The first set of permeability measurements on North Sea chalk (porosity 5%–20%) using the transient pulse decay method shows that its permeability is similar to that of siliciclastic mudrocks (10−22 to 10−18 m2) over the porosity range of 5%–20% and is as much as three orders of magnitude below the lowest measurements previously reported for chalk. Such low permeability allows these rocks to act as highly effective aquitards and allows pore pressures to increase from hydrostatic to near lithostatic. Measurements were made in nonreservoir, non–hydrocarbon-bearing chalk, which comprises >90% of the chalk succession of the North Sea in a gross interval of >1 km. Previously published permeability values for chalk from the area are predominantly from the hydrocarbon-bearing reservoirs, where the fact that the permeability is generally at least four orders of magnitude higher renders them ineffective at sustaining fluid pressures above hydrostatic. Permeability is uniformly low across a range of facies and insoluble residue and shows a weak relationship with porosity.