How much do you know about natural gas?

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In any conversation about fuel and resource consumption, you're likely to hear at least a passing reference to natural gas. Some argue it could wean the modern world off of its dependence on oil and coal. It burns cleaner than other fuel sources, and is readily available in many locations around the globe.

But what is natural gas, and how does it form? Read on, because understanding the answers to these basic questions can paint a clearer picture of the plusses and minuses of this versatile fuel source.

In its unprocessed form, natural gas is actually a mixture of naturally occurring flammable gases. The mix varies from source to source, but its chief ingredients include methane, butane and propane. Like coal and oil, natural gas can form deep underground. The remains of ancient plants and animals decay under the intense heat and pressure created by the layers of sand, silt, rock and water above them. Over time, this geologic pressure cooker causes the decaying materials' carbon atoms to break down, forming thermogenic natural gas.

Since both natural gas and oil come from the same raw material, deposits of both often appear in the same location. In fact, ancient organic matter becomes one fuel or the other depending on its depth and temperature: Deeper deposits, exposed to more of the Earth's heat and pressure, have much higher concentrations of natural gas.

But natural gas can also form in a much shorter, more immediate period of time. Just ask a livestock farmer — or the U.S. Department of Energy. Either will tell you that organic matter such as animal waste produces significant amounts of gas as it decomposes, thanks to methanogenic organisms: bacteria that eat the waste and produce methane as their own waste product.

This biogenic natural gas (or biogas) formed from the decomposition process presents a tremendous renewable resource. According to a 1998 study by advocacy group Natural Gas Vehicles for America, biogas could account for 6 percent of the natural gas used in the U.S., the equivalent of displacing 10 billion gallons of gasoline use per year as of the study date. Biogas typically contains 20 percent to 30 percent less methane than is found in thermogenic natural gas. But purification can filter and concentrate that methane, making biogenic gas useable for heating and transportation.

Techniques for extracting natural gas vary as much as the processes that create it. Thermogenic gas deposits are mined in much the same way as oil: Deep-drilled wells pump the gas to the surface, where it's purified and stored for transport. For deeper deposits, a controversial process called hydraulic fracturing, or fracking, uses a mix of drilling fluids to break through layers of gas-containing shale. This opens the door to more gas, but critics argue that it can contaminate regional water supplies without careful adherence to safety regulations.

Biogenic natural gas is often collected using enclosed chambers called digesters. These airtight chambers collect biogas from waste placed in them. Biogas from large, well-established landfills can sometimes be mined through shallow wells drilled into the mountains of trash.

However companies choose to collect natural gas, the end result for consumers is the same: This cleaner-burning fuel is becoming more widely available, offering an increasingly attractive option for consumers fed up with rising oil prices and concerned about their impact on the environment.