Oil-bearing rocks are sedimentary rocks that contain oil and gas. Shales are a type of oil-bearing rock that is composed of clay, silt, and other fine-grained sediment. They are formed from the compaction of mud and silt particles that have been deposited in layers over time. Shales are often found in areas where there is a lot of organic material, such as swamps and shallow seas. The oil and gas contained in shales can be extracted through drilling and fracking.