12.faciesexercise

8/14/2019 12.FaciesExercise

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Stanford Rock Physics Laboratory - Gary Mavko

292

A North Sea Example

A Seismic Facies ClassificationCase Study




293

A North Sea Example

PROBLEMCan we predict and characterize

reservoirs in North Sea deep-water clasticsystems using seismic data?

1 km1 km

SEISMIC AMPLITUDE MAP OF A NORTH SEA SUB-MARINE FAN




294

A North Sea Example

SEISMIC LITHOFACIESIN DEEP- WATER CLASTIC SYSTEMS

A seismic lithofacies is a seismically resolvable sedimentaryunit characterized by its:

• lithology(clay content)

• bedding configuration(massive, interbedded, chaotic)

• petrography(grain size, clay location and cementation)

• seismic properties(seismic velocities and density)




295

A North Sea Example

I

II

III

IV

V

VI

FACIES I:Gravels and conglomerates

FACIES II:Thick-bedded sandstone

FACIES III:Interbedded sandstone-shale

FACIES IV :Silty shale and

silt-laminated shale

FACIES V:Pure shale

FACIES VI:Chaotic deposits




296

A North Sea Example

Identification of Seismic LithofaciesFrom Well Logs (Field A, Well A-1)

40 60 80 100 120 140

2100

2150

2200

Gamma Ray (API)

D e p

t h ( m )

V

IVII b

II c

III

1.5 2 2.5 3 3.5 4

2100

2150

2200

Vp (km/s)

D e p

t h ( m )

V

IVII b

II c

III

V

IV




297

A North Sea Example

IIa

III

IIc IIb

IVV

Overburden

Tuff

Chalk

1.0 km

200 m

Depth = 2.0 km

2-D Geological Model




298

A North Sea Example

40 60 80 100 120 140

2.2

2.6

3

3.4

Gamma Ray (API)

P - w a v e v e l o c i t y

( k m / s )

IIb

IIcIII

IVV

Rock Physics Analysis

40 60 80 100 120 140

0.2

0.3

0.4

Gamma Ray (API)

P o r o s i t y

IIbIIc

III

IV V




299

A North Sea Example

40 60 80 100 120 140

4.5

5.5

6.5

7.5

Gamma Ray (API)

A c o u s t i c

I m p e d a n c e

IIb

IIc III

IV

V

40 60 80 100 120 1401.5

2

2.5

3

Gamma Ray (API)

V p / V s

IIb IIc

III

IV

V

Seismic Properties




300

A North Sea Example

Seismic Reflectivity Modeling(AVO)

R(θ ) = R0 + G * sin ( θ )

R e f l e c t i v i t y

0

- 0.1

0.1

II b

V IIc

IIaIII

Angle of incidence ( θ )

10 20 504030

Cap-rock = IV

0 60

2




301

A North Sea Example

Facies-GuidedReflectivity Modeling

IIb

V IV

III

IIc

Schematic facies map of sub-marine fan




302

A North Sea Example

Facies-GuidedReflectivity Modeling

Zero offset reflectivity

0.02

0.04

-0.02

0

-0.04

-0.12

-0.16

-0.08

Zero offset reflectivity+ AVO gradient

0




303

A North Sea Example

Seismic Response of Unconsolidatedand Cemented High-Porosity Sands




304

A North Sea Example

Zero-offset Reflectivity, R(0)

Relatively high R(0)(blue)

Relatively low R(0)

(yellow)




305

A North Sea Example

AVO Gradient, G

Relatively large negativegradient G (yellow)

Relatively smallgradient (blue)




306

A North Sea Example

Variability of AVO Response

Monti-Carlo, drawn from pdf, and assuming siltyshale cap rock.




307

A North Sea Example

Bivariate pdfs: R(0) vs. G




308

A North Sea Example

Facies classification usinglinear discriminant analysis

Sample to be classifiedFacies 1

Facies 2 Facies 3

R(0)

G

= Facies 2

Mahalanobisdistance

M 2= ( X − µ i )

T −1∑ X − µ i( )




309

A North Sea Example

Oil sands

Shale

Brine sands

Interbeddedsand-shales

3D seismic lithofacies prediction

12.faciesexercise

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