Density-Neutron Crossplot
Classic lithology identification using bulk density (RHOB) vs neutron porosity (NPHI).
- Sandstone: RHOB ~2.65 g/cc, diagonal trend
- Limestone: RHOB ~2.71 g/cc, steeper slope
- Dolomite: RHOB ~2.87 g/cc, flattest slope
- Gas effect: Neutron-density crossover (NPHI < density porosity)
φD = (ρma - ρb) / (ρma - ρf)
Vp-Vs Crossplot
Identifies lithology and fluid content from acoustic velocities.
- Mudrock line: Vp = 1.5Vs + 1.36 (shale trend)
- Sandstone: Above mudrock line, Vp/Vs ~1.6-1.8
- Carbonate: High Vp, low Vp/Vs ratio (<1.8)
- Fluid indicator: Deviation from mudrock line
Useful for discriminating lithology in clastic and carbonate sequences.
M-N Plot (Mineral Identification)
Combines sonic, density, and neutron logs for mineral identification:
M = (ΔtF - Δt) / (ρb - ρF) × 0.01
N = (φN,F - φN) / (ρb - ρF)
Where ΔtF=189 μs/ft, ρF=1.0 g/cc, φN,F=1.0 (fluid point)
- Quartz: M=0.82, N=0.62
- Calcite: M=0.91, N=0.00
- Dolomite: M=0.70, N=-0.03
- Anhydrite: M=0.65, N=0.00