I’m sorry for your loss. That bit of information changes things. Do you know if title to the minerals was in his name individually, a trust or a business entity? Did he leave a will?
It sounds like his estate needs to be probated if these minerals weren’t in a trust or a business entity.
First thing I would do is I’d find out as much information as you can and then seek help from an Oil and Gas Attorney licensed in the state of Texas.