ABSTRACT

Characteristically, when hydrocarbons are bombarded with high energy (>4.8 MeV) neutrons, 4.43 MeV gamma rays are created. Specific detection of these gamma rays in a non-carbonaceous reservoir would provide a direct means of identifying the presence of hydrocarbons. There are, however, interfering gamma rays lying at or near 4.43 MeV that arise from down-scattering of 6.09 MeV gamma rays created by neutrons in elastically scattering with omnipresent oxygen. Heretofore the magnitude of this interference problem has been unknown. A Monte Carlo procedure has been developed to calculate the uncollided steady-state 4.43 and 6.09 MeV gamma flux received by a detector immersed in an infinite reservoir together with a 14 MeV neutron source. These data have been combined with moments methods data and the magnitude of the interference problem determined. Analysis of data presented indicates:

  1. The interference problem posed by energy degraded oxygen gamma rays is not severe enough to make direct detection of liquid hydrocarbons by nuclear means impractical and commercial saturations can be detected in sandstones having a porosity >= 15 per cent.

  2. It is doubtful, because of carbon atom density reduction, that residual hydrocarbons can be detected. A carbon logging tool will therefore find its greatest application in work over operations, although it is not limited to this single application.

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