Where does the gas come from?
To find gas, you have to have some idea where to start looking for it. This means you have to have some idea how it formed in the first place.
Once upon a time...
This is how scientists think it happened: both oil and gas are the remains of animals and plants that were alive millions of years ago in seas or lakes. This is how we think it happened...
About 300 million years ago
Plants and animals living in the water die and sink to the bottom where they mix with sediment washed from the land. The mixture is buried under more layers of sand.
A bit later...
More layers build up, trapping and squashing the decaying remains of the plants and animals. This goes on for a very long time.
Later still... (but still millions of years ago)
Over many millions of years, the layers build up, burying the decayed remains deep under the surface. They are heated and squashed by the movement of the earth's crust and turn to oil and gas in the layers of rock built from the sediment.
Much later (now):
The movement of the earth's crust can result in rocks being bent. Oil and gas, from the decay of the plants and animals, is trapped if the layer on top of it will not let it rise upwards. Scientists look for these rock shapes because they might contain gas.