petrophysics · porosity
Gas-Corrected Neutron-Density Porosity
φ = √[(φN² + φD²) / 2] + gas flag when φD > φN + 0.04
click formula to derive ↑
Inputs
fraction
fraction
Description
Extends the standard neutron-density crossplot porosity to explicitly detect and quantify the gas effect by monitoring the φD−φN crossover. The gas saturation estimate from crossover is a quick-look indicator only and not a substitute for a full Archie Sw calculation, but it is valuable for flagging gas-bearing intervals during real-time logging.
Variables
| Symbol | Unit | Description |
|---|---|---|
| φxp | fraction | Crossplot Porosity |
| Sg* | fraction | Gas Saturation Estimate |
| φN | fraction | Neutron porosity from the log. In gas zones, φN reads anomalously low (low hydrogen index of gas), creating the characteristic gas crossover where φN < φD. |
| φD | fraction | Density porosity from the log. Gas reduces bulk density, so φD reads high in gas zones. The crossover condition (φD > φN) is the primary gas indicator from logs. |
Assumptions
- Both neutron and density logs are run over the same depth interval and are depth-matched within 0.5 ft
- No significant borehole washout or heavy mud effect that would independently shift φD or φN
- The gas crossover is due to actual reservoir gas, not a lithology artifact (e.g., anhydrite has similar crossover to gas)
Limitations
- The Sg estimate from crossover is a qualitative quick-look; it does not replace Archie or shaly-sand Sw calculations
- Crossover can appear in clean anhydrite or evaporite sections — always confirm with lithology (PEF) logs before interpreting as gas
- In gas condensate or retrograde condensate zones, the crossover may be smaller than expected due to intermediate fluid hydrogen index
Use Cases
- → Real-time gas zone flagging during drilling: Monitor φD−φN crossover in real time while drilling to quickly identify gas-charged intervals for immediate wellsite decision-making.
- → Gas column mapping in a well series: Track crossover magnitude across multiple wells to map the extent and quality of a gas column; stronger crossover generally correlates with higher free-gas saturation.
- → Identifying residual gas zones: Residual gas after a water flood or depletion shows a smaller crossover (4–8 pu) vs. commercial gas (8–15 pu) — useful in evaluating uphole bypassed-pay candidates.
Related Calculations
Region Notes
Permian Basin
Wolfcamp and Spraberry gas sands: typical crossover 4–12 pu in productive intervals; crossover > 8 pu often correlates with Sg > 50% from pressure tests.
Gulf of Mexico
Unconsolidated GOM gas sands: crossover often 8–15 pu in major gas pay; neutron needs shale correction before crossover is reliable (raw NPHI includes clay-bound water).
Eagle Ford
The tight organic mudrock matrix creates a subtle background crossover of 1–3 pu even in oil windows; true gas crossover is > 5 pu and confirmed with resistivity increase.
Haynesville
Dry gas shale: crossover 5–15 pu in productive intervals; however the high clay content inflates NPHI independently, so always apply a shale correction to NPHI before computing crossover.
Global
Always overlay φN and φD on the same track in linear scale (0–0.40 fraction) to visually identify crossover; the scaled overlay is the most intuitive gas indicator display.
References
Primary source
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