From loose grains to stiff rocks –

The rock-physics "life story" of a clastic

sediment, and its significance in QI studies

Prof. Per Avseth, NTNU/G&G Resources

The rock physics “life story” of a clastic sediment

– a teaser:

1Ksat

1Kmineral

+

K + K fluid

Age (M.yrs)200 0

Bu

rial

dep

th/T

emp

.

70 oC

Chemical compaction

Mechanicalcompaction

Tectonic uplift

100

A

BA

B

0.30 0.35 0.40

Ela

stic

Modulu

s

Po ro sity

ContactCement

Init ialSandPack

Friable

ConstantCement

Sorting variability

Chemical compaction

Mechanical compaction

Deposition

HC trends

Acoustic Impedance

Vp/Vs

B

A

Bjørlykke (2015)

Pore compressibilitystrongly affected by diagenesis!

3

Selected references:

• Helset et al., 2004: Combined diagenetic and rock physics modeling for improved control on seismic depth trends (EAGE Abstract)

• Brevik et al., 2011: Rock Physicist step out of the well location, meet geophysicists and geologists to add value in exploration analysis (The Leading Edge).

• Dræge et al. 2014: Linking rock physics and basin history – Filling gaps between wells in frontier basins (The Leading Edge).

• Zadeh et al. 2016: Compaction and rock properties of Mesozoic and Cenozoic mudstones and shales, northern North Sea (Marine and Petroleum Geology).

• Avseth and Lehocki, 2016: Combining burial history and rock-physics modeling to constrain AVO analysis during exploration (The Leading Edge).

Geology GeophysicsRock Physics

Bridge & Bottle-neck Few observablesMany parameters

Ambiguities

QI prediction

Limitations/ResolutionUncertainties

Pit-fallsMore/Better data!More knowledge!

More integration!

ComplexityNoise/Artifacts

Variability

Reducing uncertainties through integration

Case example from North Sea (Alvheim Field)

Sand and shale compaction trends

in the North Sea

Alvheim well (Kneler discovery)

Velocity jump in sst due to cementation

Friable sst

CCT

0.30 0.35 0.40

Ela

stic

Modulu

s

Porosity

ContactCement

InitialSandPack

Friable

ConstantCement

Vclay Vp Porosity

Oil leg

Burial history and subsidence curves for top

reservoir sst at Kneler well (schematic)

Burial depth/temperature

Time (M.yrs before present)

0-15-60

Present day burial

Upper Sst Unit

Lower Sst Unit

Time range of target > 70C (= cementation)

70C = ca. 2km

0

Maximum burial

DepositionPaleocene

Tilting/Uplift Miocene

Continuoussubsidence

AVO classification constrained by depth trends

(Alvheim field, North Sea)Rimstad et al. (2012):(Bayesian classification)

Avseth and Lehocki (2016)(Mahalanobis distance)

BrineOil

Gas

shale

Uncon.

Cemented

GasOil Brine

shaleCem.

Unc.

Are injectites on Volund cemented or not?

From Schwab et al. 2015

Well log data from 24/9-6

0 50 100 150

2000

2050

2100

2150

2200

2250

GR

De

pth

(m

)

0 2 4

Velocity

4 6 8 10 12

Acoustic impedance

1.5 2 2.5 3

Vp/Vs

Intra Balder (oil)

Intra Balder (brine)

Hermod

Heimdal

AI Vp/VsVs & VpGR

2000m

2250m

Rock Physics diagnostics of Paleocene sandstone units

0.15 0.2 0.25 0.3 0.35 0.4 0.451.5

2

2.5

3

3.5

4

4.5

Porosity

Vp

(km

/s)

Balder sst 1

Balder sst 2

Hermod

Heimdal

Unconsolidated sand model

Dvorkin-Nur

contact cement model

0.30 0.35 0.40

Ela

stic

Modulu

s

Po ro sity

ContactCement

Init ialSandPack

Friable

ConstantCement

Combined modeling of burial

history and rock physics

Mechanical compaction (Lander and Walderhaug, 1999)

Chemical compaction (Walderhaug, 1996), when T > 70 C

Rock physics modeling

0.30 0.35 0.40

Ela

stic

Modulu

s

Porosity

ContactCement

InitialSandPack

Friable

ConstantCement

Hertz-Mindlin (mech. comp)Dvorkin-Nur + Hashin-Shtrikman

(chemical compaction)

Slide 13

Stable packing configuration

Dep. porosity

Initial volume of pore filling material

Exponential decline factor

Effective stressSum of pore space:

Volume of Qz cement precipitated between time T1 and T2

Volume of Qz cement at time T1

Molar weight of qz Qz- precipitation rateQz- surface area

Qz- densityPorosity at Cement start Heat rate

Combined burial and rock physics modeling of porosity versus P-

wave velocity (sensitivity study)

Vp/VsAcoustic Imp.

PorosityCement volume

Rock physics and AVO modeling constrained

by burial history

Slide 15

1. Burial history

Burial depth (m)Temperature (degrees C)

Geologic Age (M.Yr)0250

Stø Fm

Deposition

Max. burial

(Present day burial)

2. Diagenetic modeling (Walderhaug)

Depth/Temperature

Onset cement

3. Rock physics modeling (Dvorkin-Nur)

Depth/Temperature

Oil

Brine OilBrine

4. AVO modeling (Zoeppritz)

+ shalecompaction

and RP Intercept+-

+

-

Gradient

Deposition Brine

Oil

Shale (Background)

Max. burial

A «typical» present day

geo-section offshore Norway

16

Quarternary sediments

Sea water

Miocene sediments

Oligocene-Eocene

Paleocene

Cretaceous

Syn-rift

Jurassic/Triassic

Prospect A

Prospect B Prospect C

Discovery

70 C

Cenozoic upliftIncreasing landward

«Restoring» geo-section to maximum burial.

Have prospects been into the frying pan?

17

The “Frying Pan”(T>70 C required for Qz-cem. )

Burial constrained AVO

modeling at “Discovery” well.(Campanian sands w/oil give AVO class III)

“Frying pan” (chemical compaction/Qz cementation)

Brine sst

Oil

Oil

Brine

Shale

Shale

Brinesand

Burial curve

70 C

Fluid trend

Compaction trendBrine

Oil

Burial constrained AVO modeling at prospect A

(Paleocene sst)

Oil-filled sst = AVO class I-IIp

19

“Frying pan”

Brine sstBrine sst

Oil Oil

OilBrine

Shale

Shale

Brinesand

Burial curve

70 C

Fluid trend

Compaction trend

Burial constrained AVO modeling at prospect B

(Paleocene sand)

Oil-filled sst = AVO class III

“Frying pan”

Brine

Oil

Brine

Oil

OilBrine

Shale

Shale

Brinesand

Burial curve

70 C

Fluid trend

Compaction trend

Burial constrained AVO modeling at prospect C

(Aptian sst)

Oil-filled sst = AVO class IIp

“Frying pan”

Brine sst

Oil

Brine sst

OilFluid trend

Compaction trend

OilBrine

Shale

Shale

Brinesand

Burial curve

70 C

RPT and AVO analysis

@Pingvin well 7319/12-1

22

Fluid trend

Porosity/Rock stiffness

shaleBrinesandGas

heterolithics

Limestone

AI

Vp/Vs AI Vp/Vs

Litho/Fluid classification

Intercept

Gra

die

nt

Gas

Gas(tight)

Burial analysis and simulated AVO signatures in Pingvin

(7319/12-1)

23

Net erosion mapfrom N. Johansen (2017)

Unc.sand

Cem.sst

Sorting

Diag.

Brine sand

Oil

OilBrine

Max burial = 1250m

Uplift = 700m

ShaleBrine

Oil

Reservoir

Cap-rock

700

The reservoir sands in Pingvin has not been buried deepenough to be cemented! Hence, great fluid sensitivity! AVO class III expected for any HC-fill.

Onset cementation = 70C

GasGas

Gas

Eocene more deeply buried in Sørvestnaget Basin:

Burial constrained AVO at well 7216/11-1S

24

Onset cementation = 70oC

OilBrineGas

Wisting example

Net uplift=1500m (Temp grad=42.5C/km)

Gas Oil Brine

Summary:

How burial controls AVO and fluid sensitivity

Pingvin

EoceneProspect 1

Eocene Prospect 2

Juraprospect

Age

depth

70oC

AVO class III

AVO class III-IV

AVO class II-IIp

AVO class II-III

Conclusions

• The present day seismic signatures will reflect depositional and burial

history, and this knowledge should be included in AVO and QI studies.

• Rock physics modeling can be combined with burial modeling for

uplift estimation, to constrain low-frequency trends, and to model

expected AVO signatures.

• Normally, bright seismic amplitudes and class II-III AVO signatures are

only associated with unconsolidated to poorly consolidated sandstones.

• There is likely a lot of «hidden» hydrocabrons (esp. oil) in consolidated

sandstone reservoirs with stiff rock frame and reduced fluid sensitivity,

that can be challenging to discover during AVO and QI studies.

28

Acknowledgements

• Thanks to Ivan Lehocki at Lehocki Geospace for contributions to

burial modeling codes.

• Thanks to MCG for AVO data on Pingvin

• Thanks to FORCE for the invitation