To say that Aggreko (LSE: AGK) has had a quarter to forget would be something of a huge understatement. The power generator provider has shed 22% of its value since the end of June, putting paid to its heady ascent enjoyed since the spring.

Share prices took a dive in August after Aggreko produced a worrisome trading update. The business advised that revenues slumped 12% during January-June, to £685m, a result that drove profit before tax 31% lower to £61m.

In particular, Aggreko warned of difficult trading conditions in North America, with the subdued oil price impacting on a number of its markets.

Signs that US drillers have been getting back to work in recent weeks should come as some relief to the business. But overall, capex budgets across the oil industry remain under severe pressure, and this could prevent Aggreko’s top line from snapping back any time soon should crude values fail to break higher.

The business is predicted to endure an 8% earnings dip in 2016, resulting in a P/E rating of 15.1 times. This sails well above the benchmark of 10 times for high-risk stocks, however, and I reckon this could lead to further share price weakness should industry news flow fail to improve.

Troubled Tullow

Fossil fuel giant Tullow Oil (LSE: TLW) has also taken a whack during quarter three, the stock shedding 17% of its value in the period. Investor appetite has wavered in oft-rocky trading as the market has absorbed increasingly-worrying supply and demand indicators from the oil sector.

Just this week the International Energy Administration (IEA) slashed its global demand growth projections by 100,000 barrels per day, to 1.3m barrels per day, and warned that “supply will continue to outpace demand at least through the first half of next year.”

Tullow Oil had already got the quarter off to a downer on the final day of June, as output issues at its Jubilee field assets in Ghana forced the firm to reduce its 2016 production forecasts.

These reduced volumes, allied with the low crude price, caused revenues to sink by more than a third during January-June, to $541m, the company advised in July. But hedging activity and cost-cutting helped the business flip to a pre-tax profit of $24m from a $10m loss a year earlier.

News that maiden oil at Tullow Oil’s blockbuster TEN project in Africa had been reached on budget and on time last month will come as great news to the firm’s investors.

But given that fears over the oil market imbalance have emerged again, I reckon Tullow Oil may struggle to gain traction in the coming weeks and months, particularly as the business already deals on a mammoth forward P/E ratio of 42.9 times.

And with the driller also languishing under a colossal $4.7bn net debt pile, I reckon Tullow Oil is a risk too far for cautious investors, in the near term and beyond.

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Royston Wild has no position in any shares mentioned. The Motley Fool UK has no position in any of the shares mentioned. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.