Lekas, M.A.; Lekas, J.M. & Strickland, F.G. Initial evaluation of fracturing oil shale with propellants for in situ retorting, Phase 2, report, May 1, 1991; (https://digital.library.unt.edu/ark:/67531/metadc1093781/m1/1/: accessed November 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .