When two or more separately-owned tracts are embraced within a development unit, or when there are separately owned interests in all or a part of the development unit, then the owners and lessees thereof may pool their interests for the development and operation of the development unit. In the absence of this voluntary pooling, the department, upon the application of any interested person, shall enter an order pooling all interests, including royalty interests, in the development unit for the development and operation thereof. Each such pooling order shall be made after notice and hearing. The applicant or applicants shall have the burden of proving that all reasonable efforts have been made to obtain the consent of, or to reach agreement with, other owners.