US oil reserves off 10.3 pct in 2008 vs yr earlier

*U.S. proved oil reserves off 10.3 percent in 2008

*Drop in proved oil reserves due to reporting rules (Adds background about fall in oil reserves)

WASHINGTON, Oct 29 (Reuters) - U.S. oil reserves plunged 10.3 percent last year to 19.1 billion barrels, largely due to government reporting requirements, the U.S. Energy Information Administration said on Thursday.

Under current reporting rules, oil companies must assess their yearly proved reserves based on what they could reasonably produce at market price on the last day of the year.

The tanking economy pushed oil prices down to levels below $33 a barrel at the end of 2008, however, making proved oil reserves much lower compared to 2007.

New rules set to take effect in 2010 will require operators to use an average of first day of the month prices throughout the year.

"Under the new rules, there would likely have been a smaller drop (or possibly even an increase) in crude oil proved reserves," the EIA said.

U.S. natural gas reserves rose 2.9 percent to 244.7 trillion cubic feet in 2008, the EIA said. (Reporting by Ayesha Rascoe; Editing by Christian Wiessner)