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Diagenetic Modifications of The Eagle Ford Formation: Implications on Chemical and Physical Properties.

Mcallister, Richard Thomas

[Thesis]. Manchester, UK: The University of Manchester; 2017.

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Abstract

This thesis investigates the impacts of diagenesis on the Late-Cretaceous Eagle FordFormation (Fmn) in south-west Texas. This was achieved utilising many techniques suchas of outcrop and core analysis, standard petrographic techniques (includingcathodoluminescence [CL] and scanning electron microscopy [SEM]), and geochemicalanalysis (x-ray diffraction [XRD], stable isotope analysis of C and O within inorganicminerals and Rock Eval pyrolysis). The bulk of diagenetic products and textures wereidentified via petrographic techniques, with geochemical analysis confirminginterpretations based on visual observations.This thesis shows the Eagle Ford Fmn is a calcareous, organic-rich mudstone containingeight distinct lithofacies, which have all been directly impacted by burial diagenesis. TheLower Eagle Ford Fmn mainly comprises of dark organic and clay-rich lithofacies whichrepresent a classic source rock with interbedded carbonate rich lithofacies. The UpperEagle Ford Fmn is organic and clay-poor, with the bulk of lithofacies carbonate dominatedand heavily cemented. An initial anoxic, open marine depositional environment whichtransitions into an oxic deepening environment is inferred during deposition of the EagleFord Fmn.Early, microbial derived redox reactions have precipitated authigenic calcite and pyritewithin the Eagle Ford Fmn. Authigenic calcite infills and preserves biogenic debris(mainly planktonic and benthic foraminifera), with pyrite framboids post-dating thecarbonate cements. Kaolinite infilling biogenic debris is also a common occurrenceindicating it is also an early diagenetic product. Smectite is converted to mixed layer I/Sand illite during deep burial processes at similar depths and temperatures to hydrocarbongeneration and expulsion. Authigenic quartz cements precipitate within primary porosityand on top of carbonate cements. Chlorite is observed as the last mineral precipitated in theEagle Ford Fmn, often pseudomorphed from kaolinite within the micritic matrix.Diagenesis has had the greatest impact on porosity distribution in the Eagle Ford Fmn. Theorganic, clay-rich lithofacies contain little intra/inter-crystalline porosity with the bulkobserved as clay-held or organic porosity. Meanwhile the carbonate-rich lithofaciescontain mainly intra-crystalline porosity.Concretions are a common feature observed in the Lower Eagle Ford Fmn outcrops. Fourconcretion types were identified and studied using a variety of petrological andgeochemical techniques. Diagenesis plays a major role in all concretions types. However,primary factors such as sea level fluctuation, sediment input and tectonic activity also havekey impacts on the formation of concretions.