Well logging method and apparatus

Claim 1

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; detecting, during a second, later discrete time interval, energy dependent distribution of gamma radiations attributable to the capture of thermal neutrons by the elements calcium and silicon in the formations and generating signals representative of calcium capture gamma rays and silicon capture gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; generating a second ratio signal comprising a ratio of said calcium and silicon capture gamma ray representative signals; and recording said first and second ratio signals as a function of borehole depth.

Claim 2

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; detecting, during a second, later discrete time interval, energy dependent distribution of gamma radiations attributable to the capture of thermal neutrons by the elements calcium and silicon in the formations and generating signals representative of calcium capture gamma rays and silicon capture gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; generating a second ratio signal comprising a ratio of said calcium and silicon capture gamma ray representative signals; and recording said first and second ratio signals as a function of borehole depth.

Claim 3

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a signal representative of the sum of said carbon inelastic gamma rays, said oxygen inelastic gamma rays, said carbon background inelastic gamma rays and said oxygen background inelastic gamma rays as indicative of the porosity of earth formations in the vicinity of the borehole; and recording said carbon and oxygen ratio signal and said porosity indicative signal as a function of borehole depth.

Claim 4

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a signal representative of the sum of said carbon inelastic gamma rays, said oxygen inelastic gamma rays, said carbon background inelastic gamma rays and said oxygen background inelastic gamma rays as indicative of the porosity of earth formations in the vicinity of the borehole; and recording said carbon and oxygen ratio signal and said porosity indicative signal as a function of borehole depth.

Claim 5

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; detecting, during a second, later discrete time interval, energy dependent distributions of gamma radiations attributable to the capture of thermal neutrons by the elements calcium and silicon in the formations and generating signals representative of calcium capture gamma rays and silicon capture gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; generating a second ratio signal comprising a ratio of said calcium and silicon capture gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a signal representative of the sum of said carbon inelastic gamma rays, said oxygen inelastic gamma rays, said carbon background inelastic gamma rays and said oxygen background inelastic gamma rays as indicative of the porosity of earth formations in the vicinity of the borehole; and recording said first and second ratio signals and said porosity indicative signal as a function of borehole depth.

Claim 6

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; detecting, during a second, later discrete time interval, energy dependent distributions of gamma radiations attributable to the capture of thermal neutrons by the elements calcium and silicon in the formations and generating signals representative of calcium capture gamma rays and silicon capture gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; generating a second ratio signal comprising a ratio of said calcium and silicon capture gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a signal representative of the sum of said carbon inelastic gamma rays, said oxygen inelastic gamma rays, said carbon background inelastic gamma rays and said oxygen background inelastic gamma rays as indicative of the porosity of earth formations in the vicinity of the borehole; and recording said first and second ratio signals and said porosity indicative signal as a function of borehole depth.

Claim 7

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formation and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; generating a ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; detecting during said discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a signal representative of a ratio of said carbon background inelastic gamma ray signal and said oxygen background inelastic gamma ray signal as indicative of the lithology of the earth formations in the vicinity of the borehole; and recording said ratio signal and said lithology indicative signal as a function of borehole depth.

Claim 8

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formation and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; generating a ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; detecting during said discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a signal representative of a ratio of said carbon background inelastic gamma ray signal and said oxygen background inelastic gamma ray signal as indicative of the lithology of the earth formations in the vicinity of the borehole; and recording said ratio signal and said lithology indicative signal as a function of borehole depth.

Claim 9

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; generating a ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a signal representative of the sum of said carbon inelastic gamma rays, said oxygen inelastic gamma rays, said carbon background inelastic gamma rays and said oxygen background inelastic gamma rays as an indicator of the porosity of earth formations in the vicinity of the borehole; generating a signal representative of a ratio of said carbon background inelastic gamma ray signal and said oxygen background inelastic gamma ray signal as an indicator of the lithology of the earth formations in the vicinity of the borehole; and recording said ratio signal, said lithology indicative signal and said porosity indicative signal as a function of borehole depth.

Claim 10

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; generating a ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a signal representative of the sum of said carbon inelastic gamma rays, said oxygen inelastic gamma rays, said carbon background inelastic gamma rays and said oxygen background inelastic gamma rays as an indicator of the porosity of earth formations in the vicinity of the borehole; generating a signal representative of a ratio of said carbon background inelastic gamma ray signal and said oxygen background inelastic gamma ray signal as an indicator of the lithology of the earth formations in the vicinity of the borehole; and recording said ratio signal, said lithology indicative signal and said porosity indicative signal as a function of borehole depth.

Claim 11

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a second ratio signal representative of a ratio of said carbon background inelastic gamma ray signal and said oxygen background inelastic gamma ray signal as indicative of the lithology of the earth formations in the vicinity of the borehole; generating a third signal representative of the sum of the total gamma ray count rate at all energies in a second, later discrete time interval during which the gamma rays present in the borehole are substantially attributable to the capture of thermal neutrons by nuclei in the earth formations in the vicinity of the borehole, as indicative of the porosity of the formations; and recording said first ratio signal, said lithology indicative signal and said porosity indicative signal as a function of borehole depth.

Claim 12

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to background radiations in energy regions near the energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a second ratio signal representative of a ratio of said carbon background inelastic gamma ray signal and said oxygen background inelastic gamma ray signal as indicative of the lithology of the earth formations in the vicinity of the borehole; generating a third signal representative of the sum of the total gamma ray count rate at all energies in a second, later discrete time interval during which the gamma rays present in the borehole are substantially attributable to the capture of thermal neutrons by nuclei in the earth formations in the vicinity of the borehole, as indicative of the porosity of the formations; and recording said first ratio signal, said lithology indicative signal and said porosity indicative signal as a function of borehole depth.

Claim 13

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; detecting, during a second, later discrete time interval, energy dependent distribution of gamma radiations attributable to the capture of thermal neutrons by the elements calcium and silicon in the formations and generating signals representative of calcium capture gamma rays and silicon capture gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; generating a second ratio signal comprising a ratio of said calcium and silicon capture gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a third ratio signal representative of a ratio of said carbon background inelastic gamma ray signal and said oxygen background inelastic gamma ray signal as indicative of the lithology of the earth formations in the vicinity of the borehole; generating a fourth signal representative of the sum of the total gamma ray count rate at all energies in said second, later discrete time interval during which the gamma rays present in the borehole are substantially attributable to the capture of thermal neutrons by nuclei in the earth formations in the vicinity of the borehole, as indicative of the porosity of the formations; and recording said first and second ratio signals, said lithology indicative signal and said porosity indicative signal as a function of borehole depth.

Claim 14

A method of investigating subsurface earth formations traversed by a borehole comprising the steps of: repetitively irradiating earth formations in the vicinity of a borehole with pulses of fast neutrons which engage in inelastic scattering interactions with formation nuclei and which are thereafter slowed to thermal energy and engage in further neutron capture interactions with formation nuclei; detecting, during a first discrete time interval associated with said fast neutron pulses, energy dependent distributions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of carbon inelastic gamma rays and oxygen inelastic gamma rays; detecting, during a second, later discrete time interval, energy dependent distribution of gamma radiations attributable to the capture of thermal neutrons by the elements calcium and silicon in the formations and generating signals representative of calcium capture gamma rays and silicon capture gamma rays; generating a first ratio signal comprising a ratio of said carbon and oxygen inelastic gamma ray representative signals; generating a second ratio signal comprising a ratio of said calcium and silicon capture gamma ray representative signals; detecting during said first discrete time interval associated with said fast neutron pulses second different energy dependent distributions of gamma radiations attributable to carbon and oxygen background radiations in energy regions of gamma radiations attributable to the inelastic scattering of fast neutrons by the elements carbon and oxygen in the formations and generating signals representative of said carbon background and said oxygen background inelastic gamma rays; generating a third ratio signal representative of a ratio of said carbon background inelastic gamma ray signal and said oxygen background inelastic gamma ray signal as indicative of the lithology of the earth formations in the vicinity of the borehole; generating a fourth signal representative of the sum of the total gamma ray count rate at all energies in said second, later discrete time interval during which the gamma rays present in the borehole are substantially attributable to the capture of thermal neutrons by nuclei in the earth formations in the vicinity of the borehole, as indicative of the porosity of the formations; and recording said first and second ratio signals, said lithology indicative signal and said porosity indicative signal as a function of borehole depth.