ABSTRACT

Low electrical resistivity measurements in organic-rich mudrocks are commonplace in highly mature zones. These low resistivity values are usually difficult to justify and lead to overestimation of water saturation when using conventional resistivity-porosity-saturation models (e.g., dual water and Waxman-Smits). Previous publications suggest that the electrical conductivity of kerogen increases when it thermally matures. This increase in thermal maturity of kerogen might contribute to low resistivity measurements in organic-rich mudrocks. However, electrical properties of kerogen within these rocks have not yet been quantified experimentally. We have introduced a technique to quantify electrical resistivity of kerogen through combined experimental and numerical approaches and quantified electrical resistivity of kerogen samples from the Haynesville and Eagle Ford Formations. We first isolated kerogen from mudrock samples using physical and chemical treatments. The isolated kerogen powder was then compressed into a homogeneous disk. Then, we synthetically matured mudrock and kerogen samples to controlled maturity levels and measured the electrical resistivity and geochemical properties of each sample. The true electrical resistivity of kerogen was then estimated by minimizing the difference between the numerically simulated and measured electrical resistivity of the molded kerogen samples. We have observed a significant decrease in the electrical resistivity of kerogen isolated from the Haynesville (i.e., up to four orders of magnitude) and Eagle Ford (i.e., up to nine orders of magnitude) Formations upon heat treatment from 300°C to 800°C. The decrease in resistivity can be reasoned by the chemical transformations of organic matter through thermal maturation. The results of solid-state C13 nuclear magnetic resonance spectroscopy and transmission electron microscopy imaging confirmed increase in graphitization and aromaticity in the kerogen samples as thermal maturity increases. Our outcomes can potentially improve interpretation of electrical resistivity logs in organic-rich mudrocks, such as enhancing well-log-based assessment of in situ hydrocarbon saturation.

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