ALBERT

Description: North Sea Central Graben chalk reservoirs are unique in that they have exceptionally high porosities at several thousand metres depth. This is partly an effect of overpressure, but, equally important are the combination of diagenesis, fracturing, and the timing of hydrocarbon generation, migration and entrapment.For chalk to serve as a reservoir, available porosity and permeability at the time of hydrocarbon migration are critical. Porosity in a normal chalk is reduced by early marine cementation, mechanical compaction and chemical compaction (i.e. dissolution and precipitation of calcite cement). To what extent these processes influence porosity evolution through time is related to the chalk’s burial history, specifically the pressure history, thermal history and the partial pressure of CO2(PCO2).This was demonstrated by modelling porosity evolution through time in a cross-section extending from crestal Valhall Field to basin areas. The modelling results show that at crestal Valhall the chalk is highly porous and only mechanically compacted. Thermal and pressure (PCO2) conditions were unfavourable for carbonate burial diagenetic cement to form prior to ≈20 Ma. At this point, oil emplacement prevented further porosity reduction, despite increasing temperatures and favourable PCO2conditions during continued burial. In basin areas, rapid burial and exposure to high temperatures combined with favourable PCO2conditions, from ≈40 Ma until present, resulted in more extensive porosity reduction by mechanical and chemical compaction.Permeability in chalk is significantly enhanced by fracturing. Chalk fractures if total stress (pore pressure + bending moment) is equal to or exceeds fracture pressure. Providing that fractures are open while hydrocarbons are migrating, these will serve as migration pathways. As for the modelled basin areas, absence of tensional fractures at the time of hydrocarbon migration prevented filling of potential reservoirs. Instead, hydrocarbons were directed laterally towards crestal Valhall where pore space and permeability enhancing fractures were available.By combining the results from the chalk diagenesis and fracture studies with hydrocarbon generation, migration and entrapment modelling, the distribution of known chalk fields and dry holes in the area was reproduced. The approach taken in the study emphasizes the necessity of a detailed understanding of the processes behind empirical observations such as porosity values in North Sea chalk reservoirs, a full evaluation of all factors having an impact on fracturing, and the integration of this understanding with basin-scale hydrocarbon migration models. Despite still unresolved questions concerning the chalk hydrocarbon system, the achievements so far are promising and encouraging for future prediction of hydrocarbon accumulations.