Then beginning roughly 390 million years ago, the organic rich rocks to the west were thrust eastward, eventually riding up and over the Silurian and Devonian reefs.  As the thrusting continued, a portion of the rocks became elevated and were eroded, forming another organic rich deposit on top of the old reefs.  This thrusting event, which is known as the Antler orogeny, set the stage for today’s oil fields by placing organic rich rocks in close proximity to the very porous reef rocks.