petrophysics · rock physics
M-N Lithology Plot
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Inputs
μs/ft
μs/ft
g/cc
g/cc
fraction
fraction
Description
Computes the M and N lithology-identification parameters from sonic, density, and neutron logs. M and N each normalize a pair of porosity-sensitive logs against a common fluid point, giving two lithology-sensitive numbers that are largely independent of porosity itself — plotting them together separates the three common matrix minerals (sandstone, limestone, dolomite) into distinct clusters on an M-N crossplot.
Variables
| Symbol | Unit | Description |
|---|---|---|
| M | — | M Value |
| N | — | N Value |
| Δt | μs/ft | Compressional sonic slowness from the log at the depth of interest. |
| Δtfl | μs/ft | Sonic slowness of the pore fluid — use 620.1 μs/m (189 μs/ft) for fresh mud filtrate, the standard M-N reference value. |
| ρb | g/cc | Formation bulk density read directly from the density log. |
| ρfl | g/cc | Density of the fluid occupying the pore space — about 1.0 g/cc for fresh mud filtrate. |
| φN | fraction | Neutron porosity from the log at the depth of interest, in limestone-matrix units. |
| φNfl | fraction | Neutron porosity of the pore fluid — 1.0 (100% apparent porosity) is the standard reference value for fresh water. |
Assumptions
- The formation is liquid-filled (not gas-bearing) — gas depresses both neutron porosity and, to a lesser extent, sonic transit time, shifting M upward and N downward away from the true lithology point
- DTfl, rhoFl, and phiNfl represent the actual pore fluid — the standard fresh-water reference values (620.1 μs/m, 1.0 g/cc, 1.0) apply only when mud filtrate is genuinely fresh water
- Borehole and environmental corrections have already been applied to DT, RHOB, and NPHI before use here
Limitations
- Gas effect shifts a data point away from its true lithology cluster on the M-N plot — cross-check against a gas-corrected neutron-density porosity (phi_nd_gas_corrected) before concluding a low-M reading indicates a light or dense mineral
- Shale shifts M-N readings unpredictably (elevated neutron porosity from clay-bound water, variable density) — apply in clean intervals only, or alongside a Vcl cutoff
- Only separates the three common evaporite/carbonate/sandstone-family minerals cleanly; complex mixed mineralogy (e.g., significant clay, anhydrite, or unusual heavy minerals) requires multi-mineral log analysis beyond this simple two-parameter crossplot
Use Cases
- → Quick-look lithology screening: Compute M and N at multiple depths and compare against known mineral points to screen for lithology changes without a full multi-mineral analysis.
- → Matrix density selection QC: Use an M-N-indicated lithology to sanity-check the matrix density (rhoMa) assumption used in phi_den or the sonic matrix slowness (DTma) assumption used in phi_sonic.
- → Secondary porosity / gas-effect flagging: A data point falling well outside the normal mineral triangle can indicate secondary (vuggy/fracture) porosity in carbonates or an unflagged gas effect worth investigating with phi_nd_gas_corrected.
Related Calculations
Region Notes
Permian Basin
Mixed carbonate-siliciclastic Wolfcamp sections benefit from M-N screening to distinguish dolomitized intervals (lower M, lower N) from clean sandstone before picking a matrix density for phi_den.
Gulf Coast
Predominantly siliciclastic sections rarely need M-N lithology screening for matrix identification, but it remains useful as a gas-effect and shale QC check alongside phi_nd_gas_corrected.
Global
M-N reference mineral points assume fresh-water mud filtrate — in saline mud systems, recompute the fluid point (DTfl, rhoFl) locally before comparing to the standard published mineral cluster coordinates.
References
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