GUIDE
Well log curves explained
In short
- Each curve has a usual mnemonic, unit and track scale.
- Resistivity is plotted on a logarithmic scale; most other curves are linear.
- Calibrating a scan correctly depends on knowing each curve’s printed scale.
| Curve | Common mnemonics | Unit | Typical track scale | Scale type |
|---|---|---|---|---|
| Gamma ray | GR | API | 0–150 | Linear |
| Spontaneous potential | SP | mV | Shifted range, e.g. −80 to +20 | Linear |
| Resistivity | ILD, LLD, RT, RES | ohm·m | 0.2–2000 | Logarithmic |
| Bulk density | RHOB | g/cm³ | 1.95–2.95 | Linear |
| Neutron porosity | NPHI, CNL | v/v or % | 0.45 to −0.15 (reversed) | Linear |
| Sonic | DT | µs/ft | 140–40 (reversed) | Linear |
| Caliper | CALI | in | 6–16 | Linear |
These are typical values only. Always read the scale printed on your own log header.
What each curve tells you
- GR separates shale (high) from cleaner sand or carbonate (low).
- SP indicates permeable beds against a shale baseline.
- Resistivity responds to fluid content: higher values suggest hydrocarbons or tight rock, lower values suggest saline water.
- Density and neutron together help estimate porosity and distinguish lithology; gas often shows as a crossover.
- Sonic relates to porosity and compaction.
- Caliper flags washouts where other measurements may be unreliable.
Why this matters for digitizing
When you digitize a scanned log you enter the left and right values of each track and choose a linear or logarithmic scale. Reversed scales (for neutron or sonic) are entered with the higher value on the left. See how to calibrate a log curve scale.
Related guides
Try the sample log in your browser · Back to the introduction