Abstract: Illustrative embodiments of the present invention include methods and apparatus for producing indications of the presence of hydrocarbons in subsurface earth formations. In one embodiment, two different pore volume indications are derived and comparably displayed. The first is primarily dependent upon the hydrogen content of the fluid in the pore space of the formation and may be derived from count rate ratios obtained between two epithermal neutron responsive detectors differently spaced from a neutron source. If thermal neutron or neutron captured gamma ray responsive detectors are used, compensation is provided for thermal neutron absorption effects. The second pore volume indication is produced directly from a measurement of the thermal neutron absorption characteristic of the formation and a value for the water component of the formation.