Core sample is a cylindrical rock plug obtained during well core drilling.

The core sample is an important geological document used for geological well section investigation (lithology, stratigraphic age, modes of rock occurrence), determination of mineral resources conditions of a field. Drilling performance is represented by continuous core recovery, that is the percentage of the core length and the length of a drilled rock interval. It is quite rare when 100 per cent of the core sample can be retrieved. The core sample recovery rate less than 60 per cent is considered as a fault in well operations.