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 is 0.
See also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology