Although Pennsylvanian black shales in Kansas have been studied extensively on outcrop for many years, less is known concerning their variability in properties in the subsurface. The black shales are correlated easily for long distances on gamma-ray logs as "hot shales," although in places they can grade laterally into less distinctive gray "phantom black shales." Nuclear log suites provide several petrophysical measurements that are a useful data source for petrologic studies in the Kansas subsurface. Spectral gamma-ray logs partition natural radioactivity between contributions by uranium, thorium, and potassium sources. The photoelectric factor is a direct function of aggregate atomic number. The neutron porosity curve is primarily a response to hydrogen content. Studies of this nuclear log information for Pennsylvanian black shales in successive cyclothems allows systematic patterns in both space and time to be discriminated. Geological interpretations can then be extended from the areally restricted outcrop to a regional framework.