This study presents new optical petrography and electron microscopy data, interpreted
in the context of previously published petrophysical, geochemical, and mineralogical
data, to further characterize the Tuscaloosa marine shale (TMS) as an unconventional
reservoir in southwestern Mississippi. The basal high resistivity zone has a higher
proportion of Type II sedimentary organic matter than the overlying TMS, indicating
it is more prone to oil generation. Optical petrography and electron microscopy reveal
a heterogeneous clay matrix with ubiquitous pyrite grains, quartz, feldspar, glaucony,
foraminifera, shell fragments, and rarer occurrences of apatite and crinoid fragments
as well as liptinite, alginite, inertinite, and vitrinite. Our petrographic observations
suggest that higher abundances of detrital quartz grains coupled with minimal authigenic
cements result in higher porosity and permeability. However, the TMS is also more
clay-rich than other unconventional shale oil and gas plays, which can impair the
effectiveness of hydraulic fracture stimulation. Thin section observations reveal
alternating clay and calcium carbonate laminae that are interpreted to reflect changes
in sediment flux. Planktonic foraminifera indicate an overlying oxygenated water column
while benthic inoceramid fragments and pervasive authigenic pyrite suggest anoxic
or dysoxic bottom water conditions. Apatite fragments in thin section suggest mixing
events and an influx of nutrient-rich sediments. Overall, these observations suggest
that a variety of paleodepositional environments occurred in the TMS and the lithofacies
diversity resulting from these small-scale depositional cycles makes it difficult
to determinatively identify areas conducive to enhanced economic hydrocarbon recovery.
