Summary

Formation of diagenetic illite by illitization of kaolins may cause significant permeability deterioration in the Statfjord Formation during burial from 3500 to 4000m. The illite formation needs potassium which is derived from dissolution of K-feldspar. If all detrital K-feldspar is diagenetically removed before burial to 3500m, there is an increased possibility for permeability preservation during burial towards depths greater than 4000m.

Diffusion transport of potassium from sandstones to shales during moderate burial (2.5-3.5 km) is probably an important mechanism for removing potassium from the sandstones, prior to the onset of kaolin illitization. In the Statfjord Formation, good permeability has been found in wells from the northern North Sea at burial depths exceeding 4000m. In these well, diagenetic kaolins are little affected by illitization and the average potassium content lower than in reservoirs with abundant illite. Similarly, the average potassium contents in the associated mudstones are higher than in shallower wells. The present study thereby suggests that potassium is removed from the sandstones during intermediate burial, before illitization commences.

Complete removal of K-feldspar before illitization has, however, not occured in all deeply buried structures, and the average potassium content is high in some deeply buried sandstones with abundant diagenetic illite and low permeability. Furthermore, the potassium content in the mudstones in one deep well is not significantly different from the potassium content in the mudstones in the shallower wells. Why potassium is not transferred from the sandstones to the shales is then an important question during permeability prediction in deep structures. One pessimistic, but likely model, is that efficient transport of potassium from sandstones to mudstones requires a water saturated pore system, and that hydro-carbon emplacement will stop the process. Hence, if this model is correct it implies that good permeability in deeply buried Statfjord Formation prospects will be encountered only if the structure is dry, or if hydrocarbon emplacement occured after burial to more than 3500m

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah