Barnett%20Shale%20Geology%20and%20Petrology

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• Barnett ShaleGeology and Petrology

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Barnett Shale Paleogeography
From Mississippian Barnett Shale Lithofacies
and depositional setting of a deep-water
shale-gas succession in the Fort Worth Basin,
Texas, by Robert G. Loucks and Stephen C. Ruppel,
AAPG Bulletin, v. 91, no. 4, p. 579-601, April
2007
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Barnett Shale Depositional Processes
From Mississippian Barnett Shale Lithofacies
and depositional setting of a deep-water
shale-gas succession in the Fort Worth Basin,
Texas, by Robert G. Loucks and Stephen C. Ruppel,
AAPG Bulletin, v. 91, no. 4, p. 579-601, April
2007
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Barnett Shale Lithology
LITHOLOGY The Barnett Shale is composed of a
combination of marine clays, primarily illite and
chlorite, detrital quartz silt, silicified and
carbonate bioclasts and fossils, interstitial
organic carbon, and phosphate. Most Barnett Shale
samples are siliceous mudstones, not shales, such
as the top photo. Many are so rich in quartz that
they are really argillaceous siltstones, such as
the lower photo. In both photos, quartz grains
are white, but some elliptical grains are
compressed silicified microfossils.
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Barnett Shale Bedding
DISTURBED BEDDING Barnett Shale sediments were
often disturbed by bottom currents created by
gravity, density, and turbidity flows. These
flows would carry shelf and slope components,
especially shallow-water fossils and bioclasts,
down to the basin floor and deposit them in the
carbon-rich muds. The density flows were strong
currents that disturbed the bottom, creating
ripple marks and disturbed, wavy bedding.
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Barnett Shale Fossils
FOSSILS Barnett Shale often contain macrofossils
and bioclasts transported from the shelf to the
basin by density and gravity currents. These
fossils are composed of their original calcite,
of recrystallized calcite or dolomite, or of
quartz after undergoing silicification. The top
photo shows bivalve mollusks (pelecypods) and
skeletal fragments. The lower photo shows crinoid
columnal bioclasts and flattened pelecypod shells.
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Barnett Shale Microfossils
MICROFOSSILS Barnett Shale samples often contain
microfossils. These have two origins. The
examples shown here are both shallow-water
foraminifera that were swept into the basin
environment by density and gravity currents. The
top photo is an agglutinated foram whose walls
were composed of calcite grains that have now
been completely silicified and compressed. The
lower photo is a uniserial foram that has been
phosphatized.
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Barnett Shale Phosphates
MARINE PHOSPHATES The Barnett Shale often
contains phosphates. Phosphates form at the edge
of a basin just seaward of the slope. Here,
upwelling creates an enormous phytoplankton
productivity which causes the underlying
sediments to become rich in organic components,
including phosphorous. The phosphates accumulate
in nodules or crusts, and phosphate can replace
other minerals or precipitate in pore space. The
orange-yellow mineral is phosphate.
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