You've started off with a strawman. What has been argued is that in an underdevelopped country (such as Mexico) rife with corruption, labor abuses and human rights abuses, wealth is kept at the top and the prosperity only comes to those directing the projects from an executive possition. Saudi Arabia, the largest producer of oil in the world has also some of the highest poverty rates in the world. The list goes on: Venezuela, Nigeria, Kuwait. The pattern is obvious and has been explored before. In underdevelopped countries, oil does not mean prosperity for the country. It means prosperity for a select few and poverty and oppression for the rest.
The second part of your post is just laughable. What percentage of those who cross the border do oil workers constitute? The majority of those who cross the border are low skilled workers working in food manufaturing, cleaning services and grounds keeping. So what would opening up Mexico to oil field work do? Nothing. Especially considering oil companies are notorious for abusing the rights of workers and not paying wages.