PetrophysicsRock Properties
Gamma Ray Log Shale Index Formula
Gamma Ray Log Shale Index calculates gamma ray shale index for rock properties workflows in petrophysics.
How engineers use this formula
Use this formula when the listed inputs (GR_cl, GR_sh, GR) are known and the assumptions behind the cited rock properties relationship match the engineering case being checked.
Assumptions
- Input values are representative for the well, reservoir, fluid, or equipment case being evaluated.
- The declared units match the field-unit constants used in the formula.
- The cited formula applies to the selected petroleum engineering workflow.
Limitations
- The calculation does not replace a full engineering model or operating procedure.
- Accuracy depends on the source correlation, assumptions, input quality, and unit consistency.
Common mistakes
- Mixing unit systems without converting the inputs.
- Using default example values as field recommendations.
- Applying the formula outside the source assumptions.
Default example
Using the default inputs, I_GR equals 0.4 fraction.
GR_clAPI
20
GR_shAPI
120
GRAPI
60
Inputs
GR_cl
APIGamma Ray Response in Clean Beds
GR_sh
APIGamma Ray Response in Shale Beds
GR
APIGamma Ray Response in Zone of Interest
Outputs
I_GR
fraction
Gamma Ray Shale Index
GR
API
Gamma Ray Response in Zone of Interest
GR_cl
API
Gamma Ray Response in Clean Beds
GR_sh
API
Gamma Ray Response in Shale Beds
Source and review
reviewedCore Laboratories. 2015. Formation Evaluation and Petrophysics, Page 63.
Source