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Some years ago I wrote two or three pages that appeared in the McGraw Hill Yearbook of Science and Technology, i.e., "Railroad Engineering." Even that source is a bit uncertain about the "origin of petroleum" to wit:

"Less than 1% of the organic matter that originates in or is transported to the marine environment is eventually incorporated into ocean sediment," and

"Most petroleum is formed during catagenesis (undefined anywhere). If sufficient organic matter is present oceanic sediments that undergo this process are potential petroleum sources. Deeply buried marine organic matter yields mainly oil, whereas land plant material yields mainly gas." (Their idea of "deeply buried" is the "out.")

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