Geophysical stratigraphy I: Will logging

Due to the lack of surface outcrop in many areas geophysical methods of correlation have been developed that exploit the methods of physics to map the physical properties of rocks, including:

Variations in those properties with depth reveal the presence of different rock types and are used to create vertical and lateral sections of rocks that can't directly be examined. There are two general approaches:

Well logging

Direct sampling: Not all well information is remote. Bentonite muds are continually circulated through the drill pipe as a coolant and lubricant for the drill bit. Rock chips are brought to surface with the mud, captured, identified and logged, creating a direct lithologic record.

Aditional information comes from devices lowered into the borehole:


What the sonde records:

Spontaneous potential (SP) log: measures changes in electrical potential between an electrode on the sonde and one at the surface. An electrical potential exists between the natural pore fluids of the rock and the drilling mud, where these com into contact through the semi-permeable barrier of porous rock. Therefore, SP logs are a measure of rock permeability:

Resistivity (R) log: measures resistivity of fluids in the surrounding rock to an applied electrical current. (Resistivity is a materials ability to oppose the flow of an electrical current.) Resistivity indicates amount of fluid in the pore spaces, therefore R logs are measures of porosity. Resistivity increases with decreasing pore space.

Sondes with different spacing between resistivity electrodes will probe the resistivity of adjacent rocks to different depths. By this means, contrasts between the resistivity of the highly conductive drilling mud and the native pore fluid of the rock can be assessed. If resistivity increases significantly with greater distance from the borehole, for example, a resistive fluid like petroleum may be present.

Examples of Spontaneous Potential logs:

Well-logging techniques all have one significant drawback: They require you to drill a borehole. There is a less expensive alternative: