225 ILCS 725/23.12
The unit area may be unitized or joined with adjoining portions of the same pool or pools, or portions thereof, including a unit area of another or other units and a new unit created for unitized management, operation and further development of such enlarged unit area upon the filing of a petition therefor, and after notice and hearing, in the same manner and on the same conditions, all as herein provided with respect to the creation of a unit in the first instance, provided always that the requisites of Sections 23.5 and 23.8 shall be applicable. Upon issuance of the order by the Director creating a new unit, the new unit so created shall thereupon supersede such existing unit or units. Such order shall not alter the proportionate allocation of unit production among the separately owned tracts included in such previously established unit area or unit areas. Any new working interest owner whose interest by such order is added to the new unit area and who becomes liable for his proportionate share of the unit expense shall not be liable for any unit expense incurred prior to such person's entry in the new unit. At the time such interest is included in the new unit area, an equipment inventory will be made in order to charge or credit such newly committed interest with its proportionate share of capital investment. An oil-in-storage inventory will be taken immediately prior to adding the newly committed interest.
225 ILCS 725/23.12