Preservation of Permeability in Deeply Buried Sandstones of the Viking Graben: Inhibition of Illitization Due to Absence of K-Feldspar

S. N. Ehrenberg, P. A. Bjorkum, P. H. Nadeau, N. E. Aase

Three areas of deeply buried gas-condensate fields in the Norwegian sector of the northern North Sea, including strata from the Brent Group and the Cook and Statfjord Formations, contain striking examples of anomalously high porosity and permeability at 3.5 to 4.1 km below the sea floor. Permeabilities in the range of hundreds of mD to several D in these reservoirs are critically dependent upon both porosity preservation and the absence of extensive illitization. Abundant kaolin (dickite) is present in the sandstones, but has not reacted to form illite because of the complete absence of detrital K-feldspar, necessary as a local source of K⁺. Several alternative hypotheses could account for the observed deficiency in K-feldspar prior to the onset of extensive sa dstone illitization (which normally occurs at 120-140°C):

1. Provenance - The sands may have been low in K-feldspar when deposited.

2. Early leaching - K-feldspar dissolved by meteoric water at shallow depth.

3. Albitization - The K-feldspar could have been altered to albite.

4. Late kaolinization - Consumption of K⁺ and introduction of H⁺ by smectite illitization and organic maturation in shales could alter K-feldspar to kaolin in interlayered sandstones.

Although we know that processes 1-3 all have important effects on Viking Graben sandstones in general, we find compelling evidence against these explanations for the lack of K-feldspar in the cores studied, leaving us with the late kaolinization hypothesis by default. In this case, however, the question arises as to why late kaolinization has prevented illitization only in these three areas and not in all deep sandstone reservoirs on the Norwegian continental shelf.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995