Banks are slashing credit lines to shale drillers, as an oil-price crash and wells that have failed to produce as much as predicted force a painful reassessment of companies’ assets.

The cuts vary from company to company, but Moody’s Corp. and JP Morgan Chase & Co. forecast a total reduction of as much as 30% to the asset-backed loans, or tens of billions of dollars. At current prices, that will be enough to tip some weaker players into bankruptcy as capital for the beleaguered industry dries up, say bankers, lawyers and energy executives.