SEQUENTIAL STRUCTURE TRANSFORMATION OF ILLITE-SMECTITE-VERMICULITE DURING DIAGENESIS OF UPPER JURASSIC SHALES FROM THE NORTH-SEA AND DENMARK

For mixed-layer clay fractions from the North Sea and Denmark, X-ray d iffractograms have been recorded for specimens saturated with Mg, Ca, Na and NH4, both air-dry and intercalated with ethylene glycol, and th e patterns have been computer-simulated with a multicomponent program. The mixed-layer fractions consist of an illite-smectite-vermiculite ( I-S-V) phase constituting similar to 90% of the fraction and a kaolini te-illite-vermiculite (K-I-V) phase. For each I-S-V, the degree of swe lling in swelling interlayers depends on both interlayer cation and gl ycolation, whereas the amount of non-swelling illite and swelling inte rlayers and the interstratification parameters are constant. Based on structural characteristics and the degree of diagenetic transformation , the samples investigated can be divided into three groups. The I-S-V of group one is predominantly detrital and has 0.69-0.73 illite, 0.26 -0.20 smectite and 0.04-0.07 vermiculite interlayers, the illite, smec tite and vermiculite interlayers being segregated. The I-S-V of group two has been diagenetically transformed and has 0.80 illite, 0.12 smec tite and 0.08 vermiculite interlayers, the vermiculite interlayers bei ng segregated whereas the illite and smectite have the maximum orderin g possible for R = 1. The I-S-V of group three has been further transf ormed during diagenesis and has 0.84 illite, 0.08 smectite and 0.08 ve rmiculite interlayers. Statistical calculations demonstrate that the I -S-V transformation can be described as a single interlayer transforma tion (SIT) within the crystallites.