Method and apparatus for obtaining real-time formation gas saturation

A method for evaluating a formation gas saturation of a subterranean formation, comprising:

• S1:
• deploying a nuclear logging tool into the subterranean formation, wherein the nuclear logging tool comprises one or more neutron source and three or more dual-function detectors configured to simultaneously detect neutrons and gamma rays
• S2:
• causing the one or more neutron sources to emit neutrons into the subterranean formation;
• S3:
• detecting neutrons and gamma rays from the subterranean formation at the three or more detectors to form electric signals;
• S4:
• separating electric signals from neutrons and gamma rays for each of the three or more detectors;
• S5:
• obtaining detector count rates of total neutrons, fast neutrons and thermal neutrons;
• S6:
• obtaining the ratios of thermal neutrons and determining the formation porosity;
• S7:
• obtaining detector count rates of capture gamma rays, and the energy spectrum from inelastic gamma rays and the energy spectrum of capture gamma rays;
• S8:
• determining the formation type and obtaining the ratio of capture gamma rays from at least two of the detectors
• S9:
• determining the formation gas saturation using formation type, porosity, the ratios of thermal neutrons and the ratios of capture gamma rays.