Petrographic analysis of closely spaced sandstone samples (GCO/DOE No. 1 well) of a sandstone/shale package from the Frio Formation (Oligocene) indicates that sandstone reservoir quality was influenced by shale diagenesis. Three alteration zones at increasing distance from the sandstone/shale contact are observed. This zonation may be explained as follows.

Organic and inorganic maturation processes modified shale fluids which, upon expulsion into the sand, resulted in the precipitation of thin, isopachous chlorite grain coatings in the contact sand. Late in the chloritization process and thereafter, unstable framework grain silicates began to dissolve within the sand. We believe that aluminum from framework grains was removed from the contact zone by mobile organic complexes. Silica released from grain dissolution reprecipitated as quartz cement. This contact zone is about 1-ft (0.3 m) thick.

As fluids passed into the second zone (about 1-ft (0.3 m) thick) the sandstone framework grain leaching continued but to a lesser degree. Kaolinite was produced from internal mass sources and from aluminum imported from the first zone. The net addition of alumina from the contact zone prevented development of quartz overgrowths in this zone and the third zone.

In the third zone, dissolution of framework grain silicates was least thorough because of greater distance from the shale and alteration appears to have been aluminum conservative.

End_of_Article - Last_Page 609------------