LONDON, March 24 (Reuters) - Traders bet half a billion dollars on the price of crude only 15 minutes before U.S. President Donald Trump announced a five-day delay to attacks on Iran's energy infrastructure that sent the market plunging, exchange data and Reuters calculations showed.

Having issued Iran with a Monday deadline to reopen the critical Strait of Hormuz or face its power plants being "obliterated", Trump's post on Truth Social at 1105 GMT on Monday unleashed a powerful selloff in oil and natural gas.

Brent crude fell as much as 15% in a matter of minutes as Trump indicated constructive talks between Washington and Tehran were ongoing, prompting investors to price in the possibility of a de-escalation that could unblock the millions of barrels of oil now choked off in the Gulf.

LSEG data shows that between 1049 and 1050 GMT, traders placed bets on 5,100 lots of Brent and WTI crude futures, worth well over $500 million, based on a Reuters calculation.

The data also shows that, in the minute in which those contracts changed hands, it was selling that dominated volumes. It was not possible to establish who traded the oil.

13 MILLION BARRELS OF OIL TRADED IN 60 SECONDS AT 1105 GMT

The roughly 2,000-lot spike in volume in Brent futures at that point was far larger than those logged earlier in the day.

But turnover was dwarfed by what followed when Trump posted. Over 13,000 lots of Brent and WTI crude futures, equivalent to 13 million barrels of oil, changed hands in the space of 60 seconds at 1105 GMT.

Brent crude crashed to around $99 a barrel from $112 before the pre-announcement trades took place, while WTI fell to $86 from closer to $99 prior to Trump's post.

The Intercontinental Exchange, on which Brent crude is traded, and CME Group, which owns the NYMEX exchange on which WTI trades, did not immediately respond to Reuters' requests for comment.

The U.S. Securities and Exchange Commission declined to comment. The White House also did not respond to a request for comment. The Commodity Futures Trading Commission was not immediately available for comment.

OIL PRICES UP MORE THAN 40% FROM PRE-CONFLICT LEVELS

With around a fifth of the world's daily oil supply cut off by the Middle East war, prices are still more than 40% higher than they were when the conflict erupted in late February.

Trading volumes and volatility have exploded. On average, in the three years leading up to the war, some 300,000 lots of Brent crude futures would change hands on a daily basis.

That amount has doubled in the last four weeks as daily volumes have hit record highs above 1 million lots, equal to a billion barrels of oil.

For now, the Brent oil price is just below $104 as uncertainty persists over the total hit to the global economy - and even over the status of negotiations, as Iran denied it was engaged in discussions with the U.S.

(Additional reporting by Samuel Indyk, Robert Harvey and Ahmad Ghaddar; Editing by Alex Lawler and Jan Harvey)