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Estimating regional pore pressure distribution using 3D seismicvelocities in the Dutch Central North Sea Graben
Available online at www.sciencedirect.com SCIENCE
ELSEVIER
~_~DIRECT e
Journal of Geochemical Exploration 78-79 (2003) 203-207
JOURNAl. OF
GEOCHEMICAL EXPLORATION vcww.elsevier.com/locate/jgeoexp
Abstract
Estimating regional pore pressure distribution using 3D seismic velocities in the Dutch Central North Sea Graben P.L.A. Winthaegen*, J.M. Verweij 1 Netherlands Institute of Applied Geoseience TNO~National Geological Survey, P.O. Box 80015, 3508 TA Utrecht, The Netherlands
Abstract
The application of the empirical Eaton method to calibrated sonic well information and 3D seismic interval velocity data in the southeastern part of the Central North Sea Graben, using the Japsen (Glob. Planet. Change 24 (2000) 189) normal velocitydepth trend, resulted in the identification of an undercompacted and overpressured zone within the shaly Lower North Sea Group. Calculated overpressures near the bottom part of the group increase from east to west, from 3.5 to 5.5 MPa, that is in the direction of increasing sedimentary loading rates in Pliocene-Quatemary times. This ability of the application of sonic and 3D seismic velocity in empirical methods to assess the spatial distribution of overpressures in such shaly units is of importance because measured pressure data are usually not available for shaly sections. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Pressure; Seismic; Velocity;North Sea
1. Introduction
Assessment o f overpressures is important in the exploration and appraisal phase o f a hydrocarbon field. TNO-NITG investigates the combination o f different methods to estimate regional pore pressure distributions. This paper focuses on the analysis of sonic and seismic velocities in the Central North Sea Graben from offshore Netherlands. Empirical pore pressure estimation methods, such as the commonly used Eaton (1975) method, provide a relation between seismic velocity and effective pressure and they enable the calculation o f pore
* Corresponding author. Tel.: +31-30-2564639; fax: +31-30256-4605. E-mail addresses: [email protected] (EL.A. Winthaegen),[email protected] (J.M. Verweij). 1 Fax: +31-30-256-4605.
pressure as a function o f overburden pressure (derived from a density log), and the observed and normal velocity. The normal velocity or inverse velocity (travel time) trend is derived from the normal compaction trend. The principle o f the applied Eaton method is that the derived pressure is the difference between the overburden pressure gradient and the ratio o f the observed velocity and normal velocity. A comparison o f seismic pressure estimation methods for shale is given by Winthaegen (2000). Empirical methods have a number o f limitations. These limitations include, for example, other causes that may explain deviations from the normal velocity trend (e.g. Bradley, 1975; Mouchet and Mitchell, 1989; Swarbrick, 2000). In general, a distinction can be made between sub-optimal velocity analysis and changes in lithology and anisotropy. In the first case, the velocity might not be well determined due to a complex subsurface conditions (faults, dipping stmc-
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0375-6742(03)00106-7
204
Abstract
tures), poor imaging (poor illumination of the subsurface) and the lack of resolution. Changes in lithology and anisotropy and changes in pore water salinity may cause a change in velocity that is not indicative for porosity and pressure. In addition, the methods assume that disequilibrium compaction is the main mechanism generating overpressures, that compaction is restricted to mechanical compaction only and that a normal-shale compaction curve can be established. Taking these limitations of Eaton's method into account, we decided to restrict the first estimation of overpressure distribution to the Lower Tertiary shales in the F blocks southeast of the Central North Sea Graben. Data available for this study included well information (e.g. sonic, density and gamma ray logs, mud weights) and a 3D seismic survey. Measured pressure data are not available for these poorly permeable shales.
2. Lithostratigraphic setting The Cenozoic North Sea Groups (_+ 1460 m) unconformably overly the Chalk Group (780 m), which in turn is on top of the Rijnland Group (165 m) (Fig. la). The Lower North Sea Group (NL; 410 m) consists, from bottom to top, of regionally deposited marine shales (370 m) and marl (40 m). The lithology of the Middle North Sea Group (NM) is predominantly shale: The Mid-Miocene unconformity divides the predominantly shaly Lower and Middle North Sea Groups (NL and NM) and the Upper North Sea Group (NU), which is increasingly sandy towards the top. The lowermost part of the Upper North Sea Group (_+ 300 m) consists in large part of deltaic sandy claystones, while the upper 400 m are sands. The lithology of the Upper North Sea Group shows considerable lateral variation in lithology.
3. Cenozoic evolution and present-day undercompaction and pressures After uplift and inversion during Late Cretaceous and Early Tertiary times, regional subsidence and sedimentation resulted in deposition "of the Lower North Sea Group on top of the eroded Chalk Group. After interruption of sedimentation by Pyrenean
uplift, marine Middle North Sea Group sediments were deposited. Only after Miocene times did major depocentres develop in the northern offshore areas. Sedimentation rates in these shifting depocentres started to increase during the Pliocene and remained high during the Quatemary, reaching values >> 100 m/ My. The total thickness of Pliocene and Quaternary deposits increases towards the north of the offshore Netherlands, and towards the Central North Sea Graben. Sedimentary loading is a major mechanism explaining present-day overpressures in the Central and Southern North Sea (e.g. Mann and Mackenzie, 1990; Verweij, 2002; Verweij and Simmelink, 2002). Japsen's (1999) study of interval velocities of the Cenozoic revealed an area of relatively low velocities for the depth of burial of the Paleogene in the northern offshore of the Netherlands (A and B blocks and northern F blocks) and the adjacent Central North Sea. Basin modelling studies suggest that normal pressures prevail in the Lower Tertiary shales in the southern offshore Netherlands (Verweij, 2002).
4. Pressure estimation Fig. lb shows the change of travel time with depth of a well in the location of investigation. A first requirement for interpreting the change of travel time with depth for estimating pressures in the Lower Tertiary is the selection of a proper normal travel time-depth trend for marine shales. Because the shallow, probably normally pressured, part of the Cenozoic section has a highly variable lithology and is dominated by sands, we decided to use the normal travel time-depth trend for marine shales of the North Sea derived by Japsen (2000). It is given by tt=460 x exp(-z/2175)+ 185, where tt is in gs/m and the depth z in m. Fig. lb indicates that the sonic travel-time depth trend for the shallow part of the well is well below Japsen's normal trend. This possibly reflects the effect of the sandy lithology (Mouehet and Mitchell, 1989). Any departure of Japsen's normal trend may result from a change in lithology/mineralogy or porosity. Two depth intervals of relatively high sonic travel time correspond to shale-dominated lithologies of the North Sea Super Group and the Rijnland Group, respectively. The sonic travel times in the Tertiary
205
Abstract
a) ,,
1 top N U s a n d
0
- 2 sand/shale
m
b)
~8 0
¢~1
Ckl
tO
to
(D
to
200
- - 3 shale 96 400 - ~ - 4 s h a l e 100 600 5 t o p NM shale 85 800 ....... 6 sand o
r 7"r"77 ?'* 1000
7 t o p NL marl
E .................8 shale 100 - - 9 shale 90 D
~" 1200 "~ 1 4 0 0
10 t o p Chalk
1600
11 chalk
1800
)
2000
ravel time
onent m.
- - - 12 marl/chalk ,
13 top Rijnland shale/marl
•"-+- 14 shale 80
2200 2400
lapsen
travel time
- "+- 15 top Schieland
Fig. 1. (a) Schematic view of the lithology of the area under investigation. For shale, the shale percentage is mentioned (behind lithology name). (b) Displayed are the dc exponent, sonic travel time, travel time trend according Japsen (1999) and the travel time derived from seismic velocity.
shales exceed the normal trend line by between approximately 1000 and 1460 m (= top Chalk Group). The maximum travel time values in this depth interval occur within the Lower North Sea Group. Application of the Eaton (1975) method, using calibrated sonic information and the Japsen trend, result in a calculated maximum overpressure in the Lower Tertiary shales of approximately 4.5 MPa. The sonic travel times in the shaly lower part of the Upper North Sea Group are below the normal trend (Fig. lb). The lithology of this deltaic sequence differs from the shales of the Lower North Sea Group, because it includes slightly to moderately sandy shales and two sand layers. The corrected drilling (dc) exponent (Fig. lb) is based on the formation drillability and is 'normalised' by the mud weight. The exponent may give an indication of the differential pressure if the lithology is constant (Mouchet and Mitchell, 1989). The drilling rate entering an undercompacted and overpressured zone may increase, and hence, the dc exponent will decrease. A contemporaneous increase of the mud weight could mask the value of the dc exponent. Fig. lb shows that the first decrease of the dc exponent starts
at 900 m (corresponding to the bottom shale layer of the Upper North Sea Group and top of the Middle North Sea Group) and is approximately constant to 1500 m. Fig. lb also includes the change of seismic travel time with depth reconstructed from a 3D survey in the vicinity of the well. In contrast to the sonic travel time data the seismic travel times fit the Japsen normal trend until depths of approximately 800 m. Assuming that the deviation of the 3D interval velocities from the normal velocity trend are representative for overpressures, we calculated a 3D pore pressure (gradient) distribution. In the calculations the hydrostatic pressure gradient is assumed to increase according to 0.01 MPa/rn. In addition, the subsurface is assumed to be isotropic and as a consequence, the seismic interval velocity is the actual vertical velocity. Fig. 2 shows the pore pressure gradient plotted for one inline and a time slice. In Fig. 2 the calculated local pressure gradient (pressure difference over a small depth interval) is shown, and is normal in the Middle and Upper North Sea Groups (approximately 0.01 MPa/m). The gradient rises to a maximum of 0.02 MPa/m in the shales
206
Abstract x direction
g
400-
.~, 8 0 0 Q
E1200>
1600-
2000-
2400-
Fig. 2. East-west seismic time migration profile (17 km) showing the estimated local pore pressure gradient. The fault on the left (at about 1700 ms) is the boundary of the inner (left) and outer graben. The interval down to 1100 ms ( 1000 m) has a normal pressure gradient of 0.01 MPa/m. The light shade in the interval 1100 1700 ms (1000-1650 m) shows an increase in pore pressure up to 0.02 MPa/m. At the top the 1550 ms time slice is displayed giving the lateral pressure gradient distribution. The dashed line shows the location of the seismic line. Also the approximate, projected well location is displayed.
of the Lower North Sea Group. Calculated overpressures in the shales located near the top of the Chalk Group vary between 5.5 MPa in the western part of the section and 3.5 MPa in the east. The overpressures increase towards the west, that is in the direction of increasing thickness of Pliocene-Quatemary sediments, i.e. in the direction of increasing sedimentary loading rates. As a consequence of lower seismic travel times (Fig. lb), the overpressure values derived from seismic data are lower than those calculated from sonic data. The occurrence of overpressures in the Lower Tertiary shales in F blocks in the southeastern part of the Dutch Central North Sea Graben, indicated by sonic, 3D seismic data and the dc component, is in accordance with mud weight data. The results of this study suggest that the region of undercompaction extends further southward than the main region identified in Japsen's (1999) study.
5. Discussion and conclusions
In the first phase of the project, the "interpretation of calibrated sonic data and 3D seismic data to reconstruct overpressure distribution has been confined to
the Tertiary shale interval. Herewith, we took into account the main limitations of the empirical estimation method of Eaton. Fig. lb clearly shows additional changes of sonic and seismic travel time and dc within the Chalk Group and the Rijnland Group. For example, the sonic travel time starts to increase in the Chalk Group at a depth of 1550 m and reaches maximum values at depths of approximately 1700-1800 m, and it increases in the Rijnland Group, reaching maximum values at 2350 m. These travel time changes require careful analysis in combination with other porosity and pressure estimating methods, such as basin modelling, taking into account burial history and different pressure generating mechanisms (second phase of the project). The analysis of sonic and 3D seismic data resulted in the assessment of regional undercompaction and overpressuring in the shales of the Lower North Sea Group in the southeastern part of the Central North Sea Graben. The estimated overpressures varied between 5.5 (west) and 3.5 MPa (east) along a westeast cross-section. Measured pressure data are generally not available for poorly permeable shaly units. The analysis of sonic and 3D seismic velocity data allow the quanti-
Abstract
tative assessment and the 3D mapping of overpressure distribution in such poorly permeable units.
Acknowledgements The authors thank their colleagues Erik Simmelink, Yheo Wong, Harald de Haan and Henk Pagnier for their contribution to the research project.
References Bradley, J.S., 1975. Abnormal formation pressure. AAPG Bulletin 6, 957-973. Eaton, B.A., 1975. Pore pressure estimation from velocity data: accounting for overpressure mechanisms besides undercompaction. SPE Drilling Conference 27488, 515-530. Japsen, E, 1999. Overpressured Cenozoic shale mapped from ve-
207
locity anomalies relative to a baseline for marine shale, North Sea. Petroleum Geoscience 5, 321-336. Japsen, E, 2000. Investigation of multi-phase erosion using reconstructed shale trends based on sonic data. Sole Pit axis, North Sea. Global and Planetary Change 24, 189-210. Mann, D.M., Mackenzie, A.S., 1990. Prediction of pore fluid pressures in sedimentary basins. Marine and Petroleum Geology 7, 55-65. Mouchet, J.E, Mitchell, A., 1989. Abnormal pressures while drilling. Elf Acquitaine Editions, Boussens. Swarbrick, R.E., 2000. The Challenge of Porosity Based Pressure Prediction. Overpressure Workshop 2000. April 4th-6th, 2000, London, UK. Verweij, J.M., 2002. Fluid flow systems analysis on geological time scales in the on- and offshore Netherlands (in preparation). Verweij, J.M., Simmelink, H.J., 2002. Geodynamic and hydrodynamic evolution of the Broad Fourteens Basin (the Netherlands) in relation to its petroleum systems. Marine and Petroleum Geology 19/3, 339-359. Winthaegen, EL.A., 2000. Pore Pressure derived from Seismic Data: Evaluation for a Mid-Norway case, EAGE 62nd Conference and Exhibition, p. 25. Extended Abstracts.
ELSEVIER
~_~DIRECT e
Journal of Geochemical Exploration 78-79 (2003) 203-207
JOURNAl. OF
GEOCHEMICAL EXPLORATION vcww.elsevier.com/locate/jgeoexp
Abstract
Estimating regional pore pressure distribution using 3D seismic velocities in the Dutch Central North Sea Graben P.L.A. Winthaegen*, J.M. Verweij 1 Netherlands Institute of Applied Geoseience TNO~National Geological Survey, P.O. Box 80015, 3508 TA Utrecht, The Netherlands
Abstract
The application of the empirical Eaton method to calibrated sonic well information and 3D seismic interval velocity data in the southeastern part of the Central North Sea Graben, using the Japsen (Glob. Planet. Change 24 (2000) 189) normal velocitydepth trend, resulted in the identification of an undercompacted and overpressured zone within the shaly Lower North Sea Group. Calculated overpressures near the bottom part of the group increase from east to west, from 3.5 to 5.5 MPa, that is in the direction of increasing sedimentary loading rates in Pliocene-Quatemary times. This ability of the application of sonic and 3D seismic velocity in empirical methods to assess the spatial distribution of overpressures in such shaly units is of importance because measured pressure data are usually not available for shaly sections. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Pressure; Seismic; Velocity;North Sea
1. Introduction
Assessment o f overpressures is important in the exploration and appraisal phase o f a hydrocarbon field. TNO-NITG investigates the combination o f different methods to estimate regional pore pressure distributions. This paper focuses on the analysis of sonic and seismic velocities in the Central North Sea Graben from offshore Netherlands. Empirical pore pressure estimation methods, such as the commonly used Eaton (1975) method, provide a relation between seismic velocity and effective pressure and they enable the calculation o f pore
* Corresponding author. Tel.: +31-30-2564639; fax: +31-30256-4605. E-mail addresses: [email protected] (EL.A. Winthaegen),[email protected] (J.M. Verweij). 1 Fax: +31-30-256-4605.
pressure as a function o f overburden pressure (derived from a density log), and the observed and normal velocity. The normal velocity or inverse velocity (travel time) trend is derived from the normal compaction trend. The principle o f the applied Eaton method is that the derived pressure is the difference between the overburden pressure gradient and the ratio o f the observed velocity and normal velocity. A comparison o f seismic pressure estimation methods for shale is given by Winthaegen (2000). Empirical methods have a number o f limitations. These limitations include, for example, other causes that may explain deviations from the normal velocity trend (e.g. Bradley, 1975; Mouchet and Mitchell, 1989; Swarbrick, 2000). In general, a distinction can be made between sub-optimal velocity analysis and changes in lithology and anisotropy. In the first case, the velocity might not be well determined due to a complex subsurface conditions (faults, dipping stmc-
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0375-6742(03)00106-7
204
Abstract
tures), poor imaging (poor illumination of the subsurface) and the lack of resolution. Changes in lithology and anisotropy and changes in pore water salinity may cause a change in velocity that is not indicative for porosity and pressure. In addition, the methods assume that disequilibrium compaction is the main mechanism generating overpressures, that compaction is restricted to mechanical compaction only and that a normal-shale compaction curve can be established. Taking these limitations of Eaton's method into account, we decided to restrict the first estimation of overpressure distribution to the Lower Tertiary shales in the F blocks southeast of the Central North Sea Graben. Data available for this study included well information (e.g. sonic, density and gamma ray logs, mud weights) and a 3D seismic survey. Measured pressure data are not available for these poorly permeable shales.
2. Lithostratigraphic setting The Cenozoic North Sea Groups (_+ 1460 m) unconformably overly the Chalk Group (780 m), which in turn is on top of the Rijnland Group (165 m) (Fig. la). The Lower North Sea Group (NL; 410 m) consists, from bottom to top, of regionally deposited marine shales (370 m) and marl (40 m). The lithology of the Middle North Sea Group (NM) is predominantly shale: The Mid-Miocene unconformity divides the predominantly shaly Lower and Middle North Sea Groups (NL and NM) and the Upper North Sea Group (NU), which is increasingly sandy towards the top. The lowermost part of the Upper North Sea Group (_+ 300 m) consists in large part of deltaic sandy claystones, while the upper 400 m are sands. The lithology of the Upper North Sea Group shows considerable lateral variation in lithology.
3. Cenozoic evolution and present-day undercompaction and pressures After uplift and inversion during Late Cretaceous and Early Tertiary times, regional subsidence and sedimentation resulted in deposition "of the Lower North Sea Group on top of the eroded Chalk Group. After interruption of sedimentation by Pyrenean
uplift, marine Middle North Sea Group sediments were deposited. Only after Miocene times did major depocentres develop in the northern offshore areas. Sedimentation rates in these shifting depocentres started to increase during the Pliocene and remained high during the Quatemary, reaching values >> 100 m/ My. The total thickness of Pliocene and Quaternary deposits increases towards the north of the offshore Netherlands, and towards the Central North Sea Graben. Sedimentary loading is a major mechanism explaining present-day overpressures in the Central and Southern North Sea (e.g. Mann and Mackenzie, 1990; Verweij, 2002; Verweij and Simmelink, 2002). Japsen's (1999) study of interval velocities of the Cenozoic revealed an area of relatively low velocities for the depth of burial of the Paleogene in the northern offshore of the Netherlands (A and B blocks and northern F blocks) and the adjacent Central North Sea. Basin modelling studies suggest that normal pressures prevail in the Lower Tertiary shales in the southern offshore Netherlands (Verweij, 2002).
4. Pressure estimation Fig. lb shows the change of travel time with depth of a well in the location of investigation. A first requirement for interpreting the change of travel time with depth for estimating pressures in the Lower Tertiary is the selection of a proper normal travel time-depth trend for marine shales. Because the shallow, probably normally pressured, part of the Cenozoic section has a highly variable lithology and is dominated by sands, we decided to use the normal travel time-depth trend for marine shales of the North Sea derived by Japsen (2000). It is given by tt=460 x exp(-z/2175)+ 185, where tt is in gs/m and the depth z in m. Fig. lb indicates that the sonic travel-time depth trend for the shallow part of the well is well below Japsen's normal trend. This possibly reflects the effect of the sandy lithology (Mouehet and Mitchell, 1989). Any departure of Japsen's normal trend may result from a change in lithology/mineralogy or porosity. Two depth intervals of relatively high sonic travel time correspond to shale-dominated lithologies of the North Sea Super Group and the Rijnland Group, respectively. The sonic travel times in the Tertiary
205
Abstract
a) ,,
1 top N U s a n d
0
- 2 sand/shale
m
b)
~8 0
¢~1
Ckl
tO
to
(D
to
200
- - 3 shale 96 400 - ~ - 4 s h a l e 100 600 5 t o p NM shale 85 800 ....... 6 sand o
r 7"r"77 ?'* 1000
7 t o p NL marl
E .................8 shale 100 - - 9 shale 90 D
~" 1200 "~ 1 4 0 0
10 t o p Chalk
1600
11 chalk
1800
)
2000
ravel time
onent m.
- - - 12 marl/chalk ,
13 top Rijnland shale/marl
•"-+- 14 shale 80
2200 2400
lapsen
travel time
- "+- 15 top Schieland
Fig. 1. (a) Schematic view of the lithology of the area under investigation. For shale, the shale percentage is mentioned (behind lithology name). (b) Displayed are the dc exponent, sonic travel time, travel time trend according Japsen (1999) and the travel time derived from seismic velocity.
shales exceed the normal trend line by between approximately 1000 and 1460 m (= top Chalk Group). The maximum travel time values in this depth interval occur within the Lower North Sea Group. Application of the Eaton (1975) method, using calibrated sonic information and the Japsen trend, result in a calculated maximum overpressure in the Lower Tertiary shales of approximately 4.5 MPa. The sonic travel times in the shaly lower part of the Upper North Sea Group are below the normal trend (Fig. lb). The lithology of this deltaic sequence differs from the shales of the Lower North Sea Group, because it includes slightly to moderately sandy shales and two sand layers. The corrected drilling (dc) exponent (Fig. lb) is based on the formation drillability and is 'normalised' by the mud weight. The exponent may give an indication of the differential pressure if the lithology is constant (Mouchet and Mitchell, 1989). The drilling rate entering an undercompacted and overpressured zone may increase, and hence, the dc exponent will decrease. A contemporaneous increase of the mud weight could mask the value of the dc exponent. Fig. lb shows that the first decrease of the dc exponent starts
at 900 m (corresponding to the bottom shale layer of the Upper North Sea Group and top of the Middle North Sea Group) and is approximately constant to 1500 m. Fig. lb also includes the change of seismic travel time with depth reconstructed from a 3D survey in the vicinity of the well. In contrast to the sonic travel time data the seismic travel times fit the Japsen normal trend until depths of approximately 800 m. Assuming that the deviation of the 3D interval velocities from the normal velocity trend are representative for overpressures, we calculated a 3D pore pressure (gradient) distribution. In the calculations the hydrostatic pressure gradient is assumed to increase according to 0.01 MPa/rn. In addition, the subsurface is assumed to be isotropic and as a consequence, the seismic interval velocity is the actual vertical velocity. Fig. 2 shows the pore pressure gradient plotted for one inline and a time slice. In Fig. 2 the calculated local pressure gradient (pressure difference over a small depth interval) is shown, and is normal in the Middle and Upper North Sea Groups (approximately 0.01 MPa/m). The gradient rises to a maximum of 0.02 MPa/m in the shales
206
Abstract x direction
g
400-
.~, 8 0 0 Q
E1200>
1600-
2000-
2400-
Fig. 2. East-west seismic time migration profile (17 km) showing the estimated local pore pressure gradient. The fault on the left (at about 1700 ms) is the boundary of the inner (left) and outer graben. The interval down to 1100 ms ( 1000 m) has a normal pressure gradient of 0.01 MPa/m. The light shade in the interval 1100 1700 ms (1000-1650 m) shows an increase in pore pressure up to 0.02 MPa/m. At the top the 1550 ms time slice is displayed giving the lateral pressure gradient distribution. The dashed line shows the location of the seismic line. Also the approximate, projected well location is displayed.
of the Lower North Sea Group. Calculated overpressures in the shales located near the top of the Chalk Group vary between 5.5 MPa in the western part of the section and 3.5 MPa in the east. The overpressures increase towards the west, that is in the direction of increasing thickness of Pliocene-Quatemary sediments, i.e. in the direction of increasing sedimentary loading rates. As a consequence of lower seismic travel times (Fig. lb), the overpressure values derived from seismic data are lower than those calculated from sonic data. The occurrence of overpressures in the Lower Tertiary shales in F blocks in the southeastern part of the Dutch Central North Sea Graben, indicated by sonic, 3D seismic data and the dc component, is in accordance with mud weight data. The results of this study suggest that the region of undercompaction extends further southward than the main region identified in Japsen's (1999) study.
5. Discussion and conclusions
In the first phase of the project, the "interpretation of calibrated sonic data and 3D seismic data to reconstruct overpressure distribution has been confined to
the Tertiary shale interval. Herewith, we took into account the main limitations of the empirical estimation method of Eaton. Fig. lb clearly shows additional changes of sonic and seismic travel time and dc within the Chalk Group and the Rijnland Group. For example, the sonic travel time starts to increase in the Chalk Group at a depth of 1550 m and reaches maximum values at depths of approximately 1700-1800 m, and it increases in the Rijnland Group, reaching maximum values at 2350 m. These travel time changes require careful analysis in combination with other porosity and pressure estimating methods, such as basin modelling, taking into account burial history and different pressure generating mechanisms (second phase of the project). The analysis of sonic and 3D seismic data resulted in the assessment of regional undercompaction and overpressuring in the shales of the Lower North Sea Group in the southeastern part of the Central North Sea Graben. The estimated overpressures varied between 5.5 (west) and 3.5 MPa (east) along a westeast cross-section. Measured pressure data are generally not available for poorly permeable shaly units. The analysis of sonic and 3D seismic velocity data allow the quanti-
Abstract
tative assessment and the 3D mapping of overpressure distribution in such poorly permeable units.
Acknowledgements The authors thank their colleagues Erik Simmelink, Yheo Wong, Harald de Haan and Henk Pagnier for their contribution to the research project.
References Bradley, J.S., 1975. Abnormal formation pressure. AAPG Bulletin 6, 957-973. Eaton, B.A., 1975. Pore pressure estimation from velocity data: accounting for overpressure mechanisms besides undercompaction. SPE Drilling Conference 27488, 515-530. Japsen, E, 1999. Overpressured Cenozoic shale mapped from ve-
207
locity anomalies relative to a baseline for marine shale, North Sea. Petroleum Geoscience 5, 321-336. Japsen, E, 2000. Investigation of multi-phase erosion using reconstructed shale trends based on sonic data. Sole Pit axis, North Sea. Global and Planetary Change 24, 189-210. Mann, D.M., Mackenzie, A.S., 1990. Prediction of pore fluid pressures in sedimentary basins. Marine and Petroleum Geology 7, 55-65. Mouchet, J.E, Mitchell, A., 1989. Abnormal pressures while drilling. Elf Acquitaine Editions, Boussens. Swarbrick, R.E., 2000. The Challenge of Porosity Based Pressure Prediction. Overpressure Workshop 2000. April 4th-6th, 2000, London, UK. Verweij, J.M., 2002. Fluid flow systems analysis on geological time scales in the on- and offshore Netherlands (in preparation). Verweij, J.M., Simmelink, H.J., 2002. Geodynamic and hydrodynamic evolution of the Broad Fourteens Basin (the Netherlands) in relation to its petroleum systems. Marine and Petroleum Geology 19/3, 339-359. Winthaegen, EL.A., 2000. Pore Pressure derived from Seismic Data: Evaluation for a Mid-Norway case, EAGE 62nd Conference and Exhibition, p. 25. Extended Abstracts.