One may restrict a search for wells drilled to a specified Geologic Unit or formation (or a range of units) penetrated at the Total Depth of a Well Event.
Select formations using the dropdown input boxes (From: and To:).
The default values for the TD Formation input boxes return every well in the PDMS:
| From: | Surface | Include wells with any formation beginning immediately below the ground surface. |
| To: | And Deeper | Include wells drilled to any formation deeper than the "From" formation or ground surface |
Example selection of wells falling within a specified range of geologic units:
| From: | Ste. Genevieve | Exclude wells with TDs in formations above the Ste. Genevieve Limestone |
| To: | And Deeper | Exclude wells with TDs in formations below the New Albany Shale |
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.