Basic petrophysics : controls on seismic...
Transcript of Basic petrophysics : controls on seismic...
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VelocititesVelocitites
Basic Basic petrophysicspetrophysics: controls on : controls on
seismic velocityseismic velocity
Not constant! Typically about 1% per decade (I.e. x10) in frequency e.g. 20Hz-20kHz � 4%
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Some of the physical properties influencing Vp and Vs
sandstone limestone dolomite
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Unconsolidated material: Dry sand 0.2 - 1.0 km/sWet sand 1.5 - 2.0 km/s
Clay 1.0 - 2.5 km/s
sedimentary rocks: Tertiary sandstone 2.0 - 2.5 km/s
Carbon.
sandstone 4.0 - 4.5 km/sChalk 2.0 - 2.5 km/s
Limestone 3.4 - 7.0 km/sSalt 4.5 - 5.0 km/s
Igneous/metamorphic
rocks: Granite 5.5 - 6.0 km/sGabbro 6.5 - 7.0 km/s
Gneiss 3.5 - 7.5 km/s
Air: 0.33 km/sWater: 1.43-1.54 km/s
Petroleum: 1.3-1.4 km/s
Lithology variations?
(Vp only)
NB -unusually low
density
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Porosity effectsPorosity effects
Porosity: Pore volume per unit volume
Not the same as permeability. A porous rock without interconnected
pores is not permeable!
http://earthnet-geonet.ca/glossary/display_term_e.php?term=porosity
Wyllie’s Time Average Equation
( )mf VVV
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11φφ −+=
Total traveltime
=Travel time
in pores
(fluid)
+Travel time
in
matrix
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Seismic Impedance
Why is impedance important?
Reflectivity depends on impedance
⇒ important for reflection surveys
Gardners Formula:
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aV=ρ
a = 0.31
ρ in g/cm3
V in m/s
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VVP P / V/ VSS ratioratio
Poisson Ratio
xx
zz
xx
yy
ε
ε
ε
εσ
−=
−=
ratio of transverse contraction strain to longitudinal extension strain
in the direction of stretching force
An alternative parameter to describe Vp/Vs characteristics
(Vs/Vp) = √[(0.5 - σ)/(1-σ)],
hence Vs/Vp = 0 - (1/√2), or Vs = 0-70% of Vp
or Vp/Vs = √2 - ∞
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Poisson’s Ratio: σ = 0 - 0.5
≈0.05 for hardest, rigid rocks � ≈0.45 for unconsolidated materials
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Pore fill effectsPore fill effects
Blocking the pore space with clay
The principal cause of seismic
attenuation in sedimentary rocks
is pore fluid movement relative
to matrix - clay fill in pore throats
can have significant effects
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Gassman’s Equations.Convert between brine, oil, and gas pore fill (at fixed pressure), based on knowledge of rock
properties with any one fill, plus ‘laboratory’ values for elastic moduli of grains and other fill types.
In routine use for exploration and for time-lapse seismics.
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Pressure effects
NB: what is pressure an analogue for?
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Shale over gas
sand-ve
-ve
≈0
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1216 m
2432 m
3648 m
4864 m
5472 m
Velocity [m/s]
610 1220 24401830 3048 3658 4267
Shale over oil
sand-ve
≈0
+ve
1216 m
2432 m
3648 m
4864 m
5472 m
Velocity [m/s]
610 1220 24401830 3048 3658 4267
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Gas-oil
contact+ve
+ve
+ve
1216 m
2432 m
3648 m
4864 m
5472 m
Velocity [m/s]
610 1220 24401830 3048 3658 4267
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Oil-water
contact+ve
≈0
≈0
1216 m
2432 m
3648 m
4864 m
5472 m
Velocity [m/s]
610 1220 24401830 3048 3658 4267
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Reminder - the key features