Mesoscopic and Microscopic Analyses of Rock Samples Utilized in Rock Mechanics Studies

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Uncompacted and experimentally compacted rock samples from Frio 'A' sandstones and Anahuac shales, Hitchcock N.E. field (Galveston County), utilized in rock mechanics studies were investigated mesoscopically and microscopically to determine fabric and grain movement. Sandstone petrography and porosity reduction were measured by point counting to estimate the degree of shortening the sandstones had undergone.

Rounded, irregular to elongate grains of quartz, feldspar, rock fragments, chlorite, and shale clasts, along with cements, form the main constituents of the Frio 'A' reservoir. Quartz and feldspars exhibit corrosion, whereas skeletal and vacuolized feldspars comprise about 2 percent of the sandstones.

The Frio 'A' reservoir consists of a stacked sequence of distributary-mouth-bar sandstones, delta plain units, and delta destructional units overlain by the transgressive Anahuac shale. Grains within these sandstones are imbricated in shallow to steeply dipping crossbeds. Steeply dipping interference banding present on Fry (1979) diagrams constructed from grain distributions in mouth-bar sandstones result from foreset bedding.


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