He is asking about how to clean the floor of his shed, not the dirt.


--James--

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If the oil has seeped into the ground, you're going to have to
dig up all the contaminated dirt, dispose of it in a legally
permissible manner (it's hazardous waste!), and replace it
with clean fill.

There are probably legal restrictions on how you go about
doing this; these restrictions vary from state to state. You
may need to report the spill to your local or state
government, get it inspected, get a permit to clean it up,
get an inspection after clean-up, and/or get certification
that you cleaned it up in a legally acceptable manner.