Evolution of porosity and geochemistry in Marcellus Formation black shale during weathering

Lixin Jin, Ryan Mathur, Gernot Rother, David Cole, Ekaterina Bazilevskaya, Jennifer Williams, Alex Carone, Susan Brantley

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Abstract

Soils developed on the Oatka Creek member of the Marcellus Formation in Huntingdon, Pennsylvania were analyzed to understand the evolution of black shale matrix porosity and the associated changes in elemental and mineralogical composition during infiltration of water into organic-rich shale. Making the reasonable assumption that soil erosion rates are the same as those measured in a nearby location on a less organic-rich shale, we suggest that soil production rates have on average been faster for this black shale compared to the gray shale in similar climate settings. This difference is attributed to differences in composition: both shales are dominantly quartz, illite, and chlorite, but the Oatka Creek member at this location has more organic matter (1.25. wt.% organic carbon in rock fragments recovered from the bottom of the auger cores and nearby outcrops) and accessory pyrite. During weathering, the extremely low-porosity bedrock slowly disaggregates into shale chips with intergranular pores and fractures. Some of these pores are either filled with organic matter or air-filled but remain unconnected, and thus inaccessible to water. Based on weathering bedrock/soil profiles, disintegration is initiated with oxidation of pyrite and organic matter, which increases the overall porosity and most importantly allows water penetration. Water infiltration exposes fresh surface area and thus promotes dissolution of plagioclase and clays. As these dissolution reactions proceed, the porosity in the deepest shale chips recovered from the soil decrease from 9 to 7% while kaolinite and Fe oxyhydroxides precipitate. Eventually, near the land surface, mineral precipitation is outcompeted by dissolution or particle loss of illite and chlorite and porosity in shale chips increases to 20%. As imaged by computed tomographic analysis, weathering causes i) greater porosity, ii) greater average length of connected pores, and iii) a more branched pore network compared to the unweathered sample.This work highlights the impact of shale-water-O2 interactions in near-surface environments: (1) black shale weathering is important for global carbon cycles as previously buried organic matter is quickly oxidized; and (2) black shales weather more quickly than less organic- and sulfide-rich shales, leading to high porosity and mineral surface areas exposed for clay weathering. The fast rates of shale gas exploitation that are ongoing in Pennsylvania, Texas and other regions in the United States may furthermore lead to release of metals to the environment if reactions between water and black shale are accelerated by gas development activities in the subsurface just as they are by low-temperature processes in our field study.

Original languageEnglish
Pages (from-to)50-63
Number of pages14
JournalChemical Geology
Volume356
DOIs
StatePublished - Oct 9 2013

Funding

We acknowledge A. Dere, A. Fornadel, J. Paul, V. Prush, C. Ebersole, J. Kaye and K. Downie for soil sample collection and analyses, and H. Gong, L. Liermann and W. Castro for ICP AES and ICP MS analyses. We acknowledge support from the Susquehanna-Shale Hills Critical Zone Observatory ( NSF-EAR-0725019 to C. Duffy, Penn State) and from the Department of Energy Grant ( DE-FG02-05ER15675 to SLB), and neutron scattering instrumentation and technical support from David Mildner at NIST-NCNR. The research using the small-angle scattering instruments at the National Institute of Standards and Technology was supported in part by the National Science Foundation under Agreement No. DMR-0944772 . DRC was supported by the Department of Energy Office of Science Basic Energy Sciences Energy Frontier Research Center — Nanoscale Control of Geologic CO 2 . GR's effort was sponsored by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Science, US Department of Energy .

FundersFunder number
Department of Energy Office of Science Basic Energy Sciences Energy Frontier Research Center
NIST-NCNR
Susquehanna-Shale Hills Critical Zone ObservatoryNSF-EAR-0725019
National Science Foundation0725019, 1239285, DMR-0944772
U.S. Department of EnergyDE-FG02-05ER15675
Basic Energy Sciences
Chemical Sciences, Geosciences, and Biosciences Division

    Keywords

    • Computed tomography
    • Neutron scattering
    • Organic matter
    • Pyrite dissolution
    • Shale gas
    • Trace metals

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