Although it might not look like it as the bear market in energy continues to stretch on, this could be a great opportunity for investors with long-term thinking.

Sure, the health of the crude market isn’t looking particularly strong at the moment. Supply continues to be an issue, with inventories in North America bumping up against record highs. Many oil sands operators are pressing on with large expansions, which could replace much of the other supply we see dropping off.

There’s also the Middle East, which hasn’t blinked. Production from that region has remained steady, with OPEC on record saying it’s comfortable waiting out this supply glut, no matter how long it takes.

But as the old expression goes, it’s darkest right before the dawn. And like in 2008 when oil peaked at nearly $150 per barrel, there are plenty of reasons to think the market has swung too far, and a return to more normal prices is in store.

If that happens, investors who buy cheap oil stocks now are going to be very happy. Because of the way the economics of the sector work, a $20 per barrel increase in the price of crude likely means the shares of these three beleaguered energy companies could double…or even more. Let’s take a closer look.

Canadian Oil Sands

Canadian Oil Sands Ltd (TSX:COS) is the 38% owner of the Syncrude oil sands project, which upgrades bitumen into something that’s very close to light sweet crude.

It’s a good business when oil prices are high, but with crude hovering around US$60 per barrel, there isn’t much room for it to extract the bitumen, upgrade it, pay to maintain all the equipment, and make a profit. The decline in the Canadian dollar has helped, but at this point the company barely breaks even. This has pushed shares down to below $10 each.

But over the long term, Canadian Oil Sands has a lot going for it. It’s sitting on massive amounts of reserves, with a decent balance sheet, and the potential to hike its current $0.05 per share quarterly dividend. Remember, just a year ago each share paid out $0.35 per quarter and shares were at $25. They could easily recover back to that level once the price of oil returns to normal.

Pengrowth Energy

It hasn’t just been the last year that’s been terrible for Pengrowth Energy Corp. (TSX:PGF)(NYSE:PGH). Shares have fallen from a high of nearly $14 in 2011 to just over $3 today, thanks to weak natural gas prices and the company slicing the dividend twice.

Management has responded to these issues in a number of ways. The company moved away from being a primarily natural gas producer, with 50% of 2015’s production expected to be oil. It also hedged 63% of 2015’s oil production at US$94 per barrel, which should be enough for it to continue to pay its still generous 7.5% yield.

The biggest issue with Pengrowth is the balance sheet. Total debt is more than $2 billion, but just $170 million of that comes due in the next two years, and the company just extended its $1 billion operating line of credit. This should be enough to get it through these lean times.

Lightstream Resources

Perhaps the riskiest oil play is Lightstream Resources Ltd. (TSX:LTS), which is trading at barely above $1 per share.

Lightstream’s story is similar to many others in the sector. It loaded up on cheap debt when times were good, and is now stuck with the Herculean task of paying it all back when the price of crude is 40% lower.

But the company is taking steps in the right direction. It paid down more than $400 million on its credit line over the past year, and has negotiated relaxed terms with lenders. And when times are good it’s a pretty profitable company, enjoying some of the highest netbacks in the sector. It just needs the price of crude to recover, and in a hurry.

Investors can play it safe in the sector and invest in the more-established energy producers, but they won’t produce nearly as much upside. For investors who don’t mind the risk, it sure looks like the reward is there with these troubled stocks.

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Fool contributor Nelson Smith owns shares of CANADIAN OIL SANDS LIMITED.