What's Oil Doing in Pennsylvania?

Hundreds of millions of years ago, northwestern Pennsylvania was a shallow sea, with mountains to the east, in what is now eastern Pennsylvania and New York. Rivers from these mountains carried sand to the ocean. This sand got reworked by waves forming sand bars, in many ways just like what we see today along the Gulf Coast of North America. When the oceans rose, less sand made it as far as western Pennsylvania. Mud, however, did make it this far west, and so it was being deposited at this time. When sea levels again fell, sand was again deposited in northwestern Pennsylvania. This formed alternating layers of sand and mud.

The diagram at the left shows the conditions of the mid-Atlantic region of the United States during the Devonian time period, about 350 million years ago. Mountains to the east (known as the Acadian Mountains) shed sediments to the west, where there was a shallow sea. Alternating layers of sand and clay deposited in this area, and reworked by the waves of the shallow ocean, created conditions that were perfect for the trapping of oil.

After millions of years of burial, heat and pressure turned these sediments into rock. Because the mud and sand alternate based on the rising and falling seas, and because the sand often got dumped in certain areas (depending on where the river entered the sea at any given time) and not in others, the sand often formed pods. These pods, blanketed in mud, make something known as a stratigraphic trap. Although throughout the Oil Creek Valley region there is the structure of an anticline, which in many parts of the world is responsible for creating the trap which holds the oil, it is these stratigraphic traps which hold the oil in northwestern Pennsylvania.

...continue learning about the geology of the Oil Regions

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