Effects of thermal maturity and chemical composition of kerogen on its dielectric constant Archana Jagadisan 1 and Zoya Heidari 1 ABSTRACT Reliable formation evaluation using borehole geophysical measurements in organic-rich mudrocks requires knowledge about the physical properties of kerogen. For instance, estimates of water-filled pore volume are significantly affected by the as- sumptions made for dielectric permittivity of kerogen, which can be influenced by thermal maturity. However, the impact of thermal maturity of pure kerogen on its dielectric properties is not yet thoroughly understood. We quantify the dielectric con- stant of kerogen samples extracted from three formations, with different levels of natural thermal maturity, and we identify the impact of thermal maturity on their dielectric properties. We first isolate kerogen from mudrock samples using physical and chemical treatments. We then synthetically mature the samples in a controlled environment and measure their dielectric con- stant (at 1 GHz) using a microwave resonator. X-ray photoelec- tron spectroscopy (XPS) monitors the variation in chemical composition of kerogen. The dielectric constant of the kerogen samples varied significantly in the range of 1.89–3.2 upon being heat treated from 25°C to 650°C. The variation in the dielectric constant is explained by the alteration in the chemical compo- sition and structure of kerogen as a result of thermal maturation. XPS measurements also showed an increase in aromatic carbon content in the kerogen samples as the thermal maturity in- creased. The documented results enable the integration of the kerogen geochemistry to the interpretation of dielectric mea- surements, which contributes to improved interpretation of di- electric logs in organic-rich mudrocks, and result in enhanced formation evaluation of these reservoirs. INTRODUCTION Log-based formation evaluation of organic-rich mudrocks can be challenging due to uncertainties associated with petrophysical proper- ties of the rock components, such as kerogen, as well as the complex rock physics of these reservoirs. For instance, one important challenge faced in the formation evaluation of organic-rich mudrocks is to reliably determine fluid saturations. Resistivity measurements have typically been used for water saturation evaluation in conventional formations. In unconventional organic-rich mudrocks, however, their application in the quantification of water saturation becomes limited due to significant uncertainties in the formation water salinity, as well as complex pore structure and fluid distribution, which lead to failure of conventional resistivity-porosity-saturation rock-physics models such as Archie’s, dual-water, and Waxman-Smits models (Archie, 1942; Waxman and Smits, 1968; Clavier et al., 1984). Compared with resistivity measurements, dielectric logs are less sensitive to water salinity, especially at high frequencies (i.e., greater than 1 GHz) (Shao et al., 2003; Seleznev et al., 2014). Thus, they have been considered to be attractive options for water saturation assessment and as alterna- tives to resistivity logs (Calvert and Wells, 1977) in the presence of uncertainty in salt concentration and at low salinities. Dielectric properties manifest as a complex permittivity. The real component of the dielectric permittivity is usually expressed as rel- ative permittivity, which is the dielectric property of a material rel- ative to that of free space. The imaginary component contains a frequency-dependent conductivity term. The complex permittivity of porous media (von Hippel, 1954) can be expressed as ε Ã ðfÞ¼ ε r ðfÞþ jε i ðfÞ¼ ε r ðfÞþ j σðfÞ 2πε 0 f ; (1) Manuscript received by the Editor 9 March 2018; revised manuscript received 12 July 2019; published ahead of production 22 October 2019; published online 30 December 2019. 1 The University of Texas at Austin, Hildebrand Department of Petroleum and Geosystems Engineering, Austin, Texas, USA. E-mail: archana.jagadisan@ utexas.edu; zoya@utexas.edu. © 2020 Society of Exploration Geophysicists. All rights reserved. D53 GEOPHYSICS, VOL. 85, NO. 1 (JANUARY-FEBRUARY 2020); P. D53–D64, 11 FIGS., 5 TABLES. 10.1190/GEO2018-0180.1 Downloaded from http://pubs.geoscienceworld.org/geophysics/article-pdf/85/1/D53/4927448/geo-2018-0180.1.pdf by University of Texas at Austin user on 14 December 2022