petrophysics · porosity
Neutron-Density Porosity
φ = √[(φN² + φD²) / 2]
click formula to derive ↑
Inputs
fraction
fraction
Description
Crossplot porosity combines neutron and density logs as a root-mean-square average, reducing the lithology and gas-effect bias present when either log is used alone. It is the standard porosity curve for mixed-lithology and gas-bearing intervals.
Variables
| Symbol | Unit | Description |
|---|---|---|
| φ | fraction | Neutron-Density Porosity |
| φN | fraction | Neutron porosity from the log, in fractional units (e.g., 0.20 for 20%). Reads high in gas zones due to the low hydrogen index of gas. |
| φD | fraction | Density porosity from the log, in fractional units. Reads low in gas zones — separation from φN (crossover) is the classic gas indicator. |
Assumptions
- Neutron and density logs are run over the same interval and are properly depth-matched
- Matrix lithology assumed in the density-to-porosity transform is representative of the formation
- Borehole conditions (no significant washout) are good enough that both logs are reliable
Limitations
- Underestimates true porosity in gas-bearing zones because neutron porosity is suppressed by low hydrogen index
- Inherits any lithology bias from the density porosity transform used upstream
- Not valid in heavy/viscous oil zones, where neutron response can mimic a gas effect
Use Cases
- → Gas zone identification: Plot φN and φD together; separation where φD > φN (crossover) flags gas-bearing intervals for further evaluation with Sw and net pay calculations.
- → Standard porosity for mixed lithology: Use as the default porosity input to Archie Sw and net pay cutoffs when lithology varies through the interval and a single-log porosity would be biased.
- → QC for density matrix selection: Large, systematic crossover across a known water-bearing zone suggests the matrix density (ρma) used to compute φD is incorrect and should be revisited.
Related Calculations
Region Notes
Permian Basin
Wolfcamp and Spraberry benches commonly show 2-4 pu of N-D crossover in gas-charged intervals; the combined φ is the default porosity input to Sw in these plays.
Gulf of Mexico
Unconsolidated Miocene/Pliocene sands require shale-corrected NPHI before crossplotting — raw crossover here can reflect clay-bound water rather than gas.
Global
In carbonates, cross-check against a sonic-derived porosity to help separate lithology effects from gas effects — limestone vs. dolomite matrix choice strongly biases φD.
References
Need geoscience support? BauerCalc is just one of the tools we build. If you're looking for independent expertise in well planning, geosteering, reservoir characterization, or opportunity evaluation, visit BauerSubsurface.com.