Orphaned wells that predate regulation often go unnoticed because their locations were never recorded.  One of the fundamental issues with orphan wells is that data concerning their existence is, by definition, inexact. Data collection in the early years was incomplete at best and many owners and operators were lost in the course of history as the industry cycled through boom and bust.

States have long recognized their duty to locate and plug orphaned wells and to work diligently to ensure the mechanical integrity of idle wells -- both to protect the environment and to minimize the state's liability due to an operator's financial insolvency. 

Links to orphan well location programs
Michigan Orphan Well Program
Venango Pennsylvania Senior Environmental Corps (PaSEC)