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Copyright ©2019 by The American Association of Petroleum Geologists.
DOI: 10.1306/13672211M1213822
5
Diagenetic Pyrite Morphology in
Mudstones of the Upper Ordovician Point
Pleasant Limestone, Appalachian Basin:
Evidence for Dysoxic Deposition
David R. Blood
DRB Geological Consulting, 4116 Marion Hill Rd, New Brighton, PA, 15066, USA
(e- mail: randy@drbgeological.com)
Steve Schlaegle, Christopher M. Hefferan, Alexa Vazquez, and Darlene McAllister
RJ Lee Group, 350 Hochberg Rd, Monroeville, PA 15146, USA (e-mails: SSchlaegle@rjleegroup.com;
CHefferan@rjleegroup.com; AVazquez@rjleegroup.com; Dmcallister@rjleegroup.com)
ABSTRACT
Organic- rich mudstones of the Appalachian Basin hold a sizable portion of the natural gas
produced in the United States. Indeed, in 2015, Pennsylvania and West Virginia accounted
for 21% of produced natural gas, driven in part by production from the Point Pleasant Lime-
stone. The critical role that unconventional reservoirs will play in future global energy use
necessitates the need for an enhanced understanding of those geological aspects that shape
and influence their reservoir architecture. Foremost among these is a clearer understand-
ing of the preservation and accumulation of organic carbon, as it is the source of hydro-
carbons, and often provides the dominant host of interconnected porosity and hydrocarbon
storage. To this end, pyrite morphology can offer insight into the redox conditions of the
bottom and pore water environment at the time of sediment deposition and early diagenesis
and can be especially useful in the analysis of deposits devoid of redox sensitive trace met-
als. Pyrite contained in cuttings and core chips retrieved from vertical and horizontal Point
Pleasant Limestone wells were analyzed by scanning electron microscope. Results demon-
strate a dearth of pyrite in the Point Pleasant (0.02–1.7% of the surface area analyzed). Pyrite
morphology is dominated by euhedral grains and masses (~80% of pyrite encountered) co-
occurring with infrequent framboids. Framboids are uniformly small (average = 4.7 µm)
with just a few examples >10 µm. The presence of small amounts of euhedral pyrite grains
and masses is consistent with accumulation under a dysoxic water column. Conversely, the
size of the framboids suggests that they formed in a water column containing free hydrogen
Blood, David R., Steve Schlaegle, Christopher M. Hefferan, Alexa Vazquez, and
Darlene McAllister, 2019, Diagenetic pyrite morphology in mudstones of the
Upper Ordovician Point Pleasant Limestone, Appalachian Basin: Evidence for
dysoxic deposition, in W. Camp, K. Milliken, K. Taylor, N. Fishman, P. Hackley,
and J. Macquaker, eds., Mudstone diagenesis: Research perspectives for shale
hydrocarbon reservoirs, seals, and source rocks: AAPG Memoir 120, p. 69–82.
14252_ch05_ptg01_069-082.indd 69 11/26/19 1:17 PM
70 BLOOD ET AL.
sulfide. Amodel invoking a lack of reactants necessary to sustain diagenetic pyrite growth
in anoxic pore waters may explain this apparent paradox. In such a case, the framboid size
distribution may reflect newly forming diagenetic framboids competing for a finite amount
of reactants resulting in a population of small framboids and few large examples. Indeed, the
low total iron/aluminum (Fe/Al) content of the Point Pleasant (average Fe/Al = 0.45) would
indicate a low delivery of reactive iron to the seafloor during Point Pleasant deposition. The
data suggests a model in which organic carbon preservation occurred by rapid burial and
removal from oxygen- bearing water. In turn, more organic- rich and potentially higher qual-
ity reservoir facies of the Point Pleasant Limestone occur in areas of higher clastic delivery
to basin.
Figure 1. Stratigraphic chart depicting the subsurface
nomenclature used in this study along with the surface
nomenclature of Kentucky and Ohio. Adapted from
McLaughlin et al. (2004) and Dattillo and Strunk (2016).
INTRODUCTION
Recent years have witnessed a significant increase in
hydrocarbon exploration of the Point Pleasant Lime-
stone and Utica Shale Formations of the Appalachian
Basin (Figure 1). Indeed, oil and natural gas produc-
tion from these formations in Ohio constituted 45%
and 58%, respectively, of the states total production in
2013 (Patchen and Carter, 2015). This production rep-
resents a significant increase from 2011, when these
units accounted for just 1% of oil and 3.5% of natural
gas produced (Patchen and Carter, 2015). Successful
exploitation in Ohio in turn has led to increased explo-
ration of the Point Pleasant and Utica in Pennsylvania
and West Virginia. This burgeoning exploration neces-
sitates an understanding of the environment in which
these deposits accumulated and how that environment
affected the generation and accumulation of hydro-
carbons. Specifically, redox conditions of bottom and
pore waters during deposition are of interest given the
often- observed covariance of anoxia and organic car-
bon preservation in marine sediments (Demaison and
Moore, 1980; Werne et al., 2002; Sageman et al., 2003).
In this chapter, we assess the role of oxygen depriva-
tion in the accumulation and diagenetic modification
of marine sediments by use of redox- sensitive trace
element concentrations and pyrite morphology.
Marine mudstones catalog a robust library of ele-
mental data that can be used to deduce temporal
variations in sediment supply, ocean hydrography,
and redox conditions at the time of deposition and
early diagenesis (Werne et al., 2002; Sageman et al.,
2003; Brumsack, 2006; Tribovillard et al., 2006; Calvert
and Pederson, 2007; Algeo and Tribovillard, 2009;
Lash and Blood, 2014). Specifically, a suite of redox-
sensitive trace elements, including molybdenum (Mo),
uranium (U), vanadium (V), and chromium (Cr), is
often used to determine the existence of bottom and
pore water anoxia and euxinia in organic- rich mud-
stones (Sageman et al., 2003; Tribovillard et al., 2006;
Rowe et al., 2008; Lash and Blood, 2014). However, in
their chemostratigraphic investigation of the Upper
Ordovician Trenton Limestone through Utica Shale
14252_ch05_ptg01_069-082.indd 70 11/26/19 1:17 PM
Diagenetic Pyrite Morphology in Mudstones of the Upper Ordovician Point Pleasant Limestone, Appalachian Basin 71
Formation. Wickstrom et al.’s (1992) subsurface inves-
tigation of the Trenton Limestone and associated strata
assigned the name Point Pleasant Formation to a suc-
cession of interbedded limestone and calcareous shale
that grades vertically and laterally from the Trenton
and underlies the Kope Formation in Ohio. Subsequent
biostratigraphic and carbonate C- isotopic studies sug-
gest that the Point Pleasant Formation of Wickstrom
et al. (1992) is not correlative with the Point Pleasant of
Orton (1873). Rather, the Point Pleasant of Wickstrom
et al. (1992) is equivalent to the Upper Ordovician
(Mohawkian) upper Grier Member through Devil’s
Hollow Member of the Lexington Limestone Forma-
tion (Figure 1; Young et al., 2015; Dattilo and Strunk,
2016). Herein, we use the term Point Pleasant in effort
to maintain consistency with previous subsurface
investigations and petroleum geology considerations
of this interval in Ohio (Cole et al., 1987; Wickstrom
et al., 1992; Drozd and Cole, 1994).
Point Pleasant deposits accumulated in a southern
subtropical subbasin defined by the Galena platform
to the northwest, the Trenton platform to the north-
east, and the Lexington platform to the southeast
(Figure 2A; Scotese and McKerrow, 1990; Young et al.,
2015). The narrow Sebree Trough connected the basin
to the Iapetus Ocean to the southwest (Figure 2A;
Kolata et al., 2001; Patchen et al., 2006; Patchen and
Carter, 2015). In the study area (Figure 2B), the Point
Pleasant comprises ~125 ft (38.1 m) of medium to dark
(Figure 1) succession in a core from eastern New York
State, Saboda and Lash (2014) demonstrate a paucity
of redox- sensitive trace elements throughout the stud-
ied interval. Indeed, Saboda and Lash (2014) illus-
trate that U and Mo are present at levels only slightly
enriched relative to crustal values, suggesting that
widespread ocean anoxia may have sequestered the
redox- sensitive trace element budget into deep ocean
muds leaving behind water masses generally depleted
in these trace metals (see Algeo, 2004). Given these
observations, a robust relationship between redox-
sensitive trace elements, redox conditions, and organic
matter preservation is lacking in the Point Pleasant
and Utica interval necessitating the need for an alter-
native assessment of redox conditions. This chapter
seeks to understand organic matter preservation by
investigating pyrite framboid diameters and redox-
sensitive trace element data to determine the nature
of bottom and pore water redox conditions during
deposition and early diagenesis of the Point Pleasant
Limestone in southwestern Pennsylvania and north-
ern West Virginia (Figure 2).
STRATIGRAPHY AND GEOLOGIC SETTING
Orton (1873) assigned the name Point Pleasant to
a series of beds exposed along the Ohio River near the
town of Point Pleasant, Ohio, directly beneath the Kope
Figure 2. Location map depicting (A) the position of the Appalachian Basin during the Late Ordovician (Mohawkian), adapted
and redrawn from Patchen et al. (2006), and (B) location of wells sampled in this study. Red circles indicate samples
obtained from horizontal wellbores, whereas green circles indicate vertical wellbore data sets. Data points with half circles
indicate both horizontal and vertical data sets on the same well.
14252_ch05_ptg01_069-082.indd 71 11/26/19 1:17 PM
72 BLOOD ET AL.
Table 1. Framboid size criteria used to define redox conditions (Bond and Wignall, 2010).
Conditions Framboid Diameters and Associated Data
Euxinic (persistently suldic bottom water) Abundant small (mean diameter = 3–5 µm) framboids; narrow size
range; few if any euhedral pyrite crystals
Anoxic (no oxygen in bottom water for extended
periods of time)
abundant small (mean diameter = 4–6 µm) framboids, including
asmall number of larger framboids; few euhedral pyrite crystals
Lower dysoxic (weakly oxygenated bottom water) Framboids 6–10 µm in diameter are moderately common; subordinate
larger framboids and euhedral pyrite crystals
Upper dysoxic (partial oxygen restriction in
bottomwater)
Large framboids are common; rare small (<5 µm diameter) framboids;
most pyrite is euhedral crystalline
Oxic (on oxygen restriction) No framboids; rare pyrite crystals
gray organic- rich (maximum total organic carbon
(TOC) = 4.5%) mudstone and interbedded limestone
shell beds. The Point Pleasant Limestone was depos-
ited in association with the drowning of the Ordo-
vician carbonate platforms as a series of alternating
mudstone and carbonate layers (McLaughlin et al.,
2004). Siliciclastic deposits of the overlying Kope/
Utica Formation represent the initial deposition of
mudstone, siltstone, and sandstone comprising the
Queenston clastic wedge (Ettenshon, 2004).
BACKGROUND
Pyrite Framboids and Use as Redox Indicators
Pyrite framboid size distribution provides a reliable
measure of the paleo- redox conditions under which
ancient marine deposits accumulated (Table 1; Wilkin
et al., 1996; Wignall and Newton, 1998). Framboids,
spherical aggregates of microcrystallites, form at
or just below the sulfide chemocline, the boundary
separating sulfide- bearing water from overlying
oxygenated water. Here, ferrous iron species, sul-
fide species, and suitable electron receptors such as
oxygen are available in enough quantity to promote
the precipitation of iron- monosulfide (FeS) miner-
als such as mackinewite and greigite (Sweeney and
Kaplan, 1973; Wilken and Barnes, 1997). The mag-
netic properties of greigite result in the rapidly form-
ing microcrystallites attracting to one another to form
spherical aggregates. Upon passing out of the zone of
Fe reduction, FeS reacts rapidly with hydrogen sul-
fide (H2S) to form pyrite (Wilkin and Barnes, 1997).
Associated euhedral pyrite crystals form more slowly
and at saturation levels less than that necessary for
the formation of diagenetic framboids. Ifbottom
water is sulfidic, syngenetic framboids may form
within the water column immediately beneath the
sulfide chemocline (Wilkin et al., 1996, 1997). In this
case, however, the dense framboids are unable to
achieve diameters much in excess of 3–6µm before
their sinking away from the chemocline terminates
their growth (Wilkin et al., 1996). A well- sorted pop-
ulation of very small framboids accumulates under
these conditions. Conversely, when the chemocline
resides at the sediment– water interface or within the
sediment, the availability of reactants limits framboid
growth. These sediments are characterized by a rather
poorly sorted population of large (>10 µm) framboids
(Wilkin et al., 1996). The presence of metabolizable
organic matter for bacteria to reduce sulfate to sul-
fide, and reactive detrital iron minerals are the pri-
mary limiting factors of sedimentary pyrite formation
(Berner, 1970, 1983).
Framboid diameter analysis can be especially use-
ful to the differentiation of deposits that accumulated
under euxinic conditions from those of more oxygen-
ated settings, especially dysoxic conditions defined by
weakly oxygenated bottom water and variable levels
of Fe reactivity (Table 1; Wilkin et al., 1996). However,
it is noteworthy that the distinction between euxinic
and suboxic bottom water is potentially less clear- cut
if framboids are growing under reactant- limited condi-
tions. For example, the abundance of small framboids
(diameter = ~4 µm) in modern sediments of the Santa
Barbara Basin that accumulated under a dysoxic water
column has been attributed to a paucity of reactive Fe
(Schieber and Schimmelmann, 2007). Morse and Wang
(1996) have suggested that anoxic and euxinic envi-
ronments promote high nucleation density of pyrite.
Under these circumstances, framboids precipitating at
multiple sites in anoxic pore water consume the lim-
ited supply of reactants resulting in a population of
14252_ch05_ptg01_069-082.indd 72 11/26/19 1:17 PM
Diagenetic Pyrite Morphology in Mudstones of the Upper Ordovician Point Pleasant Limestone, Appalachian Basin 73
to sediment represented by elevated Fe/Al (Lyons and
Severmann, 2006) in excess of average shale values
(Fe/Al = 0.55, Wedepohl, 1971; Fe/Al = 0.5, Taylor and
McLennan, 1985).
METHODS
Pyrite and Framboid Diameter Analysis
Sixty- two Point Pleasant Limestone samples were
selected for pyrite morphology analysis. The data set
includes samples obtained by EQT Production from
a core in Greene County, Pennsylvania, and drill cut-
ting chips collected from three vertical and four hor-
izontal exploration wells drilled in Greene County,
Pennsylvania, and Wetzel County, West Virginia
(Figure 2B, Table 2). Data sets from vertical well cores
and cuttings provide an assessment of elemental
data and pyrite morphology over the Point Pleasant
stratigraphic interval, whereas horizontal well data
sets provide insight into spatial changes along the
path of the wellbore within a restricted stratigraphic
interval. Measured depth (MD) denotes the position
of a sample obtained from vertical wellbores. Sample
position above the Trenton– Point Pleasant con-
tact denotes the stratigraphic position of horizontal
wellbore samples.
As a result of the brittleness of the samples used
in this analysis, samples were cut to size and epoxy
vacuum impregnated with a resin sensitive to high
pressures and capable of maintaining the structural
integrity of the sample. Samples were ground with
silicon carbide paper at increasingly finer grits and
finished with a series of diamond paste compounds.
The prepared samples were examined using a Tes-
can MIRA3 XM field- emission scanning electron
microscope (FE- SEM). Imaging was performed with
backscattered electrons to illustrate any elemental
differences that may be present and to distinguish
high atomic density pyrite from the low atomic den-
sity matrix. For each sample, a high- resolution, large-
area image survey was collected. Here, 2.042 mm2
areas were mapped with a 0.0733 µm pixel resolution
via the collection of 100 images in a 10 × 10 grid. Bulk
pyrite was quantified as the ratio of pixels of pyrite
to pixels of the entire image. Individual images were
examined by use of the IntelliSEM™ software pack-
age that permitted the identification, counting, and
classification of framboids as “normal” or “welded.”
“Welded” or “infilled” refers to framboids displaying
evidence of secondary diagenetic pyrite growth that
can infill interstitial spaces among microcrystallites,
small diagenetic framboids. In some instances, these
framboids approach the size distribution typical of
those forming in a euxinic water column (Schieber and
Schimmelmann, 2007).
Geochemical Proxies
Some portion of most marine sediment is of a detri-
tal provenance, including eolian and fluvial inputs
(Calvert and Pedersen, 2007). Aluminum is considered
the principal conservative proxy for clay mineral flux
in fine- grained clastic deposits (Arthur et al., 1985;
Arthur and Dean, 1991; Calvert and Pedersen, 2007).
Changes in grain size of the detrital flux can be rec-
ognized by consideration of variations in the relative
abundances of those elements associated with the
coarser size fraction relative to Al, including Ti and Zr
(Sageman and Lyons, 2003). Further, the conservative
behavior of Al in soil formation and weathering pro-
files favors its use as a parameter to assess authigenic
enrichment of redox sensitive elements via Al normal-
ization (Brumsack, 1989; Arthur et al., 1990).
Molybdenum and U are especially useful to
paleoenvironmental analysis of modern and ancient
oxygen- deficient marine systems for several rea-
sons. First, detrital concentrations of both elements
are low, U ~2.7 ppm and Mo ~3.7 ppm (Taylor and
McLennan, 1985). Moreover, both elements have
long residence times in seawater (~450 ka for U and
~780ka for Mo) meaning that Mo and U exhibit
nearly uniform global seawater concentrations
(Algeo and Tribovillard, 2009). Finally, inasmuch as
both elements are present in low concentrations in
marine plankton, authigenic uptake from seawater
enhanced by oxygen- deficient conditions leads to an
enrichment of Mo and U in marine deposits (Algeo
and Tribovillard, 2009).
The Fe/Al ratio has been used to aid in the recog-
nition of water column (syngenetic) pyrite formation
and, therefore, euxinic bottom water conditions (Lyons
et al., 2003; Lyons and Kashgarian, 2005; Lyons and
Severmann, 2006). The shuttling of dissolved Fe2+ as
Fe- oxyhydroxide particulates from where the chemo-
cline (the boundary separating oxygenated water
from underlying oxygen- deficient water) impinges the
seafloor enhances enrichment of bottom water in the
reactive Fe necessary for the formation of syngenetic
pyrite. The transported Fe2+ may then be sequestered
in sulfidic bottom water by precipitation of sulfide
minerals, including pyrite (Canfield et al., 1996; Lyons
and Kashgarian, 2005; Raiswell and Anderson, 2005;
Lyons and Severmann, 2006). This process gives rise
14252_ch05_ptg01_069-082.indd 73 11/26/19 1:17 PM
Table 2. Framboid size data, pyrite occurrence, and geochemical data for Point Pleasant Limestone samples.
Sample Location Framboid Data Elemental Data
Well Orientation Location Depth Distance
above
Trenton
Top
n Maximum
Diameter
Mean
Diameter
Standard
Deviaiton
25th
Percentile
75th
Percentile
Framboid
Density
Bulk
Rock
Pyrite
Framboidal
pyrite
Al Mo EF U EF Fe/Al
ft ft µm µm µm µm µm framboids/
mm2
% % %
Well A Horizontal Greene average 64.2 109 13 4.5 1.7 3.5 5.2 14 0.76 27.01 3.54 0.79 0.46 0.49
Well A Horizontal Greene 13,670 52.6 109 9 3.9 1.6 3 4.7 23 0.56 35.46 n/a n/a n/a n/a
Well A Horizontal Greene 14,540 59.7 114 9 3.9 1.4 3.1 4.6 18 0.56 33.89 n/a n/a n/a n/a
Well A Horizontal Greene 15,590 68.0 116 18 5.3 2.0 4.0 6.0 10 0.53 30.11 n/a n/a n/a n/a
Well A Horizontal Greene 16,460 76.4 95 15 5.0 1.7 4.0 5.6 5 1.37 8.56 n/a n/a n/a n/a
Well B Horizontal Greene average 67.5 127 13 5.3 1.7 4.1 5.8 9 0.12 23.54 2.98 2.90 0.10 0.47
Well B Horizontal Greene 14,390 76.1 150 19 6.1 2.3 4.8 6.7 9 0.09 4.79 n/a n/a n/a n/a
Well B Horizontal Greene 15,020 64.5 170 13 5.3 1.6 4.2 5.8 9 0.02 66.28 n/a n/a n/a n/a
Well B Horizontal Greene 16,010 56.3 58 13 5.4 1.8 4.0 5.9 2 0.19 6.96 n/a n/a n/a n/a
Well B Horizontal Greene 17,000 70.0 128 7 4.2 1.2 3.3 4.9 16 0.18 16.13 n/a n/a n/a n/a
Well B Horizontal Greene 17,990 69.4 126 16 5.0 2.1 3.6 5.8 11 0.09 16.60 n/a n/a n/a n/a
Well B Horizontal Greene 19,070 68.6 62 11 5.4 1.6 4.2 6.3 4 0.27 7.22 n/a n/a n/a n/a
Well C Horizontal Wetzel average 76.4 146 19 4.8 2.1 3.6 5.3 27 0.46 12.55 4.49 2.55 1.02 0.48
Well C Horizontal Wetzel 14,280 81.1 102 10 3.8 1.5 2.7 4.7 19 0.64 19.25 n/a n/a n/a n/a
Well C Horizontal Wetzel 15,030 73.6 210 40 4.9 3.2 3.8 5.2 53 0.44 9.34 n/a n/a n/a n/a
Well C Horizontal Wetzel 15,990 74.2 160 17 5.6 2.4 4.0 6.2 10 0.33 3.98 n/a n/a n/a n/a
Well C Horizontal Wetzel 17,010 72.6 110 10 4.7 1.4 3.8 5.2 28 0.44 17.63 n/a n/a n/a n/a
Well C Horizontal Wetzel 18,000 78.2 497 11 3.8 1.4 2.8 4.5 124 1.73 23.09 n/a n/a n/a n/a
Well C Horizontal Wetzel 18,990 78.4 436 12 4.0 1.5 2.9 4.7 109 0.86 33.05 n/a n/a n/a n/a
Well C Vertical Wetzel average n/a 103 15 5.0 2.2 3.6 5.8 21 n/a n/a 4.04 1.45 0.86 0.45
Well C Vertical Wetzel 13,030 n/a 108 11 4.8 1.7 3.6 5.8 35 n/a n/a n/a n/a n/a n/a
Well C Vertical Wetzel 13,045 n/a 114 26 5.8 3.4 3.8 6.4 23 n/a n/a n/a n/a n/a n/a
Well C Vertical Wetzel 13,055 n/a 106 11 4.9 1.8 3.6 5.7 16 n/a n/a n/a n/a n/a n/a
Well C Vertical Wetzel 13,065 n/a 84 12 4.7 1.8 3.5 5.2 12 n/a n/a n/a n/a n/a n/a
Well C Vertical Wetzel 13,075 n/a 106 11 4.7 1.5 3.8 5.5 62 n/a n/a n/a n/a n/a n/a
Well C Vertical Wetzel 13,085 n/a 63 14 5.9 2.5 4.0 6.9 9 n/a n/a n/a n/a n/a n/a
Well C Vertical Wetzel 13,100 n/a 101 11 5.1 2.0 3.7 6.1 18 n/a n/a n/a n/a n/a n/a
Well C Vertical Wetzel 13,115 n/a 13 9 5.4 1.5 4.3 6.3 2 n/a n/a n/a n/a n/a n/a
Well C Vertical Wetzel 13,130 n/a 3 10 8.1 2.7 7.0 9.6 < 1 n/a n/a n/a n/a n/a n/a
74 BLOOD ET AL.
14252_ch05_ptg01_069-082.indd 74 11/26/19 1:17 PM
Well D Horizontal Greene average 76.4 102 9 4.5 1.5 3.4 5.0 12 n/a n/a 4.49 2.55 1.02 0.48
Well D Horizontal Greene 13,880 52.3 102 10 5.0 1.7 3.7 5.7 7 n/a n/a n/a n/a n/a n/a
Well D Horizontal Greene 14,870 66.1 100 8 4.2 1.3 3.3 4.8 6 n/a n/a n/a n/a n/a n/a
Well D Horizontal Greene 15,950 80.5 103 11 4.8 1.6 3.7 5.3 11 n/a n/a n/a n/a n/a n/a
Well D Horizontal Greene 16,940 92.5 103 7 3.8 1.2 2.9 4.3 25 n/a n/a n/a n/a n/a n/a
Well D Horizontal Greene 17,930 74.3 127 8 4.0 1.2 3.1 4.7 56 n/a n/a n/a n/a n/a n/a
Well D Horizontal Greene 18,850 79.1 100 12 4.0 1.6 2.9 4.7 66 n/a n/a n/a n/a n/a n/a
Well D Horizontal Greene 19,930 79.3 105 9 3.8 1.4 3.0 4.3 93 n/a n/a n/a n/a n/a n/a
Well D Horizontal Greene 20,730 87.0 100 10 4.7 1.8 3.4 5.6 13 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene average n/a 84 10 4.4 1.6 3.3 5.1 15 n/a n/a 3.30 0.82 0.00 0.42
Well D Vertical Greene 13,475 n/a 102 10 5.0 1.6 3.8 5.9 19 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,485 n/a 103 8 3.8 1.4 2.8 4.5 7 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,495 n/a 100 14 4.3 1.9 3.1 4.8 28 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,505 n/a 30 8 4.3 1.4 3.5 5.0 7 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,515 n/a 101 12 4.2 1.6 3.1 5.0 35 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,520 n/a 65 11 4.4 1.4 3.6 4.9 7 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,535 n/a 100 11 4.6 1.7 3.2 5.4 16 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,545 n/a 100 14 5.0 2.1 3.5 5.7 11 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,555 n/a 100 13 4.0 1.8 2.7 4.6 32 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,565 n/a 69 9 4.1 1.5 3.2 4.9 5 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,575 n/a 101 14 4.3 1.9 3.4 5.2 20 n/a n/a n/a n/a n/a n/a
Well D Vertical Greene 13,585 n/a 100 11 3.9 2.2 2.2 5.1 10 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene average n/a 80 12 5.6 1.9 4.5 6.2 26 n/a n/a 3.61 0.80 0.14 0.59
Well E Vertical Greene 13,402.3 n/a 100 11 5.7 1.7 4.6 6.5 22 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene 13,417.1 n/a 19 16 5.9 2.9 4.4 5.9 3 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene 13,433.3 n/a 101 11 5.0 1.5 4.1 5.3 34 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene 13,447.2 n/a 100 11 5.8 1.5 4.8 7.0 23 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene 13,453.8 n/a 102 11 6.3 1.9 4.9 7.7 37 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene 13,457.2 n/a 101 8 4.4 1.2 3.5 5.1 35 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene 13,462.3 n/a 101 12 4.9 1.4 3.9 5.6 32 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene 13,488.4 n/a 100 9 4.5 1.4 3.4 5.3 24 n/a n/a n/a n/a n/a n/a
Well E Vertical Greene 13,508.5 n/a 100 9 4.6 1.5 3.5 5.3 20 n/a n/a n/a n/a n/a n/a
Well F Vertical Wetzel average n/a 29 13 6.9 2.3 5.3 8.1 4 n/a n/a 6.46 0.31 0.18 0.54
Well F Vertical Wetzel 12,240 n/a 29 14 6.7 2.4 5.1 7.8 4 n/a n/a n/a n/a n/a n/a
Well F Vertical Wetzel 12,250 n/a 28 17 9.2 3.3 6.7 11.1 4 n/a n/a n/a n/a n/a n/a
Well F Vertical Wetzel 12,260 n/a 41 13 5.4 2.1 4.2 6.0 6 n/a n/a n/a n/a n/a n/a
Well F Vertical Wetzel 12,270 n/a 16 9 6.4 1.6 5.1 7.5 2 n/a n/a n/a n/a n/a n/a
Well F Vertical Wetzel 12,280 n/a 19 14 5.1 2.4 4.1 5.7 3 n/a n/a n/a n/a n/a n/a
Well F Vertical Wetzel 12,310 n/a 5 6 4.4 1.5 4.0 5.1 1 n/a n/a n/a n/a n/a n/a
Well F Vertical Wetzel 12,330 n/a 8 7 4.7 1.5 4.1 5.4 1 n/a n/a n/a n/a n/a n/a
Diagenetic Pyrite Morphology in Mudstones of the Upper Ordovician Point Pleasant Limestone, Appalachian Basin 75
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76 BLOOD ET AL.
Figure 3. Scanning electron micrograph depicting normal
and welded framboids in the Point Pleasant.
and/or partially to completely overgrown framboids
(Figure 3; Wilken et al., 1996; Wilken and Barnes,
1997). Care was taken to measure framboids that
lack evidence of secondary diagenetic pyrite growth
that could misrepresent the original size of the fram-
boid. Finally, the maximum diameter was measured.
Although apparent diameters of framboids tend
to underestimate their true diameter, Wilken et al.
(1996) demonstrated the possible error in measure-
ments to be <10%. Where feasible, a minimum of
100 framboids from each sample were measured to
obtain a statistically valid sample size (see Wilkin
et al., 1996).
Inorganic Geochemical Analysis
Inorganic geochemical assessment of the samples was
performed on approximately 5 g of drill bit cuttings
and core chips that was homogenized into a powder
and pressed to form 31-mm- diameter pellets. The pel-
lets were analyzed using a tabletop Spectro XEPOS
III energy- dispersive X- ray fluorescence (ED- XRF)
using a 50-W end- window X- ray tube and a Peltier
cooled Silicon Drift Detector. The cuttings were cap-
tured at intervals ranging from 5 ft (1.5 m) to 10 ft
(3 m) MD along the vertical and 30 ft (9.1 m) in the
lateral sections of the wellbore and at one- foot inter-
vals along the core. Thirteen major elements and 20
trace elements were quantitatively measured. Data
quality control was maintained using a set of well-
characterized international geochemical reference
materials (GRMs). These internationally accepted
GRMs are chosen to matrix- match the values of the
samples analyzed to make any correction required
because of instrument performance or drifts. GRMs
chosen for thisstudy were United States Geological
Survey (USGS) Green River Shale (SGR-1), USGS mica
schist (SDC-1), National Institute of Standards and
Technology (NIST) limestone (1D), and NIST dolo-
mitic limestone (88b). These standards are of varied
lithology and sufficient to make any elemental correc-
tion required for the different formations expected in
the study.
Finally, we present elemental data as elemental
abundance in wt. % or ppm, Al- normalized ratios, and
enrichment factors (EF) where the element to Al ratio
of the sample is normalized to the element to Al ratio
of the average shale (Wedephol, 1971, 1991).
Organic Geochemical Analysis
For TOC analysis, approximately 0.10 g of ground
material was treated with concentrated hydrochloric
acid for a minimum of two hours to removed carbon-
ates. The samples were placed in a LECO crucible and
dried at 230°F (110°C) for a minimum of one hour. The
dried sample was then analyzed with a LECO 600
Carbon Analyzer with detection limits to 0.01 wt. %.
RESULTS
Greene County, Pennsylvania
Pyrite Morphology
Samples from wells A and B horizontal well bores were
analyzed for total and framboidal pyrite abundance
and framboid diameter. Samples from wells E and
D vertical pilot hole, and well D horizontal wellbore
were analyzed for framboid diameter only. Results are
presented in Table 2. The average bulk pyrite abun-
dance in wells A and B is 0.39% (range 0.02–1.37%).
The average diameter of the 3863 framboids measured
is 4.7 µm with a standard deviation (STDV) of 1.7 µm
(Figure 4). The average maximum framboid diameter
is 11.2 µm and framboid density is 21 framboids/mm2.
Finally, 23% (range 5–66%) of pyrite occurs as
framboidal pyrite in wells A and B lateral samples.
14252_ch05_ptg01_069-082.indd 76 11/26/19 1:17 PM
Diagenetic Pyrite Morphology in Mudstones of the Upper Ordovician Point Pleasant Limestone, Appalachian Basin 77
Figure 4. Plot of the mean vs. the standard deviation of
framboids in the Point Pleasant.
Figure 5. Framboid diameter data for well D with box and whisker plots depicting the mean and maximum framboid diameters
demonstrating minimal variability in mean framboid size throughout (A) the stratigraphic section and (B) along thehorizontal
portion of the wellbore. Note that although minimal, larger framboids, represented by those >10 µm, do occur in some samples.
Average framboid diameters show minimal change
throughout the Point Pleasant stratigraphic interval
(Figure 5A), not along the wellbore (Figure 5B).
Geochemistry
Three hundred and thirteen Point Pleasant samples
from Greene County wells were analyzed for elemen-
tal abundance (Table 2). The average Al concentration
is 3.6% (range 1.1–11.0%) or roughly 40% of the aver-
age shale value of 8.8% (Wedephol, 1971, 1991). There
is a strong covariance between Fe and Al as depicted
by well E (Figure 6; r2 = 0.95). The average Fe/Al for
each well ranges from 0.40 to 0.49. The average Point
Pleasant Fe/Al for the entire data set is 0.46. Molyb-
denum and U occurrence in the Point Pleasant is
minimal. Indeed, of the 313 samples analyzed, only
67 contain measurable quantities of both Mo and U,
approximately 20% of the sample population. Fur-
ther, these samples exhibit modest enrichment in Mo
and U, where EFs average 3.3 (range 0.3–7.7) and 1.4
(range 0.2–3.0), respectively.
Wetzel County, West Virginia
Pyrite Morphology
Samples recovered from well C lateral wellbore were
analyzed for total and framboidal pyrite abundance
and framboid diameter. Samples from wells C and F
vertical pilot hole were analyzed for framboid diame-
ters. Results are presented in Table 2. The average bulk
pyrite in well C is 0.70% (range 0.33–1.73%). The aver-
age diameter of 2359 measured framboids is 5.4 µm
with an STDV = 2.0 µm (Figure 4). The average max-
imum framboid diameter is 13.4 µm. Average fram-
boid density is 25 framboids/mm2 and an average of
18% (range 4–33%) of the pyrite occurs as framboidal
pyrite.
Geochemistry
One hundred and sixty Point Pleasant samples recov-
ered from Wetzel county wells were analyzed for
14252_ch05_ptg01_069-082.indd 77 11/26/19 1:17 PM
78 BLOOD ET AL.
elemental abundance (Table 2). The Point Pleasant of
Wetzel County displays slightly higher Al concentra-
tions than Greene County at 4.6% (range 2.1–7.9%)
whereas still approximately 50% depleted relative to
the average shale (Wedephol, 1971, 1991). The aver-
age Fe/Al ratio for each well ranges from 0.45 to 0.48
(average = 0.47), a marginally higher Fe content than
observed in Greene County. As with Greene County,
the Wetzel County samples display a paucity of Mo
and U. However, 67 samples demonstrate detectable
levels of Mo and U, a 100% increase over that observed
in Greene County. The average Mo EF of 4.1 (range
and 1.1–29.3) and U EF of 3.0 (range 0.26–5.7) are
slightly higher than that observed in Greene County.
DISCUSSION
The Point Pleasant Formation hosts a low concentra-
tion of pyrite (Figure 7A). Moreover, of the pyrite pres-
ent, ~75 to 80% occurs as euhedral crystal masses and
euhedral grains (Figure 7B). Oddly, the subordinate
framboidal pyrite generally occurs as very small ~5 µm
framboids (Figure 4B) with few examples >10 µm
(Figure 4B). Although framboids are marginally larger
in Wetzel County, framboid size varies little through the
stratigraphic interval of the Point Pleasant (Figure 5A).
Likewise, framboid size demonstrates little variation
along a horizontal wellbore (Figure 5B). These observa-
tions suggest that the processes of pyrite and framboid
formation remained largely consistent across the study
area throughout Point Pleasant deposition.
The morphological occurrence of pyrite and the
diameter of framboids in the Point Pleasant Limestone
imply two opposing models for the position of the
chemocline during pyrite formation. The prevalence of
euhedral grains and a lack of framboidal pyrite (Table 2)
suggest accumulation of sediments under a dysoxic to
oxic water column; an interpretation consistent with
the low occurrence of detectable Mo and U, and Fe/
Al ratios at or below average shale values. However,
the small diameter and narrow size range of analyzed
framboids (Figure 5, Table 2) is suggestive of formation
in suspension, inferring that the sulfide chemocline
resided in the water column above anoxic- sulfidic (eux-
inic) bottom waters (Figure 4; Wilken et al., 1996).
An alternate explanation for the abundance of small
framboids is a lack of those reactants necessary to the
formation of pyrite (either reactive Fe or bacterially
mediated H2S; cf. Wilken et al., 1996). The dominance
of framboids displaying a mean diameter of ~2–4 µm
forming under a dysoxic water column of the Santa
Barbara Basin was attributed to a paucity of reactive
Fe (Schieber and Schimmelmann, 2007). Here, anoxic-
sulfidic conditions exist a few millimeters below the
sediment– water interface. Reactive Fe mobilized from
Fe- rich, terrigenous dominated winter sediment enters
adjacent, Fe- poor, but organic- rich sediment deposited
during summer. The rapid production of diagenetic
framboids consumes the limited amount of Fe in the
summer sediment resulting in a population of small
framboids (Schieber and Schimmelmann, 2007).
Average Al concentrations of the Point Pleas-
ant Limestone of Greene and Wetzel counties, 3.6%
and 4.6%, respectively, are significantly less than the
average shale value of 8.8% (Wedepohl, 1971, 1991).
This indicates a significantly diminished clastic flux,
including the requisite reactive iron for the production
of diagenetic pyrite. Indeed, the covariance of Al and
Fe observed in the Point Pleasant (Figure 7) illustrates
the critical role the detrital load played in delivering
Fe to the basin. Moreover, the strong covariance of
framboids with Al (r2 = 0.74) would also suggest a rela-
tionship between clastic influx and framboid occur-
rence, where clastic delivery of reactive Fe enabled the
production of framboids (Figure 8).
The small average diameter of Point Pleasant fram-
boids and their narrow size distribution warrants
further discussion. Morse and Wang (1996) postulate
that generally narrow size range of diagenetic miner-
als reflects the supersaturation of pore fluids and con-
sequent rapid formation of multiple nuclei. In turn,
growth at nucleation points occurs at a roughly uni-
form rate until equilibrium is reached. Indeed, if all
Figure 6. Plot of Al vs. Fe in well E. Note the strong
covariance suggesting the strong role clastic influx plays in
delivering Fe to the basin. The lack of a decoupling of Fe
from Al also indicates minimal growth of authigenic pyrite.
14252_ch05_ptg01_069-082.indd 78 11/26/19 1:17 PM
Diagenetic Pyrite Morphology in Mudstones of the Upper Ordovician Point Pleasant Limestone, Appalachian Basin 79
Figure 7. Scanning electron micrographs of the Point Pleasant Limestone depicting (A) the general scarcity of pyrite and
(B)the occurrence of both euhedral (eu) small framboidal (sf, <6 µm) and large (lf, >10 µm) framboids.
framboids in a volume of sediment initiated growth
at the same time and grew at the same rate, the result
would be a framboid population of one size (Wilken
et al., 1996). However, a range in sizes of observed
framboids, as is the case in the Point Pleasant, sug-
gests framboids nucleated at different times (Wilken
et al., 1996). Under such circumstances of continuous
nucleation, however, framboids may still approach
a uniform size given that (1) framboids are generally
assumed to grow at a constant growth rate (Wilken
et al., 1996) and (2) as saturation diminishes, nucle-
ation ceases but growth continues, and therefore, the
difference in grain size between framboids of different
ages diminishes (Morse and Wang, 1996).
The simplest explanation for the small size of Point
Pleasant framboids involves the ratio of nuclei vs.
the supply of reactants. In this scenario, nucleating
framboids competing for a finite amount of reactants
exhaust the supply before framboids have an oppor-
tunity to achieve diameters typical of diagenetic fram-
boids (>10 µm). A “sphere of influence” may also
contribute to the small size of framboids (Mores and
Wang, 1996). That is, diagenetic framboids compet-
ing for finite resources have a small area from which
to pull the reactants before the supply is exhausted
because of either minimal input of the reactant into the
Figure 8. Plot of Al vs. framboid density. A strong
relationship exists between increased clastic influx,
attendant influx of reactive Fe, and the number of
framboids encountered in the samples. This suggests that
the availability of reactive Fe delivered to the basin limits
framboid growth.
14252_ch05_ptg01_069-082.indd 79 11/26/19 1:17 PM
80 BLOOD ET AL.
system or having to compete with nearby framboids.
Thus, under such conditions, diagenetic framboids in
the Point Pleasant would have exhausted the reser-
voir of reactive Fe before attaining the large sizes more
typical of diagenetic framboids (>~10 µm). Again,
it is worth noting that although framboids >10 µm
are uncommon (~2% of the framboid population,
Table 2), their presence in 44 of 62 analyzed samples
points unambiguously to the growth of diagenetic
framboids. In sum, we argue that the framboid pop-
ulation of the Point Pleasant reflects the formation of
diagenetic framboids under an oxic to dysoxic water
column wherein growth was limited by a lack of nec-
essary reactants, namely reactive Fe. Such a model
for framboid formation is consistent with the general
scarcity of pyrite and prevalence of diagenetic euhe-
dral grains (~75–80% of observed pyrite; Figure 7).
Indeed, euhedral grains are readily observed in those
sediments where less reactive Fe forms from direct
interaction with free H2S in pore waters under a dys-
oxic water column (Wilken et al., 1996). Furthermore,
U and Mo at or below average shale concentrations are
consistent with a water column that rarely achieved
the anoxic and/or euxinic conditions necessary for
their enrichment in marine sediments (Algeo and
Tribovillard, 2009).
CONCLUSIONS
The Point Pleasant Limestone hosts a pyrite popu-
lation comprising euhedral grains and subordinate
framboids. The consistently small framboids observed
likely formed in a diagenetic environment deficient in
reactive Fe, and do not imply the formation of synge-
netic framboids suspended in a euxinic water column
(cf. Wilken et al., 1996). Indeed, the strong covariance
of Fe and Al and the low amount of Al in the Point
Pleasant are consistent with such a model. Further, the
low occurrence of pyrite and lack of detectable abun-
dances of redox sensitive trace elements, notably U
and Mo, are consistent with accumulation in a dysoxic
to possibly oxic water column. Burial of organic mat-
ter and its removal from zones of oxidization likely
contributed to its preservation in these deposits.
ACKNOWLEDGMENTS
We thank Wayne Camp and Neil Fishman for their
invitation to contribute to this volume. We extend
our gratitude to Gary Lash for the many fruitful dis-
cussions, guidance, and efforts to improve this chap-
ter. This chapter benefited from the thorough review
of two anonymous reviewers. Finally, we thank EQT
Production for their permission to generate and use
the data sets in this publication.
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