Spectral gamma ray data has long been recognized for its usefulness in clay typing and rock typing, including distinguishing organic rich shales and radioactive sands from conventional shales. The use of this data has not been widespread, however, possibly because the analysis often involves point-based ternary diagrams and the results are not easily presented in a depth based log. Colour cubing is a technique for turning the portion of the total gamma ray signal that is due to potassium, uranium, and thorium into distinct and meaningful colour patterns. Colour cubing uses the red-green-blue (rgb) colour axes to depict the changing proportions of three attributes (similar to how a colour tv can create any colour from rgb mixing). Using this colour shading on the total gamma ray curve results illustrates both the total gamma and its composition.
We have found this very useful for a variety of applications. It results in a step change improvement in our ability to recognize markers whendoinginter-well stratigraphic correlation. Sediments from a common source will have a common mixture of radioactive elements, so layers of sediments rich in potassium, uranium, and thorium appear as distinct blue, red & green bands in cross sections, which greatly eases stratigraphic correlation. Whether diluted or concentrated, if the radioactive mixture does not change, the colour will remain the same, telling us the sediments are likely from the same source.
In addition,changingcolour patterns in “shales” provide a quick indication when the shale properties have changed, which may call for different shale end-point parameters when doing petrophysical analysis. In unconventional reservoirs, since toc rich sediments are often rich in uranium, the desirable zones are easily recognizable by their blue tint. The colour representation of shales is also useful for completion design for differentiating rock properties. Rocks with similar colour are likely to have similar properties. The colour patterns also make it possible to distinguish zones which are favorable for completion and fracturing, from others which are not favorable."
Michael J. Sullivan is a Formation Evaluation Advisor working for Chevron in Calgary. Mike has had previous assignments as a Cased and Open Hole Petrophysicist in Angola, Calgary, and Houston, as Team leader for Petrophysics R&D with Chevron Energy Technology Company in San Ramon, Ca., and as Reservoir Surveillance Coordinator for the Tengiz field in Kazakhstan.
He has also held a variety of positions in Petroleum and Production Engineering starting in 1979. Sullivan is a Distinguished Member of SPE, has authored several technical papers, and has been a Distinguished Lecturer with both SPE and Distinguished Speaker with the SPWLA. Email: Michael.email@example.com