The magnitude of this year's oil price plunge has been unusual, making it particularly challenging for investors to assess how falling prices will impact corporate earnings and ultimately the stocks they're invested in.

We can certainly expect companies to say a lot when they discuss their Q4 (Oct.-Dec.) financial results in January.

For now, Wall Street's stock gurus are hacking away at their earnings growth forecasts for the energy sector.

"As the price of crude oil has continued to decline during the fourth quarter (to $66.81 yesterday from $91.16 on September 30), analysts have also continued to lower earnings estimates for companies in the Energy sector in the S&P 500 for the fourth quarter," FactSet's John Butters writes.

Energy sector earnings don't look good. FactSet Currently, analysts expects S&P 500 energy sector earnings to plunge 14.6% to $9.49 per share in Q4, which compares to the 6.6% increase to $11.94 expected in September 30. According to Butters, this represents "the largest decline in the EPS estimate for the Energy sector during a quarter since Q1 2009."

The good news is that the energy sector, while big, represents just one relatively small part of the diversified collection of big US companies in the S&P 500. Indeed, analysts expect 3.4% earnings growth for the index as a whole in Q4.

As far as the top line is concerned, revenues are expected to dive 13.7% in the energy sector, while the S&P 500 as a whole sees a 1.5% increase.