Ratio of pore volume above porosity cut-off to the bulk rock volume:
| {\rm NTG} = \frac{V_{\phi>\phi_0}}{V_{\rm bulk}} |
This value is usually used as a cell property in 3D models to identify which part of the cell volume is actually contributing to the fluid flow and disqualify the cell from calculations if NTG = 0.
See also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology