AVO in carbonates, its potentialYongyi Li*(Scott Pickford, Canada)

17th Floor, 715 – 5 Ave. SW, Calgary, Canada, T2P 2X6, yli@integrageo.com Jonathan Downton (Scott Pickford, Canada)

Introduction

Using amplitude versus offset (AVO) in hydrocarbon exploration and reservoir characterization incarbonate reservoirs has long being a challenge to both seismic processing and interpretation in last twodecades. Application of AVO in carbonate reservoirs is considered as with more difficulties incomparison to in clastics. Effort has been made by a few workers (e.g., Chacko, 1989), but most problemswere solved on the bases of case by case. The challenges still remain. A few factors contribute to thedifficulties in using AVO in carbonate reservoirs. One of the major reasons is lack of understanding therelationships between the petrophysical properties of carbionates and their AVO responses. Only limitedliterature deal with carbonate rock properties (e.g., Rafavich, 1984). Even so, these studies only deal withspecific situations, which results in the difficulties in extending these studies to AVO related applications.Using the knowledge of clastic AVO directly into carbonates may not be suitable because carbonateshave their own unique rock properties.

New advances in AVO technology and better understanding carbonate rock properties may provide thepossibility for re-examining the current situation. This study intents to address both of these issues and toexplore the potential for AVO applications in carbonate reservoirs. This includes a detailed analysis ofcarbonate rock properties; general description of AVO responses in various carbonate reservoir situations;and finally to extend amplitude analysis to pre-stack seismic inversion and porosity prediction.

Rock physics analysis

Rock physical properties and their relationships are the foundation for amplitude versus offset analysis.Figure 1 illustrates the relationships among the clastic and carbonate rocks by crossplotting well logs. Itshows that carbonates distinguish themselves from clastics with high velocities and relative steady Vp/Vsratio. Further more, the carbonates has different mudrock-line (Vp = aVs + b) with larger slope incomparison with mudrock-line of the clastics. To illustrate the rock properties of carbonate rocks, re-analyzed carbonate data from Rafavich (1984) is given in Figure 2, where gas and porosity effects arehighlighted as they are the most interesting variables in prospect evaluation. The gas and wet lines joint atthe velocities with zero porosity. Direct observations are that porosity increases with decreasingvelocities. Similar to the clastic rocks the gas effect produces low P-wave velocity accompanying with amild increase of the S-wave velocity. Notice that gas effect is amplified in Vp/Vs ratio domain. Thevelocity contrast between tight and porous carbonates and the variation of the velocity due to the gaseffect are important in AVO analysis. The significant variation of velocities due to porosity and gas effectin carbonate rocks with porosity or fluid, as illustrated implies different AVO responses.

The commonly used rock properties in pre-stack seismic inversion λρ , µρ (often refer toincompressibility and rigidity) and λ/µ ratio vs. porosity φ are shown in Figure 3. Figure 3a shows thatwater saturated λρ� forms the upper bound, and gas and porosity effects result in significant drop of theλρ values that indicates the resolution of λρ is high for both porosity and gas effect. This provides thepossibility for pre-stack seismic inversion to be used in identification of these effects. In addition, therigidity is relatively unaffected by gas (Figure 3b), and λ/µ ratio is a good indicator for the gas effect butnot for porosity. The λρ, µρ and porosity have the following relationships:

AVO in carbonates

and

where Cλρ and Cµρ are pore incompressibility and rigidity, which are function of porosity, pore fluid, poretype and pore geometry. The relations for water saturated dolomite and limestone are given in Figure 3.

Amplitude versus offset analysis

To illustrate the AVO response, a few commonly encountered carbonate reservoir models with a porosityvariation up to 20% are calculated and shown in Figure 4. For the case of porous limestone overlain bytight limestone, at low porosity, amplitude remains relatively constant with offset. However, for largeporosity, amplitude decreases (dims) with offset. If the reservoir is replaced by gas-charged porousdolomite, the amplitude increases with offset. For the case of porous dolomite overlain by anhydrite, atlow porosity the amplitude is relatively constant. At high porosity, however, amplitude decreases withoffset. For the case of high velocity shale and porous dolomite, amplitude variation with offset is similarfor all porosities, the amplitude reverseing at small porosities. These AVO responses show that thepresence of porosity and gas may be identified from amplitude variation with offset.

Pre-stack seismic inversion especially the λρ and µρ method (Goodway et al., 1997) has been provedsuccessful in lithology delineation and gas identification. This method can be used in carbonate reservoiras well. Equations 1 and 2 provide a tool to use pre-stack seismic inversion for predicting porosity in acarbonate reservoir.

Conclusions

The application of AVO method in the characterization of carbonate reservoirs has great potential butfaces some challenges. With better understanding of the relationships between carbonate rock propertiesand amplitude variation with offset, we expect that more information in a carbonate reservoir could beextracted from surface seismic with the help of AVO technology.

ReferencesChacko, S., 1989, Porosity identification using amplitude variations with offset: examples from south

Sumatra, Geophysics, 54, 942-951.

Goodway, W., Chen, T., and Downton, J., 1997, Improved AVO fluid detection and lithologydiscrimination using Lame’s petrophysical parameter, CSEG Recorder, 22, No.7, 3-5.

Rafavich, F., Kendall, C. H. St. C., and Todd, T. P., 1984, The relationship between acoustic propertiesand the petrographic character of carbonate rocks, Geophysics, 49, 1622-1636.

Biographical note:Yongyi Li graduated from University of Alberta in 1992 with an M.Sc. and in 1997 with a Ph.D. both ingeophysics. He currently is a processing geophysicist R&D in Scott Pickford, Canada. His professionalinterests are primarily in the areas of rock physics and AVO analysis.

φλρλρλρC

matrix+=

11

φµρµρµρC

matrix+=

11

(1)

(2)

AVO in carbonates

Figure 1. Velocities of various lithologies and gas effect in sandstone

Figure 2. Porosity and gas effect in carbonate rocks

2.5 3.3 4.2 5.0 5.8 6.5

3.6

3.2

2.8

2.4

2.0

1.62.5 3.3 4.2 5.0 5.8 6.5

4.0 4.5 5.0 5.5 6.0 6.5

gas sand

1.60

1.504.0 5.0 6.0 7.0

1.80

1.70

Vp/V

s ra

tio

P-wave velocity (km/s)

Porosity effect

Gas effect

P-wave velocity (km/s)

d)

4.0 5.0 6.0 7.0

4.0

3.6

3.2

2.8

2.4

S-w

ave

ve

loci

ty (k

m/s

)

P-wave velocity (km/s)

Porosity effect

Gas effect

a) b)

P-wave velocity (km/s)

S-w

ave

ve

loci

ty (k

m/s

)

gas sand

coal

wet sand

carbonatesa) b)

4.0 4.5 5.0 5.5 6.0 6.5

3.4

3.2

3.0

2.8

2.6

2.4

S-w

ave

ve

loci

ty (k

m/s

)

Porosity effect

Gas effect

c)1.9

1.8

1.7

1.6

1.5

Vp/V

s ra

tio

Porosity effect

Gas effect

P-wave velocity (km/s)

2.98

2.67

2.36

2.06

1.75

1.44

120

105

90

75

60

45

30

15

0

γ ray

P-wave velocity (km/s)

Vp/V

s ra

tio

coal

wet sand

carbonates

AVO in carbonates

λρ (

GP

a x

g/c

c)

Porosity (%)

Gas effect

Porosity (%)

Tight limestone

Porous limestone

Tight limestone

Gas-chargedporous dolomite

Porosity (%)

0 5 10 15 20

200

150

100

50

0

a) b) c)a)

λ/µ

ratio

Gas effect

0 5 10 15 20

2.5

2.0

1.5

1.0

0.5

0.0Gas effect

µρ (

GP

a x

g/c

c)

0 5 10 15 20

140

100

60

20Gas effect

Saturated dolomite : Saturated limestone:1/λρ = 10-4 (7.30φ + 54.0) 1/λρ = 10-4 (8.52φ + 70.7)1/µρ = 10-4(8.75φ + 75.0) 1/µρ = 10-4(8.72φ + 115.6)

0.15

0.05

-0.05

-0.25

-0.35

Ref

lect

ion

coef

ficie

nt

Incident angle (degrees)0 10 20 30 4050

Porosity effect

Shuey 2 termapprox.Shuey 3 termapprox.

φ = 0.0φ = 0.0φ = 0.0φ = 0.0

φ = 0.1φ = 0.1φ = 0.1φ = 0.1

φ = 0.2φ = 0.2φ = 0.2φ = 0.2

0.15

0.05

-0.05

-0.25

-0.35

Ref

lect

ion

coef

ficie

nt

Incident angle (degrees)0 10 20 30 40 50

Porosity effect

Shuey 2 term approx.Shuey 3 term approx.φ = 0.0φ = 0.0φ = 0.0φ = 0.0

φ = 0.1φ = 0.1φ = 0.1φ = 0.1

φ = 0.2φ = 0.2φ = 0.2φ = 0.2

0.15

0.05

-0.05

-0.25

-0.35

Ref

lect

ion

coef

ficie

nt

Incident angle (degrees)0 10 20 30 40 50

Porosity effect

φ = 0.0φ = 0.0φ = 0.0φ = 0.0

φ = 0.1φ = 0.1φ = 0.1φ = 0.1

φ = 0.2φ = 0.2φ = 0.2φ = 0.2

Shuey 2 term approx.Shuey 3 term approx.

0.15

0.05

-0.05

-0.25

-0.35

Ref

lect

ion

coef

ficie

nt

Incident angle (degrees)0 10 20 30 40 50

Porosity effect

Shuey 2 term approx.Shuey 3 term approx.φ = 0.0φ = 0.0φ = 0.0φ = 0.0

φ = 0.1φ = 0.1φ = 0.1φ = 0.1

φ = 0.2φ = 0.2φ = 0.2φ = 0.2

Figure 4. Typical AVO responses with varied porosity

Anhydrite

PorousDolomite

High velocity shale

Porous limestone

Figure 3. Relationship between elastic modulii and porosity