Dec 30 (Reuters) - Chevron (CVX.N) on Thursday said it was postponing a scheduled January full return to office for its two largest U.S. work sites as the Omicron variant spreads across the United States. This week, the United States reported a record number of coronavirus cases based on the seven-day average, with over 290,000 new infections each day, according to a Reuters tally. Chevron, which postponed return to work plans for its San Ramon and Houston offices, joined several big U.S. employers that have reversed course in recent weeks on plans to bring employees working from home to their corporate campuses. A spokesperson for Chevron did not specify when employees would return to the office full time. The North American unit of energy firm Engie (ENGIE.PA) also was set to bring its U.S. employees back to their offices next month, but pushed the start date to late February, according to a person familiar with the matter. The company did not respond to a request for comment. In mid-December, asset management firm Fidelity Investments said it was pausing a voluntary return-to-office for employees in New England amid surging Omicron cases, while JPMorgan instructed unvaccinated staff members to work from home and moved an annual healthcare conference set to be held in January online amid concerns of the fast-spreading Omicron variant.