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The science of inferring geology from processedseismic data

Seismic data contains 2 basic elements

Arrival Time of reflection events

T = f (depth, velocity of overlying rock) Reflection, Amplitude, re!uencies

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Seismic Frequency SpectrumSeismic Frequency Spectrum

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Seventies

1971: Seismic Stratigraphy

1979: 1-D Attributes

1979: 1-D Impedance Inversion

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19!": #right Spot $echno%ogy 19!": II&S 19! : A'(

19!9: 2-D Attributes

Eighties

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199": Sub-sa%t Imaging 1992: Seismic Se*uence Stratigraphy 1994: 4-D +%o, Seismic .arameters 199 : 3-D /oherence /ube -----: 'irtua% 0ea%ity

Nineties

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Other Creative Concepts arine Airguns %oba% .ositioning System 24 #it-De%ta Sigma 0ecording 9-/ 0ecording In ormation $echno%ogy

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R"S#$%T&#'

R"S#$%T&#' *AT +A' " "AS%R" -

. hat is the resolving ability of modern seismic reflectiondata-/ The !uestion in fact has t0o parts1

A *o0 thin can a reservoir unit be and still be resolvedseismically-

3 *o0 0ide does a unit have to be laterally in order tobe seismically visible-

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R"S#$%T&#'R"S#$%T&#' *AT +A' " "AS%R"

A Thickness (or Thinness)

The critical bed thickness for seismic resolution isabout one eighth .the predominant 0avelength computed using the velocity of the bed /

&n the presence of noise and conse!uent 0aveletbroadening, this resolution po0er may decrease tobeds 0hich are one !uarter 0avelength thick

a4imum amplitude occurs at a thickness of aboutone !uarter 0avelength This is kno0n as the tuning

thickness

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Vertical ResolutionVertical Resolution

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Vertical ResolutionVertical Resolution

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ResolutionResolutionTemplates ofTemplates ofbeds of varyingbeds of varyingtop & bottomtop & bottomreflectionsreflections

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ResolutionResolutiongraph ofgraph of

beds ofbeds ofvaryingvaryingtop &top &bottombottomreflectionsreflections

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VerticalVerticalResolutionResolution

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1)

HorizontalHorizontal

Resolution-Resolution-

Fresnel ZoneFresnel Zone

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S 5$6 $I/ S IS ( 0A S

&t is the bridge that 5oins together logand seismic data, a form 0here they can becompared to the seismic data

$ogging tools have very high verticalresolution (measure properties of beds t0o feetthick) $ateral resolution is very limited A soniclog can only measure in situ attributes severalinches from the borehole and the density log has

only a slightly greater depth of penetration

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S 5$6 $I/ S IS ( 0A S

Seismic data has much more lateralresolution, but poor vertical resolution Theseismic techni!ue has difficulty in resolving bedsthat are thinner than 67 feet The difference inresolving po0er and dimension of resolution(hori8ontal versus vertical) is one of the mostimportant aspect Remember this1

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S 5$6 $I/ S IS ( 0A S

$#9S *A:" ";+"$$"'T :"RT&+A$

R"S#$%T&#' A'< ##R *#R&>#'TA$R"S#$%T&#' *"R"AS S"&S &+ <ATA*A:" ";+"$$"'T *#R&>#'TA$R"S#$%T&#' A'< ##R :"RT&+A$R"S#$%T&#'

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S 5$6 $I/ S IS ( 0A

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S 5$6 $I/ S IS ( 0A SThe synthetic seismogram can be used for the following ? Absolute hori8on identification in time

2 +alibration of affirmation of polarity and phase of the data

@ <etermine relative amplitudes of various events<etermination of interference patterns of reflections from

closely spaced events

6 "stimation of the seismic 0avelet

B "stimation of the fre!uency content various depths or times

C #ne dimensional modeling various thickness, porosities, etc

D #n $andmark 0orkstation Syntool is used for preparingsynthetic seismogram

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S"&S &+ TRA+" &':"RS&#' Techni!ue for estimating the sonic log response

from seismic trace <eveloped only after the

?EC7Fs Trade names1 Seislog, Synlog, seudo sonic,

:elocity Section, Trace &nversion, etc

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S"&S &+ TRA+" &':"RS&#' #n a sonic log 0e can read the interval transit time or

velocity of a particular interval of rock rom sonic logs

0e can1 &nfer lithology <etect facies changes <etect porosity and changes in porosity

Sometimes detect pore fluide 0ant to do same things using seismic data by

measuring a property of the rocks rather thandifferences in the property bet0een rock units

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Seismic Trace InversionSeismic Trace Inversion

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Sonic logSonic logsho ingsho ing

lo & highlo & highfrequencyfrequencycontentcontent

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2)

Soniclogandinvertedtrace

Soniclogandinvertedtrace

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S"&S &+ TRA+" &':"RS&#' inal roduct

The final product is a cross section 0ith a sonic logat each seismic trace location and contoured, color

coded velocities, acoustic impedance This crosssection sho0s lateral variation in velocity or acousticimpedance for each hori8on in the subsurface&t is easy to recogni8e patterns of stratigraphy on an

inverted section than it is on a seismic section&t is easier to detect changes in velocity in thesubsurface using the color coded velocities thanmeasuring changes in amplitude of an event on a

seismic section

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S"&S &+ TRA+" &':"RS&#'

The inverted section often makes it easierto detect1

? +hanges in lithology

2 orosity development

@ racturing of a reservoir

ore fluid in some cases

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S"&S &+ TRA+" &':"RS&#' %se in "4ploration

Seismic trace inversion allo0s us to more easilyrecogni8e these small amplitude variations

&n many e4ploration plays facies changes areindicative of e4ploration potential

The channel and even the most porous portions of thechannel can be mapped

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S"&S &+ TRA+" &':"RS&#' %se in "4ploration An increase in porosity of sand or a carbonate can be

detected 0ith trace inversion "ven the estimation of the

percent of porosity can be done <iminutive changes in velocity are difficult to detect on a

seismic section, but they are more recogni8able on theinverted section 0ith color coded velocities, acoustic

impedance This techni!ue allo0s us to recogni8e imperceptible patterns

in the seismic data that are indicative of areas 0ithsignificant hydrocarbon bearing potential

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Structural

Stratigrahy

$ithology

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Structural &nterpretation Subsurface structural map creation from the observedarrival timesStratigraphic &nterpretation

<evelop a chronostratigraphic frame0ork of cyclicgenetically related strata

attern of reflections observedodel of cyclic episodes of deposition

$ithological &nterpretation $itho logy changes

orosityractureore fluid

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Seismic stratigraphy

&nterpreting seismic data to reconstruct theStratigraphy history

&nvestigating stratigraphic problems using seismicdata The basic unit Sequence

A deposited se!uence is a stratigraphic unitcomposed of a relatively conformable succession ofgenetically related strata and bounded of its top andbase by unconformities or their correlativeconformities

Se!uence deposition is controlled by sea levelchanges, sediment supply and subsidence

rovides an ideal basis for establishing acomprehensive stratigraphic frame0ork regionally orglobally

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"ach basic unit has certain properties thatcan be used to describe it These include 1 The e4ternal geometry or shape &nternal reflection configuration The amplitude of reflection The fre!uency of the data The continuity of the reflection The aspect or appearance of theseproperties are called facies

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3AS&' A'A$GS&SFAQS hat is the tectonic style of area -

hat is the age of the formation of the basin-

hat is the nature of the basement- hat is age of the basin fill-

here are the potential source rocks-

hat is the pattern of the seismic reflections- hat is their spatial and temporal distribution- $itho facies redictions-

Are there migration path0ays-

hat are the plays and prospects-

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Ans0ers the interpreter should kno0 0hen heHshe has

seismic processed data- &s it a landH marine H combination of both survey-

hat 0as the surface geology and ho0 does it varythroughout the area-

&n the case of a marine survey, 0hat 0as the type ofcable, spacing of hydrophones, cable length and 0hatdepth 0as the cable run at -

or land data 0hat types of geophones 0ere used, 0hat

0as their spacing, and ho0 0ere they arrayed-hat 0as recorded fold of the data-

hat 0as the recording length- hat 0ere the recorded fre!uencies-

hat 0as the sampling rate-

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#3I"+T&:"S

$he ob ectives or the stratigraphicinterpretation o seismic data are:

to predict the stratigraphy to predict the occurrence o source reservoir

and sea% the migration path,ays o hydrocarbons to

the reservoirto reconstruct the pa%eogeography and

geo%ogic history

Stratigraphic interpretation

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Flo chart forFlo chart forseismicseismicstratigraphicstratigraphicinterpretationinterpretation

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Flo chartFlo chart

for !"for !"interpretationinterpretationonon#or$station#or$station