Heat the oil in a 12-inch dutch oven. Add the celery, onions, and carrots. Saute until onions are translucent, about 5 minutes. Add potatoes and cook another 10 minutes. Mix flour and milk to make a thin paste. Add the paste and the and the stock and bring to a boil. Add herbs and simmer until the potatoes and carrots are tender. Before serving, stir in chicken and corn and heat through for about 5 minutes. Season with salt and pepper to taste.