Here is the solution to Mystery Map VIII.
On Skidmore’s campus and beyond, debate rages concerning the morality, feasibility, and economic dynamics of the hydraulic fracturing of shales, or “fracking”—the process of fracturing deeply buried shales to exploit otherwise-inaccessible gas resources. In spite of heated discourse on the issue, however, many people lack a fundamental understanding of fracking’s root: the science of the shale itself. The GIS Center for Interdisciplinary Research has compiled a map that tries to set aside fracking’s politics while looking into its geologic foundation.
The Center thus constructed a map of U.S. shale basins and tight-gas basins with EIA (US Energy Information Administration) data. On the map, yellow shading marks shale basins, while striped shading illustrates tight-gas basins. Shale basins are basins in which shale has formed; tight-gas basins are areas within shale basins that contain low-permeability black shales—shales that require fracking to release natural-gas resources.
But what do black shales have to do with shale gas, and how does shale gas form?
Black shale, a sedimentary rock that forms in beds, produces shale gas as it becomes buried within Earth’s crust. The shale originates as mud that collects on the bottoms of low-oxygen, often-marine basins. Due to minimal oxygen, bottom-feeding organisms cannot survive on the basin floors, and thus do not sink into the sediments; nonetheless, dead plankton and the deceased anaerobic bacteria that would have eaten those plankton settle onto the floor, into the muddy basin-floor sediments. Those organisms constitute the organic shale material from which shale gas ultimately forms.
Over geologic time—hundreds of thousands to millions of years—additional layers of rock and sediments bury and compress the organic-rich basin-floor sediments, producing black shale. Subsurface temperatures and pressures increase with depth in the Earth; both variables foster transformation of black shales’ organic constituents to shale gas. Between two and four kilometers of depth, organic material begins to convert chemically to potentially oil-yielding compounds. As temperatures increase with depth, oil formation gives way to gas formation, which, in turn, terminates at extreme temperatures (~160°C+). Shale gas thus forms on a long-term chemical and physical continuum within Earth’s surface, and may vary locally.
Black shales have formed throughout much of Earth’s biological history. The Marcellus, for instance, a Northeast-U.S. shale-limestone subgroup in the midst of the fracking debate, formed ~400 million years ago, during the Devonian Period of Earth’s history. Tectonic processes during the Devonian created basins on the North American east coast. Moreover, warm global climates may have facilitated high sea levels and may have altered the circulation of Earth’s oceans—both of which may have contributed to the development of anoxic (<0.1 milliliter of oxygen per liter of seawater) marine basins that fostered black-shale production.
Tectonics and climate patterns have changed and fluctuated since the Devonian and other periods of geologic time. Today, therefore, many black shales—and their gases—are on land, buried or associated with diverse and extensive other rock layers.
It looks like Adam Schmelkin, Bert, Joe Marto, Britt Dorfman, Peter, Adam, and Ron Schott all guessed it right. Great work. Adam S. posted the correct answer first, so please stop by the GIS Center to claim your T-shirt!
Here is a link to the maps’ data sources. For further reading, see the following sources:
US Energy Information Administration http://www.eia.gov/analysis/studies/worldshalegas/
Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman.
Marshak, Stephen M. Essentials of Geology (Third Edition). New York, NY: W. W. Norton & Company, 2009.
Prothrero, Donald R. Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy. New York, NY: W. H. Freeman and Company, 2004.