Alberta on as its own independent state would be net worse off in that they'd be a landlocked state and this would not at all directly advance their goals of getting more of their oil product to tidewater, one of their main political grievances. British Columbia would still oppose further oil pipelines to its coast for the same reasons it has always opposed them and in fact it would become politically easier for Canada to deny such access.

So the only viable outcome really is American annexation. (Additionally not advancing the Albertan grievance of only selling oil to one customer...)