Improving the Total Organic Carbon Estimation of the Eagle Ford Shale with Density Logs by Considering the Effect of Pyrite

Shuxian Jiang,Mehdi Mokhtari,David Borrok,Jim Lee

doi:10.3390/min8040154

Shuxian Jiang, Mehdi Mokhtari + Show 2 more

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https://doi.org/10.3390/min8040154

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Abstract

Pyrite is a common mineral with a higher density than most other minerals in the Eagle Ford Shale formation. Hence, if pyrite is not considered in the total organic carbon (TOC) estimation, based on density logs, it may lead to errors. In order to improve the accuracy of the TOC estimation, we propose an updated TOC estimation method that incorporates the concentration of pyrite and organic porosity. More than 15 m of Eagle Ford Shale samples were analyzed using Rock-Eval pyrolysis, X-ray fluorescence (XRF), and X-ray diffraction (XRD). TOC, elemental concentration, and mineralogical data were analyzed for a better understanding of the relationship between the concentration of TOC and pyrite content in the Eagle Ford formation. An updated petrophysical model—including parameters such as organic pores, solid organic matter, inorganic pores, pyrite, and inorganic rock matrix without pyrite—was built using the sample data from the Eagle Ford. The model was compared with Schmoker’s model and validated with the Eagle Ford field data. The results showed that the updated model had a lower root mean square error (RMSE) than Schmoker’s model. Therefore, it could be used in the future estimation of TOC in pyrite-rich formations.

Highlights

Total organic carbon (TOC) is a fundamental parameter that is critical in evaluating the hydrocarbon generating potential of source rocks [1]
The pyrolysis results of the Eagle Ford Shale samples are presented in Appendix A. These data were used for the interpretation of the kerogen type, source rock generative potential, and degree of maturation [26,27,28,29]. It can be seen from the modified van Krevelen diagram (Figure 1) that the hydrogen index (HI)
An updated model was developed in this study for predicting TOC from bulk density data

Summary

Introduction

Total organic carbon (TOC) is a fundamental parameter that is critical in evaluating the hydrocarbon generating potential of source rocks [1]. TOC can be obtained directly from Rock-Eval pyrolysis or similar experiments in a laboratory, or it can be obtained indirectly from well logging techniques, such as density logs. The geophysical property that is most widely applied in identifying source rocks and estimating TOC is anomalously high gamma-ray values [2,3,4,5,6]. Schmoker [3] proposed a relationship between total gamma-ray intensity and organic richness in Devonian-age Appalachian shales.

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