The BHP Billiton Limited (ASX: BHP) share price crashed 3.1% to below $20 a share in afternoon trading on the ASX.

Given the size of the company, it has also played a part in taking the index – the S&P/ASX 200 (Index: ^AXJO) (ASX: XJO) into negative territory – down 0.1% at 5,426.8 points.

If commodity prices continue to fall, we could soon see BHP’s share price fall as far as $15 or below. The 52-week low of $14.06 was reached earlier this year, and it would only take a 20% fall to see the share price hit the $15 mark – not as fanciful as some readers might think.

Today’s 3.1% fall was mostly due to the falling oil price. Brent crude oil dropped 2.8% to US$47.04 a barrel, and expectations of some stability in the commodity price have been dealt a huge blow. Ongoing oversupply and low prices mean some higher cost oil production has dropped out of the market.

But as the chart below shows, active US oil rigs are beginning to increase as shale drillers become more efficient and capable of making profits even at these low oil prices.

US oil rig count Aug 2016

Source: Baker Hughes

BHP has also been hit by a fall in the iron ore price. The iron ore price fell to the lowest level in almost a month overnight at US$58.97 a tonne. It’s still up 35% so far this year, but the outlook for iron ore prices doesn’t appear to be all that positive.

Iron ore and petroleum products make up a huge portion of BHP’s revenues and earnings.

BHP is also dependent on coal and copper prices which have so far fared better, but the short to medium term outlook isn’t all that positive for many commodities.

You can also add BHP spin-off South32 Ltd (ASX: S32) and Rio Tinto Limited (ASX: RIO) to the list of stocks at risk as well.

Foolish takeaway

Bargain hunters who picked up BHP shares when they fell to $14 earlier this year have made a nice profit, but they may want to take some profits off the table. It took just two months for the BHP share price to go from trading at levels around today’s price to hit $14.

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Motley Fool writer/analyst Mike King doesn't own shares in any companies mentioned. You can follow Mike on Twitter @TMFKinga

The Motley Fool Australia has no position in any of the stocks mentioned. 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. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.