ABSTRACT

The south Texas area has several productive horizons that might well be described as "tight rocks." These tight rocks are relatively low-porosity, low-permeability sandstones found in the lower Oligocene, the Eocene, and the Upper Cretaceous Gulfian Series. They include such important producing formations as the Deep Frio, the Vicksburg, the Wilcox, the Olmos, and the San Miguel. They represent a vast amount of past and present production and future potential in the south Texas area.

The paper shows the geological column, the general areas or production and drilling activity, and the location of the several example logs presented in the paper. The more important formations are described in general. Various petrophysical parameters such as matrix density, velocity and porosity, as well as formation water resistivity, water saturation ranges, and types of production are discussed.

Various example wells are presented, showing productive zones in the various formations. The examples consist of the log (including the Dual Induction Laterolog, the Borehole Compensated Sonic Log, the Formation Density Log Compensated, and the Compensated Neutron Log) and computer-processed interpretations. These are discussed in detail along with a review of the more subtle aspects of the logs.

The paper thereby gives a general overview if the tight rocks of south Texas with examples and specific parameters and local knowledge for use in these formations.