In spite of a well-received trading update the previous session, Wood Group's stock pulled back sharply on Friday after Numis Securities downgraded its rating on the oilfield services and engineering group from 'add' to 'hold' due to oil-price headwinds.With the price of Brent crude nearing the $89-a-barrel mark on Friday morning, down over a fifth from its June peak, Numis highlighted that as a result of recent acquisitions Wood Group has become "much more levered to oil prices than we had appreciated, particularly in the US onshore market", said analyst Sanjeev Bahl.Bahl said Wood's 'maintenance' business is generating a large proportion of its earnings from short-duration capital projects, rather than long-term maintenance agreements."Whilst we remain comfortable in the fact the US onshore maintenance market will grow in 2015, we are less certain about capex given the oil price," he said.Visibility of client spend in the US shale market is low, Bahl reckons, and with oil prices so depressed there is an increased risk of a reduction in US onshore rig count associated capital expenditure.The broker left its target price of 752p unchanged for the stock, which was down 3% at 671.5p by 11:30.