File:Weathered lacustrine oil shale (North Horn Formation, Cretaceous-Tertiary; Wales Spring, San Pitch Mountains, Utah, USA) 1.jpg

English: Oil shale from the Cretaceous-Tertiary of Utah, USA.

Oil shales, also known as kerogen-rich marlstones or kerogenites, are organic-rich, fine-grained, sedimentary rocks. With burial and heating, the dark organic matter of oil shale is converted to blebs of petroleum. Petroleum then migrates out from the rock and through its sedimentary basin, usually upward and updip along fractures and bedding planes. Finally, petroleum accumulates in reservoir rocks that have significant porosity - typically sandstones.

All petroleum (oil and natural gas) in the world was originally organic matter in sources rocks such as oil shales. As global demand for petroleum continues to exceed supply (Earth is obscenely overpopulated), non-traditional sources of petroleum will increasingly be seen as attractive mining targets. America has significant oil shale deposits - the largest occurrence is the Eocene-aged Parachute Creek Member of the Green River Formation. The Green River Formation is also famous for beautifully preserved fish fossils.

Seen here is an oil shale sample from Utah's North Horn Formation, which is older than the Green River Formation.

Stratigraphy: ~middle North Horn Formation, Maastrichtian Stage to Danian Stage, uppermost Cretaceous to lowermost Paleocene

Locality: Wales Spring, ~1/3 of the way between Wales Gap (to the east) and 7803' summit (to the west) along Chicken Creek Road, eastern slopes of the Gunnison Plateau / San Pitch Mountains, northwestern Sanpete County, central Utah, USA