This is an interesting study of the K-bentonites present in the Point Pleasant Formation in the Utica Shale section in Eastern Ohio. Disclosure- I used to work for this company, but that was many years ago. They have done some great work in recent years.
K-bentonites are altered
volcanic ash layers that are very good chronostratigraphic markers. These beds
are soft, and drill quickly compared to the rock around them. They are
typically easy to point out on logs due to their gamma ray signatures, but can
be difficult to correlate one to the other across wells, which is what this
paper addresses. The goal is to correlate properly so that the ideal target
zones can be found for each area. The targeted sections are what are known as
“condensed sections,” or sections of very slow deposition of organic matter
below wave base in quiet offshore waters during highstand system tract sediment
deposition. They are condensed in time, with a few feet representing, in some
cases, millions or tens of millions of years of slow deposition.
The paper correlates eight
K-bentonites in total and focuses specifically on:
“three well-preserved and laterally traceable beds
were selected as type localities and named the Wells Crum, Shane, and
Brookfield K-bentonites.”
The K-bentonites are mainly
composed of the clays illite and smectite. These Ordovician beds are thin but
regionally extensive around the Midwest U.S. The presence of multiple
K-bentonite beds reveals a period of intense volcanism where explosive
eruptions led to widespread ash deposition.
The analysis utilized X-Ray
Fluorescence, or XRF, to chemically identify and later correlate the Point
Pleasant Formation K-bentonites from cores. The study also utilized ultraviolet
light on the cores to better distinguish the beds visually.
The study identified about eight
K-bentonites and focuses specifically on three correlatable ones near the
condensed intervals targeted for horizontal drilling to extract oil and natural
gas. They call the three type K-bentonites:
“These type K-bentonites have been named the 1) Wells
Crum K-bentonite (representing the K-bentonite near the top of the Point
Pleasant Formation), 2) Shane K-bentonite (representing the K-bentonite found
near the top of Unit 3 of the Point Pleasant Formation) and 3) Brookfield
K-bentonite (representing the K-bentonite found near the top of Unit 2 of the
Point Pleasant Formation) (Figure 3).”
Crossplots of thorium-silicon/aluminum, Th-Si/Al, and zirconium-silicon/aluminum, Zr-Si/Al, were tied to depths and presented as geochemical logs that best identify K-bentonites, as shown below.
As indicated below,
identification of K-bentonites in the cores was visually done in plain light
and ultraviolet light. The plain light identification was aided by the use of
the Munsell color chart, typically used for soils. The upper two K-bentonites,
the Wells Crum and the Shane, were found to be about 0.12 ft (1.44 inches)
thick, and the lower K-bentonite, the Brookfield, was found to be about 0.06 ft
(0.72 inches) thick.
The study utilized seventeen
samples of the three K-bentonites from seven wells. The breakdown of chemical
components revealed via XRF is shown in the table below.
Box and whisker analysis and
K-means cluster analysis were used as statistical techniques to derive chemical
signatures or fingerprints for each K-bentonite.
“Observation of the mean values indicates that some
elements, such as Ti, show distinct values that differ for each K-bentonite.”
This was used to inform the
box and whisker plots and ultimately the table and cluster map below, which
show that the three K-bentonites are geochemically distinctive and can be
correlated with adequate certainty based on geochemistry.
References:
Point
Pleasant Formation K-Bentonites of Southeastern Ohio. Jeffry D. Grigsby and
Robert B. Thomas, Sr. EMF Geoscience, Inc. September 22, 2025. Bentonite+Geochemical+Analysis+Final+Report+092225.pdf
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