Distribution of overpressures

The direct indicators of fluid flow conditions in the subsurface are the distribution of groundwater potential, and the associated distribution of groundwater overpressures.

The groundwater is overpressured if its pressure exceeds the hydrostatic pressure at a specific depth (Chapter 1). The dimensionless overpressure at a specific point is defined by (P_observed– Phydrostatic) divided by (Plithostatic– Phydrostatic) (e.g. Domenico and Schwartz, 1998). In theory, the dimensionless overpressure varies between 0 and 1.

The maximum groundwater pressure that can be maintained in the subsurface is controlled by rock failure and by fracture opening and reopening conditions. Hence, the upper limit of groundwater pressure is controlled by the minimum principal stress and the tensile strength of the porous medium (leak-off pressure corresponds to fracture initiation and can be considered as the upper bound of the minimum principal stress; Gaarenstroom et al. 1993, Grauls 1997). In the North Sea area the minimum principal stress corresponds to the minimum horizontal stress. Leak-off pressures and minimum horizontal stresses change with depth in this area (Crépieux et al. 1998, Gaarenstroom et al. 1993, Giles et al. 1999, Grauls 1997). As a consequence, the maximum value of the dimensionless overpressure will change with depth as well.

Maximum values for dimensionless overpressures are 0.45 at 1000 m depth (σ3/σz= 0.70), 0.60 at 2500 m (σ3/σz= 0.80) and 0.80 at 4000 m (σ3/σz= 0.90).

Wells have encountered highly overpressured brines and gases within Zechstein salt units, indicating that salt can hold overpressures close to lithostatic pressures (corresponding to maximum dimensionless overpressures of 1).

Figure 22 shows the distribution of the dimensionless overpressures of the ground-water in pre-Tertiary reservoir-type hydrostratigraphic units in onshore and offshore Netherlands. These values are based on pressure data from approximately 80 oil and gas wells (Cottençon et al. 1975, Fontaine et al. 1993, Lepoutre et al. 1996, RRI 1985, 1988, Stheeman 1963, Van den Bosch 1983, Van Wijhe et al. 1980, Verweij 1990a, b, Webatlas of North Sea fields) and additional information on overpressure distribution from Bloch et al. (1993), Crépieux et al. (1998), Japsen (1999) and Megson (1992).

Severely overpressured conditions occur in the Dutch Central North Sea Graben, Step Graben, Terschelling Basin and southwest of the Elbow Spit High. The dimensionless overpressures exceed 0.20. The pressures have been measured in different reservoir type hydrostratigraphic units and at different depths, and as a consequence vary widely.

Dimensionless overpressure values of at least 0.20 in northern offshore Netherlands (A and B blocks) are representative of the pressure conditions in the Chalk Group (Megson 1992, 1998, Japsen 1998). Japsen (1998) analysed interval-velocity data for the Chalk Group in the UK, Norwegian, Danish and Dutch North Sea and identified an area of negative velocity anomalies in the central and northern parts of the North Sea Basin, including the eastern A blocks, B blocks, northern F blocks and northern G blocks in offshore Netherlands (Japsen identified negative velocity anomalies in wells A11-01, B13-02, F02-01, F03-06, G17-01 and L08-02). Giesen (1995) also mapped a clear northward trend of decreasing depth-corrected interval velocities for the Chalk Group in offshore Netherlands. Japsen (1998) related these negative velocity anomalies

0 50 km

A

B

E F

G

N M

L K

P Q

S

Severely overpressured (dimensionless overpressure > 0.20 Overpressured (dimensionless overpressure 0.10 - 0.20) Slightly overpressured (dimensionless overpressure 0.05 - 0.10) Normally pressured to slightly overpressured

(dimensionless overpressure 0 - 0.05) No pressure data available in pre-Tertiary units

Figure 22 Distribution of dimensionless overpressures of the groundwater in pre-Tertiary hydrostratigraphic units in onshore and offshore Netherlands.

with undercompaction and overpressuring of the Chalk Group. He identified the area north of the A and B blocks as the major zone of Chalk overpressuring.

In the Central North Sea Graben and the Terschelling Basin severely overpressured conditions also occur in the laterally and vertically restricted syn-rift hydrostratigraphic units and the pre-rift Triassic units. The dimensionless overpressures mapped in the F blocks (Figure 22) are mainly from the Central Graben Group, and vary between 0.20 and 0.30. An exception is the dimensionless overpressure in the Central Graben Group in the F3 block (at least 0.50 at 2500 m), that approaches its maximum value.

In the southern part of the Central North Sea Graben and in the Terschelling Basin severely overpressured conditions have been encountered in the Lower Germanic Trias Group (e.g in blocks F10, F15, L2, L3, L6, L9; e.g. Crépieux et al. 1998, Fontaine et al. 1993, Lepoutre et al. 1996). This is illustrated by the pressure conditions in the F15-A Triassic gas reservoir, which forms a structural compartment located between the Central North Sea Graben, the Terschelling Basin and the Schill Grund High (Fontaine et al. 1993; Lepoutre et al. 1996). This Main Buntsandstein reservoir, which is partly affected by salt cementation, is located between two major NNE-SSW trending salt ridges, a fault to the north and overlying formations plugged by salt.

The pressure in this structurally and hydrodynamically restricted gas reservoir is approximately 67.5 MPa at 3520 m (Fontaine et al. 1993, Lepoutre et al. 1996, Matthis and Nieuwland 1999), which corresponds to a dimensionally overpressure value of approximately 0.7 which is close to its maximum value. Giles et al. (2000) also refer to fluid pressures that approach the minimum in situ stress in the nearby Terschelling Basin.

The pre-rift Upper Rotliegend Group is of reservoir quality in the area south of the Central North Sea Graben and the Terschelling Basin. Severe overpressures occur along the northern limit of the Upper Rotliegend Slochteren Formation below a thick Zechstein seal (e.g. in southern M blocks, and blocks L1, L8, K6). The overpressures dissipate rapidly in the direction of decreasing sealing capacity of the Zechstein Group, e.g. towards the Texel IJsselmeer High, and in the direction of increasing permeability of the Slochteren unit. The pressures in the Slochteren unit are near hydrostatic to slightly superhydrostatic in onshore and offshore Netherlands south of the High, where Zechstein salts are less thick and of limited areal extent.

The mapped dimensionless overpressures (Figure 22) are indicative of normal to slightly overpressured conditions in the main hydrostratigraphic units in the Broad Fourteens basin (Upper Rotliegend Group, Main Buntsandstein Subgroup, Delfland Subgroup, Vlieland Sandstone Formation) and in the Vlieland Sandstone Formation of the West Netherlands Basin.

It should be kept in mind that no pressure data were available for poorly permeable units (shales, evaporites) of pre-Tertiary age. It cannot be excluded that overpressured conditions do exist in shales and evaporites adjacent to normally pressured reservoir units.