Petrographic characterization of the Barnett Shale, Fort Worth Basin, Texas
Creators: name: Papazis, Petros Konstantinos
Publisher: name: The University of Texas at Austin
Year: 2005
Subjects: subject: Petrology--Texas--Fort Worth Basin
subject: Shale--Texas--Fort Worth Basin

description: Shale sedimentology is one of the less explored areas of sedimentary geology. Shales account for two-thirds of the sedimentary rock record and preserve large portions of Earth's history in relatively continuous successions. In addition, these rocks serve as key element of basin-scale fluid flow systems and are the primary source of materials released from water-rock interactions at depth. The Mississippian Barnett Shale of north-central Texas is of particular interest because it serves as both source rock and hydrocarbon reservoir for the Newark East Field, the largest gas field in Texas (Montgomery et al., 2005), and the target of intense current exploration interest. This study explores the petrographic characterization of the Barnett Shale by identifying the different silt-, sand-, and granule-size grain types; quantifying percentages and ratios of clay-size to silt-, sand-, and granule-size grain fractions; and determining the nature of the clay-size constituents. Samples used in this study come from four cores in Cooke, Erath, and Wise counties in north-central Texas, and from outcrop in Lampasas and San Saba counties in central Texas. An integrated approach utilizing transmitted polarized light microscopy, secondary and back-scattered electron imaging, cathodoluminescence imaging, X-ray mapping, and X-ray diffraction is applied to 76 polished thin sections. Five major lithologic groups are identified from core observations: 1) black shale relatively lacking in silt-size particles; 2) calcite-rich lithologies; 3) silt-rich black shale; 4) coarse grain accumulations that include calcite-rich and phosphate-rich varieties; and 5) concretions. Grain types for each lithology are classified based on their provenance, as either extrabasinal or intrabasinal particles. Extrabasinal particles include plant fragments and detrital silt-size quartz, feldspar, and mica. Intrabasinal particles are subdivided into calcareous and non-calcareous fossils, coated grains, and phosphatic intraclasts. Authigenic phases identified include Ca-phosphate, calcite, dolomite, ankerite, pyrite, quartz, albite (?), chlorite, barite, sphalerite, P-rich moganite, and titanium-oxide. The results of this study reveal that clay minerals are not the dominant components in these rocks, which are dominated instead by silt- and clay-size quartz, and locally, by biogenic debris and phosphatic intraclasts. This research demonstrates that the Barnett contains rich variety of lithologies that denote important environmental variations. Ultimately, documentation of the spatial distribution of these lithologic types can contribute to regional environmental/stratigraphic model that can then be further linked to mechanical properties within this important economic unit. An understanding of rock properties in the Barnett has wide-ranging implications because this little-understood rock type forms fractured-shale gas systems worldwide.