The Point Pleasant member of the Utica Shale is
a mixed siliciclastic and carbonate sequence, where it produces oil and gas in
the Appalachian Basin. The Utica above it is a siliciclastic mudstone, while
the Point Pleasant is more of a calcareous mudstone. One key to optimizing
production in the formation is to target structural settings in the Ohio
sub-basin where the highest amounts of organic matter and resulting organic
porosity are preserved. Typically, this is in structurally low topographic
settings far enough below wave base to enable higher preservation.
The use of X-Ray Fluorescence
(XRF) in determining chemostratigraphy is well-documented. It can be used to
analyze cores and drill cuttings to define ideal zones for targeting in
horizontal wells, for noting stratigraphic differences over an area and
continuity of beds, and to inform geosteering if done in real-time. As noted in
a 2021 Master’s Thesis by Barbara Kemeh:
“…chemostratigraphy reflects changes in depositional and
facies characteristics of the Utica shale and Point Pleasant Formation.”
I am utilizing Kemeh’s 2021
Master’s Thesis and Zachary Fernandez’s 2023 Master’s Thesis as the main
sources in this post.
Also from Kemeh’s
thesis:
“For this study, two cores were analyzed using a handheld
x-ray fluorescence (HH-XRF) spectrometer along with core descriptions, x-ray
diffraction (XRD) and total organic carbon (TOC) data to interpret the
depositional environment. Hierarchical clustering technique was used to
identify five chemofacies which reflect the geochemical variability present in
both cores. Six chemozones were identified and correlated using the chemofacies
coupled with stratigraphic plots of selected major elements, trace metals and
TOC. Detrital influx analysis revealed that the Utica-Point Pleasant interval
in both cores were deposited in different water depths resulting in different
amounts of terrigenous input. Paleoredox conditions revealed the Farley core
was deposited in oxygenated bottom waters which account for the depletion of
trace metals throughout the core. In the Tracker core, analysis showed that
bottom-water conditions at the time of deposition varied between anoxic and
euxinic. The Tracker core shares similar bottom-water conditions present in the
Sebree Trough in Kentucky and is believed to have been deposited in an
extension of the trough into northeast Ohio. The Farley core appears to have
been deposited outside this trough and likely in the Utica-Point Pleasant
basin. Overall the study supports the existence of different depocenters across
the area with different conditions at the time of deposition.”
It is important to know
paleoredox conditions since they affect the preservation of organic matter
beds. Chemostratigraphy can be used to help determine redox conditions, water
depth during deposition, as well as the percentage of terrigenous detrital
sediment delivered to the seafloor vs. marine sediments via carbonate
accumulation.
Kemeh begins her thesis with the note that two-thirds of the sedimentary rock record consists of mudstones. She also notes the variability between mudstones and within mudstones, often at centimeter scale. Fine-grained rocks like shales can often result in ‘condensed intervals’, which refer to thin beds that cover long time periods during mostly very slow deposition in deeper waters.
The definition of a condensed interval
given by Wikipedia is as follows:
“In sequence stratigraphy. condensed sections are strata
that are thin, yet span a large time interval. They are associated with the
maximum flooding surfaces, represented by sedimentary intervals deposited
during the maximum marine transgression.”
Zachary Fernandez, in his 2023
Master’s Thesis, noted that in sequence stratigraphy, the condensed intervals
are associated with the end of the Transgressive Systems Tract (TST) and capped
by the Maximum Flooding Surface (MFS). The condensed interval is thus the
interval with the least amount of influx of terrigenous sediment due to high
sea level. This leads to optimal preservation of organic matter. First, the
chemo facies are identified from the elemental content, then they are paired with
sequence stratigraphy depositional systems tracts.
Below, Kemeh explores the
reasons for variability. She also explains how different elements can be
proxies for local environmental conditions during deposition. Ti, Zr, K, and Al
can indicate detritus deposited during sea level transgression and regression.
Ca, Mg, and Sr are associated with carbonate accumulation. Al and K indicate
feldspars and clay. “The combined use of Mo, V,
and U is used in distinguishing suboxic environments from anoxic-euxinic ones.”
“The major influences on variability in mudrocks include
tectonic setting, water depth, oxygenation, climate, eustasy, and detrital
influx which control composition, fabric, and texture. Thus, different
lithofacies are produced from changing transport and depositional processes,
while mineralogical and total organic carbon (TOC) variations can be attributed
to proximity to sediment source.”
Zachary Fernandez noted
chemostratigraphic proxies as follows:
“…proxies for terrigenous sedimentation (Ti, Zr and Rb),
marine sedimentation (Ca and Sr), clay sedimentation (Al and K), upwelling (P),
pyrite minerals (Fe and S) and biogenic quartz (Si/Al)”
Regarding the use of
chemostratigraphy, Kemeh notes:
“Chemostratigraphy relies on identifying variations in
element concentrations through an interval and using these changes to develop a
stratigraphic characterization that is based on changes in geological features,
such as paleoclimate (Pearce et al., 2005b, Ratcliffe et al., 2010) and
provenance (Ratcliffe et al., 2007, Wright et al., 2010).”
Kemeh utilized two cores for
the XRF study: The Tracker core from Portage County, Ohio, and the Farley core
from Washington County, Ohio. She characterizes the study below:
“The focus of the study was to establish chemostratigraphic correlations and paleoenvironment proxies, such as water bottom oxygen levels, based on trace metal concentrations within the Utica shale and Point Pleasant Formation across Ohio which will help formally characterize the Utica shale within the state.”
Below, she gives elemental proxies, a cross-sectional model of deposition showing wave base and positions of the two cores, and a relative sea level curve.
Fernandez’s study analyzed 31
wells from eight counties in eastern Ohio. He used an Olympus Delta Pro
Handheld X-ray Fluorescence Spectrometer (pXRF), which is less accurate than a
tabletop XRF but adequate for determining the relative abundance of elements.
Fernandez recognized three chemofacies beginning at the base of the Point
Pleasant.
“Chemofacies 1 has the lowest
clay proxies of the entire section. It has the highest indicators of upwelling.
Redox proxies are low with a minor increase at the top of the chemofacies.
Marine sedimentation is variable, yet overall high. Terrigenous sedimentation
is variable, yet overall low. Biogenic quartz steadily increases up section.”
Chemofacies 2 is dominated by
terrigenous sedimentation.
“Clay proxies are high and steady. Redox proxies are
intermediate and steady. Marine sedimentation has significantly dropped off and
it is decreasing further. Lastly, the proxy for biogenic quartz has leveled
off. The top of chemofacies 2 is marked by another shift in sedimentation, the
decline of biogenic quartz and the rise of redox proxies.”
Chemofacies 3 “has increasing clay
proxies. The redox proxies are at their zenith in this chemofacies. Marine
sedimentation is rapidly decreasing, and terrigenous sedimentation is rapidly
increasing; however, there is some significant variability in this overall
sedimentation trend. The final attribute of this chemofacies is that biogenic
quartz is steadily decreasing.”
“Chemofacies displaying high proxies for marine
sedimentation (Ca and Sr) and upwelling (P) are denoted as the Transgressive
Systems Tract (TST). The TST may also have a high clay content (Al and K).
Chemofacies displaying high proxies for terrigenous sedimentation (Ti, Rb and
Zr) and low redox sensitive proxies (Fe and S) are denoted as the Highstand
Systems Tract (HST). Finally, chemofacies displaying high terrigenous
sedimentation and high redox sensitive elements are denoted as the Lowstand
Systems Tract (LST).”
Chemofacies 3 is where the
main oil & gas producing interval is in the Point Pleasant, more
specifically, the top of chemofacies 3, where the upper few feet have been
interpreted as a condensed interval, as shown below.
The cross-section below shows
correlation via gamma ray and resistivity logs done by EMF Geoscience. It shows
that there are good gamma and resistivity indicators for the top of the
condensed interval, which occurs at the top of OPTPL2, as correlated. Below
that is a sequence stratigraphy interpretation showing the condensed interval
at the maximum flooding surface. The third graphic below is an
isopach map of the CI, which varies from 4 to 13 feet in thickness. Unfortunately,
it is not referenced to a geographic map, so it is unclear exactly where the
thick is. I was a bit surprised at the variation in thickness, and it makes me
wonder about that variation throughout the producing area.
Fernandez notes:
“Isopach maps reveal a unique trend. That the formation
thickens to the northeast; however, the CI is thickest in the center of the
region. Many of the wells in this study land in the CI hotspot. This interval
of core was also the section that was most densely sampled for the TOC
analysis. It is apparent that there is significant interest in this interpreted
CI.”
Kemeh notes that the
organic-rich facies in the Point Pleasant have roughly 40% to 60% carbonate
content, with TOC ranging from 3% to 8% (average 4%–5%). She used the elemental
ratio of Si/Al to determine if quartz was detrital or biogenic.
Kemeh distinguished five
chemofacies, including parts of the Trenton Limestone. She grouped the
chemofacies into chemozones of certain sequences. She notes that in the Tracker
core, the rocks were deposited in more anoxic waters below wave base. The
Farley core shows more storm deposits and more oxygenated waters, often above
wave base.
Kemeh’s conclusions are given
below:
1) Chemostratigraphy is a useful method of distinguishing
between the formations in the Utica shale Play in Ohio.
2) Raw elemental curves allow identification of changes in
mineralogy at a finer scale while cluster analysis removes the burden of
meaningful pattern identification and correlations across the raw curves by
grouping samples based on the degree of similarity to one another.
3) Decreased Ca concentration and increased Al from the
Utica-Point Pleasant interval record a shift from the calcareous mudstones of
the Point Pleasant to the siliciclastic rich mudstones of the Utica shale.
4) It is assumed that the depositional environment for the
Lexington-Utica interval in the Farley core was mostly likely a carbonate shelf
to a storm-dominated deep ramp between fair weather and storm base, <131 ft
(40 m), frequently disturbed by storm currents. The Trenton-Utica interval in
the Tracker core was interpreted to be deep ramp, dominantly below storm wave
base in anoxic bottom water, >131 ft (40 m). These different conditions
support the existence of separate depocenters across the area.
5) Bottom conditions of the Tracker Core
compared to the Farley core suggests an extension of the Sebree Trough across
northeast Ohio, where oceanic conditions allowed for the enrichment of redox
sensitive trace metals indicating euxinia to anoxia.
Further studies are ongoing.
EMF Geoscience is continuing to collect XRF data regarding the condensed
interval. According to their website:
“EMF Geoscience has purchased a Vanta M Series Handheld
XRF and started collecting geochemical data from in-house Point Pleasant cores
in September 2023. The primary goal of this Chemostratographic study is to
define the Condensed Interval inside the Point Pleasant Formation.”
References:
IDENTIFYING
THE POINT PLEASANT FORMATION’S CONDENSED INTERVALS WITH XRF CHEMOSTRATIGRAPHY: A
THESIS SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE MASTER OF SCIENCE. BY ZACHARY FERNANDES. DR. JEFFRY
GRIGSBY – ADVISOR. BALL STATE UNIVERSITY. MUNCIE, INDIANA. May 2023. content
Geochemical
Characterization of the Utica Shale Play using XRF-Based Chemostratigraphy in
Ohio. Barbara Kemeh and Advisor Julie M. Bloxson. Masters Thesis. Stephen F
Austin State University. May 4, 2021. Geochemical
Characterization of the Utica Shale Play using XRF-Based Chemostratigraphy in
Ohio - Stephen F. Austin State University
Exploration
Opportunities North America: Utica – Point Pleasant Shale. Satinder Chopra,
Ritesh Kumar Sharma, Hossein Nemati, and James Keay; TGS. GeoExPro. December
10, 2018. Utica -
Point Pleasant Shale - GeoExpro
Southeastern
Ohio Pt. Pleasant: A Shale Play? Chad Cunningham, presented at Appalachian
Geological Society Meeting, Jan. 19, 2016.
Condensed
sections. Wikipedia. Condensed
sections - Wikipedia
Sequence
Stratigraphy and TOC Modeling of the Utica-Point Pleasant Interval in the
Middle Appalachian Basin. Taylor McClain. AAPG Datapages/Search and Discovery
Article #90262 ©2016, AAPG Division of Professional Affairs, Pittsburgh
Playmaker Forum, Pittsburgh, Pennsylvania, April 13, 2016. View
PDF
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