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:
- Character of pore fluids
- Well logs: that record information provided by probes that are placed down boreholes
- Seismic studies: in which physical features of subsurface rocks are approximated based on seismic wave propagation
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:
- Caliper: measures the width of the drill hole. This indicates the presence of mudrocks, which are prone to caving and sagging, hence constricting the borehole slightly.
- Sonde: A probe lowered into hole to measure various electrical and physical properties of the rocks.
- Gas detectors and gas chromatographs: measure gases in the well.
- Gamma-ray log: measures natural radioactivity of the strata
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:
- Shales and limestone nearly impermeable and have a 0 reading
- Negative deflection for sandstones (high permeability) or fluids with higher conductivity than the drilling mud (like saltwater)
- Positive deflection for fluids with lower conductivity than mud (like freshwater)
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.
- High resistivity: dense rocks with no pores (quartzite, limestone.), rocks with non-conducting fluid in their pores (like petroleum)
- Low resistivity: rocks with significant porosity (sandstone), rocks with conducting fluid in their pores (like salt water), rocks containing significant amounts of water in their crystal structure (clay-rich rocks).
Examples of Spontaneous Potential logs:
- Fluvial deposit with point bar sequence and overbank mud shows fining-upward sequence.
- Deltaic deposit grading into shoreline Coarsening upward sequence with thick sands.
- Deltaic deposit grading into distributary channel and interdistributary bay Coarsening upward sequence.
- Deltaic deposit grading into delta plain Coarsening upward sequence with thick sands.
- Regressive shoreline Coarsening upward sequence.
Dipmeter: measures resistivity in four directions. By this means, it locates contacts and identifies their dip direction, allowing identification of folds, faults, and other structure.
Gamma-ray log: measures natural radioactivity of the rock. Most gamma radiation comes from decay of 40K. Therefore, the gamma-ray log is sensitive to rocks high in K-bearing minerals (feldspars, micas, clays) including:
- feldspathic and lithic sandstones
Well-logging techniques all have one significant drawback: They require you to drill a borehole. There is a less expensive alternative: