8/18/2019 Introduccion Teoria Avo
1/17
1
Practical AVO
Introduction to AVO
8/18/2019 Introduccion Teoria Avo
2/17
2
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$bda6%u6ho 75%)
11) Theory Part , : A#I: Analy8in& uncertainty in AVO
12) Theory Part . : Ac9uisition " Processin& Concerns in AVO
8/18/2019 Introduccion Teoria Avo
3/17
3
Introduction
This tutorial is a brief introduction to the A$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;
8/18/2019 Introduccion Teoria Avo
4/17
4
A +eis$ic +ection
The fi&ure above shows a stac!ed seis$ic section recorded over the shallow
Cretaceous in Alberta;
8/18/2019 Introduccion Teoria Avo
5/17
5
+tructural Interpretation
>our eye $ay first &o to an interestin& seis$ic event between .'4 and .(4 $s;
8/18/2019 Introduccion Teoria Avo
6/17
6
*as well location
And in this case he or she would have been ri&ht? A successful &as well was drilled
at that location; The fi&ure above shows the sonic lo& inte&rated to ti$e spliced on
the section; The &as sand top and base are shown as blac! lines on the lo&;
8/18/2019 Introduccion Teoria Avo
7/17
7
@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$ic section as indicated above;
8/18/2019 Introduccion Teoria Avo
8/17
8
hat is a @bri&ht spotB
To understand @bri&ht spotsB recall the definition of the 8ero6offset reflectioncoefficient shown in the fi&ure above; R
0 the reflection coefficient is the a$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$ic
avelet
Shale
Gas Sand
8/18/2019 Introduccion Teoria Avo
9/17
9
hat is a @bri&ht spotB
The fi&ure on the ri&ht
ta!en fro$ *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;
8/18/2019 Introduccion Teoria Avo
10/17
10
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$plitude increase
with offset shown
here was predicted
by Ostrander 7lane-
!ave reflection
coefficients for gassands at nonnormal
angles-of-incidence:
Geophysics, 19"4) for
certain &as sands
7Class ' as we will
discuss later);
8/18/2019 Introduccion Teoria Avo
11/17
11
hat causes the AVO effect=
As shown above the traces in a seis$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 ;
8/18/2019 Introduccion Teoria Avo
12/17
12
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;
8/18/2019 Introduccion Teoria Avo
13/17
13
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 $atch to the
offset stac!; Also note that Poissons ratio is a function of VpHVs ratio and will be
discussed in the net chapter;
P6wave -ensity +6wavePoissons
ratio +ynthetic Offset +tac!
8/18/2019 Introduccion Teoria Avo
14/17
14
AVO Attributes
Intercept: A
*radient:
AVO Attributes are
used to analy8e
lar&e volu$es ofseis$ic data
loo!in& for
hydrocarbon
ano$alies;
8/18/2019 Introduccion Teoria Avo
15/17
15
Cross6plottin& of Attributes
One of the AVO $ethods that we will bediscussin& later in the course involves cross6
plottin& the 8ero6offset reflection coefficient 7A)
versus the &radient 7) as shown on the left;
As seen in the fi&ure below the hi&hli&hted
8ones correspond to the top of &as sand 7pin!)base of &as sand 7yellow) and a hard strea!
below the &as sand 7blue);
*radient 7)
Intercept 7A)
8/18/2019 Introduccion Teoria Avo
16/17
16
Dear Inversion
AVO Inversion
#ar Inversion
A very i$portant new toolco$bines Inversion with
AVO Analysis to enhance
the reservoir
discri$ination;
8/18/2019 Introduccion Teoria Avo
17/17
17
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;