OPEC's moves to increase production even as oil prices crashed helped push the rate of default among U.S. drillers to a record high, costing investors who bet on the shale boom billions of dollars, according to a new report by a credit rating agency.

Nearly one in three domestic oil producers that financed expansions with high-risk or junk bonds have defaulted, failing to pay investors a total of nearly $29 billion, according to Fitch Ratings. (Fitch Ratings is owned by Hearst Corp., the parent of the Houston Chronicle.) Fitch estimates that these defaults could rise to $40 billion by year-end.

Oil prices have rebounded since hitting a low of $26 a barrel in February, but they haven't risen fast or far enough to save small drillers that need more than $50-a-barrel oil to survive. Crude settled on Tuesday at $46.80 a barrel in New York.

More companies have defaulted on their debt this month, including the Canadian company Lightstream Resources, Forbes Energy Services of Alice, and IronGate Energy Services of Corpus Christi. Houston driller Halcón Resources said in May that it plans to file for Chapter 11 bankruptcy protection as part of a prepackaged deal with investors.

Fitch's list of companies that could potentially default on their debts includes local driller W&T Offshore. W&T Offshore executives were not immediately available for comment Tuesday.

In the five years leading up to the crash, the U.S. oil industry ran up more than $500 billion in risky debt to finance the shale drilling boom. But the boom pumped more oil than global markets could absorb. The result: a crippling two-year bust that's still bankrupting companies and depressing U.S. crude production.

Drillers are likely to come under more pressure as energy demand softens in key global markets and OPEC keeps the spigots open. Oil demand in China, whose rapid modernization helped drive the surge in oil prices a few years ago, slipped 2.7 percent in May to about 10.9 million barrels a day as economic activity softened, S&P Global Platts, an energy research company, said in a new report Tuesday.

Imports to the world's second-largest energy consumer plunged 41 percent compared to the same month last year, Platts said.

Meanwhile, the Organization of the Petroleum Exporting Countries reported that the group of 14 oil producing nations increased output by 264,100 barrels a day last month to nearly 33 million barrels a day. Saudi Arabia's production climbed to a near record of 10.5 million barrels a day.

"It's going to continue to weigh on the market," said Andy Lipow, president of Lipow Oil Associates in Houston. Refiners, he said, have turned the oil glut into a gasoline glut that has benefitted consumers by lowering pump prices.

But the large supplies of gasoline will moderate the demand of refiners and any increases in oil prices, Lipow said.

OPEC expects global energy demand to rise at a pace of 1.2 million barrels a day through 2017, while oil production outside the cartel declines by 900,000 barrels a day this year.

In the United States, the Energy Department expects demand for oil to grow, propelled by American drivers' thirst for gasoline. Gasoline demand has climbed to record levels and will increase the nation's oil consumption by 1 percent this year, the Energy Department projects.

But the nation's storage tanks still have plenty of oil to spare, with a stockpile of 530 million barrels that could further weigh on crude prices.

"The oil market is prepared" for the increase in demand, Lipow said. "That's why we have inventory levels 10 percent higher than last year."