This is a time of sadness for you so go slowly. There is no single source of such records which is why it is best to save all paperwork. Hopefully you will find some information in your father's records. Start with check stubs from oil companies which will list wells and have an owner number. You can contact each company and ask for current and formerly producing wells, with state and county. Or find 1099 forms from oil companies that reported income and ask for information. Look for division orders for wells which will describe acreage, wells, county and state. Look for property tax statements. Then you can go to county deed records and look or deeds, leases, assignments or other documents in your father's name to get legal descriptions. This is going to be a long process if that acreage is in many states and counties. But the more research you do yourself, the more you will learn and the less it will cost. Think of this as an adventure and mystery to solve. You can also look for unclaimed funds in state treasuries in your father's name which should identify the company which sent in the money.