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Impacts of Diagenesis Upon Reservoir Quality of the Eagle Ford Formation, Texas

Abstract

Here we document the mineralogical composition and the diagenetic alterations of the Eagle Ford Fm. We focus on mineral formation and evolution during burial through mineralogical and petrographic analysis using XRD, optical and electron microscopy, and stable isotope analysis and highlight controls upon reservoir quality. The Eagle Ford Fm is principally an organic rich marl, which experienced early diagenetic calcite precipitation as a result of organic matter oxidation and sulphate reduction. Calcite cements take the form of replacements of foraminifera tests and micrite coatings on coccolith fragments within the argillaceous matrix. Detrital smectite and illite are the main clay component, however authigenic kaolinite and chlorite are common, especially in the lower Eagle Ford Fmn. Concretions and limestone beds are major features in the lower Eagle Ford Fmn. Concretions are laterally discontinuous in outcrop, some up to 20cm thick and 1m long. The limestone beds are continuous in outcrop and are commonly >30cm thick. Optical microscopy and cathodoluminescence (CL) show calcite precipitation within the concretions is extensive and invasive and occurs early during burial. Abrupt changes to the pore water chemistry are interpreted to be caused by microbial activity and are the cause of the nodular concretions within the organic marls. Initial inorganic oxygen isotope analysis indicates similar sea surface temperature (SST) and depositional environments for the organic marls and nodular concretions, suggesting the concretions and limestones are purely diagenetic. Carbon and oxygen isotope data will be used to constrain this further. We can identify different fabrics and pore types based on mineralogy and organic matter content that can help optimise target definition, well completion and development of the Eagle Ford Fm. The lower Eagle Ford Fm is rich in organic matter (TOC~8%) and clay (both detrital and authigenic). In our study area total porosity is 5–8% and is found predominantly within mature organic matter. In the middle Eagle Ford Fmn carbonate content increases with Total porosity and organic content decreasing slightly (TOC~5%). In the middle Eagle Ford Fm porosity is found mainly between euhedral carbonate grains creating intragranular porosity. Different fabrics and pore types have a significant impact on hydrocarbon storage and permeability.