As used in this article:

1. (a) “Surface zone” means the zone which lies above a plane which is 500 feet below the surface of the land.
2. (b) “Subject land” means that area occupied by the particular described surface and surface zone for which plaintiff seeks to terminate the leasehold right of entry and occupation.
3. (c) “Lease facilities” means storage tanks, wash tanks, separators, heaters, and other facilities reasonably necessary for the production of oil or gas, including secondary recovery operations.