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Crystal stresses in the north sea from breakouts and other borehole data
Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. V o l . 3 0 , N o . 7 , pp. 1 1 1 1 - 1 ! 14, 1993
f]148-9062/93 $6.00 + 0.00 P e r g a m o n P r e s s Ltd
Printed in G r e a t Britain
Crustal Stresses in the North Sea from Breakouts and other Borehole Data S.M. C O W G I L L * P.G. M E R E D I T H * S.A.F. M U R R E L L * N.R. B R E R E T O N #
INTRODUCTION This paper describes the preliminary results of a study of crustal stress orientations in the North Sea basin, Northwest Europe. The magnitude and orientation of in situ stresses influences the propagation of faults, the opening and closing of cracks and pore fluid circulation, and hence are important to the exploitation of geothermal, hydrocarbon, and other mineral resources as well as to the understanding of broader tectonic processes. Previous work carried out in the North Sea [1,21 has either concentrated on small areas within the basin or focused on the whole area using proprietary data. Using borehole data provided by Amerada Hess, British Petroleum, the Department of Trade and Industry, Ranger Oil and Unocal we have analysed breakouts from wells over a large area of the North Sea using software (WELLOG) developed at the British Geological Survey (BGS). The data reported here samples depths from 300m to 5500m, and the chronostratigraphic age of the rocks extends from the Oligocene to the Precambrian.
through a borehole section is the rose diagram. Such diagrams may be combined into a map showing all breakouts from a selected area (Fig. I). These maps also display the summed (Total) rose indicating the average breakout direction for all the data depicted. 5*W
5*E
0°
TOTAL
.:W GlmlU Glen
.¢_
F~ulIY
~k.
W,'dn Grtmml G ~
Inn¢'lMe,Ira,* F t ~
WELLBORE BREAKOUTS
A whole plethora of data strongly suggests that the three principal stresses operating in the upper crust lie in approximately horizontal and vertical planes 13]. Unequal horizontal principal stresses imposed on near vertical boreholes commonly cause Iocalised spalling of the wallrock in a direction parallel to that of the minimum compressive horizontal stress. The resultant elongation of borehole cross-sections have been termed "breakouts' [4].
Central
~J~m
N;~rth Sea B~t~m
[~wsmgFault/~n~
METHODOLOGY The boreholes analysed in this study were drilled as exploration, appraisal or development wells and all the data were obtained from conventional tour-ann dipmeter tools. The raw data from magnetic tape was read and incorporated into the WELLOG borehole log analysis system prior to examination. The most illustrative presentation of azimuthal frequency
Gr~lx'n
Mbd-Nl~lh Sell High
5ow
0o
5~E
Scale 1:9.000.0110
Figure 1 Rose map of all the data analysed - 91 boreholes showing the general trends of the graben hounding fault zones and major crustal lineaments.
Rock and Ice Physics Laboratory, Research School of Geological and Geophysical Sciences, Birkbeck and University A further method of data presentation is the profile plot College London. Gower Street, London. WC1E 6BT, U.K. (Fig. 2). This is essentially generated by deriving average n Regional Geophysics Group, British Geological Survey, results from a 25m depth window and moving that Keyworth, Nottingham, NGI2 5GG, U.K. 1111 *
1112
ROCK MECHANICS IN THE 1990s
150. -.~
J
I--o.6 I
0.4
L-..J /
GP Detain (metres) 1900.
2000.
t 2100.
KU 2200.
\
I I~.L 2300.
JU 2400.
2500.
Figure 2: Profile plot of an example borehole. The single line represents the average azimuth of Caliper I throughout the borehole. the arrows define the length and orientation of breakouts and the dotted line depicts the 'Quality' of the breakouts relative to the largest breakout in the borehole. The bottom row repf'esents the chronostratigraphy of this borehole IGP - Palaeocene. KU - Upper Cretaceous, KL - Lower Cretaceous, JU - Upper Jurassic and JM - Middle Jurassic. window down the borehole at 5m intervals. This facility enables the influence of depth and chronostratigraphy on breakout rotation to be examined. To minimise the likelihood of asymmetric borehole elongations due to wear by drill pipes ('key seats') being confused with breakouts, a borehole deviation upper limit of 10° from vertical was set for all boreholes analysed. Following analysis, the results are stored in an O R A C L E database. Wellbore registration data. rose diagrams and profile plots are stored in tables together with basic chronostratigraphic information for each well. The profile plots stored in the database allow data from a specific chronostratigraphic interval e.g. the Cretaceous (KU and KLo Fig. 2) to be plotted as a rose diagram for that interval. Similarly. boreholes containing data from a specific chronostratigraphic interval can be selected and plotted to show all the breakout results pertinent to that interval (Figs. 3, 4 and 5). RESULTS AND DISCUSSION The far-field stresses affecting the NW Europe are thought to result from the configuration of the tectonic plate boundaries to the west and south of the region. The east to southeasterly separation of Europe from North America and the north to northeasterly movement between the African and Eurasian plates combine to produce a broadly consistent N W - S E maximum compressive stress orientation [5].
The information presented here indicates that within this context, stress orientauons in the North Sea basin are predominantly influenced by local features. Figure I gives an indication of the influence of the graben bounding fault zones on breakout orientation. The summed rose shows that the breakout directions within the area show little evidence of a dominant direction, although there is some preference for m i m m u m stresses being oriented at 037°/217 ° +_ 26 °. This is only slightly different to the minimum stress direction determined for boreholes on the U.K. mainland by Brereton and Evans 16]. There is often good agreement between breakout orientations within particular domains, for example in the Southern North Sea basin, hut the data also show that there are significant variations in orientation between different domains throughout the North Sea region (Fig. l~. In c o m m o n with previous work in the area [ 1,2] the effect of the major graben structures (Witch Ground. Viking, and Central) on breakout orientation can be clearly seen. Breakouts in boreholes in the Inner Moray Firth are aligned normal to the northern bounding fault indicating that this lineament exerts a great influence over stress orientation m its immediate vicinity (Fig. l). Breakout directions in the Witch Ground Graben (Fig. i) are generally orthogonal to the graben axis although a few breakouts can be seen to run parallel to the gratmn axis. In the southern North Sea the breakout directions are subtly changed by the influence of the Dowsing Fault Zone (DFZ). It is proposed, following Bell et al. [7], that this
ROCK MECHANICS IN THE 1990s fault acts as a free surface and deflects the stress orientations from an ENE to a NE direction across the fault. We have commenced a study into the effects of rock age on breakout orientation using the techniques described earlier. All the breakout data from each chronostratigraphic interval has been extracted from the database and plotted as rose maps, examples of which are presented as Figures 3, 4 and 5.
1113
5*W
0o
5*E
TOTAL
Cretaceous Witch Ground
,,I
Cretaceous strata in the North Sea have only been logged in the Northern part of the area (Fig. 3). 5*w
I1~
"t
5*E
O* TOTAL
3.i
5*W
O*
5*E
Scale 1:8,000,000
Figure 4: Rose map of breakout orientations from the Jurassic. Data from 47 boreholes are ploned with a mean breakout direction of 142°/322 ° -+ 22 °.
5ow
0o
5oE
Scale 1:8,000,000
Figure 3: Rose map of breakout orientations from the Cretaceous. Data from 48 boreholes are plotted with a mean breakout direction of 094"/274 ° _+ 24 °. The breakouts in these rocks show three dominant orientations (033°/212 °, 086°/266 ° and i 21 °/301 °) with a mean of 0940/274 ° (_+ 24°). Differences in sedimentary processes leading to the formation of the Cretaceous rocks may have lead to differing mechanical properties within the sediments. The two dominant directions. 086 ° and 033 ° (Fig. 3.) are almost equal in prominence and it is hypothesised that this may be related to differences in "residual' stresses in the Upper and Lower Cretaceous. Jurassic Jurassic strata have also only been logged in the Northern part of the area (Fig. 4).
The breakouts in these rocks show a much greater degree of consistency than those in the Cretaceous rocks. The Total rose shows a mean N W - S E direction (143°/323 ° _+ 22°). This mean is markedly influenced by the breakouts adjacent to the northern bounding lhult of the Inner Moray Firth as discussed previously, and this trend continues eastward into the Witch Ground Graben. Eastwards from 0 ° the trend becomes progressively dominated by N-S and E-W oriented breakouts in the Viking and Central Grabens. It is clear that the Witch Ground Grabcn is an important transition zone in the orientation of breakouts in the Jurassic of the North Sea. Permo-Triassic Permian and Triassic strata have been logged throughout the North Sea region as they tbrm important oil and gas reservoirs. The region is divided into two sub-basins by the Mid-North Sea High (MNSH) and, as a whole, has a single, dominant breakout direction which trends N E - S W (060°/240 ° -+ 21°). although local vanations do occur (Fig. 5). To the north of the MNSH the breakouts are again influenced by the same tault systems as discussed previously. As the influence of these fault systems diminishes eastwards, the breakouts become somewhat more variable in orientation from NE-SW to NW-SE. East of 0 °, in the Viking and Central Grabens the breakouts are oriented N-S and E-W. This may reflect a "residual' stress associated with the Permo-Triassic rifting of the grabens.
1114
ROCK MECHANICS IN THE 1990s 0o
I- ,
i
!
J
!
F = t
!
It
due to tectonic plate boundary forces. It is also apparent that breakouts in the relatively strong and fully iithified "basement' (pre-Jurassic) rocks show closely related orientations to those measured in maintandboreholes in the Western European stress province [5], Whereas breakouts in the weaker and softer "basin' rocks show m u c h more variability in orientation that appear to be related to intermediate scale structures within the basin.
Acknowledgements - The work reported in this paper forms part of S.M.C's Ph.D. project. The funding for this work has been provided by a NERC Research Studentship with CASE support from the BGS. This paper is publication # I I of the Birkbeek and UCL Research School of Geological and Geophysical Sciences.
,wsing Fault Zone
/.
REFERENCES
I. Alexandrowski P., Inderhaug O.H. and Knapstad B. Tectonic structures and wel lbore breakout orientation. In Tillerson J.R. and Warwersik W.R. (eds) Proceedings 33rd U.S. Symposium on Rock Mechanics. 29-37 (t992).
0o
Scale I :t~.O00.OOO Figure 5: Rose map of breakout orientations from the PermoTriassic. Data from 38 boreholes are plotted with a mean breakout direction of 0600/240 ° - 21 °. To the south of the MNSH the breakouts to the NE o f the D F Z show a dominant N E - S W orientation, re. approximately orthogonal to the fault. On the mainland side of the fault, the trend deviates towards an E-W orientation indicative of the stress refracting influence o f the DFZ. An interesting pomt with regard to the dominant breakout direction is that it is similar to that determined by Brereton and Evans [6] for the whole of the mainland
2. Klein R.J. and Ban" M.V. Regional State of Stress in Western Europe. In Stephansson O. (ed.) Rock Stress and Rock Stress Measurements. 33-44 (1985). 3. McGarr A. and Gay N.C. State of stress in the Earth's Crust. Ann. Rev. Earth. Planet. Sci.. 6, 405-436 (1978) 4. Babcock E.A. Measurement of subsurface fractures from dipmeter logs. Bull. Am. Ass. Petrol. Geol. 62, I I 1 I-I I26 (1978). 5. Brereton N.R. and M011er B. European stress: contributions from borehole breakouts. Phil. Trans. R. Soc. A 337, 165-179 (1991).
U.K. CONCLUSIONS Results from this study clearly demonstrate that the North Sea basin does not display the uniform stress orientations typical of large scale continental land masses such as Western Canada (see Bell and Gough [8]). However, the area can be divided into ' d o m a i n s ' both spatially and temporally, related to major geological structure and chron0stratigraphic interval respectively. We suggest that the stress distribution within each d o m a i n ts controlled by the superimposition o f effects due to local structure and chronostratigraphic age onto the stress field
6. Brereton N.R. and Evans C.J. Rock stress onemations in the U.K. from borehole breakouts, p.36 Report r~J the Regional Geophysics Research Group. British Geological Survey Report RG 87/14 (1987). 7. Bell J.S.. Caillet G. and Adams J. Attempts to detect open fractures and non-sealing faults with dipmeter logs. in Hurst A.. Griffiths C.M. and Worthington P.F. (eds) Geological Applications of Wireline Logs H Geological S(~ciety Special Publication No. 65, 211-220 (1992). 8. Bell J.S. and Gough D.I. Northeast - southwest compressive stress in Alberta: evidence from oil wells. Earth and Planet. Sci. Lett. 4,5, 475-482 (1979).
f]148-9062/93 $6.00 + 0.00 P e r g a m o n P r e s s Ltd
Printed in G r e a t Britain
Crustal Stresses in the North Sea from Breakouts and other Borehole Data S.M. C O W G I L L * P.G. M E R E D I T H * S.A.F. M U R R E L L * N.R. B R E R E T O N #
INTRODUCTION This paper describes the preliminary results of a study of crustal stress orientations in the North Sea basin, Northwest Europe. The magnitude and orientation of in situ stresses influences the propagation of faults, the opening and closing of cracks and pore fluid circulation, and hence are important to the exploitation of geothermal, hydrocarbon, and other mineral resources as well as to the understanding of broader tectonic processes. Previous work carried out in the North Sea [1,21 has either concentrated on small areas within the basin or focused on the whole area using proprietary data. Using borehole data provided by Amerada Hess, British Petroleum, the Department of Trade and Industry, Ranger Oil and Unocal we have analysed breakouts from wells over a large area of the North Sea using software (WELLOG) developed at the British Geological Survey (BGS). The data reported here samples depths from 300m to 5500m, and the chronostratigraphic age of the rocks extends from the Oligocene to the Precambrian.
through a borehole section is the rose diagram. Such diagrams may be combined into a map showing all breakouts from a selected area (Fig. I). These maps also display the summed (Total) rose indicating the average breakout direction for all the data depicted. 5*W
5*E
0°
TOTAL
.:W GlmlU Glen
.¢_
F~ulIY
~k.
W,'dn Grtmml G ~
Inn¢'lMe,Ira,* F t ~
WELLBORE BREAKOUTS
A whole plethora of data strongly suggests that the three principal stresses operating in the upper crust lie in approximately horizontal and vertical planes 13]. Unequal horizontal principal stresses imposed on near vertical boreholes commonly cause Iocalised spalling of the wallrock in a direction parallel to that of the minimum compressive horizontal stress. The resultant elongation of borehole cross-sections have been termed "breakouts' [4].
Central
~J~m
N;~rth Sea B~t~m
[~wsmgFault/~n~
METHODOLOGY The boreholes analysed in this study were drilled as exploration, appraisal or development wells and all the data were obtained from conventional tour-ann dipmeter tools. The raw data from magnetic tape was read and incorporated into the WELLOG borehole log analysis system prior to examination. The most illustrative presentation of azimuthal frequency
Gr~lx'n
Mbd-Nl~lh Sell High
5ow
0o
5~E
Scale 1:9.000.0110
Figure 1 Rose map of all the data analysed - 91 boreholes showing the general trends of the graben hounding fault zones and major crustal lineaments.
Rock and Ice Physics Laboratory, Research School of Geological and Geophysical Sciences, Birkbeck and University A further method of data presentation is the profile plot College London. Gower Street, London. WC1E 6BT, U.K. (Fig. 2). This is essentially generated by deriving average n Regional Geophysics Group, British Geological Survey, results from a 25m depth window and moving that Keyworth, Nottingham, NGI2 5GG, U.K. 1111 *
1112
ROCK MECHANICS IN THE 1990s
150. -.~
J
I--o.6 I
0.4
L-..J /
GP Detain (metres) 1900.
2000.
t 2100.
KU 2200.
\
I I~.L 2300.
JU 2400.
2500.
Figure 2: Profile plot of an example borehole. The single line represents the average azimuth of Caliper I throughout the borehole. the arrows define the length and orientation of breakouts and the dotted line depicts the 'Quality' of the breakouts relative to the largest breakout in the borehole. The bottom row repf'esents the chronostratigraphy of this borehole IGP - Palaeocene. KU - Upper Cretaceous, KL - Lower Cretaceous, JU - Upper Jurassic and JM - Middle Jurassic. window down the borehole at 5m intervals. This facility enables the influence of depth and chronostratigraphy on breakout rotation to be examined. To minimise the likelihood of asymmetric borehole elongations due to wear by drill pipes ('key seats') being confused with breakouts, a borehole deviation upper limit of 10° from vertical was set for all boreholes analysed. Following analysis, the results are stored in an O R A C L E database. Wellbore registration data. rose diagrams and profile plots are stored in tables together with basic chronostratigraphic information for each well. The profile plots stored in the database allow data from a specific chronostratigraphic interval e.g. the Cretaceous (KU and KLo Fig. 2) to be plotted as a rose diagram for that interval. Similarly. boreholes containing data from a specific chronostratigraphic interval can be selected and plotted to show all the breakout results pertinent to that interval (Figs. 3, 4 and 5). RESULTS AND DISCUSSION The far-field stresses affecting the NW Europe are thought to result from the configuration of the tectonic plate boundaries to the west and south of the region. The east to southeasterly separation of Europe from North America and the north to northeasterly movement between the African and Eurasian plates combine to produce a broadly consistent N W - S E maximum compressive stress orientation [5].
The information presented here indicates that within this context, stress orientauons in the North Sea basin are predominantly influenced by local features. Figure I gives an indication of the influence of the graben bounding fault zones on breakout orientation. The summed rose shows that the breakout directions within the area show little evidence of a dominant direction, although there is some preference for m i m m u m stresses being oriented at 037°/217 ° +_ 26 °. This is only slightly different to the minimum stress direction determined for boreholes on the U.K. mainland by Brereton and Evans 16]. There is often good agreement between breakout orientations within particular domains, for example in the Southern North Sea basin, hut the data also show that there are significant variations in orientation between different domains throughout the North Sea region (Fig. l~. In c o m m o n with previous work in the area [ 1,2] the effect of the major graben structures (Witch Ground. Viking, and Central) on breakout orientation can be clearly seen. Breakouts in boreholes in the Inner Moray Firth are aligned normal to the northern bounding fault indicating that this lineament exerts a great influence over stress orientation m its immediate vicinity (Fig. l). Breakout directions in the Witch Ground Graben (Fig. i) are generally orthogonal to the graben axis although a few breakouts can be seen to run parallel to the gratmn axis. In the southern North Sea the breakout directions are subtly changed by the influence of the Dowsing Fault Zone (DFZ). It is proposed, following Bell et al. [7], that this
ROCK MECHANICS IN THE 1990s fault acts as a free surface and deflects the stress orientations from an ENE to a NE direction across the fault. We have commenced a study into the effects of rock age on breakout orientation using the techniques described earlier. All the breakout data from each chronostratigraphic interval has been extracted from the database and plotted as rose maps, examples of which are presented as Figures 3, 4 and 5.
1113
5*W
0o
5*E
TOTAL
Cretaceous Witch Ground
,,I
Cretaceous strata in the North Sea have only been logged in the Northern part of the area (Fig. 3). 5*w
I1~
"t
5*E
O* TOTAL
3.i
5*W
O*
5*E
Scale 1:8,000,000
Figure 4: Rose map of breakout orientations from the Jurassic. Data from 47 boreholes are ploned with a mean breakout direction of 142°/322 ° -+ 22 °.
5ow
0o
5oE
Scale 1:8,000,000
Figure 3: Rose map of breakout orientations from the Cretaceous. Data from 48 boreholes are plotted with a mean breakout direction of 094"/274 ° _+ 24 °. The breakouts in these rocks show three dominant orientations (033°/212 °, 086°/266 ° and i 21 °/301 °) with a mean of 0940/274 ° (_+ 24°). Differences in sedimentary processes leading to the formation of the Cretaceous rocks may have lead to differing mechanical properties within the sediments. The two dominant directions. 086 ° and 033 ° (Fig. 3.) are almost equal in prominence and it is hypothesised that this may be related to differences in "residual' stresses in the Upper and Lower Cretaceous. Jurassic Jurassic strata have also only been logged in the Northern part of the area (Fig. 4).
The breakouts in these rocks show a much greater degree of consistency than those in the Cretaceous rocks. The Total rose shows a mean N W - S E direction (143°/323 ° _+ 22°). This mean is markedly influenced by the breakouts adjacent to the northern bounding lhult of the Inner Moray Firth as discussed previously, and this trend continues eastward into the Witch Ground Graben. Eastwards from 0 ° the trend becomes progressively dominated by N-S and E-W oriented breakouts in the Viking and Central Grabens. It is clear that the Witch Ground Grabcn is an important transition zone in the orientation of breakouts in the Jurassic of the North Sea. Permo-Triassic Permian and Triassic strata have been logged throughout the North Sea region as they tbrm important oil and gas reservoirs. The region is divided into two sub-basins by the Mid-North Sea High (MNSH) and, as a whole, has a single, dominant breakout direction which trends N E - S W (060°/240 ° -+ 21°). although local vanations do occur (Fig. 5). To the north of the MNSH the breakouts are again influenced by the same tault systems as discussed previously. As the influence of these fault systems diminishes eastwards, the breakouts become somewhat more variable in orientation from NE-SW to NW-SE. East of 0 °, in the Viking and Central Grabens the breakouts are oriented N-S and E-W. This may reflect a "residual' stress associated with the Permo-Triassic rifting of the grabens.
1114
ROCK MECHANICS IN THE 1990s 0o
I- ,
i
!
J
!
F = t
!
It
due to tectonic plate boundary forces. It is also apparent that breakouts in the relatively strong and fully iithified "basement' (pre-Jurassic) rocks show closely related orientations to those measured in maintandboreholes in the Western European stress province [5], Whereas breakouts in the weaker and softer "basin' rocks show m u c h more variability in orientation that appear to be related to intermediate scale structures within the basin.
Acknowledgements - The work reported in this paper forms part of S.M.C's Ph.D. project. The funding for this work has been provided by a NERC Research Studentship with CASE support from the BGS. This paper is publication # I I of the Birkbeek and UCL Research School of Geological and Geophysical Sciences.
,wsing Fault Zone
/.
REFERENCES
I. Alexandrowski P., Inderhaug O.H. and Knapstad B. Tectonic structures and wel lbore breakout orientation. In Tillerson J.R. and Warwersik W.R. (eds) Proceedings 33rd U.S. Symposium on Rock Mechanics. 29-37 (t992).
0o
Scale I :t~.O00.OOO Figure 5: Rose map of breakout orientations from the PermoTriassic. Data from 38 boreholes are plotted with a mean breakout direction of 0600/240 ° - 21 °. To the south of the MNSH the breakouts to the NE o f the D F Z show a dominant N E - S W orientation, re. approximately orthogonal to the fault. On the mainland side of the fault, the trend deviates towards an E-W orientation indicative of the stress refracting influence o f the DFZ. An interesting pomt with regard to the dominant breakout direction is that it is similar to that determined by Brereton and Evans [6] for the whole of the mainland
2. Klein R.J. and Ban" M.V. Regional State of Stress in Western Europe. In Stephansson O. (ed.) Rock Stress and Rock Stress Measurements. 33-44 (1985). 3. McGarr A. and Gay N.C. State of stress in the Earth's Crust. Ann. Rev. Earth. Planet. Sci.. 6, 405-436 (1978) 4. Babcock E.A. Measurement of subsurface fractures from dipmeter logs. Bull. Am. Ass. Petrol. Geol. 62, I I 1 I-I I26 (1978). 5. Brereton N.R. and M011er B. European stress: contributions from borehole breakouts. Phil. Trans. R. Soc. A 337, 165-179 (1991).
U.K. CONCLUSIONS Results from this study clearly demonstrate that the North Sea basin does not display the uniform stress orientations typical of large scale continental land masses such as Western Canada (see Bell and Gough [8]). However, the area can be divided into ' d o m a i n s ' both spatially and temporally, related to major geological structure and chron0stratigraphic interval respectively. We suggest that the stress distribution within each d o m a i n ts controlled by the superimposition o f effects due to local structure and chronostratigraphic age onto the stress field
6. Brereton N.R. and Evans C.J. Rock stress onemations in the U.K. from borehole breakouts, p.36 Report r~J the Regional Geophysics Research Group. British Geological Survey Report RG 87/14 (1987). 7. Bell J.S.. Caillet G. and Adams J. Attempts to detect open fractures and non-sealing faults with dipmeter logs. in Hurst A.. Griffiths C.M. and Worthington P.F. (eds) Geological Applications of Wireline Logs H Geological S(~ciety Special Publication No. 65, 211-220 (1992). 8. Bell J.S. and Gough D.I. Northeast - southwest compressive stress in Alberta: evidence from oil wells. Earth and Planet. Sci. Lett. 4,5, 475-482 (1979).