Coal, oil, and natural gas are all fossil fuels, but they have different origins. Coal mainly comes from plants that grew on land and died long ago. The source for oil and methane — the main ingredient of natural gas — is something else.
"But they all more or less have a fairly common origin in that they were once some sort of algae or some other form of sea life, like plankton for instance, that died off long ago," said Dave Yoxtheimer, a geologist at Penn State who studies the Marcellus shale, the largest shale gas deposit in the U.S.
"The Marcellus shale is about 385 million years old, when much of North America was covered by shallow sea. We were much closer to the equator, so the sea was a much more productive sea that was teeming with marine life," he said.
These tiny plankton floating around were full of the same compounds that make up life today. They had carbohydrates, fats, and protein, all of which are made up of long chains of carbon atoms. In any of these molecules, the carbon atoms act like a backbone. When the plankton die those carbon chains settle down to the bottom of the sea.
"When you get to a certain depth the sea can go anoxic, basically be devoid of oxygen, and that further preserves that marine life that settled down at the bottom," Yoxtheimer said.
These long chains of carbon are buried by silt and other dead sea life, at which point the organic matter starts to become something different.
"You get more pressure pushing down on it. The temperature builds up," Yoxtheimer said. At sufficient temperature and pressure for enough time, that carbon-rich material starts to change into kerogen, a chemical compound that marks an intermediate step between dead sea life and oil or gas. Kerogen can range in texture from something that feels like wax to a soft crumbly rock.
Yoxtheimer said this stage is similar to when a piece of cooked meat isn't quite done yet.
"You put a chuck roast in a dutch oven or a slow cooker for the afternoon. If you try to take that chuck roast out after an hour, it might be brown, but it is still going be awfully tough," he said, "but if you let it sit in there and cook slowly, its gets tender and moist and actually could be pretty good by the end of the day. It's the same kind of process."
Likewise, these dead plankton are getting "cooked" into hydrocarbons, chains of carbon surrounded by hydrogen. "Basically what you are doing is breaking the bonds and kind of chopping the hydrocarbons into smaller and smaller hydrocarbons," Yoxtheimer said.
If the process stops in the middle it yields crude oil, made up of longer hydrocarbon chains. If it keeps going, shorter chains like propane — what's used in backyard grills — can result. But the chains can get even shorter.
"Ultimately, methane is the simplest of the hydrocarbons: one carbon, four hydrogens. And so what you've done is broken everything down to more or less its basic element," Yoxtheimer said.
It is also possible to leave the oven on for too long, according to Fred Baldassare, a geologist and former investigator for the Pennsylvania Department of Environmental Protection. He said hydrocarbons left underground too long will become overcooked, which is what happened in some parts of Pennsylvania's Marcellus shale.
"In fact if you go to the northeast part of the state in the Marcellus, there is what's called a line of death, where if the operators go a little too far, their million dollar investment turns into nothing," Baldassare said.
Across this line, the rock has been under too much heat and pressure and all the methane is gone. "On one side you could be making millions of cubic feet a day of natural gas and you move over 20 miles or so and there is nothing, zero," Baldassare said.
After the gas formed, some of it leaked into underground pockets. From those traps it was easy for drillers to remove.
"In conventional oil and gas development, the natural gas migrated from the source rock and it migrated up into a reservoir, and so it became trapped," Baldassare said.
Many of those caverns full of gas or oil, at least in the northeast, have been depleted, so industry found a way to get the gas directly out of the shale.
"They are the source material, and that's the trap too," Baldassare said.
There are trillions of cubic feet of methane trapped in tiny voids underneath the U.S. The gas that's captured will ultimately get burned. When this happens, like with all fossil fuels, it produces carbon dioxide, though less than oil, and far less than coal.