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Twelve samples of Frio and Vicksburg (Tertiary) overpressured shales from Brazoria and Hidalgo Counties, Texas, were examined by both x-ray and transmission electron microscopy (TEM) techniques. TEM lattice fringes from shallower samples show mixed-phase illite/smectite (I/S) layers in random orientations relative to each other. Electron diffraction patterns of these shallower I/S layers show very diffuse basal reflections together with very pronounced turbostratic structure and streaking along z*. Electron diffraction patterns of I/S layers from intermediate depths still show z* streaking and turbostratic structure, but the basal reflections are more distinct. TEM lattice fringe images of intermediate depth samples show I/S layers arranged in a subp rallel orientation. Electron diffraction patterns of I/S layers from deeper samples generally show well-defined basal reflections, and both turbostratic structure and z* streaking are less pronounced. An electron diffraction pattern of I/S layers from one deep sample (12,490 ft, 3,800 m, calculated equilibrium temperature of 168°C, 334°F, and pore-pressure gradient > 0.7 psi/ft, 15.8 kPa/m) shows an illite 2M1 pattern, indicating complete dehydration of original I/S layers. The presence of pronounced z* streaking is thought to be due to disordered stacking of layers, which in turn is caused by diagenesis of original mixed-phase I/S layers. TEM lattice fringe images for this sample show parallel illite layers with basal spacing of approximatel 20 A.
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