PETROPHYSICAL IMAGERY OF THE OREAD LIMESTONE IN SUBSURFACE KANSAS

The Oread Limestone is recognized widely as an archtypal Pennsylvanian cyclothem that has been investigated extensively over its eastern Kansas outcrop for more than a century. Knowledge of the geology of the Oread in the subsurface has been restricted almost entirely to drill-cuttings, while wireline logs have provided the correlative framework for mapping structure and thickness. The curves of traditional logs are the time-honored medium for correlation, but the rich data of more recent petrophysical measurements are presented increasingly as image logs which portray geology in novel ways. FMI logs are conversions of multiple microresistivity curves into a high-resolution conductivity image of the borehole wall. MRI logs measure magnetic resonance relaxation times that are presented as contour map images of pore-size distribution. Natural and capture gamma-ray spectra logs estimate elemental concentrations of potassium, thorium, uranium, calcium, magnesium, titanium, aluminum, iron, sulfur, and manganese. Interpretations of these logs in the Oread in south-central Kansas present new opportunities in Pennsylvanian cyclothem research that can be integrated with conventional outcrop studies. As a case in point, log imagery of the anomalously thick and variable “Super-Plattsmouth” regressive limestone (anomalously thick and variable) in Sumner County provides intruiging insights into mound internal architecture.