I would recommend an O&G attorney. Probably many attorneys can handle documenting the actual transaction, but a specialist knows what additional questions to ask. There's frequently more than just the simple conveyance. It's what you don't know that you don't know that always gets you in trouble.

Also, a specialist shouldn't cost any more and may actually be less expensive since this is everyday stuff for them.

I recommend Stephen Moon. This (oil and gas, property/minerals) is his specialty and he's a darn good Aggie and an ethical man.

Texas Law Office of Stephen B. Moon
551 William D Fitch Pkwy, College Station, TX 77845

I think the number is 979-431-0692

Not an attorney, but I think you'll need to have the mineral interests appraised. If the appraised value is higher than the annual gift tax exclusion ($20k per recipient per year I think) the conveyance would be subject to taxes.

If that is the case it might be more efficient for your mother to leave the minerals to her grandchildren in a will, as the estate tax limit is significantly higher and if her assets are less than ~$13MM the transfer would be tax free. If she has substantial assets you could look into a generation skipping trust as well.

I could be crossing some wires, definitely talk to someone well versed in O&G and estate planning.