13 Geological history
15 Indicators of present-day and paleo fluid flow conditions
15.5 Characteristics of petroleum fluid systems
The first successful exploration well drilled by Mobil in the Broad Fourteens Basin encountered gas in Zechstein carbonates and Triassic sandstones in the P6 block in 1968. The first oil fields (Helm and Helder fields) were discovered by Union Oil in block Q1 in 1979 (Knaap and Coenen 1987).
Figure 45 shows the location of oil and gas fields in the Broad Fourteens Basin.
Most of the fields are located along the margins of the basin. Figure 46 illustrates the stratigraphic position of the source rocks and reservoir rocks.
Reservoirs and seals
Important commercial gas accumulations are present in the Upper Permian Slochteren Formation and, in addition, in Zechstein Carbonate Members and in the Triassic Main Buntsandstein Subgroup (Table 15). The principal top seal of the Slochteren reservoir consists of Zechstein evaporites (e.g. Frikken 1996, Oele et al. 1981, Roos and Smits 1983). Triassic traps in block K13 were formed during the Late Cretaceous – Early Tertiary inversion.
The commercial oil accumulations are principally reservoired in the Lower Cretaceous Vlieland Sandstone Formation, and, to a minor extent, in the Late Jurassic – Early Cretaceous Delfland Subgroup. The Vlieland Claystone Formation is the top seal for oil accumulations in blocks Q1 and K18 (De Jong and Laker 1992, Roelofsen and De Boer 1991). The majority of the current faulted anticlinal traps in the Vlieland
Sandstone Formation formed during the Late Cretaceous – Early Tertiary inversion (Dronkers and Mrozek 1991, Hastings et al. 1991, Roelofsen and De Boer 1991).
Source rocks
The dominantly gas-prone Westphalian coal measures of the Caumer Subgroup are considered to be the main source rock for gas in the basin (e.g. Glennie 1998, Oele et al. 1981, Van Wijhe et al. 1980, Van Wijhe 1987a). The source rock is of kerogen type III. Reconstructed burial histories of the Westphalian source rocks and maturation calculations indicate that source rock horizons reached maturity in large part of the basin prior to inversion (Oele et al 1981, Van der Poel 1989, Van Wijhe 1987a). Van Wijhe (1987a) showed that present-day vitrinite reflectance values reach up to 2.4%Ro, at the top of the coal measures along the pre-inversion axes of the basin, indicating present-day overmature conditions. During the Cenozoic post-inversion period gas generation took place outside the inverted part of the basin (Oele et al. 1981,Van Wijhe 1987).
The Posidonia Shale Formation, a kerogen type II source rock (Cornford 1998), is considered to be the most important source rock for oil in the Broad Fourteens Basin (De Jong and Laker 1992, Goh 1996, Roelofsen and De Boer 1991). Roelofsen and De Boer (1991) consider the bituminous shale of the Delfland Subgroup to be a secondary source rock for oil. In addition, Cornford (1998) suggests that Lower Jurassic shales may have developed oil and gas potential in the basin. Maturation studies for the Posidonia Shale Formation indicate that oil generation from this source rock started in the Early Cretaceous in the centre of the basin (De Jong and Laker 1992, Roelofsen and De Boer 1991).
Oil and gas composition
Table 15 shows the gravities of the reservoired oils. The Helm, Helder, Hoorn and Haven fields in block Q1 contain waterwashed and biodegraded oils (Roelofsen and De Boer 1991).
Gas accumulations Oil accumulations Broad Fourteens Basin
IJmuiden High Broad
Fourteens Basin
P Q
K L
O
Figure 45 Location of the oil and gas fields in the Broad Fourteens Basin
The methane content of gas reservoired in the Upper Permian Slochteren Formation in the northern part of the Broad Fourteens Basin varies on average between 85% and 95% (Gas Atlas 1998). Locally, the gas contains a high % of CO2(in K15-FB: 24% CO2, Oele et al. 1981). In the Zechstein reservoirs the gas compositions are characterised by 90 - 95% of CH4in the southern part of the basin and by a variable CH4content, between 60 - 65% and 80 - 85% (Gas Atlas 1998), and locally high values of nitrogen (20 - 30% N2) in the northern part of the basin. Gas fields in Triassic reservoirs contain variable percentages of methane: the CH4content in the reservoired gas varies between 90–95% in the southeastern part of the basin and 60 - 65% in the northern part. In these northern parts locally high values of nitrogen (20 - 30% N2) are also encountered in Triassic reservoirs.
15.6 Identified periods of active fluid flow
The physico-chemical characteristics of the rocks and fluids in the basin described above, provide information on the evolution of fluid flow conditions in the Broad Fourteens Basin. The pressures in the basin are direct indicators of present-day fluid flow conditions are. The published present-day groundwater pressures in reservoir horizons between depths of 1200 and 3900 m represent near-hydrostatic to slightly overpressured conditions. Published overpressures of the groundwater reach values
Time
Sandstone Members Delfland Subgroup
Main Buntsandstein Subgroup Carbonate Members Zechstein Group Slochteren Formation
Coal Measures Caumer Subgroup
Figure 46 Source rocks and productive reservoir rocks in the Broad Fourteens Basin
in the order of 1 - 2.6 MPa in the Upper Rotliegend Group in the northern part of the basin. The restricted number of published pressure measurements available for the permeable hydrostratigraphic units only, do not permit the detailed characterisation of a present-day groundwater flow system. Overpressure is a transient condition during basin evolution and the pressure distribution at a certain time is a reflection of the pressure-generating mechanism as well as of the dissipating mechanism (e.g.
groundwater flow). The observed near-hydrostatic pressures in the Broad Fourteens Basin indicate that the sedimentary loading of the basin after its Late Cretaceous uplift and erosion did not lead to widespread overpressuring of the reservoir units at present-day. This suggests that Tertiary and Quaternary sedimentary loading and other pressure generating mechanisms were not powerful enough to induce overpressuring and/or that the permeability and continuity of the reservoir units in question allowed groundwater flow to dissipate the overpressures.
Field Well no Reservoir unit Depth unit OWC Gravity Data
m –sea- m –sea- source
level level API ° Oil fields
Kotter K18-02 Vlieland Sandstone Formation 1782 32 1
Breeveertien Formation
Logger L16-6 Vlieland Sandstone Formation 1836 34 2
P9 P09-02 Vlieland Sandstone Formation 1973-2129 30 1
Haven Q01-08 Vlieland Sandstone Formation 1550 28 3
Helder Q01-07 Vlieland Sandstone Formation 1425 22 3
Helm Q01-03 Vlieland Sandstone Formation 1289 18 3
Hoorn Q01-16 Vlieland Sandstone Formation 1450 26 3
Gas fields
GWC m –sea-level
K8 K08-03 Upper Rotliegend Group 3334 4
K11 Upper Rotliegend Group 3188 4
K12 K12-03 Upper Rotliegend Group 3569 4
K13A K13-01 Main Buntsandstein Subgroup 1543 5
K13B K13-02 Main Buntsandstein Subgroup 1347 4
K13E K13-04 Upper Rotliegend Group 2503 5
K13F K13-05 Upper Rotliegend Group 2608 5
K14 Upper Rotliegend Group 3114 4
K15FA Upper Rotliegend Group 3410 6
K15FB Upper Rotliegend Group 3980 6
K17 Upper Rotliegend Group 2882 4
P6 Main Buntsandstein Subgroup 2750 (GDT) 4
Zechstein Group (ZEZ3C) 4
1. ECL 1983 4. Webatlas 1999
2. Goh 1996 5. Roos and Smits 1983
3. Roelofsen and De Boer 1991 6. Oele et al. 1981
Table 15 Characteristics of oil and gas fields in the Broad Fourteens Basin
The groundwaters in Lower Cretaceous and older units are chloride dominated brines (total dissolved solids in the range of 74 000 mg/l to 300 000 mg/l). Hydrochemistry indicates that, at present, there is no fresh water of meteoric origin, nor unevolved sea water present in reservoir horizons at depths of more than 1200 m. The large variations in concentration of total dissolved solids in the groundwaters and the associated variations in density of the groundwater in the basin constitute a controlling factor on present-day pressures and fluid flow conditions.
The published temperature data do not allow a hydrodynamic interpretation.
Important indicators of periods of active paleo-fluid flow in the basin are its present-day sediment diagenetic characteristics and the characteristics of its oil and gas accumulations. A number of diagenetic features of the Broad Fourteens Basin can be related to important permeability alterations in the basin and to distinct phases of fluid flow.
The indicators and the associated/inferred paleo fluid flow conditions are listed below.
Kaolin cements, and leached K-feldspar, in the Upper Permian Slochteren Formation.
— Expulsion of acid CO2-rich waters from the Carboniferous Limburg Group into the Slochteren Formation during burial (Gaupp et al. 1993, Lanson et al. 1995, 1996, Platt 1993, Rossel 1982);
— Flow period contemporaneous with the Early-rift period of basin evolution (period of burial before 165 Ma; Lanson et al. 1995, 1996).
Illite cements in the Upper Permian Slochteren Formation.
— Hydrothermal flow along fault zones (increased heat flow in basin around 155 Ma reported by Lanson et al. 1996);
— Expulsion of potassium-rich waters from Zechstein Group (Lanson et al. 1995, 1996, Leveille et al. 1997) and/or Limburg Group (Clauer et al. 1996, Gaupp et al. 1993, Platt 1993) into the adjacent Slochteren Formation;
— Both fluid flow events (hydrothermal flow and expulsion of potassium-rich waters) occurred during the main syn-rift period of basin evolution (130 - 165 Ma; from illite age dating by Lanson et al. 1995, 1996, Lee et al. 1989).
Anhydrite and barite cements in the Upper Permian Slochteren Formation.
— Flow of sulphate-rich waters from both the Zechstein Group (evaporites) (Lanson et al. 1995, 1996, McNeil et al. 1998, Sullivan et al. 1994) and the Limburg Group (McNeil et al. 1998) into the adjacent Slochteren Formation;
— Flow is contemporaneous with syn-inversion period of basin evolution (indicated by anhydrite cements in inversion-related joints: Gauthier et al. 2000; crystallisation of sulphates postdates illitisation: Lanson et al. 1995, 1996).
Calcite cements in the Z3 Carbonate Member.
— Flow of CO2-rich fluids (gases) from the Limburg Group into the Z3 Carbonate Member (Van der Poel 1989);
— Flow is contemporaneous with early rift to syn-rift period of basin evolution.
Biodegraded and waterwashed nature of oils in Lower Cretaceous Vlieland Sandstone Formation (block Q1; Roelofsen and De Boer 1991).
— Flow of oxygen-rich meteoric groundwater through the Vlieland Sandstone Formation;
— Flow occurred during subaerial exposure of the basin due to uplift and/or changes in sea level (after 90 Ma).
Secondary porosity and leached K-feldspars in the Lower Cretaceous Vlieland Sandstone Formation (De Jong and Laker 1992).
— Flushing of Vlieland Sandstone Formation with waters capable of grain dissolution, K-feldspar leaching;
— Flushing occurred after 90 Ma.
And in addition:
Gas fields in the Upper Permian Slochteren Formation, Zechstein Carbonate Members and Triassic Main Buntsandstein Subgroup (e.g. Oele et al. 1981, Van Wijhe et al. 1980, Van Wijhe 1987a).
— Petroleum fluid flow from Carboniferous source rocks into Upper Rotliegend, Zechstein and Triassic reservoirs presently located at the edges of the basin.
Flow from the source rock into the reservoirs was both vertical and lateral.
Oil fields in the Lower Cretaceous Vlieland Sandstone Formation and Late Jurassic – Early Cretaceous Delfland Subgroup (e.g. De Jong and Laker 1992, Goh 1996, Roelofsen and De Boer 1991.
— Petroleum fluid flow from Jurassic source rocks into overlying reservoirs included both vertical and lateral components.
Table 16 shows the timing of inferred paleo-fluid flow events during basin evolution.
Indicator Fluid flow event Timing Source
Kaolin cement, leached I. Lanson et al. 1995, 1996
K-feldspar in Slochteren Expulsion acid CO2-rich water from <165 Ma Rossel 1982 Formation Limburg Group into Slochteren Early rift Platt 1993
Formation Gaupp et al 1993
Calcite cements in I.
Z3 Carbonate Member Expulsion CO2-rich fluids from Limburg Early to Van der Poel 1989 Group into Z3 Carbonate Member syn-rift
Illite cements in II. Lanson et al 1995,1996
Slochteren Formation - Hydrothermal flow along faultzones 130-165 Ma Lee et al 1989 - Expulsion K-rich water from Limburg Main syn-rift Leveille et al 1997
Group and/or Zechstein Group into Clauer et al 1996
Slochteren Formation Gaupp et al 1993
Platt 1993
Anhydrite and barite III. Lanson et al 1995, 1996
cements in Slochteren Expulsion sulphate rich water from Syn-inversion McNeil et al 1998 Formation Limburg Group and Zechstein Group Sullivan et al 1994
into Slochteren Formation Gauthier et al 2000
K-feldspar leaching in IV. De Jong and Laker 1992
Vlieland Sandstone Flushing of Vlieland Sandstone >90 Ma
Formation Formation
Biodegraded and IV. >90 Ma Roelofsen and
waterwashed oils in Vlieland Flow of oxygen-rich meteoric water De Boer 1991 Sandstone Formation through Vlieland Sandstone Formation
Table 16 Present-day indicators of fluid flow events