Oil shales (kerogenites), are organic-rich, fine-grained, sedimentary rocks. Upon burial and heating, the dark organic matter of oil shale is converted to blebs of petroleum, which migrate out from the rock, migrate through the basin (typically upward & updip along fractures and bedding planes), and accumulate in reservoir rocks that have significant porosity (typically sandstones).
All petroleum (oil & natural gas) in the world was originally organic matter in oil shales. As global demand for petroleum continues to exceed supply (there's way too many people on Earth!), non-traditional sources of petroleum will increasingly be seen as attractive targets for economic exploitation. America has significant oil shale deposits. The most significant concentration in America is the Parachute Creek Member of the upper Green River Formation (lower Lutetian Stage, lower Middle Eocene). The Green River Formation is also famous for its beautifully preserved fossil fish faunas.
Oil shale (kerogen-rich marlstone; kerogenite) from the Eocene-aged Parachute Creek Member of the Green River Formation near the town of Rifle, northwestern Colorado, USA.