Twenty postburn and two control (unburned) coreholes were drilled into a horizontal in situ oil shale retort at the Geokinetics field site, Uinta Basin, Utah. The object of the investigation was to study the mineralogic changes and trace element partitioning resulting from a true in situ burn of Green River oil shale under field conditions. Minerals were examined by X-ray diffraction and optical microscopy; while elemental determinations were performed utilizing X-ray fluorescence, neutron activation analysis, and other analytical chemical techniques. Statistical significance values were used to establish relationships between 13 minerals, oil, water, and 45 elements. The complex mineral assemblage created was a result of rapid intense heating coupled with fluctuating temperatures, gas and fluid pressure, cooled at a shallow depth, composed primarily of solid solutions of silicates plus cations, primarily Ca/sup 2 /, Mg/sup 2 /, and to a lesser degree Fe/sup 2 /. Distinct zones were observed which correspond to temperature thresholds that can be distinguished by increasing degrees of silicification of the carbonate rich raw oil shale. Trace element partitioning closely parallels the mineral assemblages. Leaching studies indicate an increase in soluble salts in zones in which temperatures were only adequate to retort and carbonize the oil shale, with the synthesis of insoluble minerals in the hottest, most intensely altered zones minimizing the extractability of potentially detrimental materials from the residue of in situ combustion.