ABSTRACT

Gamma Ray Spectral Logs (spectralogs) from forty wells, drilled through the Permian Chase Group strata in Stevens County, southwestern Kansas, were analysed to assess their utility, in this particular stratigraphic and geographical situation, as a tool in the recognition of depositional environments, clay mineral identification, and shale volume estimation.

Comparison of depositional environments identified in three Chase Group cores with those interpreted from the spectrolog data in the same wells showed that spectrologs are useful in the study area in identification of major environments, but fell short in recognition of subenvironments.

Eighteen samples taken from the cores were analysed by x-ray diffraction, and the results compared with those obtained from an interpretation of the spectralog data at the same points. In these instances the spectralog only was successful in correctly discriminating among clay minerals species in zones where the minerals occurred in sufficient concentration to produce relatively high potassium and thorium spectralog readings.

Shale volume estimates were calculated using both the natural gamma ray readings of a conventional Gamma Ray log, and the spectralog thorium readings. Comparison of these calculations with the lithologies identified in core at the corresponding points indicates that, in these instances, the spectralog interpretation provides a more accurate indication of the amount of shale present.