By the beginning of the twentieth century, the Russian oil industry was dominated by companies such as Shell, the Russian General Oil Company and Tovarishchestvo Nobel, which controlled about 60% of total production. The main oil-producing regions were located near Baku and Grozny, where over 90% of all Russian oil was produced. Prior to the Russian Revolution of 1917, the largest oil production was achieved in the Baku Region in 1901, when specific tonnes of oil were produced. The decline in oil production in 1940 is explained by the presence of many old wells, whose rate of production had begun to fall by that time. In 1941–1945, technological progress in drilling and oil production played a significant role in maintaining the oil production level in the country. The western Siberian oil and gas-producing region made a decisive contribution to the high oil production levels in the former Soviet Union.