# Density–neutron crossover: a worked example

Explore a gas response while keeping lithology, calibration and display scales visible.

Danila Karnaukh | v4 | 2026-09-29

[Open the crossplot →](../../crossplot/index.html?example=density-neutron)[Open the log overlay](../../logs/index.html?example=four-track)

## What the example shows

In our fictional gas sandstone, low fluid density reduces RHOB and low hydrogen index reduces NPHI. Shale has a stronger neutron response from bound water. The oil and water zones can overlap in the porosity crossplot; the assigned FACIES curve colours the known model classes. These colours are labels supplied with the synthetic data, not an interpretation performed by the tool.

## Use compatible scales

The overlay uses RHOB from 1.9 to 2.9 g/cm³ left to right, and sandstone NPHI from 0.45 to −0.15 v/v. These conventional sandstone-compatible scales are intended for a quartz/fresh-water reference. Our model uses brine, shale and tool-response adjustments, so exact coincidence is not expected in every interval.

The crossplot uses X=NPHI from −0.02 to 0.45 and Y=RHOB from 2.75 at the bottom to 1.95 at the top. A gas-related shift can then be explored without confusing the crossplot axis direction with the log-overlay direction.

## What separation does not prove

A crossover alone does not establish gas. Check lithology, neutron matrix calibration, borehole condition, invasion and other measurements. In this example, GR and resistivity support the specified model. The tight water-filled layer at 960–968 m has high resistivity because its porosity is low.

![Synthetic density–neutron crossplot with FACIES colour scale and density increasing downward](../../examples/synthetic-gas-oil-water/density-neutron.png)FACIES: 1 shale; 2 gas sandstone; 3 oil sandstone; 4 cemented sandstone; 5 brine sandstone.

## Read the underlying references

- [SLB: Measuring Porosity Downhole](https://www.slb.com/resource-library/oilfield-review/defining-series/defining-porosity) — general density/neutron responses.
- [SLB Energy Glossary: sandstone-compatible scale](https://glossary.slb.com/terms/s/sandstone-compatible_scale) — the conventional overlay reference.

The plots here are original synthetic examples. No proprietary chartbook image or calibrated vendor interpretation grid is reproduced.

[Download the data, model and settings](../../examples/synthetic-gas-oil-water/index.html) · [Check units and calibration](../../docs/curve-units/index.html)