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Variations in mixed-layer illite/smectite (I/S) expandability within a 60-m interval of shale from the lower Frio Formation of south Texas are much greater than expected from a simple temperature-dependent burial diagenesis model. This variation is associated with differences in shale fabric that appear to have influenced rates of diagenesis rather than with differences in original composition. The section investigated consists of two distinct shale fabric types interbedded with several-meter thick beds of finegrained sandstone. Laminated and massive shales differ by the presence of either discrete laminations of silt or dispersed grains of silt, respectively.

The shales are composed of 65–80% clay minerals, with quartz, plagioclase, and calcite as the other major constituents. The predominant clay mineral is an I/S that ranges in expandability from 45 to 100%. Massive shales contain I/S that ranges in expandability from 60 to 95%. The I/S in the massive shales is on the high side of the range of expandability observed in other Gulf Coast wells of similar depth and age. In contrast, the I/S in the laminated shale contains 45 to 80% expandable layers.

Both massive and laminated shales contain little K-feldspar and nearly equal amounts of K. The entire section has about the same total K content as other shallow Tertiary Gulf Coast sections. The difference in I/S expandability between the massive and laminated shales is not reflected in their bulk K content. I/S expandability therefore does not appear to be controlled simply by the shale’s bulk composition. Shale fabric, as measured by its effect upon fluid flow properties, is a critical factor in understanding the difference in the relative extent of reaction, as measured by I/S expandability. The thin silt laminations in the laminated shale act as fluid conduits, providing both more reactants and more sites available for reaction.

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