This final part of the thesis discusses the quantitative analysis of fluid flow systems on geological timescales. It aims to provide 1. a quantitative understanding of the hydrogeological and pore pressure and groundwater flow response of the basin fill of the Broad Fourteens Basin to important aspects of its geological, geothermal and climatic evolution and an understanding of this response for the evolution of the oil and gas systems in the basin; 2. a time framework for petroleum generation, migration, accumulation and preservation in the basin (Chapter 1). In addition it addresses specific research questions that resulted from the extensive data analysis of the Broad Fourteens Basin (see Part 2, Chapter 17).
The Temispack 2D basin modelling package (versions 2.5 and 2.6, Breicip-Franlab 1997) was used to analyse the geofluid evolution along a SW-NE cross-section through the southern part of the Broad Fourteens Basin (Figure 33, Part 2). The modelling work was carried out at the Netherlands Institute of Applied Geoscience TNO – National Geological Survey.
The data analysis presented in Part 2 provided both the input data and boundary conditions required for the numerical modelling, and the necessary a priori understanding of the basinal processes and their interactions.
18.1 Principles of integrated 2D basin modelling
Temispack is a finite-volume model that uses a two-D mesh to simulate the following conditions during basin evolution (Burrus et al. 1991, Ungerer et al. 1990):
— Sedimentation, erosion and compaction. The model simulates changes in
geometrical framework of the model layers based on normal porosity-depth curves defined for each lithology, and taking into account the paleogeography and the vertical displacements of the layers.
— Heat flow and temperature. The calculation of temperature history is based on a transient heat flow equation that uses lithology and porosity dependent thermal conductivities, heat capacities and radiogenic heat production based on lithology, and takes into account the evolution of the sediment-water interface temperatures and the evolution of basal heat flow.
— Source rock kerogen maturity and petroleum generation. The input for the maturity module includes kerogen type, total organic carbon (TOC) values and temperature history from the previous module. In this study, Temispack default kerogen types and the associated kinetic model were used. The maturity module computes the transformation ratios for each source rock type and the amount of petroleum generated. Vitrinite reflectances are computed for Type IV reference kerogen.
— Fluid flow. The history of pore pressures and groundwater flow is calculated to be the combined result of sedimentary loading and unloading, the relief of the water table and, in addition, the generation of hydrocarbons. The module requires the input of the relationships between porosity and effective stress for each lithology, as well as the permeabilities derived from the lithology-related porosities by applying the Kozeny-Carman equation, and finally, the anisotropies.
— Petroleum expulsion, migration and accumulation. The petroleum migration module links the previous modules and calculates the expulsion and migration history by using a two-phase Darcy equation. It requires additional input of relative
permeability functions, petroleum density and viscosity, and capillary pressures.
P art 3 Fluid flow systems analysis of the Broad F ourteens Basin: results of integrated 2D basin modelling
The following assumptions underlying the basin modelling of the Broad Fourteens Basin have important implications for the evaluation of the previously identified processes and forces that might have influenced the overpressure and fluid flow evolution in the basin (Tables 10 and 11):
1. The cross-section is laterally constrained: no horizontal compression or extension of the basin fill;
2. The changes in porosity and pore pressure are related to changes in vertical effective stress by coupling of a water flow equation (Darcy’s Law) and a deformation equation (Terzaghi's Law; this law is treated in Chapter 1);
3. The densities of the fluids (water, oil, gas) are assumed to be constant; e.g.
the density of water is not influenced by changes in temperature or changes in concentration of total dissolved solids.
As a consequence of assumption 1 it will not be possible to assess the influence of the identified long-term changes in lateral tectonic stresses and short-term stress fluctuations (Tables 10 and 11) on the evolution of the fluid systems. Chapter 1 outlined that changes in porosity and pore pressure are related to changes in mean stress and not to changes in vertical stress only; in the modelling the changes in porosity and pore pressure are related to changes in vertical stress (assumption 2).
Chemical diagenesis and their influence on porosity and permeability evolution are not included in the modelling. However, the effects of these geochemical processes (pressure solution, cementation) are incorporated in the calibrated porosity-depth relations. Because of assumption 3 the basin modelling study cannot provide information on the influence of heating and cooling and the influence of density differences on the development of pore pressures and water flow.
It is clear that the basin modelling does not incorporate all the previously identified processes and forces influencing the pore pressure, fluid geology and fluid dynamics in the Broad Fourteens Basin (Part 2). Application of the 2D basin modelling to the cross-section in the southern Broad Fourteens Basin does allow to study quantitatively the integrated history of sedimentation, erosion, porosity, permeability, water table elevation, pore pressure and water flow in the basin. In addition, the basin modelling permits to examine the interaction of sedimentation/uplift/erosion, heat transport, maturation of source rocks, and the generation, migration and accumulation of oil and gas, the latter also in relation to the evolution of pore pressure and water flow.
18.2 Input data and boundary conditions
The present study included the assessment of a detailed conceptual model of basin history along the regional cross-section. This conceptual model provided the input data and the boundary conditions required for the modelling. Appendix 4 presents the reconstructed quantified history of sedimentation, uplift and erosion along the cross-section and gives an extensive overview of the input data and boundary conditions.
18.3 Modelling Procedure
The first step was to prepare the data using the information given in Appendix 4.
The geological cross-section (Figure 48) was divided into 42 geochronological events and 31 lithotypes, and was divided laterally into 83 vertical columns. This resulted in a mesh of 42 ×83 grid blocks at present-day. Secondly, the present-day section was
backstripped and the sedimentary history was calculated. Subsequently, this history of sedimentation, uplift and erosion in combination with the geothermal history along the cross-section from the Late Carboniferous to present-day was verified against present-day temperature, vitrinite reflectance and porosity data. This resulted, amongst other things, in the selection of the most appropriate thermal boundary conditions and parameters that were used during subsequent modelling phases. The third important step was to calibrate the permeability structure. Initially, the history of pore pressures and groundwater flow was calculated with a simple fluid flow model, including sedimentary loading and unloading and the relief of the water table as pressure-influencing mechanisms, assuming no-flow bottom and lateral boundary conditions. The results of the fluid flow modelling were verified against present-day measured pore pressures and measured or published petrophysical parameters.
Limburg Group Upper Rotliegend Group Altena Group North Sea Groups Chalk Group Rijnland Group
Zechstein Group Schieland Group
P9 P6 Q1
NE SW
Distance (km)
Depth (km)
0
1
2
3
4
5
6
0 10 20 30 40 50 60 70 80
Broad Fourteens Basin
Lower and Upper Germanic Trias Groups
Figure 48 Geological framework along cross-section through southern part of the Broad Fourteens Basin. The geological framework is based on in-house interpreted seismic sections SNST-83-02 and NNS-6, published geological cross-sections (Burgers and Mulder 1991), declassified well-data and regional information (Quirk 1993, Quirk and Aitken 1997, RRI 1988, Van Adrichem Boogaert and Kouwe 1993-1997, and Van Wijhe et al. 1980). Figure 33 shows the location of the cross-section
On the basis of this, the permeabilities were adjusted by adapting the initial anisotropies. In order to evaluate the adapted permeability structure, the pressure history was simulated again, now taking into account hydrocarbon generation as an additional pressure-influencing mechanism. For this purpose the petroleum migration and accumulation history was also simulated. The known distribution of oil and gas accumulations along the section was compared with the reconstructed migration and accumulation history. This step also included the analysis of the influence of time-dependent permeability of faults on the pressure history and the evolution of the petroleum systems. The final modelling step was to incorporate the results of the previous steps and to represent the favoured modelling scenario for the quantitative reconstruction of the evolution of temperature, source rock maturation, petroleum generation, pore pressure and water flow, petroleum expulsion, migration and accumulation. Below, the results of the integrated forward modelling are
discussed in relation to the conceptual model of the evolution of the Broad Fourteens Basin described in Part 2 (see also Verweij et al. 2000, 2001, 2003, Verweij and Simmelink 2002).