After five months of skidding crude prices, energy players and politicians who depend on oil and gas revenue are finally warning a slowdown in Canada's oil patch is on the way – one that will crimp growth and the economy.

The push came this week after OPEC decided to keep its production at 30 million barrels a day.

That move, coupled with soaring supplies from the United States and no cut back from Russia, sent the price for North American oil down 10 per cent this week to $66.15 (U.S.) a barrel. The price is off more than 30 per cent since June.

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Now, senior decision makers are preparing for a rocky future with some predicting energy projects will stall, small companies will fold and economic growth will shrivel.

"What I do believe is that you will have now a pause in some activities and there will be a muting impact on the economy," Murray Edwards, founder and chairman at Canadian Natural Resources Ltd., one of the country's largest energy firms, told The Globe and Mail on Friday.

"Projects will be deferred, capital budgets will be reduced and employment growth will not be as robust as it otherwise would have been. … That will be around so long as you have this volatile oil price; in other words, we don't have some stability at a price so guys can plan for the long-term."

The same financial impact goes for the Alberta government. Every time the price of oil sheds a dollar, Alberta loses out on $215-million (Canadian) in revenue, according to Premier Jim Prentice.

Mr. Edwards said the industry better get used to it. "I think we've got some tough slugging to go here for a period of time," he told reporters at the Bennett Jones Lake Louise World Cup Business Forum. "OPEC has clearly indicated that they're not going to reconvene for six months, that there will not be a change, and the price has dramatically fallen."

Nancy Southern, Atco Ltd.'s chief executive, said her company is already seeing the effect of low prices. "We're noticing deferrals and delays. Not just the oil sands, even the tight gas, shale gas drilling," she told The Globe and Mail at the business forum. "We thought [liquified natural gas] was going to go ahead. Everybody was very bullish on the LNG development and, with the delays on approvals and pipelines and discussions and coming to agreement with First Nations and indigenous people, I think there is a sense that maybe we better take this a little slower."

And that means her company will be under pressure.

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"From Atco's perspective we'll see a slowdown. Our business is very much tied to the resource industry, and overall economic growth in the markets that we participate in," Ms. Southern said. "It has a big impact."

Some oil sands companies continue to insist investment decisions taken today are not based on short-term price fluctuations.

"We don't look at a blip in a year like this," Lorraine Mitchelmore, president of Royal Dutch Shell PLC's Canadian unit, said Tuesday.

But oil's rapid descent has reversed assumptions used to justify investment decisions made only a short while ago, underscoring risks to profitability.

When Suncor Energy Inc. said it would build its proposed $13.5-billion Fort Hills bitumen mine last October, the Calgary-based company said it expected the project to yield a 13-per-cent rate of return. That was based on a bitumen price of $60.50 (U.S.) a barrel. On Friday, the implied value of bitumen was about $45, according to numbers crunched by University of Alberta energy economics professor Andrew Leach.

With operating costs pegged at $24 a barrel, Suncor would still make money from Fort Hills at today's prices. "They just wouldn't be making as much money," Mr. Leach said. "It's just less profitable." Fort Hills is expected to start producing oil in 2017.

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Existing oil sands operations are somewhat shielded from pain in part because the big money has already been spent building the projects and operating costs are under $40 to $45 a barrel.

The S&P/TSX capped energy index fell 2.3 per cent on Friday, as oil prices extended losses. The energy group is down 9.6 per cent in November. Brent, the global benchmark, crashed below $70 before paring losses and finishing the day down 3.3 per cent at $70.15 a barrel.

The loonie typically drops in value as the price of oil falls. That can help insulate Canadian companies because they generate revenue in U.S. dollars and pay expenses in Canadian dollars.

"But the big growth on the oil side has been out of the oil sands, and the cost structure there has been an issue, and it is not as profitable at $70," said Ian Dundas, chief executive officer of Calgary-based oil producer Enerplus Corp. "These are big, multiyear decisions, but there's no question that those economics are tighter than they have been before. And the longer we stay at this level, the more you start to see that growth moderate."

The Alberta government, under Mr. Prentice, this week pledged to write budgets that do not depend so heavily on revenue from oil and gas. The proposal makes sense in theory, but it is impossible to deny how reliant the province is upon the energy sector.

Alberta now predicts it will rake in roughly $9.34-billion (Canadian) thanks to revenue directly tied to the energy sector: royalties from bitumen, crude oil, natural gas and its by-products; bonuses and sales of Crown leases; and rentals and fees. That makes up about roughly 21 per cent of the government's expected revenue of $45-billion in 2014-2015. But the health of oil and gas prices influences more than just these categories. Corporate and personal income taxes, for example, also move up and down because of energy markets.

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The government assumed oil would trade at an average of $75 (U.S.) a barrel through the next two quarters when it revised 2014-2015 budget. The original budget, released in March under Mr. Prentice's predecessor, assumed oil would average $95.22 a barrel over the fiscal year. That budget predicted revenue directly tied to the energy industry would also account for about 21 per cent of the province's expected revenue – the same as the revised budget. In the last fiscal year, revenue directly stemming from the energy industry again made up 21 per cent of Alberta's total revenue.

The Tories said that, under their new budget strategy, they want to pay down debt, save money, or consider additional infrastructure when oil is strong, but not cut spending when prices tumble. So, even if the government shifts away from relying on oil and gas prices when creating budgets or low-ball price estimates, energy prices still will have an enormous influence on the government's ability to spend.

"To deal with the implications of, not just $75 oil at this point, but sub-$75 oil, there will be consequences for all Albertans," Mr. Prentice told reporters after speaking at a business luncheon in Calgary Friday. "The long-term strength of our province, of our energy industry, is not in doubt. But it's clear that we're going to be in a price trough for a period of time – it could exceed a year, based on some of the projections that we're seeing and which are available publicly.

"And so it's a time for caution, discipline and prudence in our public expenditures and that's what we're doing."