Abstract

Heterogeneity and anisotropy in organic-rich source rocks significantly affect the physical properties measured in well logs, such as electrical resistivity. For instance, neglecting the impact of rock heterogeneity and anisotropy (e.g., layered vs. dispersed distribution of matrix components) in interpretation of electrical resistivity logs causes significant uncertainty in well-log-based petrophysical evaluation of fluid saturations in both horizontal and vertical wells. In organic-rich source rocks, the spatial distributions of pyrite, kerogen, and water networks are more complicated compared to conventional reservoirs. Currently, none of the existing conventional resistivity models takes into account the impact of pyrite and conductive mature kerogen and their spatial distribution on directional electrical resistivity measurements.

We introduce a new parameter, called directional connectivity factor, to quantify the directional connectivity of conductive components of organic-rich source rocks. The directional connectivity is calculated using three-dimensional (3D) images obtained from organic-rich source rocks. We also calculate the directional effective electrical resistivity of porous media (e.g., horizontal and vertical resistivity) using pore-scale numerical simulations. To this end, we numerically solve Laplace's equation, taking into account the spatial distribution of all the conductive components in the formation including saline water, pyrite, and mature kerogen.

The results of numerical simulation on the synthetic cases of organic-rich source rocks show that the electrical resistivity of rocks decrease in the presence of a highly-connected kerogen network and layered pyrite distribution. The presence of pyrite, even in a small concentrations (about 4 vol%), impacts electrical resistivity of organic shale and, consequently, causes uncertainty in the estimates of water saturation by 14–35%, depending on spatial distribution of pyrite. For instance, the layered distribution of pyrite (as in the case of Woodford shale) can result in 35% overestimation of water saturation. Furthermore, we observe a measurable difference in effective electrical resistivity of organic-rich source rocks in horizontal and vertical directions, due to spatial distribution of conductive rock components including saline water, pyrite, and kerogen.

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