EM Log Response to Anisotropic Resistivity in Thinly Laminated Formations With Emphasis on 2-MHz Resistivity Devices

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Abstract
Summary: Anisotropic resistivity is not uncommon in electromagnetic logging environments. Many shale formations are known to be anisotropic. Thinly laminated sand/shale sequences behave as macroscopically anisotropic formations.We analyzed the tool response to anisotropic resistivity for each type of EM logging tool: electric resistivity logs, induction type logs, and toroidal resistivity tools.In particular, the effect of anisotropy on the logging-while-drilling 2-MHz induction-type resistivity log is discussed in detail. The effect causes differences between phase-derived and attenuation-derived apparent resistivities, between apparent resistivities from different transmitter-receiver spacings, and between apparent resistivities measured at different operating frequencies. Based on the tool response analysis, we present a method to estimate anisotropy from log data. An application to laminated sand analysis is discussed, in which the sand lamina resistivity and the net-gross ratio (N/G) are determined simultaneously.