Sounds like you're on your way!
If they are User Friendly companies, ask them for copies of your Oil and Gas Leases, Royalty Division Orders and Source Deeds (where your family acquired the interests). Those are good, basic documents to begin your files with.
Think of Mineral Interests as being under the surface.
Think of Royalty Interests as being above the surface.
There are several different types of Royalty Interests.
There are (or can be) Non-Participating Royalty Interests that are permanent, severed from the Mineral Interests either by sale or by reservation in a sale (good examples of "Source Deeds"). The "Non-Participating" part means that the owner doesn't have any Executive Rights (the rights to negotiate or sign anything). They just get Royalties from the sales of any produced oil and/or gas.
There can be Term Royalties that are the same as I just described, but that only last for a set period of time, such as the length of a lease or, in a few rare occasions, a set number of years unless production is established (typically, once production is established the "term" is interrupted until production ceases).
Then there are the Royalties that are reserved to the Lessor (Mineral Owner) that signs an Oil and Gas Lease. They last as long as the lease does, too.
There is also a hybrid Non-Participating Mineral Interest, where the owner doesn't have any Executive Rights, so they don't negotiate the lease. But they still get their share of the (Signing) Bonus Payment, and Annual Rentals (if any), in addition to their share of the Royalties from any production.
In Oklahoma, you will run into situations where the Executive Rights, Mineral Ownership, Rights to receive Bonus Payments, Rights to receive Annual Rentals and Royalty Ownership are each owned by different parties. Each sometimes owned by multiple parties.
I don't know what in the world they were thinking when they started all of that. It's a real mess!
You may be able to access the County Clerk's Records for your County via:
County Clerks Offices take care of anything that effects Real Property (Oil and Gas Interests are considered Real Property). Deeds, Deeds of Trust, Oil and Gas Leases, Judgments, Liens, anything that effects ownership of Real Property.
Everything that established or has ever effected your family's Mineral and / or Royalty Interest Ownership will be in the County Clerk's Records except for Probate Matters (Wills, etc), and County and District Court Matters.
One slight caveat about County and District Clerk's Records: Any Judgment rendered in any Court that effects the ownership of Real Property, a copy of the Judgment has to be filed in the County Clerk's Records. The Case Files or "Jackets", where you can read through the entire process of a Court Case, you have to go the the County and District Court Clerks' Offices to review.
Be forewarned, this type of research can be addictive.
Hope this helps -