f Reservoir Facies Variation in the Upper Cretaceous and Danian Chalk of the Danish Central Graben

The Upper Cretaceous and Danian Chalks in the North Sea contain reservoir facies which are part of a world-class hydrocarbon system. The examples shown highlight some of the paradoxes of these microporous fine-grained, high-porosity, low-permeability carbonate reservoirs. Total porosity in typical chalk reservoirs is in the range of 25% to 35% with the best examples up to 45%. Permeability is typically low at 0.001mD to 10mD, but is enhanced by natural or induced fractures. Most chalk reservoirs have much better porosity than is expected from burial depths. The better than expected preservation of porosity is most likely related to an early onset of overpressure in the chalk. Apparently during burial, the overlying Tertiary mudstones provided a seal through which the pore fluids of the chalk were not able to escape. The microporous nature of the pore network is related to the size of coccoliths, the calcareous nanofossils which constitute at least ninety percent of the rock. The majority of pores are found in between the coccoliths and in coccospheres.