The shales contain much higher concentration of radioactive minerals comparing to clean sands and carbonates (see Table 1 below).
This is why the most common way to quantify the shale content is the intensity of the natural gamma-ray (GR) emission.
The fist step is to normalize the actual GR-tool readings to the reference values in clean rocks
and pure shales
which is called Shale Index:
I_{GR}(l) = \frac{GR_{log}(l) - GR_m}{GR_{sh} - GR_m} |
where – along-hole depth.
The model parameters and
are calibrated for each lithofacies individually.
The Shale Index is varying between 0 (for non-shally rocks) and 1 (for pure shales) but the actual shaliness may behave non-linearly between these extremes (especially for shallow, young reservoirs).
This can be calibrated based on the available core data.
The table below summarises some popular shaliness models:
| # | Equation | Author | Rock Type | Correlation database |
|---|---|---|---|---|
| 1 | ||||
| 2 | Larionov (1969) | Tertiary Jurassic rocks | West Siberia | |
| 3 | Clavier (1971) | |||
| 4 | Stieber (1970) | |||
| 5 | Larionov (1969) | Older Rocks | West Siberia |
| Fig. 1. Different shales volume models |
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petrophysics / Reservoir Data Logs (RDL) @model
|
