South Texas has several productive zones that can be described as "tight rocks"--the relatively low-porosity, low-permeability sandstones of lower Oligocene, Eocene, and Upper Cretaceous Gulfian Series. They include such important producing formation as "Deep

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Frio," Vicksburg, Wilcox, Olmos, and San Miguel, and represent a vast amount of past and present production and future potential. Various petrophysical parameters such as matrix density and velocity, formation-water resistivity, porosity, and water-saturation ranges influence production. Identification of productive zones is assisted by use of the Dual Induction Laterolog, the Borehole Compensated Sonic Log, the Formation Density Log Compensated, and the Compensated Neutron Log, as well as hand- and computer-processed interpretations.

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