Petroleum Geology
Porosity = 47.6% Porosity = 25.96%
Lecture #7
Evolution of the sediment fill
• Burial and compaction
• Pressure and fluid flow
• Basin-scale pressure systems
Lecture: 7 © 1998 René O. Thomsen
Evolution of the sediment fill
• As accommodation space in a
sedimentary basin is formed, we will
accept that sediments will be deposited
Rift basin
• Here we will concentrate on how the
sediments - and some of their properties -
evolve during burial
• Primary properties:
– porosity, φ Syn-rift Post-rift
– permeability, k
• Secondary properties (depends on the
primary properties):
– thermal conductivity, κ
– density, ρ
• Other properties such as elastic and
mechanical properties will not be dealt
with here
Lecture: 7 © 1998 René O. Thomsen
Burial
• Sediments are deposited with an:
– initial porosity
and
– initial permeability
• During burial these properties change
– due to the weight of the overlying
sediment (overburden) the underlying
sediments compact and the porosity
decreases
– with a decrease in porosity the
permeability decreases
– during compaction, water is squeezed out
of the sediment
– if the permeability is too low to allow the
water to escape at the rate of added
overburden the fluid pressure increases
and creates over pressure
Lecture: 7 © 1998 René O. Thomsen
Porosity
• Porosity represents the void space
in a rock Theoretical primary porosity
Cubic packing Close packing
• Two kinds of porosity
– primary porosity
• an inherent, original
characteristic of the rock
Porosity = 47.6% Porosity = 25.96%
– secondary (or induced) porosity is
the result of geologic processes
most commonly due to
• solution
• recrystallization and
dolomitization
• fractures and joints
Lecture: 7 © 1998 René O. Thomsen
Pore space
Two ways of expressing pore space
Porosity: φ
pore volume
=
bulk volume
pore volume
Void ratio:e =
rock volume
Lecture: 7 © 1998 René O. Thomsen
Porosity
• The porosity change with burial can be Porosity
modeled* using a simple exponential 0 0.2 0.4 0.6 0.8 1
relationship: 0
φ ( z ) = φ 0 e − Cz 1000
where z is depth and C is a rock specific 2000
compaction constant
Schlater and Christie, 1980 3000
• Porosity depth relationships are used for 4000
reconstructing original thickness of
sediments (for backstripping) and to 5000
trace the thickness change and porosity
change through time
6000
• Burial history based on simple equation
such as the one above is called isostatic
burial history and assumes that no over 7000
pressure is building
8000
m
* Other models exists
Lecture: 7 © 1998 René O. Thomsen
Decompaction
Porosity
• The solid rock thickness, HS , can be 0 0.2 0.4 0.6 0.8 1
0
calculated at any instant using the
general equation: Area = HS H1
1000 Hs
HS = ∫ (1 − φ ( z ))dz
z1
2000
z2
HS remains constant 3000 H2
Area = HS
Hs
4000
• Using the exponential relationship the 5000
equation becomes:
6000
(
HS = ∫ 1 − φ 0 (e − Cz ) dz)
z1
z2 7000
8000
m
Lecture: 7 © 1998 René O. Thomsen
Permeability
• Permeability is a measure of the • Permeability can be modeled as a
capacity of a rock to transmit fluids function of porosity:
– reservoirs can have primary 0.2φ 3
1) Kozeny-Carman k=
S02 (1 − φ )
permeability (or matrix permeability) 2
– and secondary permeability k = permeability
• fractures
φ = porosity
• solution vugs
S0 = specific surface area of the rock, i.e., the
surface area to volume ratio using the surface
• Matrix permeability can be evaluated area of a sphere and the volume of a cube. Grain
with the use of Darcy’s law: size is equal to the diameter of the sphere and the
side of the cube
k ∂p e
C
v=− × k = k*
µ ∂l 2) Lerche
e*
where k = permeability
v = apparent flow velocity (cm/sec) k* = initial permeability
m = viscosity of the flowing fluid (cp)
e = void ratio
dp/dl = pressure gradient in the flow
direction (atm/cm) e* = initial void ratio
k = permeability of the rock (darcy) C = permeability power
Lecture: 7 © 1998 René O. Thomsen
Permeability model for sand
Permeability (mD)
1000
C = 1. 0 0
100
C = 1. 5 0
C = 2. 0 0
C = 2. 5 0
10
C = 3. 0 0
κκ==κκ*(e/e
(e/e
*)C)C C = 3. 5 0
* *
C = 4. 0 0
1
C = 4. 5 0
C = 5. 0 0
0.1
0 10 20 30 40 50
Porosity (%)
Lecture: 7 © 1998 René O. Thomsen
Compaction, pressure and fluid flow
• Sediment compaction is the reduction of Pressure (bar) ρf*g*z
pore space and subsequent release of
pore fluids
• If the escape of fluids keep pace with
compaction no over pressure builds -
normal compaction
• If the escape of fluids is slower than the
hydr
rate of added overburden pressure in
excess of hydrostatic builds and the
ostat
sediment becomes over pressured
ic pr
essur
lit
ho
conversions:
conversions:
fra
sta
pressure seal
tic
ctu
psi 2
psi==14.22
14.22kg/cm
pr
transition zone
r
kg/cm22
e li
es
Atm
Atm==0.968
0.968kg/cm
su
kg/cm2
mi
pressure
re
Atm = 0.068 psi
t
Atm = 0.068 psi 3 zone
psi/ft
psi/ft==0.433
0.433g/cm
g/cm3
bar
bar==14.5
14.5psi
psi pressure bleed
pa = 1 N/m 2
pa = 1 N/m2 2 off: lateral flow
NN==11kg*m/s
kg*m/s2
0 1000
over pressure
Lecture: 7 © 1998 René O. Thomsen
Over pressure
Compaction and porosity development can be Pressure is often given is psi/ft
coupled to development of pressure:
Terzaghi’s principle (Terzaghi & Peck, 1968) is
used to connect porosity and pressure:
Over pressure characterization
S=σ +p
where
S is the total vertical component of overburden
pressure hard
σ is the effective stress transmitted through the 0.85
solid matrix
p is the fluid pressure intermediate or moderate
Porosity and pressure is connected through:
A 0.65
pf soft or mild
e = e* 0.465
pf* normal
0.433
e = void ratio pf = frame pressure
e* = initial void ratio pf* = initial frame pressure
Lecture: 7 © 1998 René O. Thomsen
Basin scale pressure systems
• Over pressure typically develops where sedimentation rates are high and where there are
fine-grained and/or low permeability sediments present
• Large scale pressure systems are transient phenomena
• Elements of a pressure system:
– top seal
– lateral seal
– basal seal
Lecture: 7 © 1998 René O. Thomsen
Void ratio and porosity
Porosity: φ
pore volume
=
bulk volume
pore volume
Void ratio:e =
rock volume
express porosity in terms of void ratio: express void ratio in terms of porosity :
Lecture: 7 © 1998 René O. Thomsen
Porosity
Estimate porosity by point counting
on the grid.
Grid: 20x30 = 600 points
sand count =
pore count =
Porosity =
Lecture: 7 © 1998 René O. Thomsen
End of Lecture
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