You need to create a road to the left of the oil well. The reason armies can get out is because building the oil well also creates a road underneath the well. That allows units to stack in the space. But when attempting to return the units would be moving from a square with no road (rough terrain I assume) onto a partially occupied square (the oil well).
Basically, when I have the choice I do not like to build oil wells in the line of travel of my armies as they do tend to create bottlenecks like this.
When I create scenarios I really try to make sure that these sorts of resource placements are not present.
Hope that helps.
Cheers :)