Relationship of Pressure Regressions to Secondary Porosity Development: Exploration Model for Secondary Porosity Development in Deep Wilcox of South Texas

James Painter

In the deep Wilcox, as well as other deep Gulf Coast Tertiary formations, secondary porosity development is very important to reservoir quality. Four major Wilcox fields and several abandoned wells were studied with respect to shale pore-pressure gradients to show the relationship between secondary porosity and shale pore-pressure regression.

Secondary porosity is the result of chemical leaching and alteration of framework grains and clays in a rock by organic acids, fresh water, and other fluids or gasses reacting with the rock constituents. These chemicals, water, and gasses are formed in shales adjoining the sandstone reservoir rock and migrate into the adjoining aquifer sandstone, which is then leached. This release of fluids from the shale forms a pore-pressure regression which can be plotted using electric logs.

The presence or absence of a pore-pressure regression gives indications of secondary porosity development in the adjoining reservoir rocks. If a regression is documented, the risk of inadequate reservoir porosity of a prospect can be lowered.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.