59th Annual Report on Research 2014

This study documents the high spatial resolution primary sedimentary characteristics of the Marcellus shale and the relationships of those characteristics to today’s mechanical and chemical behavior of that unit. The scales of observation range from macroscopic features visible in New York State rock quarries and cores, to micron-scale features that require modern laboratory imaging, repeated for sedimentary columns separated by distances of hundreds of meters to tens of kilometers (hundreds of feet to tens of miles). The goal is to facilitate prediction of parameters that are important to minimizing the environmental impacts of fossil fuel production from shale. In addition to providing data valuable to users who range from companies to communities in the Marcellus region, our goal is to quantify variability of rock properties in terms that may be applicable in other shale systems. The availability of data has led to focus on the lower formation, the Union Springs, of the Marcellus Subgroup.

The Principal Investigator, two geological sciences graduate students, and an undergraduate student participated in the study during 2013-2014, in collaboration with three faculty members and two graduate students from Cornell’s engineering fields. New collaborations developed with faculty members at three other colleges/universities enabled collection of certain types of data. Three cores from the collection of the New York State Geological Survey were added to the Marcellus quarry columns whose samples were already in the process of analysis, such that we now have materials from 8 locations. In addition to thin-section petrography and SEM analyses, this year we obtained elemental data (XRF) and total organic carbon (TOC) for the quarry samples. Through collaboration with colleagues at the University of Oklahoma we also obtained mineralogy, density, gamma-ray, and NMR porosity for one of the columns. In collaboration with environmental engineers, our group conducted a pilot leaching experiment: a sample with 12%TOC was submerged in 0.0025M KCl aqueous solution and progressive changes in water chemistry over five weeks of rock-water interaction were determined using Inductively Coupled Plasma/Optical Emission Spectrometry. This experiment informed the design of a set of experiments to examine the relationships of sedimentary facies to leachate compositions, with a special focus on radioactive constituents, that will be completed in the next project year.

A preliminary report on results was presented as a poster at the 2014 AAPG Annual Convention, which focused on the degree of variability across length scales of three Union Springs stratigraphic columns. At four locations (separation distances 160 m, 600 m, and 39,000 m) three lithofacies were studied: a limestone member; a thicker, organic-rich mudstone member; a member containing pervasive calcite concretions. These were subdivided into 12 microfacies. Although grossly speaking the arrangement of these larger-scale lithofacies is constant among the columns, the microfacies reveal differences among the locations. Variations over 39 km distance are important in the basal limestone member.

A general premise for this study is that one can utilize knowledge of the Devonian sedimentary environments at a few locations, with appropriate statistical descriptions of variability, as a basis for prediction of changes in rock properties at locations that have not been sampled. An intermediate step is the use of well log correlations conducted in a sequence stratigraphic framework. Our initial criteria for the Devonian environmental conditions were physical sedimentology, trace fossils, and biogeochemical proxies, which other workers utilize in studies whose scale of resolution typically is ≥2 cm of stratigraphic thickness. Because our focus is inclusive of the lamina and sub-lamina scale, <1 mm, we completed a pilot study of the distribution and types of microfossils throughout one of the columns from a quarry and found that pelagic fauna dominate. The abundance found of dacryoconarid specimens led us to initiate a collaboration with a paleontologist (Skidmore College) who specializes in this family of zooplankton. We designed with him an in-depth study whose objective is to tie the details of dacryoconarid distribution and faunal turnover in dacryconarids, which are the most abundant constituent of the Union Springs, to the physical, chemical and isotopic indicators of the environmental conditions of the Eifelian Appalachian epicontinental sea. We are making the faunal and sedimentological comparisons at the natural scale at which the sediment accumulated, the laminations. The Union Springs and immediately overlying Cherry Valley limestone encompass two faunal overturns that were widespread in the Appalachian Basin, such that this study will probe the associations between the Marcellus sedimentological and geochemical properties and these paleobiological events. This work has begun.

The mechanical behavior of Marcellus mudstones, which are natural composite material, was a major focus throughout the year, represented by a paper presented at the Unconventional Resources and Technology conference. As a step toward understanding the controls on bulk mechanical properties, we collaborated with material scientists and mechanical engineers in characterizing the mechanical properties of the constituents at the sub-millimeter scale. We utilized micro- and nanoindentation procedures to analyze hardness and elastic modulus of our geological samples. We show that nanoindentation and micro-fracture experiments provide detailed information about the spatial variation of the mechanical properties in these rocks at the scale of the fundamental constituents. However, we demonstrate also that such results are highly affected by experimental conditions. For instance, micro-indentation created cracking that was not produced by nanoindentation, which brought into play additional deformation mechanisms that reduce the measured hardness measured by micro-indentation compared to that measured by nanoindentation. Furthermore, analysis of elastic modulus through nanoindentation demonstrated that compliance of the matrix in which a particle is embedded creates a false degree of variability in the elastic moduli across a suite of particles of like mineralogy. Therefore, we applied newly developed methods and show that the modulus of calcite in shale is not highly variable. However, these improved nanoindentation methods cannot be applied effectively to the smallest constituents because of grain-boundary effects. It is likely that even the mineral calcite, which is a common constituent of organic-rich mudrocks, has a degree of mechanical variability because it is derived from multiple biogenic and authigenic sources. This is a focus for ongoing nanoindentation study.