Thursday is looking like a rough day for oil stocks. Oil prices are falling, and oil stocks are following them down.

As OilPrice.com reports, Brent crude oil prices fell 2.8% through 10:30 a.m. ET today, to about $71 per barrel, while U.S.-preferred West Texas Intermediate (WTI) crude prices dropped more than 3% to about $67.50. Falling in tandem were shares of oil majors ExxonMobil (XOM -0.01%) (down 1.6%), ConocoPhillips (COP 0.03%) (down 2.2%), and Shell (SHEL 0.54%) (down 3.7%).

Why are oil prices falling?

OilPrice.com blames the OPEC+ group of nations (that's basically OPEC itself, plus Russia, Kazakhstan, Azerbaijan, and a few others) for the declines, reminding investors that the group still plans to gradually increase oil production beginning in October. Adding to the misery, reports out of China show an economy still in contraction mode, meaning demand will be falling there.

And as Econ 101 taught us: Rising supply plus falling demand equals lower prices.

Which of these two factors is having the biggest influence on oil prices? Well, China is the world's biggest importer of crude oil, so the health of its economy has an outsize effect on the price of crude globally. And according to China's National Bureau of Statistics, the local Purchasing Managers' Index is showing its worst numbers in six months, meaning the trend there is down.

On the plus side (from an investor's standpoint), Libya still isn't producing oil, which helps to blunt the effect of production growth elsewhere in OPEC. OilPrice argues that this production stoppage is keeping a "floor" under oil prices so that they can't fall too far. On the other hand, though, once Libya does resume producing and exporting oil, well, that's a future catalyst that could push oil prices down even further.

And Libya is capable of producing 700,000 barrels a day (bpd), so that'll be an even bigger catalyst than the OPEC output hike, which is starting at just 180,000 bpd.

Time to buy oil stocks?

All of which is to say, just because things look bad for oil stocks today, they could still get worse.

On the plus side, though (again, from an investor's standpoint), it's precisely these worries about future oil prices that are making oil stocks look cheap right now. Indeed, both Exxon and Conoco stocks have gotten cheaper over the past 52 weeks, while Shell stock is up only 3% -- a mere fraction of the 33% gain on the S&P 500. These stocks could be due for a turnaround.

Of the three, Conoco stock is the cheapest with a P/E ratio of less than 12, while Shell stock costs 12.1 times earnings, and Exxon, the most "expensive" of the three, still costs less than 14 times earnings.

Given my druthers, I think I'd lean toward buying Shell before the others, though. Shell's 4% dividend yield is the most generous of the three, and a full one-third bigger than Conoco's 3% payout. Shell also boasts the fastest projected earnings growth rate of these three oil stocks at better than 8%.

An 8% growth rate, 4% dividend yield, and a P/E of only 12? That's the very definition of a total return ratio of 1, and it sounds like a fair price to me.