20b_ seismic intepretation

8/18/2019 20b_ Seismic Intepretation

1/6

REU

Pittsburgh, PA

9/24/21

Tim

Carr

West Vir gin ia University

Understanding Seismic Data

Resolution (Vertical and Horizontal)

Two way time (TWT)

Time to Depth conversion

Interpretation of Reflectors

Creating a consistent interpretation

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Increase in Impedance Decrease in Impedance

Able to resolve boundaries of beds a few meters thick

1 meter

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CompressionRarefaction

A A = Amplitude

λ = Wavelength

length, ft or m

λ

P = Periodtime

Period = Time for the waveform

to travel 1 wavelength

Dp = PulseDuration

time

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10 m

Predominantly

Shale

Predominantly

Shale

Predominantly

Sand

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2/6

REU

Pittsburgh, PA

9/24/21

Tim

Carr

ExxonMobil

Although seismic data can not image small‐scale stratal units, it can image mid‐ to large

‐scale

units

Parasequences

Bed Sets

Parasequence Sets

Sequences

Beds

Lamina Sets

Lamina

Sequence Sets

The big advantage of seismic data is areal coverage

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There are many reflectors on a seismic section. Major changes in properties

usually

produce

strong,

continuous reflectors as shown by the arrow.

A seismic reflector is a boundary between beds with different properties. There may be a change

of

lithology

or

fluid

fill

from

Bed

1

to

Bed 2. These property changes cause some sound waves to be reflected towards the surface.

Bed 1

Bed 2

lower velocity

higher velocity

energy

source

signal

receiver

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ExxonMobil

Shale

Sand

Velocity = 2000 m/sDensity = 1.7 gm/cc


ReflectionCoefficient

= =I below – I above

I below + I above=

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Shale

Sand

Velocity = 2000 m/sDensity = 1.7 gm/ccI = 2000 * 1.7 = 3400

Velocity = 2400 m/sDensity = 1.8 gm/ccI = 2400 * 1.8 = 4320


=4320 - 3400

4300 + 3400=

I below – I above

I below + I above= 0.119

Of the incident energy, 12% is reflected, 88% is transmitted

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Shale

Carbonate


Velocity = 2600 m/s

Density = 2.1 gm/cc


= =I below – I above

I below + I above=

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Shale

Carbonate


I = 2000 * 1.7 = 3400

Velocity = 2600 m/s

Density = 2.1 gm/ccI = 2600 * 2.1 = 5460


=5460 - 3400

5460 + 3400=

I below – I above

I below + I above= 0.232

Of the incident energy, 23% is reflected, 77% is transmitted

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3/6

REU

Pittsburgh, PA

9/24/21

Tim

Carr

0.25 seconds

Two way time (TWT)

indicates the

time

required for the seismic wave to travel from a source to some point below the surface and back up to a receiver.

In this example the TWT is 0.5 seconds.

0.25 seconds

0

0.5

TWT

s e c o n d s

surface

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0.58 sec

m

1865

926

288

926 m

Two way time (TWT) does not equate directly to depth

Depth of a specific reflector can be determined using wells

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Seismic Shot

Borehole

Geophone

D e p t h

Check shots measure

the vertical one‐ way

time from surface to

various depths within

the well

Used to calibrate well

depths and times from a

sonic log

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Interval velocity

Integrated

Sonic log

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Use the sonic and density logs to compute an impedance ‘log’

Veloc it y Den si ty Imp ed an ce

=x

Shale

Sand

Shale

Sand

Shale

LithologyReflection

Coefficients

Calculate the reflection coefficients

*

Wavelet

Convolve our pulse with the RC series to get individual wavelets

Each wavelet’s amplitude is proportional to the RC

Synthetic

Sum the individual wavelets to get the synthetic seismic trace

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Log data Calculated

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Compare well data to seismic data

Relate horizon

tops

in

a

well with specific reflections

Synthetic Trace

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5/6

REU

Pittsburgh, PA

9/24/21

Tim

Carr

Average velocity map of reflector‐1 25 Depth map of reflector‐1 26

ExxonMobil

Structural Interpretation Faults & Folds

Subsidence & Uplift

Structural Trends

Structural Features

Stratigraphic Interpretation Unconformities

Stratal Packages

Environments / Facies / Lithologies

Ages

Using all available data (wells, seismic, outcrop, regional studies, gravity, magnetics, etc.) build a framework of present‐day structure and stratigraphy

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Determine the local geology from the subsurface images

Map faults and other structural features

Map unconformities and other major stratal surfaces

Interpret depositional environments

Infer lithofacies from reflection patterns & velocities

Predict

ages

of

stratal

units

Examine elements of the HC systems

Mitchum et al., 1977

AAPG©1977 reprinted wit h permission of the AAPGwhose permission is required for further use.

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Relate features seen in seismic to stratigraphic or structural processes

Interpretation

Seismic Image of Ancient Reef in Alberta [400 million years old]

29 Uses computer technology to interpret seismic data

Interpretation

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6/6

REU

Pittsburgh, PA

9/24/21

Tim

Carr

Salt domes pierce strata

and creating favorable

trapping geometry in the

Ekofisk Fm.

8km

Fancier stuff: Ant tracking of salt domes for fracture detection

Variance Attribute‐ Trace to trace variability in 3D seismic block

Patchawarra Surface at 1.752 sec

4 Km

Uses high tech visualization to interpret seismic data

Interpretation

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Seismic ReflectorsDifference in Impedance of UnitsImpedance Function of Density and Velocity

Resolution Limited to 10’s of meters Areal Coverage

Seismic versus Depth Well to Seismic Ties Velocity ModelConvert TWT to Depth

Seismic Interpretation Provides Earth Image

Structure and

Stratigraphy

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20b_ seismic intepretation

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