Contra Costa County could lose $50 million in tax revenue if a property tax appeals board lowers the assessment of Chevron's Richmond refinery by the same percentage it did in an appeal that must wrap up soon.

The county assessment appeals board lowered Chevron's assessment by $1.2 billion for the 2004-2006 tax years that will cost the county $12.7 million if a Sept. 2 preliminary ruling is upheld. The reduction was about $1.5 billion less than what Chevron sought. A final decision is due Dec. 15.

Chevron is appealing for the same reasons its appeal of the 2007-2009 tax years, company spokesman Brent Tippen said.

"Regrettably, the use of an inappropriate assessment methodology is placing the county, as the tax recipient, and the citizens, as the recipients of services from taxes, and Chevron, as the taxpayer, in a tough position," he said.

Contra Costa Assessor Gus Kramer, whose office did the assessments, declined to comment before the final ruling on the appeal.