Tex. Nat. Res. Code § 53.079

On the land office's written request, mailed to the lessee's address as shown on its lease or otherwise properly changed in conformity with the terms of the lease, a copy of a contract for the sale or processing of minerals leased under this subchapter and any subsequent agreement or amendment to the contract shall be filed in the land office within 30 days after the date the land office mails the written request. The land office shall treat a contract, agreement, or amendment filed in the land office as confidential unless otherwise authorized by the lessee.