Geodynamic and hydrodynamic evolution of the Broad Fourteens Basin (The Netherlands) in relation to its petroleum systems

Abstract

Data analysis in combination with 2D basin modelling is used to study the hydrogeologic and hydrodynamic response of the basin fill of the Broad Fourteens Basin to its geodynamic evolution and the significance of this response for the evolution of the petroleum systems in the basin. Data analysis resulted in the identification of the major mechanical processes involved in overpressure and fluid flow evolution in the basin (sedimentary loading and unloading, tectonic compression and extension, topography of the groundwater table) and provided a timeframework for the important permeability alterations in the basin and the distinct phases of fluid flow. Observed present-day diagenetic features and fracture characteristics in the basin indicate permeability alterations and paleo fluid flow during distinct periods of tectonic activity in the basin, e.g. during Late Jurassic rifting and Late Cretaceous inversion. The basin boundary fault zones were repeatedly reactivated during the periods of tectonic activity. The 2D basin modelling included sedimentary loading and unloading, topography of the water table and petroleum generation as pressure and fluid flow influencing mechanisms. In addition, the influence of time-dependent permeability of fault zones on the overpressure history and the evolution of petroleum systems was modelled and evaluated. The favoured modelling scenario, including a time-dependent permeability of faults, shows that syn-rift and post-rift sedimentation in combination with gas generation induced overpressured conditions in restricted parts of the basin, that is mainly in the deeper low permeable Carboniferous shales and Zechstein evaporites. The calculated pre-inversion overpressured conditions disappear during subsequent inversion of the basin. Predicted present-day pressures in the reservoir horizons are near-hydrostatic and reasonably in accordance with observed pressures. The favoured scenario also explains the known location of oil fields, and the location and saturation characteristics of the gas occurrences. The model results further show that the difference in timing of oil charging of the P9 and Q1 Lower Cretaceous reservoirs explains the observed differences in geochemical composition of the accumulated oils. The biodegraded and water-washed nature of the Late Cretaceous charged Q1 oil reservoir is consistent with the concentrated topography-induced groundwater flow through Lower Cretaceous reservoir units during the Late Cretaceous inversion period. Late charging of the Lower Cretaceous oil reservoir in the P9 area explains the non-biodegraded character of the accumulated oils. In general, this paper shows the importance of using data analysis to obtain an a priori characterization of the basin geofluid evolution and shows that paleohydrodynamic conditions and time-dependent permeability of the hydrogeological framework have influenced significantly the petroleum systems in the basin.