| Description |
The induced polarization (IP) effect, in general, is related to the complex resistivity of rocks. Modeling IP phenomena is important for developing effective methods for remote sensing of subsurface geology. The Generalized Effective Medium Theory of Induced Polarization (GEMTIP) has been derived based on the effective medium approach to the characterization of heterogeneous, multiphase, polarized medium typical in rock formations. It describes the relationships between petrophysical and structural properties of rock and the parameters of the corresponding resistivity relaxation model (Zhdanov, 2008). The parameters of the GEMTIP model are determined by the intrinsic petrophysical and geometrical characteristics of the medium: the mineralization, the matrix composition, porosity, anisotropy, and polarizability of the formations. In this paper, two igneous rock samples and three shale samples were tested by the randomly oriented ellipsoidal GEMTIP model, both two-phases and three-phases models were used in the study. Inversion routines were developed and tested using synthetic data to recover the three variables: volume fraction (f), relaxation parameters (C), and time constant (τ). Both Regularized Conjugate Gradient (RCG) method and extensive search method were implemented in the study. Complex resistivity were calculated from recorded EM data from 0.005 Hz to 10000 Hz at 33 different frequencies, detailed geologic analysis using Quantitative Evaluation of Materials using Scanning Electron Microscope (QEMSCAN) and X-ray is conducted to determine GEMTIP model parameters and help better understand the inversion results. The application of shale samples shows that the shale samples are characterized by a significant IP response, and the GEMTIP model can be applied to hydrocarbon bearing shale rocks. The results of our study show that the ellipsoidal GEMTIP model can successfully interpret the IP effect of the mineral rocks and shale samples. By comparing the two-phases with three-phases inversion results, the mineral rock samples study shows that the three-phases model can separate the different mineral sizes and different mineral types from the same sample, the shale samples show the three- phases GEMTIP model can separate the membrane polarization caused by the internal structure of the shale samples from electrode polarization caused by disseminated pyrite. The GEMTIP parameter time constant increases with increases of the grain size. Successful GEMTIP model of the rock samples provided insight into controlling factors of the IP effect. |
