The question was raised about cleaning soil contaminated with
petroleum.
If the amount of petroleum in the soil is very high, it
might be a good idea to simply dig it up and throw it away.
At least this would get rid of most of the problem. If,
however, the amount of petroleum in the soil is only
present in small amoumts, it can be cleaned by adding
and digging in a lot of composted organic material to
build a more robust micro organism population and they
will do the job for you.
Maaybe the most practical answer is to do both . . .
dig up and throw away the soil that seems the most contaminated
and dig in composted organic material to the whole general area.