WIRELINE PETROPHYSICAL CHEMOSTRATIGRAPHY OF PENNSYLVANIAN BLACK SHALES IN THE KANSAS SUBSURFACE

Pennsylvanian black shales in Kansas have been studied on outcrop for many years as the core unit of the classic Midcontinent cyclothem. These units are now considered to be highstand condensed sections in the sequence stratigraphic paradigm. The black shales are easily correlated for long distances in the subsurface 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 of rock chemistry that are a useful data source for chemostratigraphic studies in the Kansas subsurface. Spectral gamma-ray logs partition natural radioactivity between contributions by uranium, thorium, and potassium sources. Elevated uranium contents in black shales can be related to reducing depositional environments, while comparisons of the potassium and thorium contents give indications of clay mineral composition. The photoelectric factor is a direct function of aggregate atomic number and so is affected by clay mineral volume, clay mineral iron content, and other black shale compositional elements. The neutron porosity curve is primarily a response to hydrogen content. Although good quality logging data are available for many black shales, borehole washout features invalidate readings from the nuclear contact devices, while black shale zones thinner than tool resolution will be averaged with adjacent beds. Statistical analysis of nuclear log information between several black shales in successive cyclothems allows systematic patterns in both space and time to be discriminated.