Well logs

ESCI 428:Reflection Seismic Data Interpretation

Well Logs

Well Logs

• Wells provide ground truth, the gold standard• Well logs provide information on lithology and

pore fluids• Very sparse samples• 3D seismic provides denser coverage – tie the

two together• Lower a tool into the well, measurements

typically made as tool is pulled out

Common Well Log Types

• Conrad and Marcel Schlumberger – resistivity 1927

• Then Spontaneous Potential• GR first to measure through casing• Sonic, neutron porosity, density

SP

• Indicates the permeabilities of rocks in the well by measuring the amount of electrical current generated between the drilling fluid and the formation water that is held in pore spaces of the reservoir rock

• Porous sandstones with high permeabilities tend to generate more electricity than impermeable shales.

• Thus, SP logs are often used to tell sandstones from shales

Gamma Ray

• Measures radioactivity to determine what types of rocks are present in the well

• Because shales contain radioactive elements, they emit lots of gamma rays

• Clean sandstones emit very few gamma rays• Gamma ray logs are one of the most commonly

used logs for sequence stratigraphic analysis• Introduced in the late 1930s

Resistivity

• Determines what types of fluids are present in the reservoir rocks by measuring how effective these rocks are at conducting electricity.

• Fresh water and oil are poor conductors of electricity and have high resistivities

• Most formation waters are salty enough that they conduct electricity with ease. Thus, formation waters generally have low resistivities.

Well Logs

Sonic

• Measures how fast sound waves travel through rocks in the well

• Related to porosity and lithology• In general, sound waves travel faster through

high-density shales than through lower-density sandstones

Well Log Character

There are three general trends or curve shapes that can be recognized when looking at well log curves. For instance the Gamma log tracks the upward change in clay-mineral content. NB: When possible, one should use core data to confirm or augment well log analysis.

Well Log Character

Log response a variety of different clastic depositional systems after Malcolm Rider (1996).

Synthetic Seismogram

Vertical seismic profiling

Zero-offset VSP data