UPDATE 1-Shell Deer Park Tx. refinery cuts coker feed:state

(Adds details, background)

HOUSTON, Feb 4 (Reuters) - Shell Oil Co's joint venture 340,000-barrel-per-day (bpd) refinery in Deer Park, Texas, reduced feed to a coking unit on Monday after a malfunction, according to a notice filed with state pollution regulators.

A coker wet gas compressor tripped offline after an antisurge valve opened, according to the notice filed with the Texas Commission on Environmental Quality.

"Feed to the unit was reduced and the compressor was placed back on line as soon as possible to stabilize the unit and stop the flaring," Shell wrote in the notice.

A refinery representative wasn't immediately available to discuss plant operations.

The Deer Park refinery is owned on a 50-50 basis by Shell Oil and Mexico's state-owned oil company Pemex.

Shell Oil, the U.S. unit of Royal Dutch Shell Plc (RDSa.L: Quote, Profile, Research, Stock Buzz), operates the refinery. (Reporting by Erwin Seba, editing by Richard Chang)