When you discontinue use of an underground oil storage tank (UST) or when the in-ground tank is found to be compromised, the tank should either removed or secured (technically called "tank abandonment").
In-ground oil tank removal is much, much more than just digging a hole...
Establish the precise location of the existing underground oil tank (UST)
Establish location of potential nearby underground structures and utilities (leach field, sanitary sewer, storm drains, utilities, etc.) -- We call BEFORE we dig!