How does kerogen form in oil shale deposits?

Kerogen forms through the accumulation and burial of organic material, such as algae and various plant remains, at the bottom of ancient lakes and shallow seas. Over vast periods of geologic time, this organic matter undergoes compaction and alteration by bacteria within oxygen-depleted, fine-grained sedimentary environments.123

The resulting substance is a complex, insoluble mixture of hydrocarbon compounds. While some kerogen retains identifiable organic structures like algal colonies, much of it is amorphous, having been transformed by microbial activity during sedimentation. This process often occurs in quiet waters where fine clay minerals or carbonates settle, creating the characteristic laminar structure of oil shale deposits. Because coarse sediments like sand do not support the preservation of organic matter, oil shale is typically found in fine-grained rocks.123