Characterization of marl and interbedded limestone layers in the Eagle Ford Formation, DeWitt county, Texas

Abstract

The Eagle Ford Formation in Texas, USA is composed of marl and interbedded limestone sequences with intermittent layers of volcanic ash. To optimize hydrocarbon recovery from such a heterogeneous formation, it is necessary to have a better understanding of the effects of each individual marl and limestone layer on reservoir properties. The objective of this paper is to characterize and quantify properties of marl and limestone layers in the Eagle Ford Formation, in terms of spectral core gamma ray, elemental concentrations, lithology and organic richness. Moreover, the differences between these layers in micro-scale level were shown through the scanning electron microscope (SEM) images. XRF results show that concentrations of Al2O3, Fe2O3, Mo, and Zr in marl layers were more than 40% higher than in the interbedded limestone layers. The Eagle Ford samples fall into two groups in the ternary plot. Limestone layers contain calcite content ranging from 55% to 90% and clay content ranging from 5% to 30%, while marl layers contain 30%–70% calcite and around 20%–50% clay. Marl layers have better generative potential than the limestone layers in the Eagle Ford Formation. Moreover, estimation of total organic carbon of the Eagle Ford Formation using Passey's Δlog R method with a correction factor was provided and validated with lab measured data.