ABSTRACT

Rotary-drill well cuttings were examined to delineate subsurface alteration zones around 4 oil fields in southern Oklahoma. Particular attention was given to variations in color and cementing materials. Diagenetic mineral occurrences were mapped to determine mineral zonations, and the locations of the zone boundaries. Mineralized zones were compared with productive areas and structural features within the fields to determine their relationships. Petrographic analyses were conducted on sandstones exhibiting diagenetic features.

Permian sandstones which are typically red are altered to white over the crests of the Velma, Eola, and Chickasha Anticlines. This color change is due to a change in cementing materials from clay, limonite, and hematite to calcite, dolomite, and in some places, pyrite over the anticlines. Bleaching and mineralization was restricted to sandstone and was brought about by the reduction of iron oxides by hydrogen sulfide associated with petroleum and (or) generated by a reaction between hydrocarbons and sulfate ions. Hydrogen sulfide reacted with iron oxide to form pyrite and with oxygen to form sulfur. Pyrite cement occurs in zones that overlie pre-Permian faults, oil productive areas, and that are elongated along structural trends. Zone boundaries are sharp and nearly vertical. The average pyrite content of mineralized sandstones is 3 percent. Pyrite occurrences show that petroleum-bearing fluids were introduced into Permian rocks by vertical movement along high-angle normal and reverse faults that cut reservoirs at depth and intersect unconformities at the base of the Permian section.