 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 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.