NEW YORK, May 28 (Reuters) - A reported jump in weekly U.S. crude production data that set oil traders atwitter on Thursday was chiefly caused by revisions to two-month-old figures, not a surge in immediate output, a U.S. official said.
On Thursday, the Energy Information Administration's Weekly Petroleum Status Report showed field production of crude oil rose by 304,000 barrels per day (bpd) to 9.57 million bpd last week, the highest in weekly records going back to 1983. Monthly data show U.S. output peaked at 10.04 million bpd in November 1970.
While skepticism over the EIA's model-based weekly production estimates is not new, the dramatic increase still surprised traders and investors from London to Houston, causing some to question whether the years-long shale boom was petering out. Oil prices vacillated after the data, first falling before ending 17 cents higher on the day.
Robert Merriam, EIA's manager of petroleum supply statistics, cautioned against reading too much into the figures, since they are based largely on forecast models and historical data rather than real-time information - unlike data on inventories or refinery operations.
"At the end of the day, the crude production numbers are a modeled number. We don't and no one really has real time information," he said. "There's a long delay."
The latest figures were sharply higher because the EIA incorporated detailed data from states including Texas showing that production in March was higher than earlier estimated.
As a result, the agency raised its baseline for March by some 130,000 bpd, Merriam said. It then increased the growth trajectory to account for the higher-than-expected rise, adding another 75,000 bpd to last week's figure.
About a third of the increase was due to a rebound in Alaskan production, based primarily on near-real time volumes delivered to the Trans-Alaskan Pipeline System. Output rose some 95,000 bpd after falling by 112,000 bpd previously due to maintenance.
Over time the EIA's weekly figures have tended to lag behind monthly data. For instance in March, weekly data showed an average of 9.4 million bpd; but official data released separately on Thursday in its Petroleum Supply Monthly report showed field production reached 9.53 million bpd.
The comparison underscores the risk for investors of taking weekly numbers at face value.
In a note on Thursday, Citigroup said this week's data "might seem to indicate production growing at low rig counts," but added that weekly data estimates "need to be treated carefully." (Reporting by Catherine Ngai; Editing by Leslie Adler)