The amount of oil that the United States could potentially recover thanks to the shale boom is equal to about three times Iran's proven reserves, but exploration and production companies could soon have a harder time extracting black gold from the country's basins.

Getty Images

As U.S. oil prices have tumbled from their June highs, so too have the shares of many small and mid-sized players in the U.S. shale industry. Investors worry that if oil prices remain below $80—West Texas Intermediate was near $79 on Friday—some firms could see their debt loads rise and find it difficult to fund exploration and sustain production growth, analysts said.

Employment in the oil and gas sector has grown more than 72 percent to 212,200 in the last decade as technology such as horizontal drilling and hydraulic fracturing have made it possible to reach fossil fuels that were previously too expensive to extract. Read More Drilling chief calls bottom on U.S. oil

In order to fund the rapid growth, exploration and production companies have borrowed heavily. The energy sector accounts for 17.4 percent of the high-yield bond market, up from 12 percent in 2002, according to Citi Research. "A lot of these smaller companies, in particular the smaller frackers, they rely on debt, and the debt markets have been wide and open for them," Gregory Zuckerman, author of "The Frackers," told CNBC's "Power Lunch." "It not clear it's going to be (that way) going forward."

The question is whether companies can keep their balance sheets healthy and continue to grow in the face of falling prices. U.S. oil futures have fallen about 24 percent from a high of $103.66 in June.

Lower production growth Exploration and production companies are unlikely to accelerate operations in a cheap oil market because they are worried about increasing their debt load, said Michael Rowe, a researcher at energy investment and merchant bank Tudor Pickering Holt. Lower production growth will likely hurt stock prices in the short term, he said, but firms that significantly outspend their cash flows to fund expansion could have a harder time accessing credit in the future. Read More OPEC cuts oil price forecasts as 'price war' bites

"They can either drill themselves into a much worse credit profile or make the right decision for the financial health of the company and take their foot off the accelerator, so to speak," Rowe said. Investors become concerned when a company's debt reaches twice earnings on an EBITDA basis, and further scrutinize balance sheets when leverage reaches three times earnings, according to Tudor Pickering Holt. The debt load of about half of the mid- and small-cap companies Tudor Pickering Holt covers would exceed two times EBITDA or accelerate above three times earnings if U.S. crude prices consolidate at $80 next year, according to the firm's models. The Dallas-based Westwood Small Cap Value fund recently sold its position in Rex Energy after the firm's debt load reached three times EBITDA following its purchase of acreage in Pennsylvania and Ohio from an affiliate of Royal Dutch Shell. Rex did not immediately return a call seeking comment. Read More Bears surround oil as it falls below $80 a barrel

"It's a good acquisition. Unfortunately it stretches their balance sheet," said Bill Costello, senior analyst and portfolio manager at Westwood. "In this price environment, they're in a tough spot." Westwood is holding its positions in firms with one-to-one debt-to-earnings ratios, including Bonanza Creek, Synergy Resources and RSP Permian.