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Seismic data Interpretation

Petroleum Engineering Group Presentation
by

Hashani Abeygunasekara

on 1 August 2013

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Transcript of Seismic data Interpretation

Seismic data Interpretation
How seismic data are produced?
Seismic Data
Seismic data
Seismic data in HC exploration
What?
How?
Vibroseis
Earlier using Dynamite
Today using vibrose
Giant vibrator truck in which a pad in the middle of the truck rests directly on the ground to raise the truck so that the pad supports the full weight of the vehicle. The truck then vibrates to put energy into the ground.
Compressional Waves
Also called P waves, are basically the same as sound (acoustic) waves, except that P waves move through solid or liquid media as opposed to the air.
P waves represent zones of contraction and expansion.
The two-way wave travel times of P waves are the data that make up the 3D seismic surveys and 2D seismic lines that are usually used in petroleum exploration today.
Seismic Wave Propagation
P waves originate at a point source
Travel into the ground, reflected back from bedding surfaces as a function of the acoustic impedance of the beds
Acoustic impedence is a measure of the density of the rock times the velocity of the wave (acoustic velocity)
The greater the impedance contrast, the greater the strength of the reflected wave.
Image resolution
The more the details, higher the accuracy of interpretation
The contrast in seismic data needs to be due to the actual bedding of material and not due to lateral or facies changes in layers.
Constrained by the frequency of the seismic wave produced
A stratigraphic layer can only be resolved if its thickness is at least a quarter of the size of the actual wavelength of the seismic imaging equipment.
Degradation of resolution with depth
Earth itself filters seismic signals
The higher the noise level in the data, the more that the software must filter this out, which degrades the remaining necessary information.
Seismic interpretation must involve experienced geologists and geophysicists.
Variable property displays
Made of wiggle traces
Each trace represents a geophone on the surface
Peaks and troughs on the wiggle represent up and down movements of the geophone
Vertical axis of the wiggle is the two-way travel time of the shock wave
Amplitude of the peak or trough indicates the magnitude of geophone movement
Peaks face right and are colored black
Shades of gray are used to represent amplitude
Dual property display
Replaced variable density displays and "wiggle traces".
Troughs are colored red (to indicate low-velocity sands at shallow depths)
Peaks are colored black (to indicate high-velocity shales)
Transitions between peaks and troughs are not shown
The troughs are then reversed, and superimposed on the original traces so that both peaks and troughs face the same direction
Variable density are also used, where different shades of red and black indicate the amplitudes
Types of Seismic data
2D seismic data
Two-dimensional image is called a seismic line
Cross-sectional view of the earth oriented parallel to the line of geophones
3D seismic data
3D seismic volume is created by shooting a closely spaced grid of 2D lines and interpolating between the lines to create a "3-dimensional volume" of data that is also referred to as a cube.

4D seismic data
"3D sets with identical spatial configurations that are shot at different times for the purpose of examining the change in a reservoir over time"

4 dimensions:
2-way horizontal movements
vertical movements
temporal data
If the results of the interpretation seem favourable, then an exploration borehole will be drilled to see if there are hydrocarbons in sufficient quantity to become a reservoir of economic viability.
A well in a previously unexplored area is called a wildcat.
Drilling
Making sense of the seismic image, identifying and mapping geological structures that can act as oil traps is the final stage of the seismic process.
Seismic data Interpretation
To produce a seismic image of the subsurface, a seismic source must be generated and the resulting reflection data recorded by a field crew.
Seismic data Acquisition
Data are processed, where the raw data is put through many complex procedures using powerful computers and finally a seismic section is produced
Seismic data Processing
Case Study
Location
Weald,
South-East of UK
Geological History
Geological Mapping
First information comes from geological mapping of the surface
This cross section and the map only show the geology near the surface
Geophysical techniques
Geology at the surface does not necessarily show any of the rocks or structures at depth, which could well be different
Geophysical methods can help to reveal what is deep below
Stratigraphy of the area
Seismic Anatomy
Seismic
"Relating to geological survey methods involving vibrations"
Anatomy
"A detailed examination or analysis"
+
"Seismic anatomy is all about performing a rigorous analysis on all aspects of subsurface seismic data"

Well tieing
Peaks-Troughs-Zero crossings
Well tieng
This links the horizons on the seismic profile to the real lithology. Using the results of the well tie, we can identify the following horizons and rock units
A seismic profile runs from South to North with a borehole at CMP 740, labelled well tie.
Well tie position was marked on the top of the seismic section and draw a pencil line directly under that point right to the bottom of the profile.
TWT of the horizons ware marked in the well tie using a different coloured pencil for each one. The very black events are peaks, the greyer ones troughs and zero crossings are white

Identifying the faults
Identifying the Anticline
Drilling location
Conclusion
The results of seismic interpretation can be varied. It depends upon the person doing the interpretation.
Deciding where to drill is a long and complex procedure that sometimes takes many years.
Not all exploration wells are successful.
Why seismic anatomy in petroleum,,,,,???
"It narrows the uncertainty in reserve or resource assessment..."
Elements of seismic anatomy
Reflections and Refractions
Seismic velocity
The different parts of a seismic profile
Two way time(TWT)
Peaks and Troughs
Common mid point(CMP)
Depth
Reflection and Refraction
Change of the path of the seismic wave due to the variations of velocity in different subsurface materials
Seismic Velocity
"The speed at which seismic waves travel through different materials"
velocity depends on:
degree of compaction
presence of fluid
type of fluid
distinguish figures can be used to differentiate the rock type
The different parts of a seismic profile
"A seismic profile is made up of vertical wiggle traces which are displayed in two way time"
Two Way Time(TWT)
"Time taken by a surface generated seismic wave to reach a subsurface rock layer and return to the surface"
Peaks and Troughs
shows the maximum wave displacement
Common Mid Point(CMP)
The numbering system used across the top of a seismic profile is the CMP number
The general idea of this method is to acquire series of tracers which reflects from same common subsurface mid point.
Depth
"It is not that simple to interpret the depth values using TWT measurement. but a general idea can be taken by calculating an average velocity for all the subsurface rocks."
example
http://www.sub-surfrocks.co.uk/anatomyh.html
Group members
Abeygunaseka W A H A 090801M
Karunarathna A V P I 090825N
Rukshan A M M 090839K
Kankanamge L U M 090824K
Sandaruwan M W M S 090842M
Piratheep B 090863D

When?
4 stages of HC exploration
3D seismic data
3D seismic volume is created by shooting a closely spaced grid of 2D lines and interpolating between the lines to create a "3-dimensional volume" of data that is also referred to as a cube.
Issues related to Seismic data
Microseims
Seismograms can record many little waves, called microseisms.

Can be caused by:
Heavy traffic near the seismograph
Waves hitting a beach, the wind
Any other ordinary things that cause some shaking of the seismograph
Image resolution
The higher the resolution, the higher the accuracy of interpretation
The contrast in seismic data should represent the actual bedding of the rocks and not the lateral or facies changes in layers
Constrained ny the frequency of the seismic wave produced
Stratigraphic layer can be resolved if its thickness is atleast a quater of the size of the actual wavelength of the seismic imaging equipment
Degradation of resolution with depth
Earth itself filters seismic signals
The higher the noise level of the data. the more that the software must filter out
Degrades the remaining necessary information
At higher depths, noise will be much higher

Variable property display
Made of wiggle traces
Each trace represents a geophone on the surface
Peaks and Troughs on the wiggle represent up and down movements of the geophone
Vertical axis of the wiggle is the two-way travel time of the shock wave
Amplitude of the peak or trough indicates the magnitude of the geophone movement
Peaks face right and coloured in Black
Shades of Gray are used to represent amplitude
Display types
Well tieng
Seismic profile runs from South to North with a borehole drilled at CMP 740
Using the results of the well tie, we can identify the horizons and rock units by linking the horizons on the seismic profile to the real lithology
THANK YOU
SEISMIC ANATOMY
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