Information on the extent of most mineral reserves is unavailable. A delegation of Western petroleum experts who surveyed the petroleum industry at the end of 1970 made a tentative estimate that oil reserves would be exhausted in roughly eleven years at the current annual production rate of about 13 million tons. With a view to ensuring long-term crude oil supplies for the planned expansion of the domestic petroleum refining and petrochemical industries, the government has entered into economic cooperation agreements with several small petroleum-producing countries. The government has also discussed the possibility for joint exploration of offshore petroleum deposits in the Black Sea and elsewhere in the world with oil interests of various countries. In the meantime the government has been importing crude oil from Iran, Saudi Arabia, and Libya in exchange for industrial machinery and equipment. Oil imports from these countries in 1970 amounted to 2.1 million tons. The major natural gas deposits that were exploited in 1970 are located in the Transylvanian basin and outside the Carpathian arc (see ch. 3). According to Romanian officials, the annual addition to reserves has been double the volume of annual production. Gas output has expanded steadily from about 365 billion cubic feet in 1960 to 845 billion cubic feet in 1969. Natural gas has been used for electric power production in thermal plants, for space heating, and as a raw material for the chemical industry. Less than 1 percent has been exported through a pipeline to Hungary. Western observers believe that imports of natural gas from the Soviet Union may be initiated in the early 1970s. This belief is based on information that a gas pipeline to be built from the Soviet Union to Bulgaria will pass through eastern Romania, fairly close to the major port of Constanta, which is far removed from domestic sources of gas. Negotiations to this effect are not known to have taken place. Deposits of coal are small and, with few exceptions, low grade. Known reserves in 1970 were reported to include less than 1 billion tons of bituminous and anthracite coal and 3.5 billion tons of lignite. Fields at Petrosani in the Jiu Valley of the southern Transylvania Alps contain 98 percent of the bituminous coal reserves; 90 percent of the lignite reserves are located in Oltenia, in the southwestern part of the country. Open pit mining is possible in much of the lignite area.