A Re-examination Of The Chalk: From Reservoir To Seal

Mallon, Anthony J.¹, Richard E. Swarbrick² (1) Durham University, Durham, United Kingdom (2) Geoperssure Technology Ltd, Durham, United Kingdom

The first set of permeability measurements on North Sea Chalk using the Transient Pulse Decay (TPD) method shows they have nanoDarcy to 10s of nanoDarcy permeability (porosity 5-20%). This is similar magnitude to siliciclastic mudrocks and are up to three orders of magnitude below the lowest measurements previously reported for Chalk. Such low permeability allows these rocks to act as highly effective aquitards allowing pore pressures to increase from hydrostatic to near lithostatic. Measurements were made in non-reservoir, non-hydrocarbon bearing Chalk which comprises more than 90% of the Chalk succession of the North Sea in a gross interval of more than 1km. Previously published permeability values for Chalk from the area are predominantly from the hydrocarbon-bearing reservoirs where the permeability is generally at least four orders of magnitude higher rendering them ineffective at sustaining fluid pressures above hydrostatic. Permeability is uniformly low across a range of facies.

In addition, this Chalk dataset has been compared to a Danish North Sea Chalk dataset with similar appearance and porosity values. There is however a difference in permeability of several orders of magnitude. The method of measuring permeability was different as the Danish permeabity was measured by routine core analysis (RCA). Investigations have revealed that the two datasets have similar rock properties and differences are not due to regional variation. It has also been determined that TPD has more validity than RCA in measuring lower permeability chalk (<0.01mD). This suggests that much of the previously reported RCA permeability measurements may be in error.