Well logging, also known as borehole logging is the practice of making a detailed record (log) of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought tothe surface or on physical measurements made by instruments lowered into the hole.
The oil and gas industry records rock and fluid properties to find hydrocarbon zones in the geological formations intersected by a borehole. The logging procedure consists of lowering a logging tool on the end of a wireline into an oil well or hole to measure the rock and fluid properties of the formation.
Logging tools developed over the years measure the electrical, acoustic, radioactive, electromagnetic and other properties of the rock and their contained fluids. Well logging is performed at various intervals during the drilling of the well and when the total depth is drilled, which could range in depths from 300 m 8.000 m (1,000 ft to 25,000 ft) or more.