It is not uncommon in areas where there are "minerals" (gas, oil, coal, etc.) to separately sell the land (the "top" layer of the earth) to someone and yet retain the rights to whatever minerals may be underneath that top layer of dirt. People also sell rights to "air space" over their land but retain the land, retain water rights, retain easement rights to cross-over or build roads, etc. etc.
Get an attorney asap to look at the paperwork. If this person really is an XTO rep, that means that somehow, somewhere along the line, someone took care of probating your great-grandmother's estate and had title to the mineral rights put into the names of the heirs in being and all future heirs, known and unknown. On the other hand, this person could be an "heir finder" who is not being honest with you and is withholding critical information. There are companies that go out and look for the heirs of people who die "intestate" (that is, without a will) but who leave property behind. In such cases, the property is held in trust by the state in which the person died - the property "escheats" to the state. After a certain period of time, usually many years (varies from state to state, if no heirs come forward to file a claim for the property, the state gets the money and it goes into the general fund. These heir-finding companies will "help" you get your share of the property - FOR A FEE, usually a hefty percentage. Of course, they are careful NOT to give you any useful information so you could figure out for yourself in which state the distant relative died so that you could contact the Sec/State of that state directly and make a claim for the escheated assets! But in your case, if you know where your great-grandmother died and this IS an heir-finder company, it would behoove you to get an attorney to investigate further and if there is property in the custody of the Sec/State, make a claim.
So, you need to know with whom you are dealing. Turn it over to your attorney or better yet, one who specializes in leases of exploratory rights and royalties. A specialist in the field will be able to tell you whether the contract terms are kosher. If there is natural gas under that land, it's not going anywhere unless someone drills a well next store, so proceed at a deliberate pace.
BTW, this kind of windfall has never happened to me, but it did happen to someone I used to work with; she was contacted by an heir-finder company and netted about $4,000 as her share of an escheated estate - and found out about second and third removed cousins she never knew existed!