Basics of oil and gas log analysis

In most runs of a resistivity tool, the major purpose is to obtain measurements of Rt, the true resistivity of the formation. However, there are a variety of complicating factors involved which may require corrections to be made to the recorded values in order to obtain good estimates of the true resistivities. All resistivity tools are to some extent “averaging” devices that record resistivities of zones rather than resistivities of discrete points. So, for example, the resistivity of a thin resistive horizon will generally be underestimated by most tools since its reading will be partly reduced by contributions of more conductive adjacent beds.