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Evolution of Pore Systems in Eagle Ford Mudstones: Influence of Texture, Diagenesis, and Thermal Maturity

Abstract

Understanding controls on the evolution of pore networks is necessary for establishing models for porosity prediction in unconventional reservoirs such as the Eagle Ford Formation. Using light microscopy, FE SEM, cathodoluminescence, and XRD, we have investigated the influence of primary rock texture, diagenesis, and thermal maturity on pore networks in a range of Eagle Ford samples from low maturity outcrop cores (Ro: 0.4 – 0.7%) to higher maturity subsurface cores (Ro up to 1.2%). The Eagle Ford can be divided into four main facies on the basis of texture, diagenesis, mineralogy and organic content: calcite-rich laminated and bioturbated mudstones that dominate the Upper Eagle Ford (UEF) and organic-rich laminated and calcite-replaced radiolarian-bearing mudstones that dominate the Lower Eagle Ford (LEF). More than three hundred interbedded volcanic ash beds are found throughout the section in the Maverick Basin. UEF deposition was generally characterized by elevated oxygenation levels that supported bioturbation. The LEF was deposited under lower oxygenation and higher productivity conditions, which is reflected in higher organic matter (TOC) abundances. Calcite cement is the most common pore-reducing authigenic phase along with varying amounts of kaolinite, pyrite, quartz, dolomite, gypsum, and albite. Eagle Ford samples contain a diversity of pore types including intergranular (mostly between bioclasts, coccoliths and clays), intragranular (within forams and pellets), and organic matter-hosted pores. Their relative abundances and contribution to total porosity depend on rock type (depositional texture, degree of diagenesis, organic content) and burial (compaction, cementation, and thermal maturity). During early burial, compaction and diagenesis decrease the mineral matrix porosity. With increasing thermal maturity (early oil- to gas-window) organic matter-hosted pores start to evolve by the thermal transformation of labile kerogen. This results in porosity increasing in the organic-rich facies with increasing thermal maturity. Samples from our outcrop research cores have measured porosity values that vary between 10 – 20% for mudstones and reach up to 30% in volcanic ash beds. In comparison, porosity in the deeply buried samples is reduced to 2 – 15% for the mudstones and 8 −12% for the ash beds. Linking the variation in porosity to rock types, burial, and thermal maturity is the key for predicting porosity distribution.