Methods and apparatus for logging a wellbore and determining a presence or absence of a hydrocarbon bearing formation are disclosed. A sonde includes at least two gamma radiation detectors that are utilized for chlorine logging and lifetime logging in combination. Additionally, two detectors of the sonde are spaced axially from each other at different distances from the source to enable determination and compensation for various other parameters such as porosity and water flow velocity. Appropriate gating of the detectors enables sensing total counts of radiation emitted from adjacent formations and sensing of specific energy ranges of radiation when the formation is bombarded with energy. Signals from the lifetime logging enable adjustment of the chlorine logging for a borehole effect and background radiation. The detectors can include a sheath of a high capture cross-section material that interacts with neutrons to produce gamma radiation to shield the detectors.
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