Permian Basin Royalty Trust is an express trust. The Company's Waddell Ranch Properties has mineral interests in the Waddell Ranch. Its Waddell Ranch properties have proved reserves in six fields: Dune, Sand Hills (Judkins), Sand Hills (McKnight), Sand Hills (Tubb), University-Waddell (Devonian) and Waddell. As of December 31, 2016, the Waddell Ranch properties contained 794 gross (349 net) productive oil wells, 151 gross (64 net) productive gas wells and 250 gross (102 net) injection wells. Burlington Resources Oil & Gas Company LP operates its Waddell Ranch properties. In the six fields, there are 12 producing zones ranging in depth from 2,800 to 10,600 feet. Most prolific of these zones are the Grayburg and San Andres, which produce from depths between 2,800 and 3,400 feet. Also productive from the San Andres are the Sand Hills (Judkins) gas field and the Sand Hills (McKnight) oil field, the Dune (Grayburg/San Andres) oil field, and the Waddell (Grayburg/San Andres) oil field.