Determining petrophysical properties using sodium nuclear magnetic resonance (NMR) logs

A computing system comprising at least one processor, at least one memory, and one or more programs stored in the at least one memory, wherein the programs comprise instructions, which when executed by the at least one processor, are configured to perform:

• obtaining a sodium nuclear magnetic resonance (NMR) measurement of a subsurface formation;
• determining a salt concentration of fluid in the subsurface formation using the sodium NMR measurement;
• and
• determining the presence of hydrocarbons in the subsurface formation fluid based on the determined salt concentration, wherein the at least one processor performs determining a sodium index and determining a water saturation, an oil saturation, or both the oil and water saturations of the subsurface formation using the sodium index in conjunction with one or more other logs that are sensitive to the water saturation.

A non-transitory, computer-readable storage medium comprises instructions, which when executed by a processor, cause the processor to perform a method comprising:

• obtaining a sodium nuclear magnetic resonance (NMR) measurement of a subsurface formation;
• determining a salt concentration of fluid in the subsurface formation using the sodium NMR measurement;
• and
• determining the presence of hydrocarbons in the fluid based on the determined salt concentration, wherein the instructions cause the processor to:
• determine a sodium index;
• determine a water saturation, an oil saturation, or both the oil and water saturations of the subsurface formation;
• and
• use the sodium index in conjunction with one or more other logs that are sensitive to the water saturation.