What does the assessor's office say about your ownership? 1/2 or full OGM? They don't always have the correct information but that is a place to start.
Did you have a title search when you acquired the tract?
About the acreage: sometimes a recent survey shows a different acreage than the original survey which might have been in the 1800s.
Make sure that you have an attorney familiar with oil and gas leases look over your lease before you sign to help with the wording on what you are saying you own about this tract. You could ask the landman or landmen who are giving you these current offers to share what they have about your title.
I AM NOT an attorney! but a layman who has searched my own title in another county and know that sometimes it is many decades ago that a partial mineral reservation was done and subsequent leases don't mention it.