Neutron logs are used to determine moisture content and porosity; which property would they not reliably provide?

• A. Gas content
• B. Water saturation
• C. Porosity
• D. Moisture content

Answer: (A)

Neutron logs respond to hydrogen atoms in the pore fluids and, to a first approximation, to hydrogen in the pore space itself. Because water and many liquids contain a lot of hydrogen, neutron logs are very good at indicating how much hydrogen-bearing material is in the pores, which translates directly into porosity estimates and, when fluids are present, to moisture content as well. When the pores are filled with water, the hydrogen signal is strong, so the log shows higher porosity or higher moisture content. When you use neutron readings to infer water saturation, you’re relying on the idea that more hydrogen follows more pore-filling fluid and less neutron signal when pores are dry or filled with low-hydrogen materials.

Gas content, however, isn’t reliably determined from neutron logs alone. Gas changes the hydrogen signal in a way that depends on the amount and type of gas, pressure, and how the gas mixes with other fluids. This makes the neutron response ambiguous for quantifying how much gas is present. To identify and quantify gas content, you typically need additional logs (like density or resistivity) or cross-plot analysis with formation porosity. So neutron logs are excellent for porosity and moisture content, but not a dependable measure of gas content.