* Premium to U.S. crude reaches more than $8
* Commodity index reweighting increased interest in Brent
* Possibility of trading play
By Barbara Lewis and Christopher Johnson
LONDON, Jan 14 (Reuters) - Backwardation in the expiring Brent contract peaked at more than a dollar on Friday as the benchmark widened a huge premium to U.S. futures and a sharp exit from positions chopped levels of open interest.
By the close on Thursday, open interest stood at 30,491 lots, down from 61,729 lots on Wednesday and 101,817 on Tuesday -- the kind of steep descent that has characterised Brent over the last year.
Brent futures on ICE have traded higher than their lighter, higher quality equivalent U.S. futures on the New York Mercantile Exchange since August and since December have been in backwardation at the front end of the curve.
Backwardation, in which crude for early delivery is more expensive than for later delivery, is traditionally a symptom of tightened supplies.
At its widest so far on Friday, February Brent was trading more than $1.10 above the March contract.
Brent's premium to U.S. crude, also known as West Texas Intermediate (WTI), reached a session peak of $8.08, its widest since February 2009.
While Brent has become more backwardated, U.S. futures have been stuck in the opposite condition contango, and supplies at the U.S. futures delivery point in Cushing, Oklahoma, have been regarded as ample. Apart from relatively short supplies of Brent, analysts have said an increase in investment flows into the European contract have lent support.
DESIGN OR ACCIDENT?
Traders have also raised the possibility of a trading play or dealers adjusting after index reweighting this month, which shifted some investment class money into Brent and away from U.S. crude, although that remains the primary magnet for the major institutions.
"Underlying this spread is the shift of investors into Brent and out of WTI as they adjust their portfolios," said Eugen Weinberg of Commerzbank.
"But it may be that the spread at the moment reflects traders who have previously bet on the spread narrowing. They may now be being squeezed on the last day of trading. That is just speculation, but I think it possible."
The volumes of Brent, Forties, Oseberg and Ekofisk, which underpin the Brent futures contract, are higher for February compared with the previous month, loading programmes have shown.
Tamas Varga at brokers PVM Oil Associates noted the front month Brent spread, now firmly in backwardation, had begun to strengthen in October, coinciding with a rally across commodity markets spurred in part by U.S. quantitative easing.
"During this period the BFOE monthly loading schedules have varied between 1.25 million bpd and 1.11 million bpd with the latest one, February, expected to be at 1.22 million bpd," he said.
"So the relative strength in Brent does not appear to be coming from a change in North Sea supply fundamentals but rather from a change in demand."
IntercontinentalExchange said in an interview earlier this week the issue was U.S. crude weakness, rather than Brent strength.
It said it had a "position management programme", allowing it to ask members to limit excessive positions if necessary.
For its part, McGraw Hill unit Platts, an assessor of crude and product prices, in the past bolted on other North Sea grades to Brent to widen the physical basis of Brent futures and deter trading plays. It has not ruled out further reforms.