Bigfoot,
Technically speaking, minerals are included in the value of the surface. The value is $0. Once the lease is signed, you divide the property into two distinct estates. Mineral and Surface. The lease conveys the minerals (mineral fee-determinable interest) to the operator. In exchange, you now have a royalty interest.
Royalty, Overriding, and Working Interest are assessed for property taxes. We call this "mineral" interest but they are actually "variations" of mineral interest (per court cases and Attorney General opinions), to be exact. We use the term "mineral" loosely.
If you have production in Texas, you should have mineral tax accounts in your name at the appraisal district and tax office. It is only taxable if the market value is over $500 total. This is not based on actual income, but rather the market value of the income stream (discounted cash flow). They project income (including decline of production) in future years, based on previous year's average monthly sales price, and then use a discount factor back to present worth. I believe the typical estimate is using 5 years or projected income, and then include the other factors I mentioned. Bottom line, make sure if you have royalties coming in, that you have associated "mineral" (royalty) tax accounts in the county (or counties) you have production in. Sometimes we have wells that cross between counties, which results in bills arriving from one or both counties. That $500 rule sometimes eliminates the appraisal on one side. Some wells may be 90% in one county, and 10% in another. Another issue I see is that some taxing jurisdictions (like a school) may be collected by a county different than the one your minerals are in. For instance, you get a tax bill from the county your minerals are in, for all taxes except the school. Then you get a tax bill for only the school taxes from the next county over. This happens frequently. Let me know I can answer any additional questions. You can also friend request me and I can send you some PDFs and other guides that go into much greater detail.