Clay diagenesis of the source rocks from the Permian Basin

The main objective of this study is to investigate the transformation of illite/smectite mixed layer clay during burial diagenesis. Shales of Atokan and Wolfcampian ages from the Northwestern Shelf, Delaware Basin, Midland Basin and Southern Shelf were examined, using X-ray diffraction and analytical electron microscopy.

The illite/smectite (I/S) particles in Atokan shales are less mature, consist of a mixture of random, RO and Rl ordered phases with about 45 to 70% illite. The Wolfcampian shales contain R3 ordered phases with greater than 70% illite component. As diagenesis progresses, there is a gradual shift in the basal reflection towards 10 A, decrease in cation exchange capacity, change in particle morphology from a cornflake to a platy habit, change in texture of the aggregates from a smooth subrounded aggregate to a domained subrounded aggregate and an increase in both tetrahedral and octahedral substitution. The octahedral charge is largely created by Mg substitution for Al and the tetrahedral charge by Al for Si.

The Atokan shales, which have a higher percentage of smectite, octahedral Al and Fe and interlayer water within the I/S particles show a higher catalytic property than the mature Wolfcampian shales.

Based on this study a transformation-recrystallization mechanism is proposed for the conversion of smectite to illite in the coarse particles. Transformation involves partial dissolution, diffusion and substitution of ions. True mixed-layering occurs at the early stages of diagenesis as indicated by the lattice fringes and the reflections at 17 A and 26-30 A in Atokan shales. As diagenesis progresses, subgrains or packets of discrete illites are formed. These packets grow and eventually coalesce and form platy illite particles. A neoformationrecrystallization mechanism is proposed for epitaxial growths on clay aggregates.

Generally, as diagenesis progresses, iron-rich carbonates and Fe-chlorite are precipitated and illite/smectite mixed-layers transform into illite or illite/chlorite particles depending on whether K- or Fe and Mg-rich pore fluids are present, respectively. Availability of K"*" ions and permeability of shales are more important factors controlling illitization than depth, temperature or age.