Borehole Geophysics and Well Logging

Core Idea

Well logging records the physical properties of rocks along a borehole by lowering instruments into the hole and measuring continuously with depth. Common logs measure natural gamma radiation, electrical resistivity, bulk density, neutron response, and sound speed. Interpreted together, they reveal rock type, porosity, and the fluid in the pores, such as salt water, fresh water, oil, or gas. Logs also tie rock layers between wells and link surface seismic data to real rock properties at depth.

How It's Best Learned

Work with a real or sample log display, reading the tracks side by side, and mark shale and sand beds from the gamma ray curve first. Then compute porosity from a density reading and water saturation from resistivity using Archie's law, and check whether the answers agree with the other logs. Compare logs from two nearby wells to correlate the same beds and see how they thicken or thin.

Common Misconceptions

• A high resistivity reading always means oil or gas. (Fresh water and low-porosity rocks such as dense limestone or crystalline rock also give high resistivity, so resistivity must be read with porosity and lithology logs.)
• The gamma ray log measures how dense a rock is. (It measures natural radioactivity from potassium, uranium, and thorium, which is usually highest in clay-rich shales.)
• Logs measure porosity directly. (Density, neutron, and sonic tools measure physical responses that are converted to porosity using assumed values for the rock matrix and fluid.)

Explainer

Surface geophysical surveys image the subsurface from above, but their resolution falls with depth. A borehole lets you put instruments right next to the rock. Well logging does this. A tool called a sonde is lowered on a cable, called a wireline, and measurements are recorded as it is pulled up. The result is a set of curves plotted against depth. The first electrical log was run in 1927 in Pechelbronn, France, by a team working for Conrad and Marcel Schlumberger.

Each log answers a different question.

The gamma ray log measures natural radioactivity from potassium, uranium, and thorium. Clay minerals hold these elements, so shales usually read high and clean sandstones and limestones read low. Geologists use it first to separate shale from reservoir rock.

Porosity logs estimate how much of the rock is pore space. The density log sends gamma rays into the rock and counts those scattered back, which depends on electron density and so on bulk density. With known grain and fluid densities, porosity follows from a simple mixing equation. The neutron log responds mainly to hydrogen, which sits mostly in pore fluids. The sonic log measures how fast sound travels; sound moves more slowly in more porous rock.

Resistivity logs show what fills the pores. Rock grains such as quartz conduct very little electricity, so current flows through the pore water. Salty water conducts well, while oil and gas do not. In 1942, the Shell engineer Gus Archie published an empirical relation that links resistivity, porosity, and the resistivity of the formation water to water saturation. If a porous sandstone shows high resistivity, its pores are likely filled partly with hydrocarbons.

The logs are strongest together. A gas-bearing sand often shows a low gamma ray reading, high resistivity, and a "crossover" in which density porosity reads high while neutron porosity reads low, because gas holds little hydrogen. Matching patterns between nearby wells lets geologists correlate beds across a field.

Logs also connect geology to seismic data. Multiplying sonic velocity by density gives acoustic impedance, which controls seismic reflections. A synthetic seismogram built from logs ties each reflection to a known rock layer. Beyond oil and gas, well logs are used to locate aquifers, assess geothermal resources, and check rock for storing carbon dioxide underground.