Shale Volume Calculation

[Muresko]

Hi everyone,

At the moment I'm trying to figure out how to estimate shale volume by using the following formula: Vsh = (GRl - GRmin) / (GRmax - GRmin).
Regardless simplicity the formula itself, i cannot understand how to derive GRl from a GR log. A gamma ray curve is represented by tens or hundreds values of SGR in an area of interest.
How do we come to only one value from the range?

Thanks.

[ali23]

Whit this formula GRI=(GR-GRmin)/(GRmax-GRmin) you can calculate linear GRI for every depth level. You need to read GR value from the log, and chose single value for GRmin and GRmax

[Muresko]

Thank you Ali23 for your answer, I'm pretty clear with GRmin and GRmax. I cannot get how to read GRlog value from the log.
For example, we need to calculate shale volume in 100 feet section. We set shale and sand baselines to get GRmax and GRmin respectively, but what is GRlog value if gamma ray varies from 40 to 90 and then back to 40 API within the section?
Why GRlog becomes 70, not 50?

[OBond]

Dear Muresko, maybe You'd better try it with the thickness-of-the-bed-weighted average.