Methane, a.k.a. "Clean" Natural Gas

Once upon a time, oil drilling had no use for the flammable gas that accompanied the petroleum. They simply burned it in flares.
But now it is sent long distances by pipeline, and is a valuable commodity. Places like Indonesia even liquefy it and send it to more industrial places by tanker. It is then called LNG -Liquefied Natural Gas.

The gas that is available naturally is by no means clean, although it's easier to purefy than petroleum or coal.

The gas occurs naturally as methane, associated with ethane, propane, butane, and various noxious compounds of sulfur, and perhaps some carbon monoxide, carbon dioxide, and water vapor.

The pipeline pressures would turn propane and butane into liquids, a mechanical nuisance sloshing about and hitting the compressor blades. But they and ethane are well worth the trouble of condensing to liquid, for commercial sale, before being delivered to the pipeline.
The sulfur compounds are corrosive, so the "natural" gas had better be cleaned of them. In fact the pipeline contracts insist upon it.
Methane is actually a worse 'greenhouse gas' than carbon dioxide. But it is pretty silly of Exxon to tell us (on PBS, even!) that they're spending a few hundred million bucks to take out the carbon dioxide from the pipeline input stream, because for every molecule of methane used at the other end, a molecule of CO2 is produced, and two of water vapor.

Per energy unit, methane costs about half the carbon dioxide emission that carbon does. It's not the panacea that Pickens would have us believe.

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