"原生态可燃气体" can be translated as "primal/raw combustible gas" or "naturally occurring combustible gas" in English.
"Primal" or "raw" emphasizes the unprocessed, natural state of the gas.
"Naturally occurring" clearly indicates that the gas exists in its natural form without human intervention.
"Combustible gas" is the standard term for gases that can catch fire and burn.
Choose the phrase that best fits the context of your usage.