Log Interpretation
Which logging tool is the only passive logging tool commonly used in formation evaluation?
GR
Why can't a high-resistivity zone automatically be interpreted as hydrocarbons?
It could be hydrocarbons or a 0 porosity zone.
What property does a Neutron Porosity tool actually respond to?
Hydrogen content (Hydrogen Index)
Is compressional slowness higher in fast formations or slow formations?
Slow formations
What does NMR stand for?
Nuclear Magnetic Resonance
What three naturally occurring radioactive elements are primarily measured by the Gamma Ray tool?
Potassium, Uranium, Thorium
Which resistivity technology injects current directly into the formation?
Laterolog
Why do shales often appear to have abnormally high neutron porosity?
Due to bound water and hydrogen contained in clay minerals
What does STC stand for?
Slowness Time Coherence
What does PEF stand for?
Photoelectric Factor
Calculate Vshale if:
50%
Which resistivity technology is generally preferred in oil-based mud systems?
Induction Logging
In a zero-porosity limestone, what density reading would you expect?
2.71 g/cc
On a Vp/Vs vs DTCO plot, a cluster around 57 μs/ft may indicate what?
Casing Arrival
Two formations have identical porosity. One is water-bearing and one is gas-bearing. Which log pair is most likely to distinguish between them?
Density-Neutron logs (neutron-density crossover).
A zone has high GR and high neutron porosity. What lithology is most likely present?
Shale
Why does saline water typically have lower resistivity than fresh water?
Because it contains more dissolved ions and conducts electricity better.
Density is inversely proportional to which reservoir property?
Porosity
What slowness can we get from the monopole low frequency?
Stoneley Slowness
What type of wave is dispersive in dipole sonic measurements?
Flexural Wave
Name three log responses that together strongly indicate a gas-bearing reservoir.
Low GR, High Resistivity, Neutron-Density Crossover
An interval exhibits:
What can be inferred?
Very little or no invasion
Why are two or three detectors used in modern density tools?
To compensate for borehole effects and improve accuracy
How would the presence of gas effect the slowness?
Increases the compressional slowness, but does not affect the shear slowness
A zone shows:
Would you classify this as a reservoir? Why?
No. Despite high resistivity, the porosity is extremely low, indicating a tight formation rather than a productive reservoir.