healthmedicinet

Sample acquisition

Fresh exposures from Marcellus Shale outcrops (N = 11) were collected as part of research
at the Department of Energy, National Energy Technology Laboratory (NETL) in the Industrial
Carbon Management Initiative (ICMI) between May 2010 and September 2011. Four outcrops
were sampled from northern, surface exposures in New York State (NY) while the remaining
outcrop samples originated from southern, surface exposures in West Virginia (WV)
and Pennsylvania (PA). Outcrop samples were primarily from the Union Springs or Oatka
Creek members of the Marcellus Shale. Samples (N = 6) at six depth intervals (between
7,780 and 7,920 ft below ground surface) from a single core were provided by an industrial
partner, under terms of confidentiality, operating in Greene County, PA. Locations
of core and outcrop samples can be seen in Figure 1. Details of the outcrop samples, including lithologic and stratigraphic descriptions,
are given in Additional file 1: Table S1.

Figure 1. Approximate locations of outcrop (blue and red circles) and core (in Greene County, PA; colored green) samples. Outcrops were collected as fresh exposures.

While the samples studied here were not exclusively from gas-bearing members of the
Marcellus, if the REE are to be used as tracers it is important to have thorough characterization
of the REE in over- and underlying strata. Induced fractures (and therefore fluids)
often propagate “out-of-zone” and, at times, hundreds of meters vertically above and
below the perforation midpoint 22]. Therefore the variety of strata (within the Marcellus) studied here are generally
of interest for naturally occurring tracer applications.

Rare earth element abundance, correlations, and profiles

Concentrations of the study analytes are summarized in Figure 2, with sample-wise results presented in Table 1. In general, the REE varied over three orders of magnitude with 95% of all measurements
in the range between 0.324 and 75.2 ppm. As expected, the REE exhibited a “zig-zag”
pattern of abundance, consistent with the Oddo-Harkins effect 23]. Several samples were enriched—relative to world black shales 24]—in Pr, Dy, Ho, and Er, however most samples fell within the typical range for black
shales compiled by Ketris and Yudovich 24].

Figure 2. Rare earth element abundance in Marcellus Shale outcrop (open symbols; N = 12 for all analytes) and core samples (closed symbols; N = 6). Geochemical background (Bkgd.), anomalous (A
₀
), strongly anomalous (A
₁
) mass fraction ranges of world black-shales according to Ketris and Yudovich 10].

Table 1. Sample-wise results of LiBO
₂
fusion and ICP-MS analysis of Marcellus Shale samples

The REE were also highly, positively correlated in these samples. The median interelement
correlation (Spearman’s ) was 0.81 while 95% of all correlations fell between 0.47 and 0.98. The minimum observed
correlation (0.33) was between La and Y. In general, REE tended to correlate most
strongly with the nearest elements, with Sc correlating better with the LREE and Y
with the HREE (Additional file 1: Figure S3). Overall, the high correlations exhibited in these samples were consistent
with correlations determined in aqueous media 25], which was expected given the ubiquitous occurrence and coherent chemical properties
of the REE.

Rare earth element concentrations were statistically compared between core and outcrop
samples and between outcrop localities to determine if presumed weathering of outcrops
or regional variations might yield systematically different REE concentrations. Despite
the REE concentrations in the outcrop samples appearing to be more variable than in
the core samples (e.g. Eu in Figure 2), no statistically significant differences in element variability were determined
between the outcrop and core samples (Ansari-Bradley test for difference in scale
parameter; for all elements following Bonferroni–Holm corrections for multiple comparisons).
Similarly, no statistically significant differences were found in the central tendencies
of any of the REE between the two sample types (Wilcoxon rank-sum test for location
shift; for all elements, corrected for multiple comparisons). Analogous, parametric tests
(Bartlett test for homogeneity of variance and a t test) were performed, also indicating no significant differences (Additional file
1, Section: “Outcrop-core statistical comparison”).

Testing of reduced dimension variables, such as the total REE content, similarly exhibited
no differences between sample types. This could indicate that surface weathering processes
did not appreciably alter the REE composition. Alternatively, the small sample size
leads to aggregation of the samples as “outcrops” since insufficient samples were
available to compare among members of the Marcellus (e.g. Union Springs vs. Oatka
Creek). This could lead to false negative test results as inter-strata variability
could obscure variability due to weathering.

Application of the PERMANOVA test further confirmed the lack of difference between
the two sample types in bulk REE content ( from 10,000 permutations). While the apparent differences in dispersion or variance
between the types may not be detectable given the small sample size, the similarity
of medians corresponds with the findings of Chermak and Schreiber 18], where numerous, non-REE analytes agreed between core samples from different geographies
within the Marcellus.

Similar results (i.e. no statistically significant differences) were obtained for
uni- and multivariate comparisons between northern and southern outcrop samples. This
indicates that inter-regional variability of the bulk REE composition of the shale
may be less significant than intra-regional variability (i.e. at the stratigraphic
or mineralogical scale). However, the current dataset is insufficient to make meaningful,
statistical comparisons between stratigraphic groups.

REE profiles of these samples were variable, with enrichments of all REE weight classes—LREE,
MREE, and HREE—observed in PAAS-normalized patterns (Figure 3a, b). However, most samples exhibited LREE depletion (that is they had MREE/LREE
and HREE/LREE ratios 1) with MREE enrichments predominating (Figure 3b). Similarly, some samples exhibited negative Ce anomalies (Ce*  1), but most samples
had Ce and Eu anomalies near 1 (anomalies not pictured) fitting with an anoxic to
sulfidic, sedimentary environments such as those proposed for the Marcellus Shale
16], 20], 26]. No statistically significant differences were observed in REE patterns as either
sample type (core vs. outcrop) or sample locality (North vs. South). Taken together,
these results imply that variability in the REE profiles of the Marcellus Shale is
dominated at the mineral scale.

Figure 3. Post-Archaean Average Shale (PAAS) normalized REE profiles for Marcellus Shale outcrop
and core samples. a Representative REE profiles of samples from this study exhibiting HREE-, MREE-, and
LREE-enrichment. b Averaged, element ratio biplot. Points summarize the averaged, PAAS-normalized, interelement
ratios for each sample. The square, diamond, and triangle in (b) correspond to the REE profiles plotted with matching markers in (a) while circles represent all remaining samples. Filled symbols (N = 7) indicate core samples, while open symbols (N = 11) indicate outcrops. The generic order of normalized REE weight ranges (HREE,
MREE, LREE) are given in grey (e.g. “H  ML” in the upper right portion of the plot); see Stolpe et al. 35] and Noack et al. 25] for further interpretation.

Crystalline mineralogy determined by XRD

The results of semi-quantitative XRD analyses are presented in Figure 4. The predominant crystalline mineral phases identified in these samples were quartz
(classified as a major phase in 14 of 15 samples analyzed and as minor in 1 of 15),
illite (10/15 major, 3/15 minor, 1/15 trace, and 1/15 non-detect), pyrite (2/15 major,
7/15 minor, 1/15 trace, and 5/15 non-detect), and calcite (6/15 major, 2/15 minor,
and 7/15 non-detect). These results agree with the compilation of Chermak and Schreiber
18], who found other Marcellus samples to be predominantly phyllo- and tecto-silicates,
while other gas shales (such as the Antrim and Eagle Ford) were more carbonaceous.

Figure 4. Summary of semi-quantitative XRD analysis of samples in this study. Outcrop samples
are denoted by their location and core samples as “C-depth below ground surface”.
Sample “1-DGLS” is a core sample, but was collected from an unspecified depth. Additional
information regarding these samples is found in Additional file 1: Table S1. (Top) Example diffraction patterns with major matched peaks for two samples. (Bottom) Heat map of semi-quantitative XRD analyses for samples in this study. Model spectra
are referenced in Additional file 1: Table S4.

Comparisons among diffraction spectra and hierarchical cluster analysis of these spectra
for all samples are found in Figure 5. The cluster analysis shows some potential differences between core and outcrop samples
as four of six core samples cluster strongly (along with one outcrop). However, the
PERMANOVA test indicates no statistically significant differences between the XRD
patterns of either cores or outcrops (P  0.1 from 10,000 permutations). If the two
disparate core samples, “1-DGLS” and “C-7907” are removed from the analysis, a slightly
significant PERMANOVA result is achieved (P  0.05 from 10,000 permutations). The
primary mineralogical difference between these two core samples and the other cores
is the inferred presence of a major calcite phase (Figure 4). These samples (“1-DGLS” and “C-7907”) also exhibited the greatest REE profile fractionation:
in Figure 3a, b these samples are the green triangle and the blue square, respectively, which
exhibit significant profile fractionation.

Figure 5. Comparison of X-ray diffraction patterns for samples in this study. Outcrop samples
are denoted by their location and core samples as “C-depth below ground surface”.
Sample “1-DGLS” is a core sample, but was collected from an unspecified depth. Additional
information regarding these samples is found in Additional file 1: Table S1. (Left) Background-subtracted XRD spectra for samples in this study. Intensity was vertically
scaled to display all samples simultaneously. (Right) Cluster dendrogram of XRD patterns. Clusters and linkage heights calculated via
an average, unweighted distance algorithm. Intersample distances calculated as one
minus the inter-sample Spearman’s correlation coefficient over the range of 10°–45°.

Similarly, the results of cluster analysis provide little confidence in discernable,
mineralogical differences between the regionalized outcrop samples. PERMANOVA testing
confirms this observation, with no significant differences as a function of location
(P  0.5 from 10,000 permutations). However, the apparent lack of regionality (with
respect to these mineralogical and elemental analyses) may be an artifact of sample
size (as other geochemical parameters are known to be highly, regionally variable
in the Marcellus play 5], 7]) or may arise from the pooling of samples from unique strata.

Relationships between REE profiles and mineralogy

The Mantel test was used to test for correlation between intersample distances calculated
as a function of REE abundance and XRD spectra correlations. A moderate, positive
correlation was observed (Spearman’s  = 0.53, P  0.001), indicating that differences in the crystalline mineralogy of
the samples is a significant control on REE profile variability. This hypothesis was
further explored by applying Wilcoxon tests to both the degree of fractionation metric
and the total REE content using the semi-quantitative XRD results for each mineral
as the predictor variable.

In this analysis, the presence of major illite or calcite phases was shown to have
significant, contrasting effects on the REE abundance and fractionation (Figure 6). Total REE abundance showed a strong positive correlation with illite-enriched samples
(P  0.005). The Hodges–Lehmann estimator (HL) indicates that samples with a major
illite phase had approximately 98 ppm more total REE (95% CI: 39–158 ppm) than samples
without a major illite phase. Additionally, samples with major illite phases were
between 14 and 112% less fractionated than samples without a major illite phase (HL
95% CI; P  0.01). The latter finding seems intuitive as the degree of fractionation
is calculated relative to a composite of clayey shales (PAAS). This result also indicated
that the bulk of the REE concentration is likely found in the illite (or other clay)
phases of the samples or in trace phases correlated with the clays. The mechanism
of REE occurrence (i.e. sorbed or structurally incorporated) in these phases is not
immediately elucidated, as the separate mineralogical fractions of the shale were
not directly analyzed. Abanda and Hannigan 27] found that approximately 70% of the total REE content was likely associated with
the silicate/clay fraction of Utica shale samples.

Figure 6. Distribution of total REE and degree of fractionation in samples with and without
major fractions of illite or calcite. Distributions are depicted as standard boxplots41], where the thick, black line depicts the median; the boxed range represents the 25th–75th percentile, or inter-quartile range (IQR); and the thin whiskers denote all measurements within 1.5 times the IQR above or below. Individual values
of all observations are shown as blue dots. Statistically significant differences were found via the Wilcoxon rank sum test
(H
₀
: no difference in medians, P  0.05) between groups for both variables and both minerals,
as discussed in the text.

Conversely, samples with more calcite were between 54 and 400% more fractionated (HL
95% CI; P  0.005), with 6–120 ppm less total REE than samples without a major calcite
phase (HL 95% CI; P  0.05), corroborating the conclusions drawn regarding differences
in core samples, where dissimilar samples had a significant calcite fraction (Figures 3, 4). LREE-depletion has been observed in carbonate fractions of shales, potentially
being excluded from the crystal lattice while MREE and HREE, with more similar ionic
radii to Ca, are coprecipitated 27].

These postulates are supported by analyzing correlations between the major elements
of the shale (i.e. Al, Ca, Fe, K, Mg, Na, and Si; reported for the outcrop samples
studied here by Dilmore et al. 28]) and the total REE content as well as the degree of REE-profile fractionation (Additional
file 1: Figures S4, S5). Namely, strong positive correlations were observed between total
REE content and Al, Fe, K, Mg, and Na. This supports the hypothesis of total REE correlating
with clay phases. Given the general abundance of these elements in all geologic media,
substantial conclusions cannot be drawn on this data alone. However, Ver Straeten
et al. 17] utilized related multivariate statistics to infer mineral inputs into the Devonian
Appalachian Basin. Similar to Condie 29], no correlation was observed between total REE and P, indicating that minor phosphate
minerals, which can be strong REE accumulators 30], 31], did not contribute significantly to the REE content of these samples.

The implications of these hypothesized mineral associations can be related to the
potential for these shales to release REE during hydraulic fracturing. Since the REE
may be structurally bound within the clays (as opposed to sorbed at surface sites)
32], it is possible that produced water REE profiles will not resemble those of the bulk
shale. Yan et al. 33] found the REE to reside predominantly in the fine-grained fraction of a glacial till,
clayey aquitard, but associated evenly between seven mineral fractions (elucidated
through sequential leaching); the REE profiles of the adsorbed and exchangeable cations
fraction, which were MREE- to HREE-enriched accounting for 9–10% of the total REE
in those samples 33], most closely resembled the majority of profiles observed here. Conversely, the more
readily soluble fractions (such as the carbonates, which often produce LREE-depletion
27]) may be responsible for REE profiles observed in produced waters, which could be
used for source identification in the event of brine intrusion or waste spillage.
More study is necessary to determine the release mechanisms of the REE under conditions
relevant to hydraulic fracturing and solid waste disposal.