Geology
Most of the oil produced in northwestern Pennsylvania was formed in sandstone reservoir rocks at the boundary between the Mississippian and Devonian rock layers. Over time, the oil migrated toward the surface, became trapped beneath an impervious layer of caprock, and formed a reservoir. The presence of upwards-curving folds in the caprock called anticlines, or sometimes an inversion of an anticline called a syncline, greatly varied the depth of the reservoirs, from around 4,000 feet (1,200 m) to just beneath the surface.
The majority of the oil wells in the vicinity of the Oil Creek valley tapped into a sandstone formation known as the Venango Third sand. The Venango Third contained large volumes of oil under high pressure at only 450 to 550 feet (140 to 170 m) below ground level. Other oil-producing formations in the area were "the Venango First and Second, the latter often prevailing after the Third sand was lost."
Read more about this topic: Oil Creek State Park