Crude-oil prices plunged more than 5% on Monday to trade near $30 a barrel, making the specter of bankruptcy ever more likely for a significant chunk of the U.S. oil industry. Three major investment banks— Morgan Stanley , MS 0.28 % Goldman Sachs Group Inc. and Citigroup Inc. C 1.56 % —now expect the price of oil to crash through the $30 threshold and into $20 territory in short order as a result of China’s slowdown, the U.S. dollar’s appreciation and the fact that drillers from Houston to Riyadh won’t quit pumping despite the oil glut. As many as a third of American oil-and-gas producers could tip toward bankruptcy and restructuring by mid-2017, according to Wolfe Research. Survival, for some, would be possible if oil rebounded to at least $50, according to analysts. The benchmark price of U.S. crude settled at $31.41 a barrel , setting a 12-year low. […]