ABSTRACT

Throughout East Texas, there are occurrences of unusually high-permeability beds in the Travis Peak and Cotton Valley Formations that have low resistivity even when charged with oil and gas. In most cases, these beds have retained primary porosity due to suppression of quartz cement by clay coatings that surround the grains in a conductive sheath. The clay-coated sandstones comprise a diagenetic facies that must be identified because commonly applied open hole well log analytical parameters cannot be used.

The clay-coated grain facies is detectable with a cross plot of gamma ray and spontaneous potential overlain by resistivity. When gamma ray is plotted on the horizontal axis and spontaneous potential is plotted on the vertical axis, a triangular field of data points results. Data points that fall within the bounds of the triangle represent rock that has undergone normal diagenesis, is quartz cemented, and has low permeability. Data points outlying the hypotenuse represent uncemented beds with open pore throats and have high permeability. Outlying data points associated with resistivities of less than or equal to 1.5 ohm-meters are usually in the clay-coated grain facies.

Zones flagged in this manner can be expected to have a cementation exponent in the range of 1.6 to 1.8, permeability in double-digit millidarcies, and be a potential low-resistivity pay.