HOLY Sh*t!
5'x5' area? I had a small accident during my last oil change
I immediately poured in HOT water and put some liquid dish washer soap (few bottles of AJAX) and started scrubbing. I did this for at least an hour. Then, washed it off with a another bucket of hot water. Then, thanks to mother nature, it rained 6 days straight. Once everything dried up, I have NO oil stain. BUT your case is way more severe. I think on my home computer I have a link saved up for a good concrete oil stain cleaner. I will try to remember to post it here.