Select the word that best fits the definition the remains of these form oil and natural gas

This question has been answered before, here's what the answer stated:

"Oil and natural gas result from the decomposition of microscopic marine plants and animals which lived in the geological past times, tens of millions years ago. When these creatures died they accumulated on the sea floor and got covered under new layers of sediments. As they become buried deeper and deeper, heat and pressure began to rise transforming this biomass into oil and gas"

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