Abstract

Interpretation of electrical resistivity logs in organic-rich source rocks has been challenging for petrophysicists. Conventional resistivity-porosity-saturation models (e.g., Archie's, Dual-Water, and Waxman-Smits equations) assume that saline water is the only conductive part of the formation. However, this assumption is not reliable in organic-rich source rocks in presence of highly mature organic matter, clay, and pyrite. Previous experimental studies have shown that aromaticity of kerogen increases with an increase in thermal maturity, which might lead to conductive behavior of kerogen. In this paper, we investigate the effect of conductive kerogen on electrical resistivity of organic-rich source rocks.

First, we investigate the reliability of conventional resistivity-porosity-saturation models in the assessment of fluid saturations in organic-rich source rocks using well logs and core measurements in the Haynesville shale-gas formation. We then numerically simulate electric field, electric currents, and electrical resistivity of pore-scale rock images in organic-rich source rocks. We quantify the effect of (a) volumetric concentration of kerogen and (b) kerogen conductivity on electrical resistivity of rocks using pore-scale numerical simulations.

Well-log interpretation of the Haynesville shale-gas formation showed that conventional resistivity-porosity saturation models underestimate hydrocarbon saturation by 20% – 40% in the zones with high volumetric concentration of kerogen. In this paper, we showed that the error in estimates of hydrocarbon saturation can be due to the impact of kerogen on electrical resistivity measurements. The pore-scale numerical simulations confirmed that conventional resistivity-porosity-saturation models can cause 8% – 16% relative error in estimates of fluid saturations in organic-rich source rocks in the presence of 10% – 30% volumetric concentration of kerogen, respectively. Furthermore, we showed that in addition to volumetric concentration and electrical conductivity of kerogen, the spatial connectivity of kerogen-water network has a significant impact on electrical resistivity of organic-rich source rocks.

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