Most grades slipped in light trade on the U.S. domestic cash crude market yesterday after government data showed burgeoning oil stocks, dealers said.

On Tuesday, industry group the American Petroleum Institute (API) showed crude supplies 32.2 million barrels, or 11 percent, above last year - the biggest yearly surplus since August 1998.

Main domestic sweet Light Louisiana Sweet/St. James traded at 35 cents premium to West Texas Intermediate/Cushing, down 5 cents from Tuesday.

Sister sweet Heavy Louisiana Sweet/Empire was valued at 10/18 cents over the benchmark, down 7 cents.

WTI postings plus traded early in the day at $2.73 versus $2.80/2.82 on Tuesday. West Texas Sour/Midland was valued at $1.32/1.25 under WTI/ Cushing from $1.30/1.27 discount the previous day.

Deepwater sours such as Mars were valued early Wednesday at WTI/Cushing minus $2.00/1.90, 10 cents lower than previous day levels.

Poseidon was pegged at $1.55/1.30 under the benchmark, from $1.40/1.30 discount the previous day. WTI/Midland was talked at WTI/Cushing minus 23/22 cents.

Eugene Island was pegged at minus 45/35 cents from minus 40/35 cents to WTI/Cushing. Bonito was valued down 10 cents at 30/10 cents discount to the benchmark. WTI/Cushing was valued at $18.74/18.80 Wednesday at midday.

Meanwhile, Nymex crude futures sank deeper midsession yesterday as U.S. government data showed crude stocks rose significantly last week after refineries slashed runs due to poor profit margins.

At 12:17 p.m. EST (1717 GMT), Nymex crude for March delivery traded at $18.64 a barrel, down 94 cents and just above its session low of $18.56.

Traders said the next critical support level is in the $18.45/18.50 area, which the market would likely test shortly.

Nymex February gasoline slumped 2.59 cents to 54.10 cents a gallon, just off its session low of 53.90 cents. Nymex February heating oil was also headed south, losing 1.80 cents to 50.25 cents a gallon after diving to 49.90 cents, the session low.

In London, IPE Brent crude futures dropped steeply in late afternoon trade yesterday, extending earlier losses after bearish U.S. government stock data showed a hefty build in crude stocks.

By 1740 GMT March Brent was 74 cents weaker at $18.50 a barrel, having broken through the $18.50 support earlier with a low of $18.38. Around 37,000 lots traded on the contract.

"It's under pressure and looks to go down and test support at around $18.25," said an IPE trader. "If it breaks that it'll look very bearish," he added.

February gas oil also slumped, closing $5.75 a tonne lower at $155.75.