A Bechtel unit won a big government contract.

Bechtel Petroleum, an affiliate of the giant San Francisco construction company, has been chosen to manage the Naval Petroleum Reserve in California for almost $900 million over the next five years, the Energy Department said. Bechtel Petroleum will replace Williams Bros. Engineering Co. of Tulsa, Okla., manager of the reserve since 1975. The reserve in Kern County consists of two fields, Elk Hills, the second-largest single field in the United States, and Buena Vista.