If the combination neutron-density log is good for telling oil from gas, how do you tell oil from water?

If the combination neutron-density log is good for telling oil from gas, how do you tell oil from water?

• A. Neutron-Density Log
• B. Resistivity Log
• C. Gamma Ray Log
• D. Sonic Log

Answer: (B)

The key idea is that fluids in the rock pores control electrical conduction. Water, especially saline formation water, conducts electricity well, while oil is a poor conductor. A resistivity log directly measures how strongly the rock resists electrical current, so it tracks which fluid fills the pores. When the pore space is water-filled, resistivity is relatively low; when oil fills the pores, resistivity is higher. Archie’s relationship ties resistivity to porosity and water saturation, so higher resistivity indicates lower water saturation and thus hydrocarbons. In an oil–water case, this contrast is the most straightforward way to distinguish the two fluids: oil-bearing zones show up with higher resistivity than water-bearing zones. Gas can also raise resistivity, but the oil vs water distinction relies most cleanly on the resistivity response to water saturation. Other logs (gamma ray, neutron-density, sonic) provide lithology, hydrogen content, or velocity information, but they don’t separate oil from water as directly or reliably as a resistivity measurement does.

The key idea is that fluids in the rock pores control electrical conduction. Water, especially saline formation water, conducts electricity well, while oil is a poor conductor. A resistivity log directly measures how strongly the rock resists electrical current, so it tracks which fluid fills the pores. When the pore space is water-filled, resistivity is relatively low; when oil fills the pores, resistivity is higher. Archie’s relationship ties resistivity to porosity and water saturation, so higher resistivity indicates lower water saturation and thus hydrocarbons. In an oil–water case, this contrast is the most straightforward way to distinguish the two fluids: oil-bearing zones show up with higher resistivity than water-bearing zones. Gas can also raise resistivity, but the oil vs water distinction relies most cleanly on the resistivity response to water saturation. Other logs (gamma ray, neutron-density, sonic) provide lithology, hydrogen content, or velocity information, but they don’t separate oil from water as directly or reliably as a resistivity measurement does.