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