Quantifying Electrical Resistivity of Isolated Kerogen from Organic-Rich Mudrocks using Laboratory Experiments

Abstract

Abstract Reliability of conventional resistivity-porosity-saturation models (e.g., dual water and Waxman-Smits) is questionable in organic-rich mudrocks, which often leads to overestimation of water saturation. Previous publications showed that the interpretation of electrical resistivity logs for such rocks is challenging because of the presence of highly mature kerogen and pyrite connected networks that can influence electrical resistivity measurements. However, the electrical properties of kerogen in these rocks have not yet been quantified experimentally. Separation of kerogen from mudrocks and subsequent removal of pyrite from the kerogen, both requirements for successful laboratory experiments, remain challenging. In this paper, we quantitatively evaluate the electrical resistivity of mature kerogen using an experimental approach. We first isolated kerogen from mudrock samples, using physical and chemical techniques and removed pyrite by treating the sample with acidic chromium chloride solution under nitrogen (N2) atmosphere. The isolated kerogen powder sample is then compressed to form a homogeneous consolidated disk using a mold frame designed for this purpose. Furthermore, we synthetically matured both mudrock and isolated kerogen samples by heat-treating. Then, we measured the electrical resistivity and geochemical properties of each set of molded samples at different maturity levels. The actual electrical resistivity of kerogen was estimated by minimizing the difference between the numerically simulated and measured effective electrical resistivity of the molded kerogen samples. We successfully isolated kerogen from Haynesville and Eagle Ford mudrock samples. X-ray fluorescence (XRF) measurements confirmed the absence of carbonates, silicates, and pyrite in the isolated kerogen samples. We observed a significant decrease in electrical resistivity of isolated kerogen samples from the Haynesville (i.e., up to four orders of magnitude) and the Eagle Ford (i.e., up to nine orders of magnitude) formations upon elevating the heat-treatment temperature from 300°C to 800°C. The decrease of electrical resistivity at high maturity levels could be attributed to the presence graphite-like sheets and/or appearance of aromatic components in the organic matter. The outcomes of this paper can potentially improve interpretation of electrical resistivity logs in organic-rich mudrocks, which can lead to enhanced well-log-based assessment of in situ hydrocarbon saturation.