Hundred of millions of years ago. when our planet was already formed. The planet was completely covered with water. Oceans were not as deep as they are today, but only a few hundred feet deep but thousand of miles long.
As time passed, green sea moss began to grow in the shallow areas.
Since there was no sea life, like fishes or other type of sea life, to eat or disturb the moss, it grew very thick.
After a while, this moss broke away and slid down in the deepest vallies. As more time passes this moss will be hunded of feet deep and thousands of miles long.
The moss at the lower depth start to decompose into a black slimy goo, as more moss come on top, great pressure of moss and water cause cracks in the botton of the vallies where this thick slimey goo began to fill the empty hollows in the earth where it was "cooked" by the heat and roation of the earth for a long period of time.
This is how I think crude oil was formed in the earth and not by decomposing trees and animals.
Please comment.