Shale Porosity (%)0 10 20 30 40 50 60

4000Dep

th (m

)

0

1000

2000

3000

5000

6000

Dep

th (ft)

13,120

0

3280

6560

9840

16,400

19,680

70

1 Storer in v. Engelhardt (1970)2 Larsen & Chilingar (1983)3, 4 Magara (1978)5 Chilingarian (1983)6 Dickinson (1953)7 Proshlyakov (1960)8 Meade (1966)9 Athy (1930)10 Hosoi (1963)11 Hedberg (1936)12 Magara (1968)13 Ham (1966)14 v. Engelhardt (1970)15 Bryant et al. (1975)16 Skempton (1970)17 Bjorlykke (1983)18 Sclater & Christie (1980)19 Bredehoeft et al. (1988)20 Neglia (1979)21 PetroMod (IES)22 McKenzie Delta mudstone (Issler, 1992)23 This Study

4

15

3

18

18 6

6

9

10

1

11

8

7

7

16

21

17

25

5

12 13

19

14

21

20

22

22

Sandstone Porosity (%)0 10 20 30 40 50 60

4000

Dep

th (m

)

0

1000

2000

3000

5000

6000

7000

Depth (ft)

13,120

0

3280

6560

9840

16,400

19,680

22,96015

31216

11

2

12

76

10

4

14

9

5

13

3 4

8

15

16

1

1 v. Engelhardt (1960)2,3,4 Chilingarian (1983)5,6 Chilingarian & Wolf (1976)7,8 Stephenson (1977)9,10,11 Fuchtbauer (1974)12 Blatt (1983)13 Holland et al. (1980)14 Thomson (1982)15 Sclater & Christie (1980)16 IES 17 This study

17

17

17

Selected References

Asquith, G., 1982, Basic Well Log Analysis for Geologists: Methods in Exploration Series - Number 3: Tulsa, Oklahoma, American Association of Petroleum Geologists, 216 p.

Hearst, J. R., Nelson, Philip H., and Paillet, Frederick L., 2000, Well Logging for Physical Properties, A Handbook for Geophysicists, Geologists, and Engineers, Second Edition: New York, John Wiley & Sons Ltd, 483 p.

Issler, D. R., 1992, A new approach to shale compaction and stratigraphic restoration, Beaufort-Mackenzie Basin and Mackenzie Corridor, Northern Canada: American Association of Petroleum Geologists Bulletin, v. 76, p. 1170-1189.

Poelchau, H. S., Baker, D.R., Hantschel, T., Horsfield, B., and Wygrala, B., 1997, Basin simulation and design of the conceptual basin model, in D. H. Welte, Horsfield, B., and Baker, D.R., ed., Petroleum and Basin Evolution: New York, Springer-Verlag, p. 3-70.

Raiga-Clemenceau, J., Martin, J.P., and Nicoletis, S., 1988, The concept of acoustic formation factor for more accurate porosity determination from sonic transit time data: The Log Analyst, p. 54-60.

–200 –180 –160 –140 –120 –100 – 80 – 60 – 40 – 20 0.2

.3

.4

.5

.6

.7

.8

.9

1

1.1

1.2

1.3

1.4

1.5

Time (m.y.)

Vit

rin

ite

refle

ctan

ce (R

o %

)

Shublik Fm. - Tulaga Well

Low

er C

reta

ceo

us

Un

con

form

ity

Fort

ress

Mtn

./To

rok

Toro

k B

ott

om

Set

s &

Clin

ofo

rms

Toro

k To

pse

ts &

Nan

ush

uk

Gp

.

Co

lvill

e Fm

.

Sag

avan

irto

k Fm

.

Map showing erosion since the end of Brookian sedimentation. The map was created by W.M. Burns from erosion estimates made for each well shown, using compaction curves defined for sand, silt, and shale. These erosion estimates are incorporated into burial and thermal history modeling.

Summary plots showing the wide range of compaction curves in the literature (modified from Poelchau et al., 1997). The black dashed lines show the compaction curves defined in this study for sand (#17) and for shale (#23). The closest published analog to our shale curve is from a study of McKenzie Delta mudstones (shale #22, Issler, 1992). These mudstones compact along a trend very similar to that of the North Slope shale until about 10,000 ft. Below that depth the curves diverge to a maximum porosity difference of 7.5%.

Different compaction curves can result in significant differences in calculated thermal maturities. Vitrinite reflectance is calculated here for the Tulaga well using 1) the compaction curves defined in this study (in black) and 2) default shale compaction curve in BasinMod, chosen to illustrate the effects of a significantly different curve. Sandstone curves are the same in both cases. The calculations were made with Basin2.

Thermal maturity of the Shublik Fm. (in terms of vitrinite reflectance) is shown increasing with time. This figure provides a different view of how compaction curves can affect model calculations of thermal history and timing of source rock maturation.

In this example, use of this study's shale and siltstone compaction curves leads to the prediction (in black) that the Shublik Fm. would enter the oil window at about 87 m.y., during deposition of the Colville Gp. The same calculation (in orange) made using BasinMod default compaction curves predicts that Shublik would enter the oil window (Ro > 0.6%) at about 105 m.y., during deposition of the Torok Fm.

Summary

Compaction curves specific to a study area can improve the accuracy of basin models when adequate sonic and gamma ray logs are available to define them. The curves determined for the North Slope have allowed geologically reasonable erosion estimates for individual wells and a regional erosion map that is consistent with present day topography, i.e., greatest erosion in the Brooks Range, decreasing northward toward the coastline. The compaction-derived erosion estimates are generally more modest than estimates made using vitrinite reflectance. We believe they may be more reliable because the well log data are more continuous and internally consistent than the vitrinite data, and because the 'recycling' of dispersed vitrinite is thought to be significant on the North Slope. Finally, the compaction curves defined here improve the accuracy of our model calculations of thermal history and timing of maturation of each of the source rocks.

Estimated Erosion -- North Slope, Alaska

Compaction Curves from the Literature

Model Ro(%) values using Shale + Silt compaction curves from this study

.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

Dep

th (k

m)

Shublik Fm. - Tulaga Well

Kekiktuk Conglomerate

Basement

Kayak Shale - Endicott Gp.

Lisburne Gp.

Sadlerochit Gp.

Shublik Fm.

Hue Shale

Fortress Mtn./Torok

Torok Topsets &Nanushuk Gp.

Torok Bottom Sets & Clinoforms

Colville Gp.

Kingak Shale

Pebble Shale

Model Ro(%) values using Shale + Silt compaction curves from this study

Model Ro(%) values using BasinMod Shale and Silt compaction curves

Model Ro(%) values using BasinMod Shale and Silt Compaction curves

Effect of Compaction Curves on Thermal Maturity Calculations

Limitations

The compaction curves defined here are based on sonic-porosities for the Cretaceous and Tertiary Brookian section. The curves do reasonably well (errors generally < 5%) at predicting porosities in Brookian rocks through the Upper Cretaceous Colville Group and its equivalents. However, in certain wells in the Nanushuk and Torok Fms. (mid-Cretaceous), and in the Canning Fm. (Upper Cretaceous-Tertiary), distinct intervals of typically 1000-1500 ft show sonic-derived porosities uniformly 2-5% lower than the porosities predicted from our curves.

The discrepancies may be due to differences in lithology and/or diagenetic history. A volcanic component in the Upper Cretaceous and Tertiary sediments, including bentonites and potassium-rich clays, may alter their compaction properties relative to the older, volcanic-poor sands and shales. In addition, several mid-Brookian unconformities have been noted in seismic sections, but are not widely documented across the North Slope. Geographically, we have found it difficult to predict which wells will show the offset in porosity. Work is ongoing to better define the wells that are affected and the reason for the discrepancy.

Comparison of Model Compaction Curves

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Shale Porosity

Dep

th (f

t)

This Study

Basin2 Shale

BasinMod Shale

IES (Poelchau et al., 1997)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Sand Porosity

This study

Basin2 Sand

BasinMod Sand

IES (Poelchau et al., 1997)

Comparison of default sand and shale compaction curves from several basin modeling packages with the sand and shale curves defined in this study.

Dep

th (f

t)

0

Oil

Win

do

w(0

.6<

R o%

<1.

2)

0 .2 .4 .6 .8 1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

Vitrinite Reflectance (Ro %)

U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

OPEN-FILE REPORT 03–329Sheet 3 of 3, Version 1.0

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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