The U.S. government faces shortcomings in producing its oil-inventory data, according to internal Department of Energy documents, casting doubt on figures that affect the production and prices of the world's most important industrial commodity.

The documents, obtained through a Freedom of Information Act request, expose several errors in the Energy Information Agency's weekly oil report, including one in September that was large enough to cause a jump in oil prices, and a litany of problems with its data collection, including...