The blowout in the premium of Brent crude to Dubai to the highest in nearly seven years may reverse when major Asian buyers such as China and India start filling strategic oil reserves.

This is because the oil likely to be bought for storage will mainly be heavy, sour grades from the Middle East, for which Dubai is a benchmark.

The question is really whether there will be enough extra incremental demand from Asia for filling storage tanks to boost the prices of heavy grades relative to the lighter Brent oils, and also whether this demand will be smooth or come in fits and starts.

The International Energy Agency estimates that China will add 169 million barrels to its strategic reserves in the second phase, due to be filled by 2013, and about another 228 million barrels in the third phase by 2016.

India’s first strategic reserves programme is for 39 million barrels by 2013, and the total calls for 110 million barrels, although the timeframe on this is woolly.

This means additional oil demand of an average of 240,000 barrels a day (bpd) between now and 2016 from those two countries.

In addition, the IEA estimates commercial storage in China. India, Malaysia, Indonesia, South Korea and Singapore will expand by 320 million barrels, equating to 180,000 bpd over the next five years.

Taken together, this implies an additional 420,000 bpd of oil going into Asia up until 2016 merely to fill storage, never mind any actual demand growth from stronger economies.

This doesn’t sound like a huge amount of oil in the context of Asia demand of about 28 million bpd and it may well be possible to ship these volumes without putting undue pressure on Middle Eastern supplies.

But it is the marginal barrel of oil that sets the price and in that context an extra 420,000 bpd over five years may make quite a difference.

Of course, not all the oil has to come from the Middle East, but the assumption is that it will mainly be heavier grades, as this is what the new refining capacity in China and India is set up to process.

Saudi Arabia is China’s biggest supplier of crude, exporting about 1 million barrels a day, or just under 20 percent of the total imports by the world’s biggest consumer of commodities and world’s No. 2 oil importer.

There is a fair chance that storage needs will boost demand for heavy oil from places like Venezuela.

The other factor to consider is that part of the reason the Brent-Dubai spread has widened recently can be put down to market factors, and these can also either reverse or amplify.

Brent has been boosted by the loss of Libya’s nearly 2 million bpd of light crude.

While this output looks to be gone for the foreseeable future, by now buyers should have adjusted and there may be some winding back of the scramble to secure supplies of Brent-grade oil.

The Brent-Dubai spread also jumped after Saudi Arabia’s unilateral decision to pump more crude, which is of the heavier grades, after OPEC failed to reach agreement to boost output.

The risk in calling for a spread to narrow on the basis that extra demand from strategic reserves in Asia is that the process of filling tanks is unlikely to be smooth.

It’s hard to imagine the same volumes being bought month-by-month, it’s more likely that there will be periods of extra demand followed by periods of normal consumption.

This makes it more likely that volatility in the spread will increase as the market tries to fathom what volumes are being bought and for what purpose.