Re: Gamma ray log only has values of 30. How to normalize?
Russian Gamma Ray logs are usually recorded in uR/h (micro-Roentgens per hour) rather than the familiar Western API units. They may also be recorded in counts per minute. According to the book "Russian Style Formation Evaluation" edited by Bob Harrison (published by the London Petrophysical Society), the approximate conversion between uR/h to API units is as follows:
1 uR/h ~ 10 to 15 API units, depending on the type of detector used (Geiger counter or scintillation detector).
Therefore, you will need to know the exact tool type used to make the conversion.
Interpretation of Russian style logs is never straightforward, as you need to know a lot of tool, recording and calibration parameters.
However, if what you want is to make a comparison of the Russian
GR log profile with other types of Western GR logs, you can use an approximate conversion unit.
Furthermore, if you are going to compute shale volume from the Russian GR log, you don't even need to convert to API units. You can just use the recorded log and choose the appropriate GR Max and GR Min values to compute the shale volume.
One of the pitfalls of using the GR log to determine a field wide cutoff for reservoir (sand) and non-reservoir (shale) is that most of the time an oil or gas field can have different GR logs recorded by different contractors using different tools at different times in the field. Therefore, you will not have a consistent GR profile with corresponding min and max values for your purpose.
The usual way to get around this problem is to normalize the GR logs, after making all the necessary borehole and environmental corrections on the respective GR logs.
This needs a lot of input data, such as bit size, hole size (from caliper logs), mud weight, mud type (KCl mud), etc which may not readily available.
That is the reason some petrophysicists do not recommend normalization of GR logs.
They recommend to compute shale volume for each GR log using its appropriate GR Min and GR Max values. In a way, this is also a normalization process, which normalizes the GR log into a dimensionless curve, ranging between 0 and 1.
Then a single cutoff the field can be easily determined.
However, even in this case, it will be necessary to at least do the hole size and mud weight corrections, because the GR counts in a washed out shale section can be suppressed by the mud volume around the tool that it can be mistaken as a sand or sandy interval.
Regards,
Ko Ko Kyi
