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Resistivity logs measure electrical potential in rocks to determine their composition and are used by the oil industry to find natural reservoirs. Different types of equipment are used, and resistivity is affected by factors such as porosity and salinity. Resistivity is also used in scientific studies to learn about the Earth’s history.
A resistivity log is a data collection system used to judge the electrical potential in rocks. Because different types of rocks have different electrical conductivity, resistivity logs can provide insight into the various compositions of rock formations buried beneath the surface. These logs are often used by the oil industry to find natural reservoirs of oil by a process called formation assessment.
Resistivity is expressed in Ohm or Ohm/meter. It is often expressed on a logarithmic scale because each area studied usually has a range of resistivity. Also, the amount of current recorded depends on the area being studied. Larger areas will have less dense currents, so to determine the ohm/meter measurement, the recorded current density is divided by the area being tested. Since electric current cannot penetrate far into the ground, boreholes are drilled to facilitate the study of formations further from the surface.
Two different types of equipment used to obtain resistivity logs are laterologs and induction coils. Laterologists rely on information obtained from electrodes, which send electric current into the rock, placed at various locations along the hole. Induction coils rely on the magnetic fields produced by the coils and are used less frequently in formation assessment.
Several factors contribute to the electrical conductivity of rocks. Porosity and salinity increase electrical conductivity. Conversely, non-porous rock and areas consisting primarily of clay reduce it. Oil and gas have high resistivity and can read 50-500 ohms on the resistivity register, while water will only have a reading of 1-10 ohms. The logs themselves look a lot like seismograph or lie detector readings: a series of curvy lines on a piece of paper.
In oil drilling, an oil geologist studies resistivity and other logs to see if the amount of oil found near new wells is likely to be worth further drilling. Once a well is drilled, an oil geologist will be called in to analyze the amount of oil in the vicinity. These geologists, however, don’t just look at the resistivity log, as non-porous rocks and salt water both have high resistivity but petroleum does not. Other logs, such as gamma rays and porosity logs, are also examined in the process.
Resistivity is also used in scientific studies to find out more about the geology of the Earth. In lithostratigraphy, the study of the layers of the Earth, scientists use resistivity to gain information about the layers of the Earth. By studying the composition of rocks created over the millennia, scientists can draw conclusions about the history and evolution of the Earth.
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