"Petroleum production in Japan proper and Formosa declined gradually during World War II because of the concentration of effort on rehabilitation and exploitation of the East Indies oil fields. Production from Japanese concessions in north Sakhalin ceased at the Katangli field in 1939 and at the Okha field in 1943. Exploration practically stopped in Japan proper and Formosa during World War II, but was maintained in Manchuria until the capitulation of Japan. Exploration in both south and north Sakhalin ceased just prior to World War II. Intensive drilling and lack of exploration in Japan proper during World War II has left Japan with practically no undrilled reserves in sight."