High-yield bonds, also known as junk bonds, issued by U.S. energy companies have come under selling pressure in recent weeks as oil prices have fallen to lows not seen since mid-2009.

Bloomberg says investors in these bonds are facing $11.6 billion in losses at the moment. Energy company-related debt issued in the past three years alone – about $90-billion worth – is down on average about 13% since oil hit its 2014 high in June.

That doesn't compare favorably with the average junk loss in the $1.3-trillion junk bond market during that period of 2.2%. Overall for the year, the total return on energy junk bonds is -5.5%, while the average high-yield bond total return is 3.08% in 2014.

This selloff means that the average energy junk bond now yields more than 8.5%! That is the highest yield seen since July 2010, up from an average 5.67% yield in June 2014.

Investors seem to be asking no questions, just selling these bonds. I think a closer look at the situation is warranted.

Bonds Turn Sour

Much of the expansion in the domestic shale energy industry was funded by high-yielding bonds issued by companies such asSandRidge Energy (SD), Energy XXI (EXXI), and Halcón Resources (HK).

So many of those bonds have been issued that the junk bond market in the energy sector has doubled in size since 2008. According to Dealogic, that expansion continued early this year, leading to energy-related junk bonds accounting for 18% of the total high-yield issuance in 2014.

Today, these bonds make up 17% of the benchmark Bank of America Merrill Lynch High-Yield Bond Index.

But there's a problem. Lower oil prices – which are down 37% year to date – are squeezing many of the smaller U.S. exploration and production companies in the shale gas and oil industry. The squeeze is being felt by players in the most expensive regions to operate in, where companies need at least $60 a barrel to break even.