Rock "traps" produce petroleum (OG-13)
Indianas oil wells produced nearly 5 million barrels of oil during 1978. Most of this oil comes from rocks of Mississippian age (300 to 350 million years old) underlying southwestern Indiana. Those porous limestones and sandstones are commonly saturated with water. If oil is present, it rises to the highest parts of the porous formations because it is lighter than water. Oil-bearing traps are capped by impervious rock, generally shale, which prevents the oil from escaping until the cap rock is pierced by the drill. The illustration above shows how oil accumulates along faults (left), on domes and anticlines (center), and in stratigraphic traps (right). Most of Indiana's oil occurs in various stratigraphic traps. The inset drawing shows a generalized view of oil occurrence in sandstone.

Our Hoosier State Beneath Us: Oil and Gas