The U.S. shale oil collapse has now gotten so bad that it is beginning to impact other areas of the economy. In fact, for shareholders of BOK Financial Corporation (BOKF), Cullen/Frost Bankers, Inc. (CFR) and Texas Capital BancShares Inc (TCBI), the energy slump is already making a meaningful impact on the regional banking business.

As U.S. oil companies struggle to remain solvent, regional banks with high exposure to energy assets have found themselves in a tricky situation.

BOK Financial Corporation (BOKF)

BOKF shares are already down 4.3% year-to-date, and WTI’s push to new 13-year lows drove the bank to increase loan loss provisions for Q4 2015 to $22.5 million. Prior to the January adjustment, BOK had guided for Q4 loan loss provisions as low as $3.5 million.

“It’s going to get worse before it gets better,” Raymond James analyst Michael Rose said of the environment for energy-exposed regional banks like BOKF. However, the firm sees potential buying opportunities opening up in the space. Rose notes that many of the energy loans are some of the most conservatively-underwritten loans on the books.

“I think we’re setting up for a tremendous buying opportunity in a lot of these names because these banks are not going to lose money,” Rose said in January.

Unfortunately, Standard & Poors and Moody’s aren’t as optimistic when it comes to BOKF. In early February, S&P lowered its long-term issuer credit rating for BOKF and reiterated a negative outlook. Moody’s recently placed BOK Financial on review for a credit downgrade as well.

Currently, BOKF has the second-highest energy loan composition among all U.S. regional banks at 19.4%.

Cullen/Frost Bankers, Inc. (CFR)

CFR is another name that fell victim to both an S&P ratings cut and a Moody’s downgrade review within the last month. So far this year, the stock is down 3.1% on investor fears that its high energy exposure could cripple the bank when defaults start rolling in.

According to Barclays Capital, CFR is one of two banks that have the most concerning energy exposure at the moment.

“We think their energy reserves are comparatively low necessitating fairly material builds over the next few quarters,” Barclays said in mid-February.

CFR currently has the third largest energy loan composition among regional banks at 15.3%.

Texas Capital BancShares Inc (TCBI)

TCBI is another name that has been making 52-week lows in the past couple of weeks on fears that it does not have adequate capitalization to withstand a coming wave of energy defaults. TCBI has the fifth-highest energy loan composition at 10.2%. Not only is the bank included among both the aforementioned S&P downgrades and the Moody’s downgrade reviews, it is the other mystery bank (in addition to CFR) that Barclays fears does not currently hold enough energy reserves.

Although all three of the banks mentioned above were downgraded a single notch by S&P, TCBI’s credit rating now stands lowest of all. BOKF’s rating now sits at BBB+, CFR’s stands at A- and TCBI’s now comes in at only BB+.

While TCBI stock’s valuation (10.6 forward PE) is now currently near five-year lows, S&P believes these three banks have a long road to any potential recovery. The firm expects that loan losses for energy-exposed banks will continue to rise in coming years, even if energy prices rebound from multi-year lows.

As of this writing, Wayne Duggan did not hold a position in any of the aforementioned securities.