Pennsylvania - Shale Gas
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Pennsylvania – Shale Gas
I undertook this work to answer questions for myself and to help others understand more about the practice of shale gas extraction, commonly referred to as “fracking.” The location is in North-Central Pennsylvania, and the view is looking eastnortheast. This image represents a depth of three miles and a width of eight. A close inspection reveals five well sites (pads) on the land surface with shale gas wells installed in the Marcellus Shale.
The geologic setting: the bottom metamorphic layer formed over 1,000 million years ago within the eastern margin of the North American. 500 million years later the remaining sedimentary layers in the image were deposited during a 160 million year time span all while landmasses were coming together to form one supercontinent named Pangaea. A number of tectonic plates collided and attached to the eastern margin of North America. Each time, the crust was pushed upward forming a mountain chain, and then the mountains were worn down by erosion, sending vast amounts of sediment into an inland sea and the Appalachian Basin. When the next land mass collided, the same cycle repeated. Three of these cycles of collision and mountain building events (orogenies) occurred between 520 and 360 million years ago (mya) and are named the Taconic, Acadian, and Alleghenian orogenies.
The sedimentary layers found beneath the site include limestone, shale, and sandstone among others. In addition, fossils are found in certain rock layers such as trilobites, brachiopods, and ferns that help to date the rock. Some sediment, including the Marcellus shale, contained tiny plant and animal remains containing carbon and hydrogen compounds. Burial, heat, and pressure converted the organic matter into oil and gas. Today, gas that formed in this manner is being pumped from the Marcellus Shale, located one mile deep and shown in this image.
Groundwater and shale gas extraction: Two drinking water wells, 150- and 200-feet deep, were identified on the left side of the image. The risk of chemicals reaching groundwater comes from above and below the ground surface. Above ground, at the well site/pad, an average of 4 million gallons of water is mixed with chemicals and injected under pressure to fracture or “frack” the shale. Afterwards, an average one million gallons of wastewater is returned to the surface. Below ground a protective surface well pipe casing and cement grout seal is installed to protect the aquifer. A spill of 6,000 gallons of “frac flowback” wastewater was reported at one of the well site/pads located in this image. It flowed into a wetland that discharges to a stream.
Because it is not possible to represent features such as the shale gas and/or water well pipe at the scale shown on this website, an enlarged detail of the image is shown to the left.