Browsing by Subject "Eagle Ford Group"
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Item A facies-scale chemo-lithostratigraphic composite profile of Del Rio claystone through Austin Chalk deposition, late Cretaceous, central Texas, USA(2016-05) Gabb, Kyle Christopher; Fisher, W. L. (William Lawrence), 1932-; Rowe, Harry; Hentz, Tucker FLate Cretaceous deposition (Comanche/Gulfian Series) across the San Marcos Arch of central Texas is characterized by both aluminosilicate- and carbonate-dominated mudrock successions. Twelve drill cores were recovered from the 2-acre construction site of the AT&T Executive Education and Conference Center on The University of Texas at Austin main campus. Thirteen detailed chemo-lithofacies were defined through a combination of visual description and elemental analysis utilizing energy-dispersive X-ray fluorescence (ED-XRF). Elemental analysis was undertaken at a 0.25-ft sampling interval to provide a record of facies-scale variability. A composite profile from two cores was created by identifying the boundary between the top of the Buda Limestone and bottom of the Eagle Ford Group and then splicing the records together, thus maximizing the length of the composite core to 174 ft and incorporating the Del Rio Claystone, Buda Limestone, Eagle Ford Group, and Austin Chalk formations. Lithofacies within the four formations were defined based on Dunham’s (1962) carbonate and Potter’s (1980) shale classifications. Important sedimentary structures and mineralogical compositions were identified and added as modifiers to the facies description. The shifts between the carbonate formations and the clay-rich formations are caused by changes in sea level, productivity, anoxia, and reworking by bottom currents. The Buda Limestone and Austin Chalk are identified to range in composition from limestone to marl, as the matrixes of the formations are composed of alumo-siliceous mud. Covariance of silicon with aluminum and titanium (proxies for detrital inputs) suggests that the silica is dominantly of terrigenous origin, rather than of biogenic origin. The enrichment of redox elements (molybdenum and vanadium) indicates the occurrence of anoxia during deposition of the Eagle Ford Group. The increased presence of volcanic ash beds within fossil-rich section of the Eagle Ford Group indicates that nutrients supplied by volcanism led to increased primary productivity, triggering depleted oxygen levels and anoxic bottom waters. Whereas sea level was likely the driving force that caused changes within the Del Rio Claystone, Buda Limestone, and Austin Chalk, productivity and anoxia are the drivers of transitions within the Eagle Ford Group.Item The Eagle Ford formation of Travis County, Texas(1925-06) Green, Guy EmmettItem Lithofacies, biostratigraphy, chemostratigraphy, and stratal architecture of the Boquillas Formation and Eagle Ford Group : comparison of outcrop and core data from Big Bend National Park to Maverick Basin, Southwest Texas, USA(2015-05) Fry, Kathryn O'Rourke; Loucks, R. G.; Frébourg, Gregory; Kerans, Charles; Fisher, WilliamThe late Cretaceous-aged Eagle Ford and Boquillas Formations were deposited on the Texas paleoshelf during a major transgressive sequence wherein organic-rich mudrocks accumulated across the paleoshelf. This study investigates lithofacies; biostratigraphy, chemostratigraphy, and stratal architecture of the Eagle Ford and Boquillas Formations to characterize the depositional environment present during deposition, as well as define and describe the Cenomanian-Turonian (C-T) stage boundary and the Oceanic Anoxic Event II (OAE2). Comprehensive, high-resolution data sets compare geologic and geochemical interpretations of subsurface Eagle Ford Group conventional core from the Maverick Basin and chrono-synchronous Boquillas Formation outcrop strata from Big Bend National Park. Results from core and outcrop show a dynamic depositional environment regularly influenced by bottom-currents, debris-flows, and deposition during anoxic bottom-water conditions. Elemental and biostratigraphic data show that the water-column was stratified -- surface-waters experienced high levels of primary productivity while deeper waters were anoxic to euxinic. The Eagle Ford strata are divided into a lower and upper group defined geochemically by the appearance of a titanium-rich chemofaceis correlative to massive argillaceous claystone. This change in deposition has been defined to occur concurrently with the C-T boundary (identified biostratigraphically) and OAE2 (identified chemostratigraphically and isotopically). The OAE2 as documented within both cores shows an 'oxygenated' anoxic event, wherein burrowing and low molybdenum are documented during the positive δ¹³C isotope excursion. Outcrop and core data comparison demonstrate a similar depositional system between Big Bend National Park and Maverick Basin; bottom-currents, debris-flows, and periodic anoxia are all documented within both sections, however, further investigations are needed to correlate the sections.Item Pore network variation identified through NMR analysis : Eagle Ford Group, South Texas, USA(2015-05) Shultz, James Marion; Tinker, Scott W. (Scott Wheeler); Hammes, Ursula; Smye, Katie; Fisher, WilliamMudrock porosity is associated with both organic and inorganic matter, and hydrocarbons are found in both. The upper Eagle Ford Group is dominated by inorganic porosity, while the lower Eagle Ford Group is considered to have more organic-hosted porosity related to its high organic content. The differences in organic and inorganic pore types play a large role in the effectiveness of pore networks. This study investigates Eagle Ford Group mudrock pores through the use of nuclear magnetic resonance (NMR) in order to more accurately describe porosity. Laboratory-based NMR has been used to measure the fluid content and pore volume of mudrocks affordably and nondestructively. Although NMR is versatile, it has limitations, in that pore-throats and pores must be fluid filled in order to be detected. However, calibration to Mercury Injection Capillary Pressure (MICP) measurements yields interpretable and valuable results. Using a total of 28 core samples from three wells in Karnes and Maverick Counties, South Texas, this study examines how pore networks differ vertically within the succession. The lithology and facies vary vertically within each well and laterally between wells. The facies groups defined with visual core examination were then modeled with wireline logs. While the facies groups show significant overlap in petrophysical character measured by NMR, scanning electron microscopy (SEM), total organic carbon (TOC), x-ray fluorescence (XRF), and x-ray diffraction (XRD), the samples taken from the same facies group nonetheless have similar MICP intrusion corrected porosities. When samples are compared without regard to facies groups, both MICP and TOC strongly correlate with NMR. While NMR detects differences in the pore network including the relative amount of porosity associated with clay sized particles between the upper and lower Eagle Ford; and SEM shows that much of the upper Eagle Ford organic porosity appears to be associated with migrated organic matter. Vertically segregating the Eagle Ford by facies groups that can be modeled by wireline log, while difficult, with further sampling, may prove useful to play-wide mapping.