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Petrographic and microprobe analyses of geopressured shales from the Oligocene Frio Formation, Brazoria County, Texas, provide direct evidence of hydraulic fracturing and significant fluid flow in the shales. The optical continuity of the fracture margins and their higher Al/Si ratios indicate that the fracture margins have undergone more complete alteration of smectite to illite than the interiors of the fracture blocks. Crosscutting relationships between calcium sulfate and the organic material that fills the fractures indicate that calcium sulfate was emplaced before the organic material, and thus was precipitated in the subsurface. The calculated fracture permeabilities of the shales (on the order of 10-13 m2) are similar in magnitude to that of he Frio sandstone reservoirs, which suggests that these fractured shales could support flow comparable to that in the sandstones. Fracture-fill deposits or collapse could, in turn, reduce this flow. In addition, the more extensive alteration of the fracture margins suggests that fluid in these shales flowed preferentially through the microfractures rather than through the matrix.
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