Entries existing on February 7, 1925, and allowed prior to April 1, 1924, under the Stock Raising Homestead Act of December 29, 1916 (Thirty-ninth Statutes at Large, page 862) [43 U.S.C. 291 et seq.], for land withdrawn as valuable for oil or gas, but not otherwise reserved or withdrawn, are validated, if otherwise regular: Provided, That at date of entry the land was not within the limits of the geologic structure of a producing oil or gas field.