The Geologic Unit or formation penetrated at the Total Depth of a Well Event.
The term "formation" is used in the common petroleum industry vernacular and does not necessarily denote formal stratigraphic nomenclature.
In the past, some IGS geologists would designate a formation nearly reached as the TD Formation, on the logic that anyone interested in a particular formation might also be interested in information about the overlying unit. Although this practice is no longer followed, the PDMS contains some wells handled in this way. No systematic effort has been made to eliminate this minor inconsistency.