16.1 Conceptual model of geodynamic and hydrodynamic evolution
The indicators of fluid dynamic and permeability conditions of the Broad Fourteens Basin (Chapter 15 and 14) in combination with the identified processes influencing these conditions – as reconstructed from the data on the geodynamic, climatic and sedimentary geologic evolution of the basin (Chapter 14, Tables 9 and 10) – are the basis of the conceptual geofluid model. Figure 47 schematically illustrates the conceptual model for four important phases of basin evolution.
— Main syn-rift period. Syn-rift deposition of the Schieland Group – concentrated in the rapidly subsiding basin – induces cross-formational vertical upward flow of fluids in the shallow and relatively permeable part of the basin (shallow subsystem of burial-induced flow; Chapter 1), while in the deeper parts fluid flow directions are stratigraphically controlled (intermediate subsystem of burial-induced flow;
Chapter 1; Figure 47a). Hydrothermal flow along deeply penetrating active normal faults enters permeable units (e.g. the Slochteren Formation) in the basin during periods of extensional tectonic activity in the early part of the main syn-rift phase (section 13.3). In addition, fluid flow through the temporarily active and permeable fault and fracture zones in the basin drains the relatively permeable units in particular. During syn-rift times, water tables and topography-induced flow systems develop in the uplifted basin flanks and adjacent highs.
— Post-rift period. Continuous subsidence and sedimentation during the post-rift phase of basin evolution (Figure 47b) maintain cross-formational vertical upward flow of fluids in the shallow part of the basin and stratigraphically controlled flow in its deeper parts. At the end of the post-rift phase the continuous burial and associated compaction of the hydrostratigraphic units favour the development of overpressured conditions in the deepest parts of the basin, especially in the poorly permeable units such as Carboniferous shales and Zechstein evaporites.
In addition, the gradual build-up of regional compressional stresses may influence the distribution of overpressure and fluid flow in the basin. This effect was not studied in detail in the present study. It was studied in a separate project involving geomechanical modelling of compressive stress and overpressure in the Broad Fourteens Basin (Simmelink et al. 2001).
— Syn-inversion period. During the subsequent syn-inversion phase of basin evolution (Figure 47c) different mechanisms operate to dissipate the inferred post-rift overpressured conditions in the central part of the basin: sedimentary loading as a pressure-producing mechanism is absent, there is erosional unloading, the geometrical framework is changing, and there is increased permeability of fault zones during tectonic activity. The elongated island-like character of the inverted central part of the basin and the topographic relief of the ground surface favour topography-induced fluid flow and the development of a freshwater lens (Verweij 1990). The geometrical framework of the inverted basin favours deep infiltration of meteoric water, e.g. the inversion-related tilting of geological units exposes different permeable units to the inflow of meteoric water. Extension fractures in the uplifted infiltration area enhance deep infiltration. Bouw (1999) studied
the syn-inversion development of a freshwater lens in the northern (salt-dominated) part of the basin by applying a density-dependent groundwater flow model. The results of her different modelling scenarios show that the main factor influencing the development of a freshwater lens is the distribution of permeability; predicted near steady-state flow conditions are established within 4 My; the maximum depth of the fresh-salt water interface is 1200 m (for a high permeability scenario). In the conceptual model the dynamic permeability of repeatedly reactivated faults, and the associated fracture zones allows fluid flow during inversion tectonic activity (co-seismic pulsed episodes of fluid flow; Figure 47c). Meanwhile, subsidence and sedimentation continue to influence the pressure distribution and fluid flow in the platform area adjacent to the basin.
— Post-inversion period. Figure 47d shows that at present the main mechanism influencing the pressure distribution and fluid flow in the basin is assumed to be the sedimentary loading during the Late Tertiary and Quaternary. The regional compressive stress field is assumed here to exert a static influence only (Chapter 14).
16.2 Conceptual model of hydrodynamic evolution in relation to the evolution of petroleum systems
The hydrogeological and hydrodynamic response of a basin fill to its geodynamic evolution may exert a direct and indirect influence on all subsystems of the petroleum systems in the basin (Verweij 1993, 1997; Table 17). In the Broad Fourteens Basin,
Groundwater system Petroleum system
Distribution groundwater potential gradient — Expulsion (≈ distribution overpressure gradient) — Migration pattern
— Migration losses
— Entrapment
— Hydrocarbon-water contacts
— Preservation, remigration
— Sealing capacity rocks and faults (pressure sealing, water drive leakage)
Overpressures — Sealing capacity rocks and faults
(hydraulic fracturing, fault failure)
— Porosity and permeability rocks (undercompaction)
Active groundwater flow — Porosity and permeability rocks and faults (through rock-water interaction)
— Petroleum composition (geochemical water-petroleum interaction: e.g.
compositional fractionation, waterwashing;
biochemical water-petroleum interaction:
biodegradation)
— Petroleum generation (by influencing temperature distribution through forced convection of heat)
— Petroleum migration (gas in solution) Table 17 Possible direct and indirect influences of the groundwater system on the petroleum system
0
5000
Depth (m) Extensioal
tectonic forces Infiltration meteoric water Sedimentary loading
Infiltration meteoric water
SW NE
Continuous gravity-induced flow Flow induced by sedimentary loading Co-seismic pulsed episodes of fluid flow Faults and fractures: seismogenic permeability
Reduction of permeability of Rotliegend and Triassic reservoirs because of illitisation a. Main syn-rift phase of basin evolution
Gradual buildup regional compressional
stress-field
b. Post-rift phase of basin evolution 0
5000
Depth (m)
SW NE
Flow induced by sedimentary loading Sedimentary
loading
Pulsed tectonic
forces Sedimentary
loading Unloading
&
infiltrating meteoric water Sedimentary
loading
SW NE
Continuous gravity-induced flow Flow induced by sedimentary loading Co-seismic pulsed episodes of fluid flow
Fracture permeability related to uplift/folding and unloading Reversed faults & associated fractures: seismogenic permeability c. Syn-inversion phase of basin evolution
0
5000
Depth (m)
the identified rapidly changing permeability of fault and fracture zones and changing of dips of the hydrostratigraphic units during the main rift period and the syn-inversion period affect both the groundwater and the petroleum systems. The largest impact of groundwater conditions on rock permeability and petroleum migration, accumulation and preservation can be expected in zones of active groundwater flow, in zones where large groundwater potential gradients exist and in severely overpressured zones (Chapter 1). Such zones are generated by large net driving forces for groundwater flow that in turn are related to, for example, a large topographic relief of the water table, high sedimentary loading rates or high tectonic loading rates.
The analysis indicated that the major groundwater flow events active during the development of the Posidonia Shale oil system were the topography-induced flow and the co-seismic flow events during the syn-inversion period and, to a minor extent, the flow induced by sedimentary-loading in Pliocene-Quaternary times. Both topography-induced flow and co-seismic flow are able to influence the migration, accumulation and remigration of oil. Assuming that the active topography-induced groundwater flow actually reached the realm of the oil system, the groundwater flow may also have affected the oil composition by waterwashing and biodegradation.
During post-rift times the oil system was probably not affected by overpressures that – according to the conceptual model – developed in the deeper parts of the basin, and were induced by sedimentary loading.
The development of the gas system started with the deposition of the Coal Measures of the Limburg Group during Late Carboniferous times and, as a consequence, all
Regional compressional
stress-field
North Sea Groups Chalk Group Rijnland Group Schieland Group
d. Post-inversion phase of basin evolution (present day) Stratigraphy
Flow induced by sedimentary loading
Faults
SW NE
Altena Group
Lower and Upper Germanic Trias Groups Zechstein Group
Upper Rotliegend Group 0
5000
Depth (m)
Sedimentary loading
Sedimentary loading
10 km 0
Figure 47 Overview of the processes influencing fluid flow conditions during the different phases of basin evolution of the Broad Fourteens Basin: a. Main syn-rift phase; b. Post-rift phase;
c. Syn-inversion phase; d. Post-inversion phase (present-day). The geological cross-sections of the post-Paleozoic sequences are modified from the balanced cross-sections presented by Huyghe and Mugnier (1995)
inferred major groundwater flow events since that time may have influenced the gas system. Gas generation started prior to inversion, but there is no information on its exact timing in the published literature. Assuming that the gas generation process was already active during main syn-rift times, the changing permeability framework and the co-seismic flow event will have influenced gas expulsion and migration in the basin. In contrast to the oil system, the deeper gas system was probably in the realm of the inferred overpressured zone that was induced by continuous sedimentary loading during post-rift times. The gradients of overpressure might have influenced the expulsion of gas from the source rock as well as the sealing capacities of the cap rocks. The combined influence of a rapidly changing permeability framework and co-seismic flow during syn-inversion might have favoured the remigration and accumulation of gas in the basin and possibly also the escape of gas from the basin.