Marathon oil was purchased by ConocoPhillips, where and how do I report this?

Why are you trying to enter anything about that into your tax return?  What are you trying to do?

You should have received at least a K-1 for ConocoPhillips.

• Post this as it is.

You may have gotten one for Marathon Oil.

• If so, post it
• It should be posted as a final return.

If you did not get a K-1 from Marathon Oil

• It may have carried over from last year
• Don't delete it
• Post it as a final return, even if everything else is -0-