Carbon/oxygen well logging method and apparatus

A method for determining a characteristic of a formation through which a well bore passes, the method comprising,

• irradiating the formation with a pulse of fast neutrons from a source in the well bore;
• generating from a detector near to said source a first signal representative of the inelastic gamma rays emanating from said borehole and said formation,
• generating from a detector far from said source a second signal representative of the inelastic gamma rays emanating from said borehole and said formation,
• comparing the first signal with a composite inelastic spectrum made up of spectra of postulated components, to produce a first estimate of the formation constituents,
• comparing the second signal with a composite inelastic spectrum made up of spectra of postulated components, to produce a second estimate of the formation constituents, and
• determining from said first and second estimates said characteristic of the formation.

A method for determining a borehole corrected carbon-oxygen ratio of a formation traversed by a borehole comprising the steps of,

• bombarding the formation with a pulse of fast neutrons from a neutron accelerator in the well bore,
• obtaining inelastic gamma ray spectra from near and far detectors positioned from said source which result from inelastic collisions between said fast neutrons and materials in and of said borehole and said formation,
• comparing representations of said near and far inelastic gamma ray spectra with postulated near and far spectra representations to produce estimates of amounts of carbon and oxygen as measured by said near detector and estimates of amounts of carbon and oxygen as measured by said far detector, and
• determining a borehole corrected carbon-oxygen ratio of said formation by combining said estimates in a predetermined relationship.

A well logging apparatus for determining a characteristic of a formation traversed by a borehole, comprising:

• an elongated housing sleeve,
• a source of fast neutrons mounted in said housing sleeve,
• a near detector disposed in said housing sleeve longitudinally at a first distance from said source for producing signals representative of gamma rays produced during inelastic nuclear reactions between said neutrons and materials around said source,
• a far detector disposed in said housing sleeve longitudinally at a second distance from said source greater than said first distance for producing signals representative of gamma rays produced during inelastic nuclear reactions between said neutrons and materials around said source,
• means responsive to said near and far detector signals for generating a signal representative of said formation characteristic, and
• means for enhancing the sensitivity of said near gamma ray detector to gamma rays produced during said inelastic nuclear reactions between said fast neutrons and materials in and of said borehole.

Well logging apparatus for determining a characteristic of a formation traversed by a well bore comprising,

• a housing sleeve adapted for translation in said well bore,
• a source of fast neutrons mounted in said housing sleeve,
• means, including a scintillation crystal and phototube assembly means disposed in said housing sleeve longitudinally from said source, for producing signals representative of gamma rays produced during nuclear reactions between said fast neutrons and materials in and of said well bore and said formation,
• means for reducing the effects of magnetic fields in said well bore on said signals producing means, and
• means responsive to said produced signals for generating a signal representation of said characteristic of said formation.

Well logging apparatus for determining a characteristic of a formation traversed by a well bore comprising,

• a housing sleeve adapted for translation in said well bore,
• a source of pulsed fast neutrons mounted in said housing sleve,
• means, including near gamma ray detector disposed in said housing sleeve, longitudinally from said source, for producing near detector signals representative of gamma rays produced during inelastic nuclear reactions between said fast neutrons and materials in and out said well bore and said formation.
• means, including far gamma ray detector disposed in said housing sleeve longitudinally further from said source than said near gamma ray detector means, for producing far detector signals representative of gamma rays produced during inelastic nuclear reactions between said fast neutrons and materials in and of said borehole and said formation,
• means responsive to said near detector signals and said far detector signals for generating a signal representative of a characteristic of said formation substantially corrected for the gamma rays produced by nuclear reactions between said fast neutrons and materials in and of said borehole, and
• enhancing means for increasing the sensitivity of said near gamma ray detector means to gamma rays produced during said inelastic nuclear reactions between said fast neutrons and materials in and of said borehole.