23 History of pore pressures and groundwater flow
23.1 History of pore pressures
Figures 75 to 79 illustrate the development of overpressures in subsequent phases of basin evolution. Figure 80 shows the history of overpressures in the Carboniferous Limburg Group, the Permian Slochteren Formation and Zechstein Group in the central part of the basin.
Early-rift phase
At the end of the early-rift phase, that is at the end of a prolonged period of regional sedimentation and before the start of gas generation in Carboniferous source rocks, minor overpressured conditions of 1 - 3 MPa occurred in the shaly Carboniferous formations and the Zechstein evaporites (Figure 75). Because of uplift and erosional unloading during mid-Kimmerian tectonic phase, pressures returned to near-hydrostatic values in all hydrostratigraphic units, with the exception of the pressures in a small part of the Carboniferous Baarlo Formation at 42 km along the cross-section. This formation maintained overpressures of 2 MPa.
Main syn-rift phase
Following erosion, syn-rift deposition of the Delfland Subgroup was concentrated in the rapidly subsiding basin and induced minor overpressures in the deeper poorly permeable units in the central part of the basin (at 40 - 44 km: in Baarlo Formation Pex= 3 - 3.8 MPa, in Ruurlo Formation and Zechstein evaporites Pex <– 2.5 MPa).
Pressures remained near-hydrostatic in the Slochteren Formation. During the early part of the main syn-rift phase (156 - 140.7 Ma) the fault zones of increased permeabilities were able to dissipate the overpressures in the relatively permeable units (Figure 76).
Scenario: • no flow bottom and lateral boundaries
• gas system < gas generation
• faults
• constant groundwater density Overpressure in MPa
< 1 2 4 6 8 10
P9 P6 Q1
Broad Fourteens Basin
SW NE
70 Distance (km)
0 10 20 30 40 50 60
Depth (m) 4000
1000
2000
3000
6000 5000 0
7000
Figure 75 Predicted distribution of overpressures at the end of the early-rift phase of basin evolution (at 156 Ma; modelling scenario P3)
Overpressure in MPa
<1 2 4 6 8 10
Scenario: • no flow bottom and lateral boundaries
• gas system: gas generation phase
• faults
• constant groundwater density
Depth (m)4000
1000
2000
3000
6000 5000 0
7000
P9 P6 Q1
Broad Fourteens Basin
SW NE
70 Distance (km)
0 10 20 30 40 50 60
Figure 76 Predicted distribution of overpressures during the main syn-rift phase of basin evolution (at 140.7 Ma; modelling scenario P3)
Scenario: • no flow bottom and lateral boundaries
• gas system: main phase of gas generation and expulsion
• faults
• constant groundwater density Overpressure in MPa
<1 2 4 6 8 10
Depth (m)4000
1000
2000
3000
6000 5000 0
7000
P9 P6 Q1
Broad Fourteens Basin
SW NE
70 Distance (km)
0 10 20 30 40 50 60
Figure 77 Predicted distribution of overpressures at the end of the post-rift phase of basin evolution (at 74 Ma; modelling scenario P3)
Scenario: • no flow bottom and lateral boundaries
• gas system: decreasing expulsion
• increased fault permeability
• constant groundwater density Overpressure in MPa
Depth (m) 4000
1000
2000
3000
6000 5000 0
7000
P9 P6 Q1
Broad Fourteens Basin
SW NE
70 Distance (km)
0 10 20 30 40 50 60
<1 2 4 6 8 10
Figure 78 Predicted distribution of overpressures during the syn-inversion phase of basin evolution (at 65.9 Ma; modelling scenario P3)
Overpressured conditions built up in the Carboniferous formations along the entire cross-section during the Early Cretaceous syn-rift period because of continued differential subsidence and sedimentation and gas generation in the Carboniferous source rocks. At 125 Ma, maximum overpressures of 7 MPa occurred in the Ruurlo and Baarlo Formations in the central part of the basin (at 40 - 44 km). During this period the fault zones were poorly permeable again and, as a consequence, the overpressures in the Slochteren Formation could not dissipate from the central part of the basin (between areas P6 and Q1): Pex<– 3 MPa. In addition, the modelling predicted minor overpressures in Triassic units south of the P6 area.
Post-rift phase
The results of the modelling indicated that post-rift regional subsidence and sedimentation in combination with the continuous generation of gas extended the overpressured area in the basin. Figure 77 shows the predicted distribution of overpressures in the basin at pre-inversion time. The most significantly overpressured part of the section was the area south of P6: here minor overpressures were predicted in the Jurassic shales (1 MPa); in the Triassic (RB and RN), Zechstein and Slochteren Formations (Pex<– 4 MPa); maximum overpressure values were predicted in the Baarlo Formation (Pex= 11 MPa), Ruurlo Formation (Pex= 10 MPa) and Maurits Formation (Pex= 5 MPa). In the central part of the basin the Triassic to Carboniferous formations were overpressured (Triassic Pex<– 1.7 MPa, Zechstein Pex= 4 - 8 MPa, Slochteren Formations Pex= 4 MPa; Ruurlo Formation Pex<– 6 MPa) and Baarlo Formation Pex<– 8.7 MPa). The model results predicted overpressuring in the northern platform area as well: Zechstein Group Pex= 4 - 5 MPa in the Zechstein Group and Pex= 6.5 MPa in the Slochteren Formation. These maximum overpressures in poorly permeable units at the end of the post-rift period are well below minimum in-situ stresses (for
Scenario: • no flow bottom and lateral boundaries
• gas system: decreased expulsion
• faults
• constant groundwater density Overpressure in MPa
Depth (m)4000
1000
2000
3000
6000 5000 0
7000
P9 P6 Q1
Broad Fourteens Basin
SW NE
70 Distance (km)
0 10 20 30 40 50 60
<1 2 4 6 8 10
Figure 79 Predicted distribution of overpressures at present-day (modelling scenario P3)
example: the maximum overpressure in the Baarlo Formation at >– 5000 m is Pex= 11 MPa, corresponding to a pressure of 61.5 MPa; the lithostatic pressure at 5000 m is approximately 115 MPa, which for a minimum to vertical stress ratio at these depths of >– 0.8 (Grauls 1997) corresponds to a minimum horizontal stress of 92 MPa).
The overpressure distribution during this pre-inversion time clearly shows the difference in hydraulic behaviour between the poorly per-meable and perper-meable units. The overpressures showed large lateral variations in the poorly permeable Carboniferous and Zechstein units.
For example, the overpressures varied between 6 and 11 MPa in the Baarlo Formation south of P6, and between 4 and 8 MPa in the Zechstein Group between P6 and Q1. These lateral variations in overpressures in the Carboniferous and Zechstein units show that in these poorly permeable units the groundwater flow did not equilibrate the overpressures. In contrast, the relatively permeable Slochteren Formation permitted lateral flow of groundwater and the
associated redistribution of overpressures: the overpressure values in the permeable Slochteren Formation showed little variation laterally: Pex≅4 MPa.
The modelling accounts for the influence of regional subsidence and sedimentation in combination with the continuous gas generation on the distribution of overpressures in the basin. The gradual build-up of regional compressional stresses at the end of the post-rift phase (Part 2) may also have influenced the distribution of overpressures and groundwater flow in the basin. The Temispack programme does not allow incorporation of this effect in the modelling. It was studied in a separate project involving the geomechanical modelling of compressive stress and overpressure in the Broad Fourteens Basin (Simmelink and Orlic 2001, Simmelink et al. 2001).
Inversion phase
The combined effects of the absence of sedimentation as a pressure-generating mechanism, decreased rates of gas generation, erosional unloading, a changing geometrical framework and increased permeabilities of the fault zones during the Late Cretaceous inversion phase, was the complete disappearance of overpressured conditions in the central inverted part of the basin (Figure 78). One of the repercussions of the changed geometry of the basin fill was an increase in the drainage area, of the P6 permeable fault zones. During the subsequent period of non-deposition in the Early Paleocene the overpressures also dissipated in the northern platform area due to continued absence of pressure-generating mechanisms in the modelling.
Age (Ma)
Variscan Pre-and early rift Main syn-riftPost-riftSyn-inversionPost-inversion
0
Figure 80 The calculated history of overpressures in the Ruurlo Formation, Slochteren Formation and Zechstein Group in the central part of the basin (at 50 km)
Post-inversion phase
Post-inversion deposition of the Lower North Sea Group, in combination with some gas generation in selected parts of the basin, induced the return of overpressured conditions in the basin. The overpressures predicted in the southern part of the basin were: Baarlo Formation Pex= 8 MPa, Slochteren Formation Pex<– 1.5 MPa, Triassic Formations Pex<– 2MPa; and in the central part of the basin: Baarlo Formation Pex= 2 - 4.7 MPa, Slochteren Formation Pex= 2 MPa.
Eocene–Oligocene uplift and erosion and the subsequent period of non-deposition or only minor sedimentation resulted in approximately near-hydrostatic conditions in the modelled cross-section in the period between between Oligocene and Miocene.
Increasing sedimentation rates in Pliocene and, especially, in Quaternary times, resulted in present-day mild overpressures in the poorly permeable deeper parts of the basin (Figure 79). The maximum overpressures predicted in the central part of the basin were: Zechstein Group Pex= 6.5 MPa, Slochteren Formation Pex= 3 MPa;
Baarlo Formation Pex= 6 MPa.
The history of pore pressure outlined above is based on a modelling scenario including a no-flow boundary condition on the northern side of the cross-section.
However, deep erosion northeast of the cross-section during the Early Cretaceous could have created a lateral escape route for groundwater in the permeable Slochteren Formation and Triassic units. To study this effect the history of overpressure and groundwater flow was also simulated with an open northern boundary. Figures 81 (pre-inversion) and 82 (present-day) show modelling results assuming such a boundary. The overpressures predicted in the Limburg, Upper Rotliegend and Zechstein Groups are significantly lower in the area north of Q1 in comparison with those predicted in the closed boundary simulations.