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Geomechanical Characterization of a Host Rock for Enhanced Geothermal System in the North-German Basin
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 191 (2017) 158 – 163
Symposium of the International Society for Rock Mechanics
Geomechanical Characterization of a Host Rock for Enhanced Geothermal System in the North-German Basin M. Alber*, C. Solibida Ruhr University Bochum, Engineering Geology/ Rock Engineering, Universitätsstr. 150, D-44780 Bochum, Germany
Abstract The North-German Basin offer at depth of approximately 5000 m sufficiently high temperatures for efficient and economic heat extraction by geothermal wells. The permeability of the carboniferous strata at that depth is very low so that conductive pathways have to be created by hydraulic fracturing. For some planned projects we conduct the characterization of the carboniferous strata as exposed in “analogous outcrops”. The geological setting for fracking is demanding, as the strata comprise strong sandstone, shale and weak coal seams. All necessary petrophysical properties will be collected by extended testing. The regional horst-andgraben system allows for mapping in outcrops and extrapolate the structural content towards the rock mass at depth. ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017. Peer-review under responsibility of the organizing committee of EUROCK 2017 Keywords: enhanced geothermal system; rock chractacterization; structural characterization
1. Introduction The North German Basin (NGB) is together with the Upper Rhine Valley and the southern Molasse basin the preferred area for deep geothermal wells in Germany Several commercial (Neustadt / Glewe) and research projects (GeneSys; Groß Schönebeck) have been developed. The prognosis of temperatures at depth for Germany is shown in Figure 1. Some planned projects focus on the western part of the North Germany Basin and due to the fact that permeability of the target horizons at depth is very low fracking will be executed to create conductive pathways in the otherwise dry and hot rock. This type of heat mining is called Enhanced Geothermal System EGS which aims at producing electricity.
* Corresponding author. Tel.: +49-234-32-23296; fax: +49-234-32-14120. E-mail address: [email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017
doi:10.1016/j.proeng.2017.05.167
M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
Fig. 1. Estimates of temperatures at depth 5000 m [1], black dots = sampling area, open circle = EGS locations.
Fig. 2. Tectonic setting near the EGS projects (green circle). Sampling was executed in the horst-structures (blue crosses). Modified after [2].
The red spots in Figure 1 suggest a temperature of around 180° C at a depth of 5000 m. This information seems to be valid since many oil and gas wells were drilled and still are operated in the NGB. We focus in this paper on the area near the border to the Netherlands, on the utmost western part of the NGB. Here several commercial projects are aiming for drilling deep wells and for executing multiple fracking from horizontal drillholes for obtaining high flow rates. This area is well known from exploration on hydrocarbon, 3D seismic lines have been executed and some deep wells have been drilled for exploitation. 1.1. Geological setting The area close to the Netherlands is characterized by many graben and horst structures (Fig. 2). The target horizon for most projects is the Upper Carboniferous, i.e. Westfal C/D at a depth of around 5000 m. On top of the target horizon there is Permian Zechstein salt. Here it is of particular interest that the in-situ stresses are decoupled by Permian Zechstein salt [3]. Mesozoic and quaternary sediments complete the hanging rocks. As shown in Figure 3, the target rocks (Westfal C/D) crop out just 30 km from the area where EGS projects may take place. Thus, it was possible to characterize relevant rocks from “analogous outcrops”. The rocks from Westfal C/D comprise argillaceous rocks (shale, sandstone and conglomerate) as well as coal found in numerous seams. The tectonic setting is that of an extensional regime with mainly normal faults leading to graben-and-horst structures. The implications of the geological setting for borehole stability and hydraulic fracturing are manifold. The major horizontal stress VH changes directions above and below the Permian salt [3] and the difference between vertical stress VV and major horizontal stress VH increases below the salt change. The rock types change frequently according to the sequential sedimentation which leads in case of vertical fracs to the difficulty to propagate a fracture e.g. in stiff argillaceous rocks with interbedded soft coal seams of several m in height. Also present are numerous, mainly steep-dipping small faults, which might get re-activated by the fracking process. In summary, the knowledge about the widely varying properties of the rocks and discontinuities is necessary for a successful exploitation of heat at depth.
159
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M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
Fig. 3. Sketch of the horst of Carboniferous age at Ibbenbüren that served as “analogous outcrop” for the adjacent target horizon at 5000 m depth. Modified after [2].
2. Rock mechanical characterization After mapping and sampling in several quarries the rock characterization comprised tests as summarized in Table 1. All tests were executed according to ISRM suggested methods [4]. Table 1. Mean values of tested rock types. Specimen
Sst W 1
Sst W 2
Sst W 3
Sst W 3 NS
Kng W 1
Kng W 2
Dry density ȡd [g/cm³]
2.36
2.33
2.30
2.29
2.21
2.27
Ust P 1 2.65
Grain density ȡs [g/cm³]
2.68
2.68
2.66
2.66
2.66
2.65
2.74 -
Effecitve porosity n [%]
11.94
13.25
15.91
16.55
16.0
13.62
Void ratio e [-]
0.14
0.15
0.19
0.20
0.19
0.16
-
Compression wave velocity VP [km/s]
3.104
3.043
2.903
3.061
2.919
2.922
2.581 0.916
Shear wave velocity VS [km/s]
1.872
1.810
1.782
1.870
1.611
1.636
Tensile strength ıt [MPa]
3.8
3.6
4.3
4.2
2.7
2.7
16.4
Compressive strength ıu [MPa]
59.8
50.0
68.1
64.6
37.1
45.3
174.9
Young's modulus Estat [GPa]
7.0
7.7
9.6
9.5
6.9
9.9
13.3
Poisson's ratio Ȟstat [-]
0.15
0.14
0.23
0.22
0.33
0.29
0.11
Material constant mi (Hoek-Brown) [-]
14.2
16.1
18.9
15.9
15.8
17.0
10.1
Cohesion c (Mohr-Coulomb) [MPa]
7.6
8.5
6.8
8.6
4.7
5.6
26.2
Friction angle ij (Mohr-Coulomb) [°]
52.1
52.9
53.3
52.2
49.7
51.8
54.6
Hydraulic conductivity kf [m/s]
1.2E-08
4.1E-08
4.7E-09
1.9E-08
4.6E-08
3.7E-08
-
Permeability K [m²]
1.2E-15
4.2E-15
4.3E-16
1.7E-15
4.2E-15
3.4E-15
-
A small part of the test results are given in Figures 4-7. The uniaxial compressive strength as well as the tensile strength of the rocks at target horizon show a wide variety. Coal with an UCS of about 10 MPa is the weakest rock, the sandstones are of medium strength and the shales range up to UCS of 140 MPa. It is clear that the sandstones sampled at the surface are weathered and that stronger rock should be expected at depth. The shale offers distinct anisotropy and will further be tested in directions relevant to fracturing. Overall, the ratio of compressive-to-tensile strength is 10-15:1. The coal is the softest rock with a Young’s Modulus of about 2 GPa (Fig. 5). The stiffness contrast between the shale and coal (Eshale / Ecoal | 7/1) may be problematic for fracking as this ratio has an influence on the fracture height [5]. The ratio might be even higher when considering the low stiffness of an entire coal seam. From multi-stage triaxial test the strength envelopes using the Hoek-Brown failure criteria for intact rock is given in Figure 7. The strength contrast is obvious and the coal may be close to failure at in-situ stresses at that depth.
M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
Fig. 4. Boxplots of UCS for 4 rock types.
Fig. 5. Boxplots of Young’s Modulus for 4 rock types.
Fig. 6. Boxplots of Brazlian tensile strength of 4 rock types
Fig. 7. Hoek-Brown strength envelopes or different rock types.
The triaxial tests on specimen containing a suitable discontinuity yielded the friction angle of M = 17° (Fig. 8). This is quite low compared to direct shear tests executed on sandstone discontinuities [7]. As stated above, the sampled rocks at surface suffered some weathering and the value M = 17° may be seen as the lower limit for shear strength.
Fig. 8. Shear strength of discontinuities in sandstone from triaxial tests.
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M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
3. Field observations The geological features at the outcrops show many local normal faults, horst-and graben structures (Fig. 9) as well as coals seam of up to 4 m in height (Fig. 10). Faults end often within layers and were probably developed during Carboniferous age. Is may thus be assumed that those faults are present at depth too.
Fig. 9. Local horst-graben structures.
Fig. 10. Faults, coal seams and strong sandstones.
4. Implication of results Induced or triggered seismicity from frac operations are unwanted and detrimental to any EGS project. It is known from numerous researches that only existing discontinuities or faults are the geologic structures that lead to seismic events with perceptible magnitudes ML > 2. It is of interest to estimate how close the existing discontinuities / faults are to failure. With the approach by [8] it is possible to connect the ratio V1 / V with a discontinuities’ friction angle and its orientation to the major principal stress: ı1 ı3
=
1+ȝ·tanș 1 - ȝ·tanș
(1)
Here is P = tan M and T is the angle between the discontuity / fault and the orientation of the major principal stress, repectively. The state of stress at depth at the area under investigation may be estimated from [3]. The major principal stress V1 is the vertical stress VV with magnitude 120 MPa and V3 = Vh | 70 MPa. The ratio V1 / V is then 1.7. The caculations are shown in Figure 11. It may be concluded from Figure 12 that only discontinuities / faults with friction angles M < 15 ° are critical at certain orientations T to the major principal stress. However, when pressurized frac fluid enters the discontinuities / faults the ratio of effective V1/V3 becomes higher and more discontinuities / faults are prone to fail and may lead to seismic events (Fig. 12). 5. Discussion and conclusion This paper discusses basic rock and rock mass properties sampled from outcrops in the same lithology as may probably be encountered at deep geothermal target horizons in the W’ North German Basin. The results from the extensive lab program indicate that the strength and the stiffness contrasts of the different rock types may pose some problem for generating the desired fracture geometries. The mapping at the outcrops showed several discontinuity sets and local graben-horst structures that might get activated when frac fluid will be injected at elevated pressures.
M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
Fig. 11 Application of Sibson’s [8] approach to estimate the allowable angle T between the major principal stress, here VV, and a fault plane with given friction angle.
Fig. 12. Increase of the effective stress ratio V1eff/V3eff through injection pressure.
Acknowledgement This work was performed under research contract FKZ 0324138A funded by the German Federal Ministry for Economic Affairs and Energy. References [1] LIAG, http://www.liag-hannover.de/online-dienste-downloads/downloads/digitale-karten.html. Access 22/09/16. [2] F. Kockel, R. Baldschuhn, Osning Tektonik – einst und jetzt. Brandenburgische Geowiss. Beitr. (2002) 77–84. [3] T. Röckel, C. Lempp, The State of Stress in the North German Basin (in German), Erdöl Erdgas Kohle 119 (2003) 73–80. [4] R. Ulusay, J. Hudson. The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring:1974–2006. [5] H.A.M. van Ekelen, Hydraulic Fracture Geometry: Fracture containment in layered formations, SPE/AIME 22 (1982) 341–349. [6] M. Alber, J. Schwarz, Experiments on Low-friction Discontinuities from Carboniferous Strata in the Ruhr Mining District, in: EUROCK 2015 & 64th Geomechanics Colloquium, Schubert (ed.), 2015, pp. 529-53. [7] M. Alber, Strength of Faults - A Concern for Mining Engineers? in: Proc. EUROCK2013: Rock Mechanics for Resources, Energy and Environment – Kwasniewski & àydzba (eds) ,Taylor & Francis Group, London, 2013, pp. 545–540. [8] R.H. Sibson, A note on fault reactivation. J Str Geol 7 (1985) 751–754.
163
ScienceDirect Procedia Engineering 191 (2017) 158 – 163
Symposium of the International Society for Rock Mechanics
Geomechanical Characterization of a Host Rock for Enhanced Geothermal System in the North-German Basin M. Alber*, C. Solibida Ruhr University Bochum, Engineering Geology/ Rock Engineering, Universitätsstr. 150, D-44780 Bochum, Germany
Abstract The North-German Basin offer at depth of approximately 5000 m sufficiently high temperatures for efficient and economic heat extraction by geothermal wells. The permeability of the carboniferous strata at that depth is very low so that conductive pathways have to be created by hydraulic fracturing. For some planned projects we conduct the characterization of the carboniferous strata as exposed in “analogous outcrops”. The geological setting for fracking is demanding, as the strata comprise strong sandstone, shale and weak coal seams. All necessary petrophysical properties will be collected by extended testing. The regional horst-andgraben system allows for mapping in outcrops and extrapolate the structural content towards the rock mass at depth. ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017. Peer-review under responsibility of the organizing committee of EUROCK 2017 Keywords: enhanced geothermal system; rock chractacterization; structural characterization
1. Introduction The North German Basin (NGB) is together with the Upper Rhine Valley and the southern Molasse basin the preferred area for deep geothermal wells in Germany Several commercial (Neustadt / Glewe) and research projects (GeneSys; Groß Schönebeck) have been developed. The prognosis of temperatures at depth for Germany is shown in Figure 1. Some planned projects focus on the western part of the North Germany Basin and due to the fact that permeability of the target horizons at depth is very low fracking will be executed to create conductive pathways in the otherwise dry and hot rock. This type of heat mining is called Enhanced Geothermal System EGS which aims at producing electricity.
* Corresponding author. Tel.: +49-234-32-23296; fax: +49-234-32-14120. E-mail address: [email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017
doi:10.1016/j.proeng.2017.05.167
M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
Fig. 1. Estimates of temperatures at depth 5000 m [1], black dots = sampling area, open circle = EGS locations.
Fig. 2. Tectonic setting near the EGS projects (green circle). Sampling was executed in the horst-structures (blue crosses). Modified after [2].
The red spots in Figure 1 suggest a temperature of around 180° C at a depth of 5000 m. This information seems to be valid since many oil and gas wells were drilled and still are operated in the NGB. We focus in this paper on the area near the border to the Netherlands, on the utmost western part of the NGB. Here several commercial projects are aiming for drilling deep wells and for executing multiple fracking from horizontal drillholes for obtaining high flow rates. This area is well known from exploration on hydrocarbon, 3D seismic lines have been executed and some deep wells have been drilled for exploitation. 1.1. Geological setting The area close to the Netherlands is characterized by many graben and horst structures (Fig. 2). The target horizon for most projects is the Upper Carboniferous, i.e. Westfal C/D at a depth of around 5000 m. On top of the target horizon there is Permian Zechstein salt. Here it is of particular interest that the in-situ stresses are decoupled by Permian Zechstein salt [3]. Mesozoic and quaternary sediments complete the hanging rocks. As shown in Figure 3, the target rocks (Westfal C/D) crop out just 30 km from the area where EGS projects may take place. Thus, it was possible to characterize relevant rocks from “analogous outcrops”. The rocks from Westfal C/D comprise argillaceous rocks (shale, sandstone and conglomerate) as well as coal found in numerous seams. The tectonic setting is that of an extensional regime with mainly normal faults leading to graben-and-horst structures. The implications of the geological setting for borehole stability and hydraulic fracturing are manifold. The major horizontal stress VH changes directions above and below the Permian salt [3] and the difference between vertical stress VV and major horizontal stress VH increases below the salt change. The rock types change frequently according to the sequential sedimentation which leads in case of vertical fracs to the difficulty to propagate a fracture e.g. in stiff argillaceous rocks with interbedded soft coal seams of several m in height. Also present are numerous, mainly steep-dipping small faults, which might get re-activated by the fracking process. In summary, the knowledge about the widely varying properties of the rocks and discontinuities is necessary for a successful exploitation of heat at depth.
159
160
M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
Fig. 3. Sketch of the horst of Carboniferous age at Ibbenbüren that served as “analogous outcrop” for the adjacent target horizon at 5000 m depth. Modified after [2].
2. Rock mechanical characterization After mapping and sampling in several quarries the rock characterization comprised tests as summarized in Table 1. All tests were executed according to ISRM suggested methods [4]. Table 1. Mean values of tested rock types. Specimen
Sst W 1
Sst W 2
Sst W 3
Sst W 3 NS
Kng W 1
Kng W 2
Dry density ȡd [g/cm³]
2.36
2.33
2.30
2.29
2.21
2.27
Ust P 1 2.65
Grain density ȡs [g/cm³]
2.68
2.68
2.66
2.66
2.66
2.65
2.74 -
Effecitve porosity n [%]
11.94
13.25
15.91
16.55
16.0
13.62
Void ratio e [-]
0.14
0.15
0.19
0.20
0.19
0.16
-
Compression wave velocity VP [km/s]
3.104
3.043
2.903
3.061
2.919
2.922
2.581 0.916
Shear wave velocity VS [km/s]
1.872
1.810
1.782
1.870
1.611
1.636
Tensile strength ıt [MPa]
3.8
3.6
4.3
4.2
2.7
2.7
16.4
Compressive strength ıu [MPa]
59.8
50.0
68.1
64.6
37.1
45.3
174.9
Young's modulus Estat [GPa]
7.0
7.7
9.6
9.5
6.9
9.9
13.3
Poisson's ratio Ȟstat [-]
0.15
0.14
0.23
0.22
0.33
0.29
0.11
Material constant mi (Hoek-Brown) [-]
14.2
16.1
18.9
15.9
15.8
17.0
10.1
Cohesion c (Mohr-Coulomb) [MPa]
7.6
8.5
6.8
8.6
4.7
5.6
26.2
Friction angle ij (Mohr-Coulomb) [°]
52.1
52.9
53.3
52.2
49.7
51.8
54.6
Hydraulic conductivity kf [m/s]
1.2E-08
4.1E-08
4.7E-09
1.9E-08
4.6E-08
3.7E-08
-
Permeability K [m²]
1.2E-15
4.2E-15
4.3E-16
1.7E-15
4.2E-15
3.4E-15
-
A small part of the test results are given in Figures 4-7. The uniaxial compressive strength as well as the tensile strength of the rocks at target horizon show a wide variety. Coal with an UCS of about 10 MPa is the weakest rock, the sandstones are of medium strength and the shales range up to UCS of 140 MPa. It is clear that the sandstones sampled at the surface are weathered and that stronger rock should be expected at depth. The shale offers distinct anisotropy and will further be tested in directions relevant to fracturing. Overall, the ratio of compressive-to-tensile strength is 10-15:1. The coal is the softest rock with a Young’s Modulus of about 2 GPa (Fig. 5). The stiffness contrast between the shale and coal (Eshale / Ecoal | 7/1) may be problematic for fracking as this ratio has an influence on the fracture height [5]. The ratio might be even higher when considering the low stiffness of an entire coal seam. From multi-stage triaxial test the strength envelopes using the Hoek-Brown failure criteria for intact rock is given in Figure 7. The strength contrast is obvious and the coal may be close to failure at in-situ stresses at that depth.
M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
Fig. 4. Boxplots of UCS for 4 rock types.
Fig. 5. Boxplots of Young’s Modulus for 4 rock types.
Fig. 6. Boxplots of Brazlian tensile strength of 4 rock types
Fig. 7. Hoek-Brown strength envelopes or different rock types.
The triaxial tests on specimen containing a suitable discontinuity yielded the friction angle of M = 17° (Fig. 8). This is quite low compared to direct shear tests executed on sandstone discontinuities [7]. As stated above, the sampled rocks at surface suffered some weathering and the value M = 17° may be seen as the lower limit for shear strength.
Fig. 8. Shear strength of discontinuities in sandstone from triaxial tests.
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162
M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
3. Field observations The geological features at the outcrops show many local normal faults, horst-and graben structures (Fig. 9) as well as coals seam of up to 4 m in height (Fig. 10). Faults end often within layers and were probably developed during Carboniferous age. Is may thus be assumed that those faults are present at depth too.
Fig. 9. Local horst-graben structures.
Fig. 10. Faults, coal seams and strong sandstones.
4. Implication of results Induced or triggered seismicity from frac operations are unwanted and detrimental to any EGS project. It is known from numerous researches that only existing discontinuities or faults are the geologic structures that lead to seismic events with perceptible magnitudes ML > 2. It is of interest to estimate how close the existing discontinuities / faults are to failure. With the approach by [8] it is possible to connect the ratio V1 / V with a discontinuities’ friction angle and its orientation to the major principal stress: ı1 ı3
=
1+ȝ·tanș 1 - ȝ·tanș
(1)
Here is P = tan M and T is the angle between the discontuity / fault and the orientation of the major principal stress, repectively. The state of stress at depth at the area under investigation may be estimated from [3]. The major principal stress V1 is the vertical stress VV with magnitude 120 MPa and V3 = Vh | 70 MPa. The ratio V1 / V is then 1.7. The caculations are shown in Figure 11. It may be concluded from Figure 12 that only discontinuities / faults with friction angles M < 15 ° are critical at certain orientations T to the major principal stress. However, when pressurized frac fluid enters the discontinuities / faults the ratio of effective V1/V3 becomes higher and more discontinuities / faults are prone to fail and may lead to seismic events (Fig. 12). 5. Discussion and conclusion This paper discusses basic rock and rock mass properties sampled from outcrops in the same lithology as may probably be encountered at deep geothermal target horizons in the W’ North German Basin. The results from the extensive lab program indicate that the strength and the stiffness contrasts of the different rock types may pose some problem for generating the desired fracture geometries. The mapping at the outcrops showed several discontinuity sets and local graben-horst structures that might get activated when frac fluid will be injected at elevated pressures.
M. Alber and C. Solibida / Procedia Engineering 191 (2017) 158 – 163
Fig. 11 Application of Sibson’s [8] approach to estimate the allowable angle T between the major principal stress, here VV, and a fault plane with given friction angle.
Fig. 12. Increase of the effective stress ratio V1eff/V3eff through injection pressure.
Acknowledgement This work was performed under research contract FKZ 0324138A funded by the German Federal Ministry for Economic Affairs and Energy. References [1] LIAG, http://www.liag-hannover.de/online-dienste-downloads/downloads/digitale-karten.html. Access 22/09/16. [2] F. Kockel, R. Baldschuhn, Osning Tektonik – einst und jetzt. Brandenburgische Geowiss. Beitr. (2002) 77–84. [3] T. Röckel, C. Lempp, The State of Stress in the North German Basin (in German), Erdöl Erdgas Kohle 119 (2003) 73–80. [4] R. Ulusay, J. Hudson. The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring:1974–2006. [5] H.A.M. van Ekelen, Hydraulic Fracture Geometry: Fracture containment in layered formations, SPE/AIME 22 (1982) 341–349. [6] M. Alber, J. Schwarz, Experiments on Low-friction Discontinuities from Carboniferous Strata in the Ruhr Mining District, in: EUROCK 2015 & 64th Geomechanics Colloquium, Schubert (ed.), 2015, pp. 529-53. [7] M. Alber, Strength of Faults - A Concern for Mining Engineers? in: Proc. EUROCK2013: Rock Mechanics for Resources, Energy and Environment – Kwasniewski & àydzba (eds) ,Taylor & Francis Group, London, 2013, pp. 545–540. [8] R.H. Sibson, A note on fault reactivation. J Str Geol 7 (1985) 751–754.
163