Commodity prices, including oil, are under pressure as we near the end of 2013. Significant headwinds persist, which include rising interest rates and a stronger U.S. dollar. Also a new potential source of stress may emerge in the form of the Iranian nuclear agreement.

With all this in mind, you might want to know which energy companies are most vulnerable to falling oil prices in the United States. Some, such as ConocoPhillips (NYSE:COP), are at greater risk, which is why investors should keep oil prices in mind when considering investing in the energy sector.

Several headwinds as 2013 draws to a close
West Texas Intermediate crude oil sits at $93 per barrel in the U.S., which represents its lowest point since July. There are several contributing factors to oil's decline -- namely, rising interest rates that have strengthened the U.S. dollar and taken a bite out of most commodities. And now, the Iranian nuclear agreement threatens to drag oil down even further.

After all, relief from crippling sanctions means Iran can once again contribute to global supply. This is fairly significant, since Iran was the sixth-largest producing member of OPEC last month.

Things might not get better for West Texas Intermediate in the near future, either. The Federal Reserve may decide to taper its massive stimulus efforts at one of its next few meetings, which would put further pressure on commodities.

If this occurs, consider ConocoPhillips one of the most vulnerable of the U.S. oil giants. That's because Conoco is much less integrated than its peers. Conoco spun off its refining business, so it's got a lot to lose from falling oil prices. ConocoPhillips advises investors of the following annualized net income sensitivity: For every $1 per barrel change in West Texas Intermediate, the company's earnings stand to fluctuate between $30 million and $40 million.

Don't look now, refining might actually be a tailwind
For most of 2013, refining operations were killing integrated majors such as ExxonMobil (NYSE:XOM) and Chevron (NYSE:CVX). Tightening spreads between Brent crude, the international benchmark, and West Texas Intermediate meant crimped refining margins. Since refining is a significant piece of Exxon's and Chevron's businesses, this has had devastating effects so far this year.

ExxonMobil is still reeling from its last quarterly report, which showed that its downstream operations, which include refining, saw earnings collapse 81% in the most recent quarter. This has had a pronounced effect on the entire company this year. Exxon's earnings per share are down 27% through the first nine months of 2013.

By comparison, Chevron has held up better than Exxon. Chevron's U.S. and international downstream segments generated $1.8 billion in earnings through the first nine months of the year, down 45% from the same period last year. In all, Chevron's net income is down 13% through the first three quarters.

Now, however, the tide may actually turn. As West Texas Intermediate crude falls, Brent crude remains resilient. As a result, Brent's spread over West Texas Intermediate recently reached its widest point in the last eight months. If this trend continues, it should present relief for refining activities, which might actually serve to boost Exxon's and Chevron's earnings in the near future.

The Foolish conclusion
Falling oil prices can only serve to suppress profits for Big Oil. ExxonMobil, Chevron, and ConocoPhillips are certainly not immune. At the same time, steady prices for the international oil benchmark would increase refining profitability. In future quarters, refining would no longer serve as a drag on integrated majors' profits. As a result, ExxonMobil and Chevron would at least benefit from their more diversified business models.

The flip side is that ConocoPhillips, which has held up much better than Exxon or Chevron so far this year, would see its fortunes reverse. That's because it spun off its refining unit and wouldn't benefit from an improved climate for refining. Going forward, this represents a key consideration for energy investors as 2013 draws to a close.

Bob Ciura has no position in any stocks mentioned. The Motley Fool recommends Chevron. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.