Mexican revenues from the state-run Petroleos Mexicanos oil monopoly, known as Pemex, will plummet by about 50 percent this year from 1987 because of falling world oil prices, a government study released Monday said. The study, conducted by the Cabinet-level Commission on Foreign Trade in Petroleum, blamed the Organization of Petroleum Exporting Countries for flooding the world oil market and sending prices into a tailspin. Between Aug. 24 and Oct. 5, world oil prices plunged by an average of $3 to $4 a barrel. The average price of Mexico's oil has dropped to just $9 a barrel.