HETEROGENEITY OF MICROLITHOLOGY IN A CRETACEOUS EAGLE FORD GROUP HORIZONTAL CORE

A horizontal core in the Cretaceous Eagle Ford Group of south Texas provides an opportunity to study small-scale lithological heterogeneity in mudrocks. The core is 182 ft long and spans ~4.5 ft of section. True vertical depth is ~10,970 ft. Calculated vitrinite reflectance is ~0.9%. 21 Ar-ion-milled samples from 6 different vertical cuts along the length of the core have been examined. Visual examination of the core showed variation from a calcareous mudstone through wackestone to packstone with variation being related to the number of foraminifera present and the presence or absence of lamination. Thin grainstone lags with numerous foraminifera are also present.

On a microscopic scale, the rock can be divided into three domains; coccolith-rich lenses (peloids), argillaceous seams, and foraminifera. The coccolith-rich peloids are calcite-rich and have interparticle pore spaces filled with migrated bitumen. The argillaceous seams anastomose around the peloids and are predominantly clay minerals with quartz and feldspar. The ratio of argillaceous material to peloids varies between laminae.

Visually there is variation in the abundance of foraminifera. SEM examination shows another variation; chamber fill includes calcite, kaolinite, bitumen and rarely pyrite and quartz. From sample to sample and from foraminifera to foraminifera, the proportion of fill material changes; some samples have more calcite, some more kaolinite, some more bitumen. One sample examined was from a millimeter-scale grainstone lag layer. The foraminifera chamber fill for this layer was dominantly calcite and calcite cement was present between foraminifera.

The core has been scanned with handheld XRF instruments to map elemental variations. The ratio of foraminifera fill present seems to be sufficient to alter the bulk chemistry of the sample to a measurable degree in more foraminifera-rich layers; Al and Si values are increased by kaolinite, and Ca values are increased by calcite.

An unexpected observation was the presence of numerous silt-sized euhedral apatite crystals. These have a strong resemblance to apatite crystals seen in an ash bed from near the base of the Cretaceous Austin Chalk. This suggests that a significant amount of the argillaceous and siliceous material in these samples is admixed volcanic ash.