[chadwimc]

Not to burst your bubble... You might want to smell the ooze. I bet somebody's septic tank leech line is broken. Flush some dye marker tabs down the commode to see if it comes from the folk's house...

[The Grim Freeper]

That was the second thing I thought of, after oil, but there is no smell. It is not sewage.

The ground dried a bit and I just went back to look at it again. It seeped back into the ground, but it is still black in those patches where it had oozed up before--just not glistening.

[Prophet in the wilderness]

They should get a survey history of the properly.
Is it a individual home ? or in a development ?
What was there BEFORE the house(s) were built ?
Are there any archives in the county library or county register to check on the history of the property ? to check and make sure it was not some kind of industry there before the house(s) were built ? Need to make sure it might not be old drums of ( what ever ) oil underground.
If it is truly a crude oil reserve under the properly, get the rights, permits, certificate for Mineral Rights as soon as possible.
Find a HONEST ( if that is possible in todays world ) lawyer.

[pointsal]

Is there a gas station upgradient? This stuff travels on the surface of the water table. Before you think you've hit the legal jackpot, it would be wise to start walking one or two blocks and be sure you know who/what your neighors are doing or have done.

[BJungNan]

Not to burst your bubble... You might want to smell the ooze. I bet somebody's septic tank leech line is broken. Flush some dye marker tabs down the commode to see if it comes from the folk's house...

That's a lot more likely. And hope he does not put his hand in it anymore until he finds out.