Ignoring the presence of pyrite can lead to errors in the estimation of Total Organic Carbon (TOC) since pyrite has significantly higher density and conductivity compared to other minerals in shale formations. This study aims to improve the accuracy of estimating TOC from well log data by accounting for the pyrite effect in Eagle Ford shale. To this end, more than 50 feet of preserved cores samples from the Eagle Ford were analyzed using laboratory pyrolysis, X-ray fluorescence (XRF), X-Ray Diffraction (XRD), and spectral core gamma system.
Since there is significant vertical heterogeneity in the Eagle Ford shale, parameters such as TOC, pyrite content, Gamma ray intensity, content of Fe and S, and concentrations of U, Th, and K were analyzed on a fine scale in the Upper and Lower Eagle Ford respectively. Analysis of laboratory TOC data were applied to calibrate TOC data using geophysical well logs methodologies. Pyrite data from XRD analysis were used to find the relationship between pyrite and organic matter and to determine the effect of pyrite on well logs. Well-log-based TOC calculation methods were improved by considering pyrite as an adjustable parameter in equations. Schmoker's (1983) four-component system rock model and Alfred and Vernik's (2013) two-pore system model are two representatives of density-log-based TOC calculation methods. Based on these two models, a new petrophysical model considering pyrite and organic porosity was developed.
In this research, empirical correlation between TOC and pyrite was explored. Changes of Fe and S concentrations with depth and Gamma ray intensity was determined. The trends of Fe sand S contents matched Gamma ray intensity very well in the depth range from 13790 ft to 13825 ft. Empirical relationships were found between TOC and Gamma ray intensity, TOC and Uranium, respectively. Furthermore, a new petrophysical model considering pyrite and organic porosity was validated with TOC and density data from shale formations. The proposed model improves the estimation of TOC calculation in Eagle Ford formation by the incorporation of pyrite effect.