Introduccion Teoria Avo

8/18/2019 Introduccion Teoria Avo

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Practical AVO

Introduction to AVO


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Table of Contents

1) Overview of the AVO process

2) Theory Part 1 : oc! Physics " #luid eplace$ent %odelin&

') Theory Part 2 : AVO Theory " %odelin&

() or!shop Part 1: The Ostrander %odel " Colony *as +and

,) Theory Part ' : AVO Analysis on eal -ata

.) or!shop Part 2: AVO Analysis of 2- " '- Channel +and

/) Theory Part ( : AVO Case +tudies

0) or!shop Part ': *ulf Coast ercises

3) or!shop Part (: lastic I$pedance

14) or!shop Part ,: 5a$bda6%u6ho 75%)

11) Theory Part , : A#I: Analy8in& uncertainty in AVO

12) Theory Part . : Ac9uisition " Processin& Concerns in AVO


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Introduction

This tutorial is a brief introduction to the A$plitude Variations with

Offset or A$plitude Versus Offset 7AVO) $ethod;

e will briefly review how the interpretation of seis$ic data has

chan&ed throu&h the years;

e will then loo! at why AVO was an i$portant step forward for the

interpretation of hydrocarbon ano$alies;

#inally we will show why the AVO response is closely lin!ed to theroc! physics of the reservoir;


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A +eis$ic +ection

The fi&ure above shows a stac!ed seis$ic section recorded over the shallow

Cretaceous in Alberta;


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+tructural Interpretation

>our eye $ay first &o to an interestin& seis$ic event between .'4 and .(4 $s;


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*as well location

And in this case he or she would have been ri&ht? A successful &as well was drilled

at that location; The fi&ure above shows the sonic lo& inte&rated to ti$e spliced on

the section; The &as sand top and base are shown as blac! lines on the lo&;


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@ri&ht spotsB

ut this would have been a luc!y &uess since structure alone does not tell you that a

&as sand is present; A &eophysicist in the 13/4s would have based the well on the

fact that there is a @bri&ht spotB visible on the seis$ic section as indicated above;


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hat is a @bri&ht spotB

To understand @bri&ht spotsB recall the definition of the 8ero6offset reflectioncoefficient shown in the fi&ure above; R

0 the reflection coefficient is the a$plitude

of the seis$ic pea! shown; Dote also that the product of density and P6wavevelocity V is called acoustic i$pedance;

1122

1122

0

V V

V V R

ρ ρ

ρ ρ

+

−=

+eis$ic

raypath

Interface atdepth E d

1V

1

2V

2

t

eflection at ti$e

t = 2d/V 1

Geology Seismic Surface

+eis$ic

avelet

Shale

Gas Sand


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hat is a @bri&ht spotB

The fi&ure on the ri&ht

ta!en fro$ *ardner et al;

7Formation velocity and

density - The diagnostic

basics for stratigraphic

traps: Geophysics, 1974)

shows that there is a bi&

difference between shaleand &as sand velocity at

shallow depths; The

paper also shows that

density and velocity are

approi$ately related by

the e9uation

= 0.23 V 0.25

Thus we would epect a

bi& reflection coefficient

or @bri&ht spotB for

shallow &as sands;

-ifference between shale and &as

sand velocity at shallow depth;


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The AVO $ethod

Fnfortunately @bri&ht

spotsB can be caused

by litholo&icvariations as well as

&as sands; This lead

&eophysicists in the

1304s to start

loo!in& at pre6stac!

seis$ic data; Thea$plitude increase

with offset shown

here was predicted

by Ostrander 7lane-

!ave reflection

coefficients for gassands at nonnormal

angles-of-incidence:

Geophysics, 19"4) for

certain &as sands

7Class ' as we will

discuss later);


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hat causes the AVO effect=

As shown above the traces in a seis$ic &ather reflect fro$ the subsurface at

increasin& an&les of incidence ; The first order approi$ation to thereflection coefficients as a function of an&le is &iven by addin& a second ter$

to the 8ero6offset reflection coefficient:

θ θ 20

sin)( B R R +=

θ1

θ2θ

3

Surface

Reflector 1

V P1

V S1

2V

P2V

S2

B is a &radient ter$ which produces the AVO effect; It is dependent on

chan&es in density P6wave velocity V P and +6wave velocity V S ;


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hy is +6wave velocity i$portant=

As Gust shown the

&radient ter$ is dependent

on density P and +6wave

velocity; The reason that

+6wave velocity has such

an i$pact is shown on the

left where P and +6wavevelocity are shown as a

function of &as saturation

in the reservoir; Dote that

P6wave velocity drops

dra$atically but +6wave

velocity only increasessli&htly 7why=); This will

be discussed thorou&hly

in the net chapter;


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AVO %odelin&

ased on AVO theory and the roc! physics of the reservoir we can perfor$ AVO

$odelin& as shown above; Dote that the $odel result is a fairly &ood $atch to the

offset stac!; Also note that Poissons ratio is a function of VpHVs ratio and will be

discussed in the net chapter;

P6wave -ensity +6wavePoissons

ratio +ynthetic Offset +tac!


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AVO Attributes

Intercept: A

*radient:

AVO Attributes are

used to analy8e

lar&e volu$es ofseis$ic data

loo!in& for

hydrocarbon

ano$alies;


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Cross6plottin& of Attributes

One of the AVO $ethods that we will bediscussin& later in the course involves cross6

plottin& the 8ero6offset reflection coefficient 7A)

versus the &radient 7) as shown on the left;

As seen in the fi&ure below the hi&hli&hted

8ones correspond to the top of &as sand 7pin!)base of &as sand 7yellow) and a hard strea!

below the &as sand 7blue);

*radient 7)

Intercept 7A)


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Dear Inversion

AVO Inversion

#ar Inversion

A very i$portant new toolco$bines Inversion with

AVO Analysis to enhance

the reservoir

discri$ination;


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Conclusions

+eis$ic interpretation has evolved over the years fro$ strictlystructural interpretation throu&h @bri&ht spotB identification to directhydrocarbon detection usin& AVO;

In this course we will elaborate on the ideas that have been presented

in this short introduction;

As a startin& point the net chapter will discuss the principles of roc!physics in $ore detail;

e will then $ove to AVO $odelin&;

#inally we will loo! at AVO analysis on real seis$ic data;

In each case we will first loo! at the theory and then perfor$ awor!station ea$ple;

Documents

Introduccion Teoria Avo