AMMONIUM FIXATION DURING ILLITE-SMECTITE DIAGENESIS IN UPPER JURASSICSHALE, NORTH-SEA

Claystones of Kimmeridgian-Volgian-Ryazanian age are the main source r ock for oil in the Central Trough, North Sea. During oil generation, i llite layers form in illite-smectite (I-S) and the illite-smectites be come short-range (IS) ordered and eventually long-range (ISII) ordered . The formation of illite layers involves dehydration and fixation of interlayer cations, K+ and NH4+. The content of soluble NH4 and K in c ore samples and the content of NH4 and K in I-S were determined, the N H4 by an isotope dilution method. In the Central Trough, soluble K+ de creases with depth, whereas soluble NH4+ increases with depth. From ap proximately 3 km, the ratio NH4/K increases with depth, to an average of approximately 0.5 at 4.4 km. In I-S, the ratio NH4/K shows a simila r increase with depth. The NH4/K ratio increases significantly in I-S during ordering. It is concluded that NH4 fixation in I-S may be of ge neral importance during burial diagenesis of oil source rocks.