What three well logs can be combined to determine the mineral composition of a formation?

What three well logs can be combined to determine the mineral composition of a formation?

• A. Resistivity, acoustic, and gamma ray
• B. Density, gamma ray, and resistivity
• C. Acoustic, neutron, and density
• D. Neutron, porosity, and density

Answer: (C)

Determining mineral composition relies on how rocks differ in density, elastic properties, and hydrogen content. The acoustic (sonic) log measures how fast sound travels through the formation, which depends on rock stiffness and cementation; this helps distinguish rock types such as carbonates from clastics because they have different elastic properties. The density log gauges bulk density, which, when combined with porosity estimates, reveals the mineral mass and how much of the rock is void space; different minerals (calcite, dolomite, quartz, clay) have distinct densities, so this log helps separate minerals. The neutron log is sensitive to hydrogen content, highlighting porosity and pore fluids, and it responds differently to various minerals and pore types, providing another axis of discrimination. Using these three together, you can plot and compare responses in ways that highlight lithology: each rock type tends to occupy a characteristic region in density–neutron and in sonic responses, making it possible to identify mineral composition more reliably than with any single log. The other options rely more on fluid indicators or on redundant porosity information, which are less direct for pinning down mineralogy.

Determining mineral composition relies on how rocks differ in density, elastic properties, and hydrogen content. The acoustic (sonic) log measures how fast sound travels through the formation, which depends on rock stiffness and cementation; this helps distinguish rock types such as carbonates from clastics because they have different elastic properties. The density log gauges bulk density, which, when combined with porosity estimates, reveals the mineral mass and how much of the rock is void space; different minerals (calcite, dolomite, quartz, clay) have distinct densities, so this log helps separate minerals. The neutron log is sensitive to hydrogen content, highlighting porosity and pore fluids, and it responds differently to various minerals and pore types, providing another axis of discrimination.

Using these three together, you can plot and compare responses in ways that highlight lithology: each rock type tends to occupy a characteristic region in density–neutron and in sonic responses, making it possible to identify mineral composition more reliably than with any single log. The other options rely more on fluid indicators or on redundant porosity information, which are less direct for pinning down mineralogy.