Neutron-Density Crossplot
Plot NPHI vs RHOB with sandstone, limestone, and dolomite reference lines. Identify lithology and detect gas effects.
Log Readings
Likely Lithology
--
Density Porosity (Limestone Matrix)
--
Gas Flag
--
Crossplot — NPHI vs RHOB
How this was calculated
Lithology lines: Based on standard Schlumberger chart CP-1 reference points. Each line connects the matrix point (zero porosity) to the 100% porosity point (fluid point).
Matrix points: Sandstone (NPHI=-0.02, RHOB=2.65), Limestone (0.0, 2.71), Dolomite (0.02, 2.87).
Fluid point: NPHI=1.0 (or effectively the high-porosity end), RHOB=1.0 (freshwater).
Gas flag: Triggered when NPHI is significantly less than density-porosity (neutron-density separation), indicating light hydrocarbon or gas effect.
Lithology ID: Determined by proximity to reference lines using perpendicular distance.
Related Tool
Vshale Calculator
Compute shale volume from gamma ray using 5 methods.
Calculate Vshale →Related Tool
Pickett Plot
Determine Rw and cementation exponent from porosity-resistivity data.
Open Pickett Plot →Need help with petrophysical analysis, lithology identification, or AI-driven log interpretation?
Book a free strategy call →Understanding the Neutron-Density Crossplot
The neutron-density crossplot is one of the most widely used petrophysical tools for lithology identification and porosity determination. By plotting neutron porosity (NPHI) against bulk density (RHOB), the interpreter can distinguish between sandstone, limestone, and dolomite, as well as detect the presence of gas or light hydrocarbons.
Each pure lithology follows a characteristic line on the crossplot, connecting its matrix point (zero porosity) to the fluid point (100% porosity). Points that fall between lithology lines indicate a mixture of minerals. Points that plot above and to the left of the sandstone line (lower NPHI, lower RHOB) often indicate gas effect, where the light hydrocarbon causes the neutron tool to read low and the density tool to read low.
This crossplot assumes a simple two-mineral or single-mineral system with freshwater-filled pores. In real wells, shale content, heavy minerals, salt mud, and complex fluid mixtures can shift data points away from the reference lines.
All calculations run in your browser. Built by Groundwork Analytics. Get in touch or email info@petropt.com.