Diagenetic Structural Transformations in North Sea Jurassic Illite/Smectite

Abstract

Abstract--The Kimmeridgian-Volgian(-Ryazanian) claystone is the principal oil source rock in the troughs of the North Sea. Randomly (R0) ordered mixed-layer illite/smectite (US) appears to have transformed to RI-(IS) or R3-(ISII) ordered simultaneously with oil generation. The proportion of illite layers in I/S increased to 95% d,aring diagenesis in these claystones. Exceptions are three samples of I/S of probably bentonitic origin; these have apparently changed during diagenesis to R0-0rdered I/S containing 40-50% illite layers. Fine fractions of the claystones dominated by I/S were analyzed by ZTAl and 29Si magic-angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopy, ~TFe Mrssbauer spectroscopy, and X-ray powder diffraction (XRD), and for total chemical composition. Si/AI ratios determined from MAS-NMR agree closely with those calculated from total chemical analysis; however, tetrahedral and octahedral AI occupancies were most accurately determined by NMR. An increase in the percentage of illite layers and in the ordering of the I/S was accompanied by fixation of K ~ and NH4 + in the I/S, by tetrahedral AIfor-Si substitution, and by octahedral Al-for-(Mg + Fe) substitution, resulting both in an increase of charge in the 2:1 layers and in a migration of charge from octahedral to tetrahedral sheets. The I/S of probably bentonitic origin had a larger tetrahedral and a smaller octahedral charge than expected from its content of illite layers. MAS-NMR showed a significantly higher content of tetrahedral AI (most likely in smectitic sites) than expected from the percentage ofillite layers calculated from XRD. Correspondingly, XRD of K+-saturated and glycolated specimens showed that several smectite layers possessed a significant charge. A constant b-dimension of the I/S and the presence of a significant charge in the smectite layers suggest that a transformation of smectite to illite layers in the I/S by tetrahedral Al-for-Si substitution followed by interlayer cation fixation and interlayer contraction is the most probable genesis for the I/S investigated.

Reaction Mechanisms of Smectite Illitization Associated with Hydrothermal Alteration from Ponza Island, Italy

“…The R ---0 to R = 1 transition in Ponza I-S occurs relatively abruptly at --45% I (Figure 4), which is consistent with a DC mechanism. The R = 0 to R = 1 transition at Ponza contrasts with a more gradual R = 0 to R = 1 transition at 65% I as observed in shale (Bethke et al, 1986) and bentonite (Inoue et al, 1990), where an SST mechanism was proposed (Inoue et al, 1990;Lindgreen et aL, 1991). The progressive changes in cation ordering ( Figure 7A), layer charge ( Figure 8D), and isotopic composition which also occur over this interval are explained by the transformation of turbostratic, cv, anhedral plates to tv(?…”

“…Pollard (1971) proposed a mechanism where A1 and Si diffuse through the interlayer, followed by tetrahedral-sheet distortions as A1 replaces Si. Others studies suggest an SST mechanism but do not propose specific models (Hower, 1981;Hunziker et al, 1986;Inoue et al, 1990;Lindgreen et al, 1991).…”

“…48, No. 6 , 2000 Reaction mechanisms of smectite illitization from Ponza island, Italy 61 1 (Eberl et al, 1990), and still others favor an SST mechanism (Hunziker et al, 1986;Lindgreen et al, 1991;Clauer et al, 1995). Several studies emphasize that different mechanisms may apply in different geologic environments: an SST or local DC mechanism describing illitization in shale and bentonite (Inoue et al, 1990;Ahn and Peacor, 1986; Li et al, 1997) and a DC or Ostwald ripening mechanism in hydrothermal settings (Yau et al, 1987;Inoue et al, 1988;Christidis, 1995).…”

Transmission Electron Microscopy Study of Conversion of Smectite to Illite in Mudstones of the Nankai Trough: Contrast with Coeval Bentonites

“…Abercrombie et aL (1994) noted that the rate of illitization is controlled by silica activity, which in turn is controlled by the rate of precipitation of silica. I-S formation parallels that of maturation of organic material (e.g., Lindgreen et al, 1991). Small (1993) showed that the organic acids evolved during early diagenesis can be critical to rates of illitization, through their control on solubility of A1.…”

Comparison of the Mineralogical and Chemical Composition of 2 Shales from the Western Canada Sedimentary Basin and the United States Gulf Coast

“…More recently, diagenetic changes in finegrained sediments have been documented, most strikingly in the case of mixed-layer I/S, at the single crystal structure or clay package scale by transmission/ analytical electron microscopy (Ahn and Peacor 1986;Yau et al 1987;Freed and Peacor 1992) or nuclear magnetic resonance spectroscopy (Lindgreen et al 1991;Schroeder 1993). Differences in shale mineralogy or clay mineral composition either downhole or within a given stratigraphic unit commonly are interpreted as indicating that a diagenetic reaction has taken place.…”