Atlantic Richfield Company (ARCO), former American petroleum corporation that was headquartered in Los Angeles and was bought in 2000 by the giant BP Amoco (later BP PLC).

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The Atlantic Richfield Company was created in 1966 by the merger of Richfield Oil Corporation and Atlantic Refining Company. Atlantic Refining, whose predecessor firms date back to the 1850s, was incorporated in 1870 and, after 1892, became one of the eastern companies of the Standard Oil Trust. After the U.S. Supreme Court’s dissolution of the Standard Oil group in 1911, Atlantic Refining again became independent, with its headquarters in Philadelphia. Richfield, the product of several mergers in the first two decades of the 20th century, formally began in 1911 as a refining company financed jointly by the Los Angeles Oil and Refining Company and the Kellogg Oil Company (two of the several companies that were to merge under the Richfield name).

Under chief executive officer Robert O. Anderson, the new Atlantic Richfield Company made the first discovery of oil in Alaska’s Prudhoe Bay in 1968, and it was one of the main developers of the Trans-Alaska Pipeline in 1975–77. A further merger in 1969 brought in the refining and petrochemical capacities of Sinclair Oil Corporation. Sinclair had been formed in Kansas in 1916 by Harry F. Sinclair (1876–1956), who had made his first big oil strike in Oklahoma in 1907.

In 1977, as part of an attempt to diversify out of a dependence on petroleum, Atlantic Richfield bought the Anaconda Company, a miner and processor of copper, aluminum, and uranium and a manufacturer of copper and aluminum products. It also acquired significant coal-mining operations in the American West and Australia and expanded into the manufacture of solar panels for solar energy.

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During the 1980s and ’90s the company reversed its diversification efforts in order to refocus on its historical strength in petroleum. By the end of the 20th century, Atlantic Richfield had sold off all or most of its minerals, coal, petrochemical, and solar energy assets. It had petroleum operations in all parts of the United States as well as in Indonesia, the North Sea, and the South China Sea. The company also owned and operated transportation facilities for liquid petroleum, including pipelines and tankers. The acquisition of Atlantic Richfield by BP Amoco in 2000 for $27 billion doubled the British company’s share of natural gas in Prudhoe Bay and made BP the second largest oil company in the world.