Partial Termination. At the end of the primary lease term, this Lease shall be continued in full force and effect as to the entire Leased Premises if prior to the end of the primary lease term Lessee has initiated, and after the end of the primary lease term, continues to prosecute drilling and completion operations with a cessation of no more than 180 days between completion of one well and the commencement of drilling operations on the next well. After the end of the primary lease term, when more than 180 days elapses after a drilling or completion operation ends without another drilling operation commencing, then this Lease terminates for all portions of the Leased Premises except for each Spacing Unit acreage that contains: (a) a producing well, but only so long as such well continues to produce oil, gas, or other associated hydrocarbons in paying quantities; (b) a well that has produced but is shut-in under the terms of the Lease agreement; however, a shut-in well under the terms of this Lease will only hold the Spacing Unit as defined in this Section, not the entire original Leased Premises; or (c) a well that is undergoing rework operations under the terms of the Lease; however, reworking under the Lease will only hold the Spacing Unit as defined in this Section, not the entire original Leased Premises. A Spacing Unit is defined as any amount of acres approved by the Railroad Commission of Texas to obtain maximum allowable production for an individual well in the area of the Leased Premises. If no Spacing Unit for the area has been established, then a Spacing Unit will be defined by the statewide rules of the Railroad Commission of Texas. If this Lease is terminated as to a portion of the Leased Premises in accordance with this Section, Lessee shall nevertheless continue to have the right to lay pipelines, to build roads and to have full rights of ingress and egress on, over and across all lands originally covered by this Lease until this Lease terminates as to all lands.