This paper is the result of an investigation made to determine an empirical relationship between Gamma Ray curves and core analysis permeability values for wells in the Denver-Julesburg Basin. It is proposed to show that an empirical relationship does exist and to present information and curves to substantiate the belief that we can derive permeabilities from Gamma Ray data. This type of information can be a valuable aid in application to Basin workovers and completion activities.
The majority of Gamma Ray surveys are being taken for correlative information for use in conjunction with perforating of oil and gas pay intervals. All the service companies are now using the Scintillation type instrument. However, a number of currently producing wells have been surveyed with the Ionization type instrument; and in order to make this study comprehensive, both types of equipment are included in this investigation. This work is based on the theory that the permeability of the "D" and"J" sands of the Denver-Julesburg Basin is a function of the clay cementing material found in the pay zone pore space. The idea is not a new one as C.L. Rabe did the preliminary work, and he came to the conclusion that permeabilities could be related to Gamma Ray data. Using Rabe's recommended method for correcting Gamma Ray logs for drag, sensitivity changes, thin bed effects, and converting to a common scale was found to be entirely laborious. For practical reasons, it was decided to check and see if an empirical relationship could be established without going through all the corrections mentioned.
Development of Permeability Curves
All the core analyses of wells that have been surveyed with the Ionization type instrument were compared for similarities in deflection changes and changes of permeabilities. In order to expedite this comparison, the core analyses were plotted on transparent paper using the same footage scale as the Gamma Ray log. It soon became apparent that plotting of these permeabilities would not correlate too well with the Gamma Ray curves. Therefore, it was decided to calculate the average weighted permeability of each foot of the core analysis.