have anomalously high porosity & permeability
Intact muds and shales have negligible k
Muds and Shales Sands and Sandstones
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OverOver--Compacted CasesCompacted Cases
mud
clay
mud-stone
shale
0 0.25 0.50 0.75 1.0
clay & shale,“normal” line
sands &sandstones
depth
porosity
4-8 km
Over-compaction is also a complex
function of geological history, mineralogy,
erosion, T, t…
Strata “over-compacted” from previous deeper burial…
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Seismic AttributesSeismic Attributes
� Lower porosity → higher vP, vS, Q�e.g.: quartz sand at 28% vs. 35% porosity
�And of course greater strength, E, lower Cc, etc.
� Greater stress → higher vP, vS, Q�Hertz-Mindlin theory : v ∝ (σ′)1/6
� In the lab, reality is closer to v ∝ (σ′)0.22-0.27
� Greater density → higher vP, vS, Q�e.g.: halite versus anhydrite, both with φ ≈ 0
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Seismic Velocity and RocksSeismic Velocity and Rocks……
vP – velocity – kft/s
vP – velocity – km/sSource unknown
Increasing diagenesis
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RPDT (Rock Physics Depth Trends)RPDT (Rock Physics Depth Trends)
Porosity Density vP & vS (m/s)
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Anomalous Rock Properties ZonesAnomalous Rock Properties Zones……
Porosity Density vP & vS (m/s)
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Using RPDT DataUsing RPDT Data
� Combine well log data (lithology, density, etc.) with normal compaction trends models
� Identify departures from the models
� Interpret the reason for the departure…�Anomalous cementation event
�Uplifting and erosion effects
�Overpressure effects
� Infer other rock properties from identifying the reason for the anomalous data…
� Based on calibrated empirical relationships
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What About Thermal Properties??What About Thermal Properties??
� Thermal conductivity –κ – somewhat affected by porosity, by a factor of 2-3 maximum
� Thermal capacity – c – relatively insensitive to burial = ƒ(solid and liquid composition)
� Coefficient of thermal expansion –βT –somewhat affected by the porosity
� Thermal properties are also affected by the mineralogy…�Halite has the highest κ, very highβT
�Most rocks lie in relatively narrow ranges
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Thermal Conductivity, BasaltThermal Conductivity, Basalt
κ–
Wm
-1K
-1
porosity - φ Cla
user
and
Hue
nges
, AG
U, 1
995
Vesicular basalt can have a wide range of porosities
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Sandstone Thermal Conductivity Sandstone Thermal Conductivity
κ–
Wm
-1K
-1
porosity - φ Cla
user
and
Hue
nges
, AG
U, 1
995
Region of practical interest
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Finally Finally –– Mineralogical ChangesMineralogical Changes
� Gypsum becomes anhydrite�CaSO4⋅2H2O → CaSO4 + liberated 2H2O
�Generally, because of high solubility, φ → 0
� With great depth, smectite undergoes change�Smectite → Illite + SiO2 +H2O
�Large shrinkage associated with this
�Leads to Q-I shales
�Leads to intense fracturing of shales
�Leads to low lateral stresses as well
�Below 5-6 km, no more smectite!
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SummarySummary
� We can build robust expectations about the geomechanical behavior of rock with depth
� Diagenesis, even mineralogical changes
� Depth of burial and time at depth
� Degree of compaction (effective stress, p)
� Mineralogy, clay content…
� Amount of erosion, tectonic stressing
� Drilling parameters, and so on…
� Part of the GEM – Geomechanics Earth Model