Just inherited Sec 3 Block 106 T&STL RY. county survey

A friend of mine just inherited some interests in Pecos County. I was hoping someone could give me some insight into the area. I’m also curious about " below the depth of 3500 ft from the surface". This was noted in the conveyance document. Any help would be greatly appreciated. I believe Chevron has a well on the property that pays a small amount.

Section 3, Block 106 out of the T&STL Ry. Co. Survey, Abstract 3726, Pecos County, Texas below the depth of 3,500 feet from the surface of the ground, being 651.4 acres of land, more or less,

They acquired “deep rights,” which are much more valuable than anything above 3,500’.