What happened

Shares of seven of the largest U.S. independent oil and gas exploration and production companies (or E&Ps, for short) rose more than 14.5% in September, marking the best month many of them have had this year. Energy industry stocks in general handily beat the S&P 500, which only rose 1.9% for the month.

These top performers include ConocoPhillips (NYSE:COP), up 14.6%; Devon Energy (NYSE:DVN), up 16.9%; Apache Corporation (NASDAQ:APA), up 17.9%; Concho Resources (NYSE:CXO), up 18.7%; Noble Energy (NASDAQ:NBL), up 19.3%; Anadarko Petroleum (NYSE:APC), up 19.4%; and Hess (NYSE:HES), up an amazing 20.5%.

Wooden surface on which are a pair of small oil barrels, a stack of bills, a small pool of black liquid, a stack of pennies, and a piece of paper with graphs.

The stock market was happy with the oil industry in September, but these independent drillers are still underperformers for the year. Image source: Getty Images.

So what

As independent E&Ps, all seven of these companies' stock prices had had a rocky year through August, with price drops ranging from ConocoPhillips' comparatively modest 13.7%, all the way to Anadarko's disastrous 41.7%: 

APA Chart

APA data by YCharts.

August wrapped up just as Hurricane Harvey hit the Houston area, where much of the nation's oil and gas infrastructure is located. This caused further uncertainty in already fragile oil and gas markets. 

Luckily, although Hurricane Harvey had a major impact on Houston and its residents, the region's oil and gas infrastructure was largely spared. Companies headquartered in Houston like Apache -- one of my favorite stocks in this sector -- and Noble were able to restore operations quickly. 

Of course, by far the biggest factor in these stocks' rise is the price of oil. When the price of oil moves, independent E&P stocks tend to be affected first and most significantly. So it's hardly surprising that as the price of crude rebounded after Hurricane Harvey, these companies' stock prices followed suit. 

Now what

In spite of their September gains, all of these companies' share prices are down for the year, with the notable exception of Concho Resources, whose pure-play Permian Basin portfolio and solid balance sheet have investors excited about potential growth. And other than the mighty ConocoPhillips, which is down just 1.4%, all of the others are still down about 20% or more for the year. Even Hess, with its outstanding balance sheet, and the once-promising Devon Energy haven't been able to avoid the 2017 slump that's affected all of these companies.

The bottom line is, until oil prices can stay significantly above $50 per barrel for a sustained amount of time, there will still be a lot of volatility among E&Ps. There may also be quite a bit of upside here, but one good month doesn't make a trend. Investors in this space should still proceed with caution.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.