“The debt that fueled the US shale boom now threatens to be its undoing. Drillers’ debt ballooned to $235 billion at the end of the first quarter, a 16 percent increase in the past year, even as revenue shrank. The problem for shale drillers is that they’ve consistently spent money faster than they’ve made it, even when oil was $100 a barrel. The companies in the Bloomberg index spent $4.15 for every dollar earned selling oil and gas in the first quarter, up from $2.25 a year earlier, while pushing U.S. oil production to the highest in more than 30 years.

“The question is, how long do they have that they can get away with this,” The companies with the lowest credit ratings “are in survival mode.”

Thomas Watters, oil and gas credit analyst at Standard & Poor’s in New York