"Recent history suggests that oil price downturns tend to be short and measured in months, not years," Westwood said.

"There is plenty to suggest that, this time, it could be even shorter: rapid supply erosion is likely along with a healthy boost to GDP for net importers.

"Both high and low oil prices present opportunities for well-managed, well-financed companies that have a long-term view, as the oil and gas industry is not a short-term game. But the window of opportunity may well be very short before the next cycle begins."

He said the current downturn should be no surprise, given there have been seven significant price cycles since 1970 along with a few minor ones in between.

"The reasons for the fall in Brent crude prices from $US115.19 in June to below $85 last week are well documented, as is the realisation that OPEC is now defending market share, rather than a minimum price," Westwood said.

"That said, the nature of the oil business is very different now compared to after the 2008 crash.

"It should be noted that 70% of the additional production that has come on stream since 2005 is unconventional. Much of this, of course, is from US shales - this is not cheap oil (mostly needing prices of $60-80 to be commercially viable) and decline rates are rapid - without ongoing drilling the current production capacity will be quickly eroded."

As in the past, the present downturn could well be a great buying opportunity - and in global exploration and production, the national oil companies tend to rule - and Westwood said they could indeed continue to invest, with China a high spender and India's ONGC (Oil and Natural Gas Corporation) planning a massive $180 billion foreign production acquisition spree.

He expects a similar trend in oilfield services.

"Acquisition opportunities are likely to present themselves for strategic and private equity buyers, as was the case in the last downturn," he said.

"Even without oil demand growth, vast numbers of new wells will need drilling worldwide each year to counter natural decline rates, probably some 80,000 in 2014 and more as demand grows again, boosting the demand for oilfield services."