Mont. Code § 82-11-209
The board may provide by an order for the unit operation of a pool or pools or parts of a pool or pools that embrace a unit established by an order of the board made prior to February 13, 1969. The order, in providing for the allocation of unit production, must first treat the unit area previously established as a single tract, and the portion of the unit production allocated to that tract must then be allocated among the tracts included in the previously established unit area in the same proportions as those specified in the previous order. A new owner whose interest by the order is added to the unit area and who becomes liable for the owner's proportionate share of the costs of unit operations is not liable for any unit operating costs incurred prior to the person's entry in the unit. At the time the interest is included in the unit, an equipment inventory must be made in order to charge the newly committed interest with its proportionate share of capital investment at its current value. An oil-in-storage inventory must be taken immediately prior to adding the newly committed interest.
§ 82-11-209, MCA