The devastating drop in sales revenue comes as a result of the dramatic drop in oil prices over the last 18 months and is expected to continue well into the year for most of the companies on the list. The losses averaged to about $1 billion each from struggling oil companies EOG Resources, Devon Energy, Linn Energy, and SandRidge Energy.

Of course, there are significant differences between the 40 companies listed. Eighteen of the 40 companies were sorted into what the EIA referred to as the “high loss group,” or HLG, who reported 2015 losses in excess of 100 percent of their equity. The remaining 22 were non-HLG companies. The EIA research notably excluded some of the biggest oil companies in the world, such as ExxonMobil and Chevron, both of which made profits last year but a vastly smaller amount than they were used to. The report also only focused on onshore-only U.S. petroleum companies.

CNN Money reported that the amount of debt accrued by the companies in question played a decisive role. Companies with high debt had their revenues driven down 21 percent last year as opposed to a 6 percent loss by other companies.

“The analysis also revealed that the companies most vulnerable to losses were oil producers with too much debt taken on during the boom years. The 18 U.S. oil companies that reported the biggest losses were saddled with $57 billion in long-term debt. These big losers also had an alarming average long-term debt-to-equity ratio of 99%, the EIA said. By comparison, the remaining 22 companies that posted milder losses had total debt of $40 billion, or 58% of equity.”

When cash flow is essentially shut down to a trickle, like what has happened since about mid-2014 with oil prices hovering around $40 a barrel, companies with a lot of debt find it harder to make their interest payments. Oil producers may also suffer from debt accrued from risky investments, for example exploration of potential oil fields that turned out to be unprofitable.

“It seems those companies that were most highly leveraged were also the ones taking the riskiest bets. That’s what has come back to hurt them the most,” Matt Smith, ClipperData’s director of commodity research, told CNN.

Even though oil prices have risen somewhat from $26 to around $42 a barrel, Wall Street is bracing itself for more heavy losses to the oil industry this year. One prime example is SandRidge Energy, a shale oil driller that admitted last month it’s considering filing for bankruptcy.

The Oklahoma City-based company is carrying a burden of $3.6 billion of debt. If it goes under, it would potentially become the biggest North American-based oil company to sink during the current oil crisis.

Amidst these concerns, the oil industry is also facing a process called “redetermination,” according to the EIA report, which could see their ability to get credit from banks suffer, undermining their ability to drill and increasing the chances of bankruptcy.

“Many companies are currently undergoing a redetermination of their credit limits, and those with high leverage or lower quality assets could face significant curtailment of their access to short-term credit that could in turn constrain their drilling and completion operations, with direct implications for their production from new wells in the near term.”

However, CNN reported that bankers have not yet come down hard on their oil clients, likely encouraged by the recent rebound in oil prices. In fact, the Wall Street Journal reported that U.S. oil prices just hit a record high for 2016 at $44.05 a barrel, although some analysts predict that the global production glut will undermine further price gains and that we might see a sharp market correction in oil prices very soon.