On a gamma ray log, a sandstone bed reads low and the shale above it reads high. Why?
AShale is denser than sandstone
BShale contains more clay minerals and potassium-bearing minerals, which carry more of the naturally radioactive elements potassium, thorium, and uranium
CSandstone is saturated with oil, which blocks gamma rays
DGamma rays travel faster through shale
The gamma ray tool counts natural radiation from potassium-40 and the uranium and thorium decay series. Clay minerals hold these elements, so shales usually read high, while clean quartz sandstone and pure carbonates read low.
Question 2 Short Answer
A sandstone has a matrix density of 2.65 g/cm³, the pore fluid has a density of 1.00 g/cm³, and the density log reads 2.32 g/cm³. What is the porosity?
Think about your answer, then reveal below.
Model answer: Porosity = (2.65 − 2.32) ÷ (2.65 − 1.00) = 0.33 ÷ 1.65 = 0.20, or 20 percent.
The bulk density is a mix of solid grains and fluid in proportion to porosity. Solving the mixing equation for porosity gives this standard density-porosity formula.
Question 3 Multiple Choice
Using Archie's law with a = 1, m = 2, and n = 2, a formation has porosity 0.20, formation water resistivity 0.05 ohm-m, and measured resistivity 10 ohm-m. What is the water saturation?
AAbout 12 percent
BAbout 35 percent
CAbout 65 percent
DAbout 100 percent
Archie's law gives Sw² = (a × Rw) ÷ (φ^m × Rt) = 0.05 ÷ (0.04 × 10) = 0.125, so Sw ≈ 0.35. About 35 percent of the pore space holds water, and the rest, about 65 percent, holds hydrocarbons.
Question 4 True / False
Sonic and density logs from a well can be combined to build a synthetic seismogram, which helps match reflections in surface seismic data to specific rock layers.
TTrue
FFalse
Answer: True
Sonic velocity times density gives acoustic impedance. Changes in impedance between layers produce reflections, so a reflection series built from logs can be compared with the seismic section at the well.