Fourteen natural fission reactors are known to date in three different ore deposits located in the Franceville basin. The most famous and the first discovered are those in the Oklo uranium ore deposit. Petrographic, geochemical, and oxygen isotope studies have been performed in an attempt to answer these questions: (1) what are the main characteristics of the uranium ores in which fission reactions took place? (2) what are the processes that created such exceptionally high-grade ores? and (3) what are the effects of such fission reactions on the host rocks?The ore which hosts the fission reactions at Oklo is characterized by a gangue of clay minerals, whereas the normal ore of Oklo consists of sandstones. These clay minerals are the result of hydrothermal alteration of sandstone; hydrothermal circulation was induced by the heat produced by fission reactions. The reactor ores are further characterized by very high uranium contents (20-60%) and high uranium accumulation. By means of silica depletion and by uranium enrichment of the normal ore (without uranium depletion), hydrothermal convective circulation, induced by the fission reactions, created very high grade ore next to the reactors. Hydrothermal processes are thus critical to the propagation of the fission reactions.