What well log can be used to determine both the porosity and lithology of a reservoir?

What well log can be used to determine both the porosity and lithology of a reservoir?

• A. Gamma ray log
• B. Resistivity log
• C. Neutron log
• D. Density log

Answer: (B)

Density logs are used to estimate porosity directly because they measure how much solid rock is present in a given volume. Porosity can be calculated from bulk density by comparing the measured density to the known densities of the rock matrix and the pore fluid. For example, using a typical matrix density and the density of the pore fluid, you can compute porosity from the equation that relates bulk density, matrix density, and pore-filling density. But density logs also carry lithology clues: different rock types have different matrix densities, so the overall density response helps distinguish sandstone from limestone and other lithologies. In practice, a density log not only gives porosity, it also provides lithology indicators through the observed bulk density signature, which is why it’s the log that can serve both purposes effectively. The other logs specialize in one aspect: gamma ray mainly tracks clay content (lithology); neutron logs emphasize hydrogen content (porosity) with less direct lithology discrimination; resistivity mainly informs hydrocarbon saturation and pore geometry, not porosity or lithology on its own.

Density logs are used to estimate porosity directly because they measure how much solid rock is present in a given volume. Porosity can be calculated from bulk density by comparing the measured density to the known densities of the rock matrix and the pore fluid. For example, using a typical matrix density and the density of the pore fluid, you can compute porosity from the equation that relates bulk density, matrix density, and pore-filling density. But density logs also carry lithology clues: different rock types have different matrix densities, so the overall density response helps distinguish sandstone from limestone and other lithologies. In practice, a density log not only gives porosity, it also provides lithology indicators through the observed bulk density signature, which is why it’s the log that can serve both purposes effectively. The other logs specialize in one aspect: gamma ray mainly tracks clay content (lithology); neutron logs emphasize hydrogen content (porosity) with less direct lithology discrimination; resistivity mainly informs hydrocarbon saturation and pore geometry, not porosity or lithology on its own.