From a July 2010 NYT article about tax breaks for oil companies: "Some of the tax breaks date back nearly a century, when they were intended to encourage exploration in an era of rudimentary technology, when costly investments frequently produced only dry holes. Because of one lingering provision from the Tariff Act of 1913, many small and midsize oil companies based in the United States can claim deductions for the lost value of tapped oil fields far beyond the amount the companies actually paid for the oil rights."
Other tax breaks were born of international politics. In an attempt to deter Soviet influence in the Middle East in the 1950s, the State Department backed a Saudi Arabian accounting maneuver that reclassified the royalties charged by foreign governments to American oil drillers. Saudi Arabia and others began to treat some of the royalties as taxes, which entitled the companies to subtract those payments from their American tax bills. Despite repeated attempts to forbid this accounting practice, companies continue to deduct the payments. The Treasury Department estimates that it will cost $8.2 billion over the next decade."
Wow, it seems the US did all kinds of stupid shit in the name of fighting communism.