Geology of the Marcellus Shale
Geology of the Marcellus Shale: A Paleontological Perspective on a Modern Resource
For more information, see The Marcellus Papers
The Marcellus Shale is a black shale formation extending deep underground from Ohio and West Virginia northeast into Pennsylvania and southern New York. It has long been known that the Marcellus Shale holds natural gas deposits; however recent technological advances and commodity price increases have made recovering these deposits very attractive to natural gas companies.
- Why is there so much interest in the Marcellus Shale?
- How was the Marcellus Shale formed?
- Why is the Marcellus Shale an “unconventional resource”?
- Is the Marcellus Shale geologically unique?
Why is there so much interest in the Marcellus Shale?
credit: Dept of Energy
This map shows the geographic extent of major natural gas basins that are located in nonconventional shale sources. Some of the most promising gas shales that have already begun active production have been the Barnett Shale and the Haynesville Shale. The Utica Shale, also in NY, is a smaller reserve than the Marcellus Shale, and extends into Canada.
Importantly, you can see how much larger the extent of the Marcellus Shale is in comparison to many of the other untraditional shales in the US. Only a portion of the entire extent of any gas shale is ideal for drilling. Therefore, there is the potential for a significant amount of gas to be extracted from the large Marcellus Shale basin.
How was the Marcellus Shale formed?
Shale is rock formed from very fine grained to clay-like sediments that have been compressed over time. The Marcellus Shale is the basal unit of the Devonian Hamilton Group, which is famous in New York state for beautiful fossil preservation of marine organisms. The reason that the Marcellus Shale is rich in natural gas is that when the shale was deposited as a mud, it was full of tiny pieces of organic material. The same heat and pressure that turned the mud into shale also ‘cooked’ the organic material, creating natural gas.
The above images show, on the left, the extent of the Marcellus Shale. The contour lines show the thickness of the Marcellus Shale in that area. Pink contour lines represent 50 ft of thickness, green contour lines show that the Marcellus Shale is 100 ft in that area, purple contour lines indicate 150 ft of thickness, and red contour lines represent 200 ft of thickness of the shale. The blue lines represent the places where the Marcellus Shale outcrops and we can see it on the surface.
On the right is a paleogeographic map of the NE US during the Devonian, about 386-390 million years ago, right around the time that the Marcellus Shale was deposited. You can see arches and basins rolling across the continent. The basins, or lowest parts, were below sea level at the time and were flooded by the ocean. So, NYS was once a sea, thriving with ancient life and occasionally suffering from underwater landslides. Some organisms living in that ancient sea were covered with sediment and fossilized.
The deeper parts of the sea collected more sediment and organic material, which is why the thickness and organic carbon content of the Marcellus Shale vary around the basin. This can be seen in the map on the left, which shows different thicknesses of Marcellus Shale. The swatch of really thick Marcellus Shale, noted by the green and purplish contour lines, is the area in which natural gas companies are particularly interested. Shale with a higher organic carbon content tends to yield more natural gas.
Why is the Marcellus Shale an “unconventional resource”?
Shale is an impermeable rock, meaning that it is very hard for fluids like water and gas to penetrate and move through the rock. This is in contrast to sandstone, which is where many individual water wells are drilled into. Being impermeable means that it is very hard to extract gas from the Marcellus Shale, since the gas is trapped in the rock and has very few pathways to flow through.
New technological advances, coupled with increased demand and cost of natural gas, has made it economically feasible to gas companies to begin extracting natural gas from ‘tight shales’ like the Marcellus Shale. Extracting gas from the Marcellus Shale deposit is accomplished through a combination of deep, horizontally drilled wells and a process called hydrofracturing, which creates small fractures in the shale deposit by pumping water into the rock. Sand in the water holds open the small fractures created by hydrofracturing. These fractures provide a path for the gas to get to the well for extraction.
See video of process
Is the Marcellus Shale geologically unique?
Most gas drillers estimate that, even with today’s technology, they will only be able to successfully extract around 10% of the natural gas locked in a ‘tight shale’ like the Marcellus Shale. One aspect of the Marcellus Shale that is somewhat different from most other known tight shales is that it contains very small spherules of pyrite, or fool’s gold, because of the chemistry of the ancient sea environment in which the shale was deposited. The pyrite spherules may provide additional natural pathways that are not present in some other ‘tight shales.’ Therefore, hydrofracturing the Marcellus Shale might connect more pathways than in other shales, increasing the natural gas yield from the average 10%.
The Marcellus Shale has also experienced a lot of fracturing during geological events such as mountain building. Because of its location, it is thought that it might be more fractured than many other ‘tight shales.’ Fractures in a rock provide natural pathways for gas to travel through. Hydrofracturing in intended to create such fractured pathways in the rock, so the addition of natural fractures is again hypothesized to increase the yield of natural gas from the Marcellus Shale in comparison to other ‘tight shales.’ In summary, the Marcellus Shale is one of the largest natural gas basins in the United States. It is a 'tight shale,' but its unique geologic features (e.g, its fractures and the presences of pyrite) lead some to suggest that it might produce more gas
than other such units.