Hydrogenation of Estonian oil shale and shale oilKogerman, Paul; Kopvillem, Jaan1932 https://www.ester.ee/record=b2170142*est

On the chemistry of the Estonian oil shale "kukersite" : a monographKogerman, Paul1931 https://www.ester.ee/record=b4272601*est

Physical properties of Estonian shale oilsKogerman, Paul; Kõll, A.1930 https://www.ester.ee/record=b1455301*est