Geology of Seismic Attributes and Their Application

7/31/2019 Geology of Seismic Attributes and Their Application

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Geology of Seismic

Attributes and theirApplication


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Types of Attributes

Pre-Stack Attributes

Post-Stack Attributes


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AVO

Amplitude Vs Offset

Amplitude versus offset (AVO) oramplitude variation with offset is thegeneral term for referring to thedependency of the seismic attribute,

amplitude, with the distance betweenthe source and receiver (the offset).

AVO analysis is a technique that

geophysicists can execute on seismicdata to determine a rocks fluid content,


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Application in Oilindustry

Rising AVO is typically pronounced in oil-bearing sediments, and even more so ingas-bearing sediments.


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Caveat

An important warning is that theexistence of abnormally rising or fallingamplitudes can sometimes be caused

by other factors, such as alternativelithologies and residual hydrocarbons ina breached gas column

Not all oil and gas fields are associatedwith an obvious AVO anomaly


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Direct Hydrocarbon

Indication


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What is a Seismic Line ?

The seismic reflectivity depends on theacoustic impendance at the interface oftwo strat.

The acoustic impedance is a product ofvelocity and density of the strata orsimply rocks .

Each rock have a characteristic densityand velocity property .

Example coal, shale, Sanstone,

Limestone .


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Thus, a seismic reflection line, in a seismicsection may represent an unconformity planeor bedding/plane having more or less uniformvelocity and density.

Across such unconformity or bedding planevelocity and density changes.

A seismic reflection line may or may not behorizontal depending on the deposition ofstratas (horizontal, subhorizontal, continuous,

slightly inclined, inclined, highly inclined )below the sub_surface and their velocity anddensity properties .


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Seismic refections line generally followsunconformities and bedding surface or plane.

The strata above an unconformity are youngerthan the strat present below it.

So the seismic line has chrono (time)stratigrapic significance.

Stratal surface/planes may be bedding planes.

Ex,(bedding planes separting sanstone andshale).

Ex(bedding plane separating sandstone andshale).


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The bedding plane may have sharp, gradationalcontact depending on depositional conditions.

Sandstone and shale have different velocityand density and thus, may form adistinguishable reflecting surface and resultantseismic line.

Unconformities may be a surface of erosion ornon deposition.

Ex(sandstone and shale with an unconformity

between them).

Ex(limestone ans shale with an unconformitybetween them)


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The unconformity between sandstone andshale or between limestone and shale as wellas sanstone/limestone and shale itself has adifferent velocity and density.

Unconformity generates more prominentreflectivity contrast and thus, their resultant

seismic lines.


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au on w e a adocumentation and

InterpretationMany times unwanted causes alsogenerates seismic lines and otherattributes due to noise, poor quality of

data by virtue of survey and Dataprocessing technique Limitations.

So correlation and validation with welllog and core data is essential.


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Seismic Attributes

Seismic attributes are such characteristicsproperty of seismic data which helps to extractinformation of geological significance andinterest.

Seismic line

Configuration

Amplitude Velocity

Frequency

Continuity


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Seismic Line

Seismic line itself being an attribute providesinformation on :

Unconformity and stratal/bedding surfaces.

Specific sequence, formation and their overalldepositional pattern in space and time.

Many times promently helps to extract

structural information like folding, faulting ofthe region.


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Configuration

Configuration provides information on :

Pattern of deposited sediments likeoverall geomertry, structural

companents.

Depositional processes, palaeo-depositinal surfaces,

Configurations may be parallel,prograding, chaotic,etc. depending onthe geometry pattern of depositedsediments.


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Amplitude

Amplitude provides information on :Sediments and their fluids by virtue of their

velocity and density contrast.

Amplitude change suggest in the nature ofrocks and fluids (oil,gas and water).

Commanly used amplitude based indicators, in

hydrocarbon industry, are Bright spots, Dimspots, and Flat spots.


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Bright spot : it is associated with strongamplitudes contrast across lithologies and theirfluids (oil, gas, water) content.

Dim spot : it is associated with weak amplitudecontrast across lithologies and their fluids (oil,gas and water) content.

Flat spot : Associated with fluid contact.

These should be apply with caution as manytimes they are by virtue of changes in litho logy

and not fluids.


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Velocity

Velocity provides information on :

Lithology

Porosity Fluid content() oil, gas and water).

Changes in velocity gives information

about vertical and lateral changes inlithology, porosity and their fluidcontent.


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Frequency

Frequency provides information on :

The thickness of sequences.

Number of strata. Fluid content (oil, gas and water).


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Continuity

Continuity of seismic line providesinformation on :

Continuity of strata.

These may suggest uniformly depositedsediments.

Fluids content within sediments (oil, gas

and water).


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Aggregates attributescharacteristicsAggregates attributes characteristics

provides information on :

Structural and geological settings.

Sediments, lithology, porosity and theirfluid content.

Depositional environment.

Sequence stratigraphy.

Geometric pattern of sediments.


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Seismic resolution

Seismic resolution (vertical andhorizontal) and reliability seismic datawith the well log, well and core data.

Integrating seismic data with well logand core data helps to extract detailinformation about rocks, fluids,depositional paleo-enviroment,stratigraphic and structural betweendrilled wells of a block or region utilizingdiscussed seismic attributes.


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Application

Hydrocarbon industry (conventional andunconventional like coal and bedmethane).

Underground mine design anddvelopment (sedimentary rocks andsedimentry hosted coal, dolomite hosteduranium deposits).

Engineering geological works likehydropower comstruction, tunnel designand development in sedimentry rocks.

Geology of Seismic Attributes and Their Application

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