Chemical and mineralogic processes related to the migration of natural gases are of importance for petroleum geology. An unusual alteration of sedimentary formations by hydrogen sulfide-bearing natural gases in western Merced County, California, has been described. The alteration created an acidic medium (with pH values ranging from 0.5 to 4.5) containing sulfur, sulfides, sulfates, and black and white discoloration.

A peculiar layer of pisolitic sandstone was noted in the altered area near the boat-launching ramp on the southeastern shore of the O'Neill Reservoir. Individual pisolites 2-15 mm in diameter are present as an undulatory layer in a bed of light-gray, fluvial sandstone of the Tulare Formation, below the Pleistocene Corcoran Clay Member. X-ray diffraction of the sandstone cement yielded abundant alunite.

Data collected during studies of iron sulfide concretions from the same locality yielded some information on origin of pisolites. An aluminum sulfate-bearing gel was repeatedly obtained by solution of concretions in concentrated nitric acid. Aluminum was produced by decomposition of alumosilicates with sulfuric acid derived from oxidation of sulfides cement.

Alunite pisolites probably originated by decomposition of alumosilicates occurring in sandstone by reaction with sulfuric acid created by oxidation of hydrogen and/or iron sulfides in excavations. Natural gases in the area, following joints, tend to cause dome-shaped uplifts in overburden, which explains the undulatory occurrence of pisolitic layers. Their position could be controlled by a capillary uplift.

End_of_Article - Last_Page 961------------