The thermal rock history of any area is manifested in the color of the recovered kerogen, or dispersed organic matter, including spores and pollen. Thermally induced color differences of kerogen viewed in transmitted light can be directly related to oil, gas, and areas barren of commerical hydrocarbon accumulations. The same technique is useful in outlining areas favorable for oil and gas exploration. Laboratory heating experiments provided useful color standards, which, when compared with fossil residues, permit ranking kerogen alteration on a 1 to 5 thermal index scale. Subsidiary evidence that alteration and devolatilization has taken place in the darker colored organic residues is their higher reflectivity. Hydrogenation experiments by the U.S. Bureau of Mines have revealed chemical differences between the structured and amorphous type of kerogen, as well as a decrease in solubility as coal rank increases. Utilization of kerogen color change reveals that the economic basement below which no commercial oil or gas might be found is yet unknown on the Gulf Coast. The largest gas accumulations within the Arkoma Basin of the midcontinent are in areas of strong thermal alteration as determined by kerogen analysis of surface shales and subsurface coal samples.