A wellbore only conveyance is where the grantor or an oil company only transfers rights to a specific well, but no others. could be two or more wells, but no others. In other words, the grantor would deed the royalties or rights to the Jones No. 1 well, but not the Jones no.2 well or any future well that gets drilled. Could also go the other way, being the Jones No. 2 well or a future well, but not the Jones no. 1 well.
Not very common among mineral owners. Pretty common among sophisticated oil companies who produce from multiple wells.
As to the cimarex wells you’re speaking of, I would need a lot more information.
Typically, in a wellbore only conveyance, you cannot add a lateral or a horizontal well. But, it would depend on the language of the conveyance/deed/assignment.