This thesis investigates the impacts of diagenesis on the Late-Cretaceous Eagle Ford Formation (Fmn) in south-west Texas. This was achieved utilising many techniques such as of outcrop and core analysis, standard petrographic techniques (including cathodoluminescence [CL] and scanning electron microscopy [SEM]), and geochemical analysis (x-ray diffraction [XRD], stable isotope analysis of C and O within inorganic minerals and Rock Eval pyrolysis). The bulk of diagenetic products and textures were identified via petrographic techniques, with geochemical analysis confirming interpretations based on visual observations. This thesis shows the Eagle Ford Fmn is a calcareous, organic-rich mudstone containing eight distinct lithofacies, which have all been directly impacted by burial diagenesis. The Lower Eagle Ford Fmn mainly comprises of dark organic and clay-rich lithofacies which represent a classic source rock with interbedded carbonate rich lithofacies. The Upper Eagle Ford Fmn is organic and clay-poor, with the bulk of lithofacies carbonate dominated and heavily cemented. An initial anoxic, open marine depositional environment which transitions into an oxic deepening environment is inferred during deposition of the Eagle Ford Fmn. Early, microbial derived redox reactions have precipitated authigenic calcite and pyrite within the Eagle Ford Fmn. Authigenic calcite infills and preserves biogenic debris (mainly planktonic and benthic foraminifera), with pyrite framboids post-dating the carbonate cements. Kaolinite infilling biogenic debris is also a common occurrence indicating 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 hydrocarbon generation and expulsion. Authigenic quartz cements precipitate within primary porosity and on top of carbonate cements. Chlorite is observed as the last mineral precipitated in the Eagle Ford Fmn, often pseudomorphed from kaolinite within the micritic matrix. Diagenesis has had the greatest impact on porosity distribution in the Eagle Ford Fmn. The organic, clay-rich lithofacies contain little intra/inter-crystalline porosity with the bulk observed as clay-held or organic porosity. Meanwhile the carbonate-rich lithofacies contain mainly intra-crystalline porosity. Concretions are a common feature observed in the Lower Eagle Ford Fmn outcrops. Four concretion types were identified and studied using a variety of petrological and geochemical techniques. Diagenesis plays a major role in all concretions types. However, primary factors such as sea level fluctuation, sediment input and tectonic activity also have key impacts on the formation of concretions.

Grain assemblages in organic-rich mudrocks of the Eagle Ford Formation of South Texas are assessed to determine the relative contributions of intra- and extrabasinal sediment sources, with the ultimate goal of producing data of relevance to prediction of diagenetic pathways. Integrated light microscopy, BSE imaging, and X-ray mapping reveal a mixed grain assemblage of calcareous allochems, biosiliceous grains (radiolaria), quartz, feldspar, lithics, and clay minerals. Dominant fossils are pelagic and benthic foraminifers and thin-walled and prismatic mollusks; echinoderms, calcispheres, and oysters are present. Early-formed authigenic minerals, including calcite, kaolinite, dolomite, albite, pyrite, quartz, and Ca-phosphate, some reworked, add to the overall lithologic heterogeneity. Point counting of images produced using energy-dispersive X-ray mapping in the SEM provides observations at a scale appropriate to classifying the mudrocks based on the composition of the grain assemblage, although grains and other crystals of clay-size cannot be fully characterized even with the SEM. Each sample is plotted on a triangle, whose vertices correspond to terrigenous and volcanic grains (extrabasinal components), calcareous allochems, and biosiliceous grains. As a subequal mix of grains of intrabasinal and extrabasinal origins the detrital grain assemblage of the Eagle Ford, presents a formidable challenge to the task of lithologic classification of this unit, as neither conventional limestone nor sandstone classifications can be readily applied. The abundant marine skeletal debris in the Eagle Ford is accompanied by abundant calcite cementation and the dissolution and replacement of biosiliceous debris is accompanied by authigenic quartz, suggesting that mudrock grain classification has potential for yielding diagenetic predictions. / text

Scanning electron microscopy of Ar-ion milled samples from the Eagle Ford Formation, South Texas shows that the character and abundance of porosity changes significantly across burial conditions as a result of compaction, cementation, bitumen generation, and generation of secondary porosity within organic matter (OM). Samples displaying a range of compositions and maturities are imaged and quantified to provide insight into the effects of these processes. Porosity in low-maturity samples (Ro~0.5%) is volumetrically dominated (0.1% -12.5% bulk volume, average 6.2%) by relatively large, mostly interparticle, primary mineral-associated pores (median sizes range 35.9-52.7 nm). Larger pores are generally associated with coccolith debris that is commonly aggregated into pellets. Porosity and pore size correlate directly with calcite abundance and inversely with OM volumes. OM is dominantly detrital kerogen "stringers" that range in size and have spatial distributions and character suggestive of detrital origin. Destruction of primary porosity in low-maturity samples has occurred due to compaction of ductile kerogen and clays and, to a minor degree, as a result of cementation and infill of early bitumen. Smaller, secondary OM-hosted pores (median size range 11.1-14.9 nm) volumetrically dominate porosity (0.02%-3.6% bulk volume, average of 1.36%), in most high-maturity samples (Ro~1.2%-1.3%). Mineral-associated pores are present, but are typically smaller (median size range from 20.3-40.6 nm) and less abundant (0.0%-10.0% bulk volume, average of 2.5%) than at low maturity. Abundant mineral-associated porosity is present locally in samples where incursion of primary pore space by bitumen has not occurred. OM within high-maturity samples is distributed more evenly throughout the rock fabric, occupying spaces similar in size and morphology to primary interparticle pores, coating euhedral crystals (probable cements), and filling intraparticle porosity. These observations, and positive correlation between calcite and OM volumes (OM-hosted pore volume included) in samples with dominantly OM-hosted pore networks, suggests that a large portion of OM within high-maturity samples is diagenetic in origin and has filled primary pore space. Destruction of primary porosity in high-maturity samples has occurred through cementation, bitumen infill, and, possibly greater compaction. Additional porosity, however, has been generated through maturation of OM. / text