Date: January 18, 2023
Location: Hollytree Country Club, 6700 Hollytree Dr, Tyler, TX 75703

At the January East Texas Geological Society Luncheon Meeting Giovanni Zanoni of Premier Corex presents:

Clay mineralogy and diagenesis of the Blossom Sand, Panola County, Texas – implication for depositional environment and reservoir quality prediction

The Late Cretaceous Blossom Sand of the Austin Group, typically found at shallow depths (~ 2,000 ft), is one of the most prolific reservoir formations in the East Texas basin.

Despite its notoriety and wide lateral extension, there is relatively little consensus on the origin of its sediments, clay mineralogy, and depositional setting. Early studies provided incomplete descriptions, mainly describing the formation as a fine to medium grained quartz and glauconitic sandstone. A more recent study challenged the presence of glauconite and rather interpreted the major clay phase as illite. Most of the research done on this formation lacked an adequate analytical methodology such as proper X-ray diffraction (XRD), x-ray fluorescence (XRF), and scanning electron microscopy (SEM).

Hence, to shed more light on the nature of this sandstone, a dozen samples were analyzed using the afore-mentioned methods, with an emphasis on clay mineralogy. Moreover, the combination of the various analyses provided insights into the depositional environment and diagenetic history of the Blossom Sand.

Results of our study revealed that the major minerals found in this formation include quartz, feldpsars, calcite, and a complex mixture of clay minerals. XRD and SEM-EDS confirmed the presence of illite-smectite as well as odinite-chlorite coatings as major clay phases. Despite the literature stating that glauconite is common, we discovered that the myriad of green pellets are actually clays from the verdine facies (i.e. odinite-chlorite), optically similar to glauconite and forming in an estuarine setting from iron-rich precursors. The Blossom Sand is composed of two major sand facies, (1) bioturbated argillaceous very fine to fine sandstone and (2) massive bioturbated sandstone, the latter being richer in odinite-chlorite whilst the former contains more illite-smectite. According to SEM images, odinite-chlorite occurs both as pellets and coating whilst illite-smectite only occurs as grain coatings. The sandstone grain size, sedimentary structures, sporadic shallow marine fossils, and clay mineralogy all suggest a nearshore depositional environment. The formation of clay grain coatings during early diagenesis is known to be responsible for the preservation of primary porosity with depth by covering potential nucleation sites and inhibiting the growth of quartz cement. This information, along with the nature of porosity and permeability in sandstone reservoirs like the Blossom Sand can improve success in oil and gas exploration, secondary recovery, or carbon capture and storage.

On a final note, although the Blossom Sandstone may be a suitable candidate for CO2 storage, it should be noted that due to its high proportion (>20%) of chemically unstable minerals (feldspar, carbonates, and clays), their dissolution due to CO2 injection and storage could potentially increase reservoir permeability by an order of magnitude, thereby affecting the geomechanical and tensile strength of the formation. Therefore, an experimental study investigating the amount of CO2 to be injected (and at what pressure) is required to maintain and preserve borehole integrity.

Giovanni Zanoni

Giovanni Zanoni joined Premier Corex in Summer 2021 as Sr. Technical Advisor, clay mineralogist, and XRD analyst for the Houston Branch. Giovanni oversees X-ray Diffraction operations at Premier and provides his expertise in clay mineralogy. Prior to joining Premier, he contributed to creating a brand-new Clay Mineralogy Laboratory at Texas Tech University (Lubbock, TX), where he was the primary XRD analyst. He collaborated in various research projects leading to several scientific publications involving clay mineralogy.

Giovanni is originally from Switzerland where he received a Bachelor’s and Master's degree in Earth and Environmental Sciences from the University of Geneva (Switzerland). He will be defending his doctoral research on shale diagenesis from TTU in 2023.

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