How does buried organic matter become kerogen?

The formation of kerogen begins with the accumulation of organic remains—primarily single-celled planktonic plants and animals—at the bottom of ancient aquatic environments.1

Contrary to the idea that kerogen is a simple byproduct of decay, it is the result of specific geological and biological processes known as the immature stage of hydrocarbon formation. The process occurs as follows:1

• Rapid Burial: The remains of these organisms are buried rapidly within fine-grained sediments, which protects them from complete decomposition and preserves the organic material.1
• Diagenesis: Through a series of biological, chemical, and physical changes called diagenesis, the organic detritus undergoes bacterial alteration.1
• Conversion: Anaerobic microorganisms decompose the organic matter in the absence of free oxygen, leading to chemical rearrangements that transform the detritus into a dark-colored, insoluble substance known as kerogen.1

During this stage, the only hydrocarbon produced in significant quantities is biogenic methane. Over long periods of geologic time, this kerogen serves as the primary precursor for the eventual generation of petroleum.12