Abstract

This paper reviews existing K-Ar dates from diagenetic illites from the Brent Group and presents new age data from 11 wells from across the East Shetland Basin.

K-Ar ages from diagenetic illites record illite growth in various parts of the Brent Group of the East Shetland Basin from about 60 Ma until 17 Ma. Diagenetic illites from the Cormorant field yield ages, which when combined with burial history simulation, suggest substantial illite growth from 75°C. This temperature is believed to be close to the temperature for the start of illite growth in the Brent Group. Other illite dates, when combined with burial history simulations, demonstrate continued illite growth to temperatures in excess of 110°C. Etive Formation samples of wells in which the overlying Kimmeridge Clay Formation source rocks are mature for oil generation, show a strong correlation between the K-Ar age of the finest size fraction of diagenetic illite and the calculated time at which the overlying source rocks reached a vitrinite reflectance of 0.62%. Data from other formations do not follow this trend and frequently give ages which are considerably older than those from the Etive Formation. It is considered likely that many of these older dates may arise as a result of contamination by detrital illites which are likely to be less common in the highly reworked sands of the Etive Formation barrier complex.

No recent (< 10 Ma) K-Ar dates have been obtained from diagenetic illites of the water zone of any wells so far studied in the Brent province. It is also apparent from many wells where diagenetic illites from both the hydrocarbon and water zones have been dated, that illite growth apparently stops in the water zone relatively soon after hydrocarbon emplacement. Furthermore, the time span for illitization, suggested by dating different illite size fractions of the same sample, is much shorter than that suggested by comparison of the range of temperatures illite apparently grew at and burial histories of Brent Group wells. Consideration of these points has led the authors to conclude that illite begins to form at a temperature of about 75°C probably at quite a low reaction rate. The process of oil migration at somewhat higher temperature perturbs the static aqueous pore fluid medium and increases effective water-to-rock ratio by fluid transport. This results in increased rates of illite growth. Subsequent to the peak of oil generation, fluid flow rates decrease, with a concomitant decrease in illite-forming reaction rates. Any attempt to isolate illite formed at lower or higher temperatures will inevitably also sample illite from the most rapid period of growth and so yield a false and reduced time span of illite formation and apparently old ages for most recently formed illite in the water zone.