Resistivity Logs for Dummies

Resistivity log pair (right side)

Our staff petroleum geologist stops by every once in a while to complain about clueless freelancers who write fluffy bull about oil and the oil business. He's been particularly wrathful lately about a string of WiseGEEK.com posts attempting to explain the processes involved in formation evaluation (see list¹). In order to humor the guy, we've gone back to that particular well, singling out freelancer and "English and Professional Writing/Editing" graduate (is there really such a degree?) Angie Bates, whose ignorance of her topic is obvious  in "What is a Resistivity Log?" a post that WiseGEEK, in their infinite dumbassery, has niched at AboutMechanics.com.

Bates opens by explaining for all that,

"A resistivity log is a data gathering system used to judge the electrical potential in rocks."

No, Angie, that's not what a resistivity log is. That's not even what a resistivity tool is: the "electrical potential" in rocks is measured by the SP (spontaneous potential) log; resistivity measures the resistance of the rock adjacent to a borehole to the flow of electricity. Now that we have the basic definition out of the way, let's see what other nonsense Bates put to paper...

• "...resistivity logs can give information about the various compositions of rock formations buried beneath the surface." –Well, yeah, but geologists are a lot more interested in the fluids within pore spaces in those rocks.
• "Resistivity is ... frequently expressed in a logarithmic scale because each area studied usually has a range of resistivity. " – Duh.
• "Also, the amount of current registered is dependent on the area that is being studied." – We wanted to say, "No, it's not!" but we had no idea what this is supposed to mean.
• "Since electrical current cannot penetrate far into the ground, boreholes are drilled to aid in the study of formations farther below the surface." – Ummm, Angie? That's not why those boreholes are drilled. Idiot.
• "Porousness and salinity increase the electrical conductivity. Conversely, nonporous rock and areas made up of mostly clay decreases it." – Why is Angie suddenly talking about conductivity?²
• "...nonporous rocks and salt water both have a high resistivity but no oil..." – No, Angie, salt water has low resistivity. Fresh water is poorly conductive.

It's crystal clear that Bates has no idea what she stumbled into here, so let us tell you. Potential reservoir rocks are porous, and those holes are filled with fluids: water (usually saline), crude oil, natural gas. Rocks do not conduct electricity, but the fluids in them will conduct it to varying degrees. By studying the resistivity of rock layers at different distances from a borehole, a geologist can estimate the amount of oil present in a layer and where the different fluids are found in that layer.
Angie wasn't done, though. She finished by telling her readers that,

"In lithostratigraphy, the study of the strata of the Earth, scientists use resistivity to gain information about the Earth's layers..."

...which contains a kernel of truth. Not a lot, just a kernel; but nowhere near enough to negate the sort of misinformation already written down by our Dumbass of the Day. So for now, our geologist can go  back to his office.

¹  Here for your reading pleasure are B. Turner's "What Is Gamma Ray Logging," Paul Reed's "What Is Sonic Logging," Dee S. pretending to explain "What Is Formation Geology," Larry Ray Palmer trying to answer, "What Is Formation Evaluation," and Mary McMahon botching "What is Density Logging"...
² We know, even if Bates doesn't: conductivity is the reciprocal of resistivity.

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