Deployment	Borehole
Log Name	GR
Measured Property	Intensity of natural gamma ray emission from subsurface rocks
Sensor Type	Scintillation counter
Units
SI	R /s
Oil Metric	μ R /hr = 2.8 ·10^-10R /s
Oil Field	GAPI = 4.2^·10^-10R /s = 1.5^μ R /hr

* – for scintillation detectors

Applications

Sample Logs

(1)	$\begin{array}{l}\displaystyle GR = (1-\phi_e) \cdot GR_{m} + \phi_e \cdot (s_w \cdot GR_w + s_o \cdot GR_o)\end{array}$

(2)	$\begin{array}{l}\displaystyle GR_m = \sum_i GR_{mi} \cdot V_{mi}/V\end{array}$

where

$\begin{array}{l}\phi_e\end{array}$	effective porosity
$\begin{array}{l}GR_w\end{array}$	natural gamma radioactivity of formation water
$\begin{array}{l}GR_o\end{array}$	natural gamma radioactivity of formation oil
$\begin{array}{l}GR_m\end{array}$	natural gamma radioactivity of rocks
$\begin{array}{l}GR_{mi}\end{array}$	natural gamma radioactivity of i-th mineral
$\begin{array}{l}V_{mi}/V\end{array}$	volume share occupied by i-th mineral

The typical GR values for popular minerals is brought on Table 1.

Table 1. GR values for popular minerals
	Rock Type	GR, API
1	Halite (NaCl)	0
2	Coal	0
3	Limestone	5 – 10
4	Sandstone	10 – 20
5	Dolomite	10 – 20
6	Shale	80 – 140
7	Mica	100 – 170
8	Silvite (KCl)	500