Shale Diagenesis: An Important Factor in Development of Secondary Porosity

David A. McMahon, Jr.

Abundant secondary porosity due to the dissolution of feldspar grains, volcanic rock fragments, and carbonate cement has been observed in thin-section studies of reservoir sandstones in the McAllen Ranch field in south Texas. Secondary porosity develops in fluid flow pathways within the sandstones. Potentiometric profiles indicate that within the overpressured Oligocene strata, head gradients cause pore fluids to flow from the shales into the sandstones and toward major growth faults that bound the fields. The dissolution reactions responsible for the secondary porosity require that the pore fluid be acidic. Shale diagenesis provides an important source of hydrogen ions for these reactions.

The transformation of smectite clay to illite has been well documented in the Oligocene shales in this area. The smectite to illite reaction (assuming mobile Al⁺³) is discussed as well as the reduction of Fe⁺³.

Mass balance calculations in the study area indicate that the amount of acid produced by these reactions within the adjacent shales can easily account for the amount of secondary dissolution porosity observed in the sandstones. The exploration and exploitation of these types of sandstone reservoirs can be improved by a better understanding of the processes involved in the formation of secondary porosity.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.