A CHEMICAL, XRD, AND AL-27 MAS NMR INVESTIGATION OF MIOCENE GULF-COAST SHALES WITH APPLICATION TO UNDERSTANDING ILLITE-SMECTITE CRYSTAL-CHEMISTRY

This study assesses the distribution of Al and Fe in mixed-layer illite/smectites (I/S) in shales undergoing burial diagenetic changes, using evidence from Al-27 NMR, XRD, and chemical analyses. Samples studied include a sequence of mixed-layer I/S (ranging from 40% to 68% illite layers) in shales from a well located in the Caillou Island Oil Field, Terrebonne Parish, Louisiana, as well as synthetic mica-montmorillonite (Syn-1), Silver Hill illite (IMt-1), K-benonite (ISMt-1), an Fe-bearing muscovite, phengitic muscovite, and a randomly interstratified mixed-layer I/S with 50% illite layers. Using a simplified model, where Fe3+ isomorphously substitutes randomly for [6]Al in the dioctahedral 2:1 structure, the Al-27 NMR signal intensities are examined with regard to the paramagnetic deshielding effect of the Fe3+. The rapid decrease in paramagnetic deshielding with distance allows for a spherical ''wipeout'' model with a radius of 6 angstrom, oyer which there is complete effective paramagnetic line broadening (i.e., Al within the sphere is not ''seen''). Using the average dimensions of a dioctahedral mica, the expected relative intensities of the octahedral and tetrahedral Al signal are determined as a function of Fe2O3 content. Observed Al-27 signals, normalized per unit weight of Al2O3 and relative to the lowest Fe-bearing phase, show a clear trend of decreasing intensity with increasing Fe2O3 content. Normative fitting of oxide data to structural formulae reveals a similar trend of decreasing Al-27 intensity with increasing fraction of dioctahedral site occupied by Fe3+. Agreement between the observed Al-27 intensities of low Fe-bearing 2:1 phyllosilicates and Al-27 intensities predicted using the wipeout model indicate regular ordering of Fe and Al within the low Fe-bearing phases. However, observed Al-27 intensities for the I/S specimens fall into a region where the amount of Al seen is in excess for the given X(Fe), thus indicating segregation of Al and Fe domains. The second order quadrupole effect for the [6]Al site in the I/S fraction of shales decreases very slightly with increasing depth and percent of illite in the I/S, but not enough to effect site quantitation. Quantitative apportionment of elements into the I/S phase of the <0.2 mum fraction using NMR constraints shows directly a trend of increasing number of [4]Al sites and no change in the number of [6]Al sites with in degree of illitization for samples from the Gulf Coast diagenetic environment. Stoichiometry indicates an approximate 1:1 substitution of tetrahedral Al for Si over the 40-68% range of illite in I/S examined.