Christine Jones, age 13, of Montgomery, Alabama, for her question:
Does buried oil ever seen up to the surface?
This is a very logical question. We would expect the earth's oil to seep to the surface because it is one of the many light
weight substances, much more buoyant than the crustal rocks. When given a chance, lighter substances naturally seep or float above denser materials. Most petroleum reservoirs are deep underground, sealed there below dense layers of rock. But in some places the rocks above are cracked or thin. There the oil does indeed seep to the surface.
Most petroleum is buried deep underground because it had to remain undisturbed for many millions of years. Geologists suspect that it is a fossil residue from ancient marine plants and animals. Small creatures formed deposits of oozy slime on the ocean floors and later these were buried by crustal upheavals. Heat, pressure and ages of time caused chemical changes and the primeval fossils became petroleum.
It seems likely that the mixture had to be processed deep underground, sealed from contact with the air. While left undisturbed, it had no chance to seep through the dense rocks above it. But the earth's crust is well known for its restless upheavals. No doubt in the past, buried oil deposits were exposed when crustal formations were changed by earthquakes and mountain making. And what happened in the remote past can be repeated in more recent times.
We know of several places the rocks above oil deposits are weak and thin. There the buoyant oil seeps to the surface and comes in contact with the air. It is, of course, a mixture of hydrocarbons, similar to the biochemicals in living cells. Some of these ingredients are light and volatile. They quickly evaporate and go off into the air. Others follow and the residue becomes thicker. In time, the only ingredients left behind are tacky tars.
The asphalt deposits of Venezuela were formed from petroleum seepages. So was the tar pit near Trinidad in the Caribbean. Similar oil seepages also formed deposits of gilsonite and uintaite in Colorado and Utah. In fact, all the world's asphalt, pitch and tar products were formed by such oil seepages.
The most famous one is the La Brea tar pit that belongs to Los Angeles. It has been there since the ice ages, acting as a booby trap to catch unwary animals. Their fossiled bones were preserved in the gummy tar and now give us a picture of what prehistoric animals once roamed the neighborhoods.
Today the oil seepages at La Brea look somewhat like moist puddles on the ground, just as they did in ages past. When animals mistook them for water, they were trapped in gummy tar. The fossil remains of saber toothed tigers, giant sloths and bears and other extinct animals have been excavated. Many specimens are on display at the tar pits where so many creatures met their doom.