I would think they can do that as soon as they like. Most of us are or were at will employees who can be let go at any time for any reason except for a very short list of legally prohibited reasons like gender or race. You can be let go and replaced at any time just because they don't like you. If, instead, you were laid off and received some benefit other than your final pay check you are ahead of about 99% of the workers in America. Chevron has a long history of using periodic layoffs to trim the ranks rather than implementing effective management processes for hiring, coaching, accountability for poor performance, and moving out people who are retired in place, unpleasant, or otherwise don't fit through no fault of their own.
It might be unfair and it might s---, but I sincerely doubt if it is illegal.