Some people think that the oil exists in the big oil field. In fact, most oil reserves in the tiny crack between rock or sand. The crack is too tiny to see with the naked eye. It can examine under microscope, or can get the voids volume by testing. Generally, the percentage of volume of oil rock space to rock is 10%-35%. The professional use the porosity and permeability to measure the reservoir stratum is good or not. If the porosity is big, the amount of oil is large. If the permeability is high, the oil is easy to flow or be exploited.
The oil is generated by kerogen. After it is generated, the oil is absorbed by residual kerogen in the first. When the amount of oil generated is too much to absorb by the kerogen, the oil will flow into the space of rock strate. The pressure in stratum increases when the oil is generated much enough which can vent the oil and gas to higher porosity stratum. Sometimes, the high pressure can make the stratum broken which speed up the oil and gas discharge. (First migrate) When the oil and gas flow to the stratum with high porosity, they can move fast along the formation to shallower stratum. This is the second migrate. The oil and gas will stay up when they encounter oiltight stratum in migration. They collect in the confining bed from above down slowly. This kind of rock which can both store liquid and make the liquid flow in it is called reservoir stratum. The oil pool formed when the oil and gas become more and more. This kind of oil gather style is just like sponge absorbing water. Just because there are reservoir rocks, and trap structure, oil can settled quietly in underground, waiting for the arrival of the scouts.
If the oil constantly, continue to collection until the confining bed is filled the complementary oil and gas will overflow by the below excessive mouth and migrate to shallow place through the other way. Most oil and gas residual in the stratum can not escape out the fate of swallow by bacteria. Only a few oil and gas can serve the human race. The oil and gas’ ingredients will be changed seriously and the oil and gas become heavier and more sulfur because biochemical interaction. This kind oil is not liked by human.