Indirect Evidence of Secondary Porosity in Sandstones

R. E. Larese, Edward D. Pittman

Secondary pores originating from dissolution of framework grains are easily identifiable, but secondary pores formed by dissolution of carbonate cement are less definitive. Removal of carbonate cement produces secondary intergranular pores that commonly mimic primary intergranular pores. Direct evidence of this dissolution event may be absent or sparse. Petrologists, however, often make a subjective interpretation based on slight evidence that the intergranular porosity is all secondary.

Comparison of laboratory and natural dissolution of calcite suggests that dissolution is commonly surface controlled and progresses through various stages to ultimately produce growth-like ridges in some places. As such, care must be exercised when utilizing the SEM to distinguish growth versus dissolution origins.

When carbonate cement overlies other authigenic phases, it commonly modifies the idiomorphic form of the earlier phases. If patchy calcite-cemented sandstone occurs, a sample can be subjected to laboratory leaching using buffered acetic acid to remove the calcite and expose the earlier authigenic minerals. SEM comparison of textures of these pre-calcite authigenic minerals with the same minerals in naturally porous adjacent rock will aid in interpreting the origin of the porosity.

Indirect evidence of the occurrence of a dissolution event is sometimes present. Carbonate minerals commonly precipitate within and expand detrital mica flakes. Upon dissolution of the carbonate, the mica flakes remain open as evidence that carbonate was once present. Also, fossils commonly are partially replaced by pyrite. After dissolution of the calcareous fossil, the pyrite may subtly define the grain.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.