Oregon Revised Statutes Mineral Resources § 520.310

(1) The governing board of the State Department of Geology and Mineral Industries by order may provide for the unit operation of a pool or pools or parts thereof that embrace a unit area established by a previous order of the board. The order, in providing for the allocation of unit production, shall first treat as a single tract the unit area previously established, and the portion of the unit production so allocated thereto shall then be allocated among the separately owned tracts included in the previously established unit area in the same proportions as those specified in the previous order.

(2) An order may provide for unit operations on less than the whole of a pool where the unit area is of such size and shape as may reasonably be required for that purpose, and the conduct thereof will have no adverse effect upon other portions of the pool.