Complex pore geometry and composition as well as heterogeneous and anisotropic behavior of organic-rich source rocks significantly affects physical properties of the rock measured by well logs such as electrical resistivity and dielectric permittivity. These physical properties are used to estimate in situ petrophysical properties of the formation such as hydrocarbon saturation. Conventional methods for assessment of hydrocarbon saturation include interpretation of (a) electrical resistivity logs through resistivity-porosity-saturation models (e.g., Archie’s equation and the dual-water model) and (b) dielectric permittivity measurements using volumetric techniques such as the Complex Refractive Index Model (CRIM). In the application of these approaches to formations with complex pore structure and mineral composition such as organic-rich formations, the impact of complex pore-structure (e.g., kerogen porosity and inter-granular pores), pyrite, and conductive mature kerogen have not been taken into account. The aforementioned limitations cause significant uncertainty in estimates of water saturation.
The method proposed in this paper improves the assessment of hydrocarbon saturation using combined interpretation of dielectric and electrical resistivity measurements. We start with pore-scale numerical simulations of electrical resistivity and dielectric permittivity of fluid-bearing porous media to investigate the impact of structure of pore and matrix constituents on these measurements. The inputs to these simulators are three-dimensional (3D) pore-scale rock images. We then introduce an analytical model that combines conductivity and permittivity measurements for assessment of water-filled porosity and hydrocarbon saturation. We applied the new method on actual sandstone and synthetic organic-rich source rock samples. We observed an improvement in estimates of water-filled porosity compared to conventional methods in both cases of conventional and unconventional rock samples. This improvement was more significant in the case of organic-rich source rocks with complex pore structure. In the case of synthetic organic-rich source rock samples, the simulation results confirmed that not only the pore structure, but also spatial distribution and tortuosity of water, kerogen, and pyrite networks, affect the dielectric permittivity and electrical resistivity. Taking into account these parameters through the joint interpretation of dielectric and electrical resistivity measurements significantly improves assessment of hydrocarbon saturation.
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