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By far the most important use of resistivity logs is the determination of hydrocarbon-bearing versus water-bearing zones. Because the rock's matrix or grains are nonconductive and any hydrocarbons in the pores are also nonconductive, the ability of the rock to transmit a current is almost entirely a function of water in the pores. As the hydrocarbon saturation of the pores increases (as the water saturation decreases), the formation's resistivity increases. As the salinity of the water in the pores decreases (as Rw increases), the rock's resistivity also increases. A geologist, by knowing (or determining) several parameters (a, m, n, and Rw), and by determining from logs the porosity (ϕ) and formation bulk, or true, resistivity (Rt), can determine the formation's water saturation (Sw) from the Archie equation:

Resistivity logs produce a current in the adjacent formation and measure the response of the formation to that current. The current can be produced and measured by either of two methods. Electrode tools (also called galvanic devices or, for presently available versions, laterologs) have electrodes on the surface of the tool to emit current and measure the resistivity of the formation. Induction tools use coils to induce a current and measure the formation's conductivity. These two types of tools have many variations, which are summarized in Table 5.1. In many cases, it is desirable to use both electrode and induction tools to produce a single resistivity log. For example, an electrode device might be used to measure

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