GUIDE
What is a well log?
In short
- A well log is a record of measurements made down a borehole, plotted against depth.
- Common logs include gamma ray, resistivity, density, neutron, sonic, SP and caliper.
- Logs exist as scanned images (raster) or as digital curve data such as LAS files.
The basic idea
When a well is drilled, instruments are lowered on a cable or run with the drill string and measure the formations along the borehole. The measurements are recorded against depth and drawn as curves in side-by-side tracks. Geologists and engineers use them to identify rock types, pick formation tops, estimate porosity and fluid content, and correlate between wells.
Common log types
- Gamma ray (GR): natural radioactivity, in API units. Clean sandstone and carbonate read low; shale reads high.
- Spontaneous potential (SP): a small voltage, in millivolts, that separates permeable beds from shale when the mud and formation water differ in salinity.
- Resistivity: how strongly the formation resists electric current, in ohm·m, usually on a logarithmic scale. Hydrocarbon-bearing and tight rock read higher than saline water-filled rock.
- Bulk density: in g/cm³, used with the rock's matrix density to estimate porosity.
- Neutron porosity: responds to hydrogen content. Compared with density it helps identify lithology and gas.
- Sonic: the slowness of sound through the rock, in microseconds per foot, related to porosity and compaction.
- Caliper: the hole diameter, which shows washouts and affects the quality of other logs.
Raster logs and digital logs
Older logs are often held as paper prints, scans or PDFs. These are raster logs: a picture of the curves, with no numbers behind them. Modern logs are delivered as digital data, most commonly in the LAS format. To analyze a raster log with software you first convert the scan into digital curves. See raster vs digital well logs for the trade-offs.
Reading a log
- Find the depth scale and the units (feet or metres, measured depth or subsea).
- For each track, read the left and right scale values and check whether the scale is linear or logarithmic.
- Look for changes in log character that mark a change of rock type, which is how formation tops are picked.
- Compare curves: for example, low gamma ray with high resistivity can indicate a clean, possibly hydrocarbon-bearing zone, but always confirm with other data.
This guide is a general introduction. Interpretation depends on the basin, tool types and borehole conditions.
Related guides
Try the sample log in your browser · Back to the introduction