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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)

kilo

met

ers

kilo

met

ers

Hoa

ng P

, Avs

eth

PA

, Urs

in B

, van

W

ijnga

arde

n 20

7 F

O32

, EA

GE

69th

Con

f,

Lond

on 2

007

con

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Anomalous Rock Properties ZonesAnomalous Rock Properties Zones……

Porosity Density vP & vS (m/s)

kilo

met

ers

kilo

met

ers

Hoa

ng P

, Avs

eth

PA

, Urs

in B

, van

Wijn

gaar

den

207

FO

32, E

AG

E 6

9th

Con

f, L

ondo

n 20

07 c

on

Hoa

ng P

, Avs

eth

PA

, Urs

in B

, van

W

ijnga

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n 20

7 F

O32

, EA

GE

69th

Con

f,

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on 2

007

con

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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