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Lateral Heterogeneity of Microscale Lithology and Pore Development in an Upper Cretaceous Eagle Ford Group Horizontal Core


A horizontal core from the Eagle Ford Group of south Texas provides a rare opportunity to study lateral heterogeneity of lithology, TOC, and pore development in an argillaceous calcareous mudrock from the subsurface. The core is 182 ft long, spans between 2 and 4 ft of section, and true vertical depth is ~10,970 ft. TOC varies from 3.4 to 5.4% and averages 4.4%. Calculated vitrinite reflectance is ~1.1%. Ar-ion-milled samples (21) from 6 vertical cuts along the core were examined. Macroscopic variations in lithology are related primarily to foraminiferal abundance and degree of lamination. Thin grainstone lags with numerous foraminifera are common but generally discontinuous.

On a microscopic scale, the rock can be divided into three major domains; coccolith-rich peloids, argillaceous seams, and foraminifera. Peloids contain interparticle pore spaces filled with organic matter interpreted to be migrated bitumen. Argillaceous seams anastomose around the peloids and are predominantly clay minerals with quartz, albite and minor organic matter. The ratio of argillaceous seams to peloids varies between laminae and between samples.

Visually there is variation in the abundance of foraminifera. SEM examination shows chambers filled with calcite, kaolinite, and bitumen, along with minor pyrite, dolomite and quartz. From sample to sample and from foraminifera to foraminifera, the proportion of fill material differs; some samples have more calcite, some more kaolinite, some more bitumen. Chamber fill in grainstone lags is dominantly calcite and calcite cement is present between foraminifera. Variation in the abundance of pyritized radiolaria suggests local differences in the amount of dissolved silica available for quartz cement. Significant variations in the amounts of silt-size dolomite and albite occur between samples.

These rocks have a complex pore system. Within bitumen in peloids and foraminifera chambers there are typically two sizes of organic-matter pores: µm-scale spherical to slightly elongate pores and more numerous nm-scale pores. The larger pores appear to be connected by the smaller, more numerous, pores. Within the argillaceous seams surrounding the peloids are minor nm-scale interparticle pores. As pore abundance is influenced primarily by the local ratio of peloids to argillaceous seams, lateral variations in the composition of the mudrock drive variations in the pore system.