Seal Beach-based Clean Energy Fuels Inc., backed by legendary oilman T. Boone Pickens, has upped the size of its pending public offering to $345 million from $287.5 million, according to an updated federal filing.

The offering stands to be one of the larger ones for Orange County in recent years.

Clean Energy, which provides natural gas for buses and other fleet vehicles, plans to stage an "OpenIPO" where the price and allocation of shares will be set in an auction done by the underwriters and other securities dealers.

WR Hambrecht & Co. and Simmons & Co. are the underwriters.

The number of shares to be offered, a price range or timetable weren't disclosed in Tuesday's filing.

Clean Energy said it wants to use the proceeds to build a liquefied natural gas plant, add natural gas fueling stations, buy natural gas vehicles for sale to customers, as well as for general business purposes.

The company bills itself as the largest provider of natural gas for vehicles in North America. Clean Energy serves fleets of trash and other trucks, buses, shuttles, taxis and police cars.

Pickens is Clean Energy's principal shareholder, founder and a director. He owns about 73% of the company.