Abstract: Controls on Precipitation of Illite in Reservoir Sandstones; Examples from the North Sea and Gulf Coast Basin

K. Bjorlykke, P. Aagaard

Formation-water analyses from North Sea and Gulf Coast basin reservoirs provide important constraints on burial diagenetic processes. Thermodynamic calculations show that aluminum solubilities are very low (<1 ppm at 130°C) even in the presence of organic acids. Illite, therefore, forms mainly at the expense of dissolving aluminous minerals like smectite and kaolinite. The distribution of these precursor minerals and potassium feldspar is, in most cases, the key to prediction of illite precipitation in reservoir rocks.

The potassium values of the formation waters are always lower than K-feldspar saturation during burial diagenesis (>1.5 km burial depth) suggesting that K-feldspar is continually being dissolved. The potassium concentrations indicate in most cases supersaturation with respect to illite. This is evidence that illite may fail to form, probably because aluminum is not available. In the North Sea Basin, potassium concentrations approach equilibrium with kaolinite and illite at higher temperatures (140°C) indicating that illitization of kaolinite at that point keeps up with K-feldspar dissolution. In the subsiding parts of the Gulf Coast Basin, where kaolinite and smectite are being heated, the potassium values are clearly lower than in the uplifted parts of the basin where illite s not forming due to cooling. This suggests that the potassium concentrations are to a large extent controlled by the rate of illite precipitation, which again depends on the availability of aluminum from dissolving Al-minerals.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994