Methods and apparatus for determining formation lithology by gamma ray spectroscopy

Claim 3

The method of claim 1, wherein step (d) includes determining the relative inelastic yield for magnesium.

Claim 4

The method of claim 2, further comprising the step of: (g) deriving from the relative inelastic magnesium yield, in accordance with a predetermined relationship relating the relative inelastic magnesium yields determined in calibration apparatus having formation materials of known composition to the magnesium content of the formation materials, an estimate of at least one of the volumetric content of dolomite and the concentration of magnesium in the irradiated formation.

Claim 5

The method of claim 2, further comprising the step of forming a ratio of the relative inelastic yields for magnesium and calcium as an indicator of the degree of dolomitization of the irradiated formation.

Claim 6

The method of claim 1, wherein step (d) includes determining the relative inelastic gamma ray yield for iron.

Claim 7

The method of claim 5, further comprising the step of: (h) deriving from the relative inelastic iron yield, in accordance with a predetermined relationship relating the relative inelastic iron yields determined in calibration apparatus having borehole regions of known iron content, an estimate of at least one of the volumetric content and the concentration of iron in the irradiated formation.

Claim 8

The method of claim 6, wherein the estimate of the iron content of or the iron concentration in the irradiated formation is derived by subtracting from the relative inelastic iron yield of step (d) the relative inelastic iron yield for a borehole region of substantially the same iron content as the irradiated borehole.

Claim 9

The method of claim 2 or 5, wherein the energy range of gamma rays detected in step (b) is from substantially 1.0 MeV to substantially 7.5 MeV.

Claim 10

The method of claim 1, further comprising the steps of: (g) detecting gamma rays resulting from said thermal neutron capture reactions; (h) generating an energy spectrum of said detected capture gamma rays; (i) comparing said detected capture gamma ray energy spectrum with a composite spectrum, including a combination of standard capture gamma ray spectra for elements postulated to have contributed to said detected capture gamma ray spectrum and including at least one of silicon and calcium, to determine the relative capture elemental yields in said composite spectrum for each of said postulated elements contributing to the capture spectrum; (j) where silicon is included among the postulated elements in steps (d) and (i), deriving from (1) the relative inelastic silicon yield and the inelastic gamma ray sensitivity for silicon and (2) the relative capture silicon yield and the capture gamma ray sensitivity for silicon an inelastic/capture sensitivity ratio R Si for silicon; (k) where calcium is included among the postulated elements in steps (d) and (i), deriving from (1) the relative inelastic calcium yield and the inelastic gamma ray sensitivity for calcium and (2) the relative capture calcium yield and the capture gamma ray sensitivity for calcium an inelastic/capture sensitivity ratio R Ca for calcium; (l) utilizing at least one of said ratios R Si and R Ca to provide a calibration ratio R; and (m) combining the calibration ratio R and the relative capture elemental yield and the capture gamma ray sensitivity for at least one other element to derive the calibrated capture elemental yield X of said at least one other element.

Claim 11

The method of claim 9, wherein step (l) comprises deriving the calibration ratio R by using the more statistically precise of R Si and R Ca or by averaging the values of R Si and R Ca .

Claim 12

The method of claim 9, wherein step (m) comprises deriving the calibrated capture elemental yield X for said at least one other element by combining the calibration ratio R and the relative capture elemental yield C X and the elemental capture gamma ray sensitivity S CX in accordance with the relationship X=RC X S CX .

Claim 15

The method of claim 12, wherein step (g) comprises deriving a calibrated capture elemental yield X for at least each of the elements specifically attributable to the irradiated formation and not used in determining the ratio R.

Claim 16

The method of claim 13, further comprising the steps of: (h) deriving a normalization factor from the sums of (1) the products of the relative inelastic elemental yields for elements specifically attributable to the irradiated formation and their respective inelastic gamma ray sensitivities and (2) the products of the calibrated capture elemental yields for elements specifically attributable to the irradiated formation and their respective capture gamma ray sensitivities; and (i) deriving the volumetric fraction of at least one element in the irradiated formation by dividing the product of (1) the relative inelastic yield and the inelastic gamma ray sensitivity for said at least one element or (2) the calibrated capture yield and the capture gamma ray sensitivity for said at least one element by the normalization factor.

Claim 17

Apparatus for investigating an earth formation traversed by a borehole, comprising: (a) means for repetitively irradiating an earth formation with bursts of high energy neutrons, which neutrons undergo inelastic scattering reactions and thermal neutron capture reactions with constituents in the borehole and the formation and produce gamma rays as a result thereof; (b) means for detecting gamma rays resulting from said inelastic scattering reactions and said thermal neutron capture reactions; (c) means for generating an energy spectrum of the detected inelastic scattering gamma rays; (d) means for comparing said detected inelastic gamma ray energy spectrum with a composite spectrum, including a combination of standard inelastic gamma ray spectra for elements postulated to have contributed to said detected inelastic gamma ray spectrum and including at least one of silicon and calcium, to determine the relative inelastic elemental yield in said composite spectrum for each of said postulated elements contributing to the inelastic spectrum; (e) where silicon is included among the postulated elements in step (d), means for deriving from the relative inelastic yield for silicon, in accordance with a predetermined relationship relating the relative inelastic silicon yields determined in calibration apparatus having formation materials of known composition to the silicon content of the formation materials, an estimate of at least one of the volumetric content of sandstone and the concentration of silicon in the irradiated formation; and (f) where calcium is included among the postulated elements in step (d), means for deriving from the relative inelastic yield for calcium, in accordance with a predetermined relationship relating the relative inelastic calcium yields determined in calibration apparatus having formation materials of known composition to the calcium content of the formation materials, an estimate of at least one of the volumetric content of limestone and the concentration of calcium in the irradiated formation.

Claim 18

Apparatus for investigating an earth formation traversed by a borehole, comprising: (a) means for repetitively irradiating an earth formation with bursts of high energy neutrons, which neutrons undergo inelastic scattering reactions and thermal neutron capture reactions with constituents in the borehole and the formation and produce gamma rays as a result thereof; (b) means for detecting gamma rays resulting from said inelastic scattering reactions and said thermal neutron capture reactions; (c) means for generating an energy spectrum of the detected inelastic scattering gamma rays; (d) means for comparing said detected inelastic gamma ray energy spectrum with a composite spectrum, including a combination of standard inelastic gamma ray spectra for elements postulated to have contributed to said detected inelastic gamma ray spectrum and including at least one of silicon and calcium, to determine the relative inelastic elemental yield in said composite spectrum for each of said postulated elements contributing to the inelastic spectrum; (e) where silicon is included among the postulated elements in step (d), means for deriving from the relative inelastic yield for silicon, in accordance with a predetermined relationship relating the relative inelastic silicon yields determined in calibration apparatus having formation materials of known composition to the silicon content of the formation materials, an estimate of at least one of the volumetric content of sandstone and the concentration of silicon in the irradiated formation; and (f) where calcium is included among the postulated elements in step (d), means for deriving from the relative inelastic yield for calcium, in accordance with a predetermined relationship relating the relative inelastic calcium yields determined in calibration apparatus having formation materials of known composition to the calcium content of the formation materials, an estimate of at least one of the volumetric content of limestone and the concentration of calcium in the irradiated formation.

Claim 19

The apparatus of claim 15, wherein the means for comparing includes determining the relative inelastic yield for magnesium.

Claim 20

The apparatus of claim 16, further comprising: (g) means for deriving from the relative inelastic magnesium yield, in accordance with a predetermined relationship relating the relative inelastic magnesium yields determined in calibration apparatus having formation materials of known composition to the magnesium content of the formation materials, an estimate of at least one of the volumetric content of dolomite and the concentration of magnesium in the irradiated formation.

Claim 21

The apparatus of claim 16, further comprising means for forming a ratio of the relative inelastic yields for magnesium and calcium as an indicator of the degree of dolomitization of the irradiated formation.

Claim 22

The apparatus of claim 15, wherein the comparing means includes means for determining the relative inelastic gamma ray yield for iron.

Claim 23

The apparatus of claim 19, further comprising: (g) means for deriving from the relative inelastic iron yield, in accordance with a predetermined relationship relating the relative inelastic iron yields determined in calibration apparatus having borehole regions of known iron content, an estimate of at least one of the volumetric content and the concentration of iron in the irradiated formation.

Claim 24

The apparatus of claim 20, wherein the means for deriving the estimate of the iron content of or the iron concentration in the irradiated formation comprises means for subtracting from said relative inelastic iron yield the relative inelastic iron yield for a borehole region of substantially the same iron content as the irradiated borehole.

Claim 25

The apparatus of claim 16 or 19, wherein the energy range of gamma rays detected by said detecting means is from substantially 1.0 MeV to substantially 7.5 MeV.

Claim 26

The apparatus of claim 15, further comprising: (g) means for generating an energy spectrum of said detected capture gamma rays; (h) means for comparing said detected capture gamma ray energy spectrum with a composite spectrum, including a combination of standard capture gamma ray spectra for elements postulated to have contributed to said detected capture gamma ray spectrum and including at least one of silicon and calcium, to determine the relative capture elemental yields in said composite spectrum for each of said postulated elements contributing to the capture spectrum; (i) where silicon is included among the postulated elements in steps (d) and (h), means for deriving from (1) the relative inelastic silicon yield and the inelastic gamma ray sensitivity for silicon and (2) the relative capture silicon yield and the capture gamma ray sensitivity for silicon an inelastic/capture sensitivity ratio R Si for silicon; (j) where calcium is included among the postulated elements in steps (d) and (h), means for deriving from (1) the relative inelastic calcium yield and the inelastic gamma ray sensitivity for calcium and (2) the relative capture calcium yield and the capture gamma ray sensitivity for calcium an inelastic/capture sensitivity ratio R Ca for calcium; (k) means for utilizing at least one of said ratios R Si and R Ca to provide a calibration ratio R; and (l) means for combining the calibration ratio R and the relative capture elemental yield and the capture gamma ray sensitivity for at least one other element to derive the calibrated capture elemental yield X of said at least one other element.

Claim 27

The apparatus of claim 23, wherein the calibration ratio R comprises the more statistically precise of R Si and R Ca or the average of the values of R Si and R Ca .

Claim 28

The apparatus of claim 23, wherein the means for combining includes means for deriving the calibrated capture elemental yield X for said at least one other element by combining the calibration ratio R and the relative capture elemental yield C X and the elemental capture gamma ray sensitivity S CX in accordance with the relationship X=RC X S CX .

Claim 29

Apparatus for investigating an earth formation traversed by a borehole, comprising the steps of: (a) means for repetitively irradiating an earth formation with bursts of high energy neutrons, which neutrons undergo inelastic scattering reactions and thermal neutron capture reactions with constituents in the borehole and the formation and produce gamma rays as a result thereof; (b) means for detecting gamma rays resulting from said inelastic scattering reactions and from said thermal neutron capture reactions; (c) means for generating an energy spectrum of the detected inelastic scattering gamma rays and an energy spectrum of the detected thermal neutron capture gamma rays; (d) means for comparing said detected inelastic gamma ray energy spectrum with a composite spectrum, including a combination of standard inelastic gamma ray spectra for elements postulated to have contributed to said detected inelastic gamma ray spectrum and including at least one of silicon and calcium, to determine the relative inelastic elemental yield in said composite spectrum for each of said postulated elements in the inelastic spectrum; (e) means for comparing said detected capture gamma ray energy spectrum with a composite spectrum, including a combination of standard capture gamma ray spectra for elements postulated to have contributed to said detected capture gamma ray spectrum and including at least one of silicon and calcium, to determine the relative capture elemental yield in said composite spectrum for each of said postulated elements in the capture spectrum; (f) means for deriving from at least one of (1) the relative inelastic silicon yield and the relative capture silicon yield and their respective gamma ray sensitivities and (2) the relative inelastic calcium yield and the relative capture calcium yield and their respective gamma ray sensitivities a calibration ratio R; and (g) means for combining the calibration ratio R and the relative capture yield and the capture gamma ray sensitivity for at least one element postulated to have contributed to the detected capture gamma ray spectrum to derive a calibrated capture elemental yield X for said at least one element.

Claim 30

Apparatus for investigating an earth formation traversed by a borehole, comprising the steps of: (a) means for repetitively irradiating an earth formation with bursts of high energy neutrons, which neutrons undergo inelastic scattering reactions and thermal neutron capture reactions with constituents in the borehole and the formation and produce gamma rays as a result thereof; (b) means for detecting gamma rays resulting from said inelastic scattering reactions and from said thermal neutron capture reactions; (c) means for generating an energy spectrum of the detected inelastic scattering gamma rays and an energy spectrum of the detected thermal neutron capture gamma rays; (d) means for comparing said detected inelastic gamma ray energy spectrum with a composite spectrum, including a combination of standard inelastic gamma ray spectra for elements postulated to have contributed to said detected inelastic gamma ray spectrum and including at least one of silicon and calcium, to determine the relative inelastic elemental yield in said composite spectrum for each of said postulated elements in the inelastic spectrum; (e) means for comparing said detected capture gamma ray energy spectrum with a composite spectrum, including a combination of standard capture gamma ray spectra for elements postulated to have contributed to said detected capture gamma ray spectrum and including at least one of silicon and calcium, to determine the relative capture elemental yield in said composite spectrum for each of said postulated elements in the capture spectrum; (f) means for deriving from at least one of (1) the relative inelastic silicon yield and the relative capture silicon yield and their respective gamma ray sensitivities and (2) the relative inelastic calcium yield and the relative capture calcium yield and their respective gamma ray sensitivities a calibration ratio R; and (g) means for combining the calibration ratio R and the relative capture yield and the capture gamma ray sensitivity for at least one element postulated to have contributed to the detected capture gamma ray spectrum to derive a calibrated capture elemental yield X for said at least one element.

Claim 31

The apparatus of claim 26, wherein the means for combining derives a calibrated capture elemental yield X for at least each of the elements specifically attributable to the irradiated formation and not used in determining the ratio R.

Claim 32

The apparatus of claim 27, further comprising: (h) means for deriving a normalization factor from the sums of (1) the products of the relative inelastic elemental yields for elements specifically attributable to the irradiated formation and their respective inelastic gamma ray sensitivities and (2) the products of the calibrated capture elemental yields for elements specifically attributable to the irradiated formation and their respective capture gamma ray sensitivities; and (i) means for deriving the volumetric fraction of at least one element in the irradiated formation by dividing the product of (1) the relative inelastic yield and the inelastic gamma ray sensitivity for said at least one element or (2) the calibrated capture yield and the capture gamma ray sensitivity for said at least one element by the normalization factor.

Claim 35

The method of claim 29, wherein step (b) includes determining the relative inelastic yield for magnesium.

Claim 36

The method of claim 30, further comprising the step of: (e) deriving from the relative inelastic magnesium yield, in accordance with a predetermined relationship relating the relative inelastic magnesium yields determined in calibration apparatus having formation materials of known composition to the magnesium content of the formation materials, an estimate of at least one of the volumetric content of dolomite and the concentration of magnesium in the irradiated formation.

Claim 37

The method of claim 30, further comprising the step of forming a ratio of the relative inelastic yields for magnesium and calcium as an indicator of the degree of dolomitization of the irradiated formation.

Claim 38

The method of claim 29, wherein step (b) includes determining the relative inelastic gamma ray yield for iron.

Claim 39

The method of claim 33, further comprising the step of: (e) deriving from the relative inelastic iron yield, in accordance with a predetermined relationship relating the relative inelastic iron yields determined in calibration apparatus having borehole regions of known iron content, an estimate of at least one of the volumetric content and the concentration of iron in the irradiated formation.

Claim 40

The method of claim 33, wherein the estimate of the iron content of or the iron concentration in the irradiated formation is derived by subtracting from the relative inelastic iron yield of step (c) the relative inelastic iron yield for a borehole region of substantially the same iron content as the irradiated borehole.

Claim 41

The method of claim 30 or 33, wherein the energy range of said gamma ray energy spectra is from substantially 1.0 MeV to substantially 7.5 MeV.

Claim 42

The method of claim 29, further comprising the steps of: (e) generating an energy spectrum of thermal neutron capture gamma rays resulting from said neutron irradiation; (f) comparing said capture gamma ray energy spectrum with a composite spectrum, including a combination of standard capture gamma ray spectra for elements postulated to have contributed to said capture gamma ray spectrum and including at least one of silicon and calcium, to determine the relative capture elemental yields in said composite spectrum for each of said postulated elements contributing to the capture spectrum; (g) where silicon is included among the postulated elements in steps (b) and (f), deriving from (1) the relative inelastic silicon yield and the inelastic gamma ray sensitivity for silicon and (2) the relative capture silicon yield and the capture gamma ray sensitivity for silicon an inelastic/capture sensitivity ratio R Si for silicon; (h) where calcium is included among the postulated elements in steps (b) and (f), deriving from (1) the relative inelastic calcium yield and the inelastic gamma ray sensitivity for calcium and (2) the relative capture calcium yield and the capture gamma ray sensitivity for calcium an inelastic/capture sensitivity ratio R Ca for calcium; (i) utilizing at least one of said ratios R Si and R Ca to provide a calibration ratio R; and (j) combining the calibration ratio R and the relative capture elemental yield and the capture gamma ray sensitivity for at least one other element to derive the calibrated capture elemental yield X of said at least one other element.

Claim 43

The method of claim 37, wherein step (i) comprises deriving the calibration ratio R by using the more statistically precise of R Si and R Ca or by averaging the values of R Si and R Ca .

Claim 44

The method of claim 38 wherein step (j) comprises deriving the calibrated capture elemental yield X for said at least one other element by combining the calibration ratio R and the relative capture elemental yield C X and the elemental capture gamma ray sensitivity S CX in accordance with the relationship X=RC X S CX .

Claim 47

The method of claim 40, wherein step (e) comprises deriving a calibrated capture elemental yield X for at least each of the elements specifically attributable to the irradiated formation and not used in determining the ratio R.

Claim 48

The method of claim 41, further comprising the steps of: (f) deriving a normalization factor from the sums of (1) the products of the relative inelastic elemental yields for elements specifically attributable to the irradiated formation and their respective inelastic gamma ray sensitivities and (2) the products of the calibrated capture elemental yields for elements specifically attributable to the irradiated formation and their respective capture gamma ray sensitivities; and (g) deriving the volumetric fraction of at least one element in the irradiated formation by dividing the product of (1) the relative inelastic yield and the inelastic gamma ray sensitivity for said at least one element or (2) the calibrated capture yield and the capture gamma ray sensitivity for said at least one element by the normalization factor.

Claim 51

The method of claim 43, wherein the estimate of the iron content of or the iron concentration in the irradiated formation is derived by subtracting from the relative inelastic iron yield of step (d) the relative inelastic iron yield for a borehole region of substantially the same iron content as the irradiated borehole.

Claim 52

The method of claim 43, wherein the energy range of gamma rays detected in step (b) is from substantially 1.0 MeV to 7.5 MeV.

Claim 53

Apparatus for investigating an earth formation traversed by a borehole, comprising the steps of: (a) means for repetitively irradiating an earth formation with bursts of high energy neutrons, which neutrons undergo inelastic scattering reactions and thermal neutron capture reactions with constituents in the borehole and the formation and produce gamma rays as a result thereof; (b) means for detecting gamma rays resulting from at least said inelastic scattering reactions; (c) means for generating an energy spectrum of the detected inelastic scattering gamma rays; (d) means for comparing said detected inelastic gamma ray energy spectrum with a composite spectrum, including a combination of standard inelastic gamma ray spectra for elements postulated to have contributed to said detected inelastic gamma ray spectrum, to determine the relative inelastic elemental yield in said composite spectrum for at least iron; and (e) means for deriving from the relative inelastic iron yield, in accordance with a predetermined relationship relating the relative inelastic iron yield determined in calibration apparatus having borehole regions of known iron content, an estimate of at least one of the volumetric content and the concentration of iron in the irradiated formation.

Claim 54

Apparatus for investigating an earth formation traversed by a borehole, comprising the steps of: (a) means for repetitively irradiating an earth formation with bursts of high energy neutrons, which neutrons undergo inelastic scattering reactions and thermal neutron capture reactions with constituents in the borehole and the formation and produce gamma rays as a result thereof; (b) means for detecting gamma rays resulting from at least said inelastic scattering reactions; (c) means for generating an energy spectrum of the detected inelastic scattering gamma rays; (d) means for comparing said detected inelastic gamma ray energy spectrum with a composite spectrum, including a combination of standard inelastic gamma ray spectra for elements postulated to have contributed to said detected inelastic gamma ray spectrum, to determine the relative inelastic elemental yield in said composite spectrum for at least iron; and (e) means for deriving from the relative inelastic iron yield, in accordance with a predetermined relationship relating the relative inelastic iron yield determined in calibration apparatus having borehole regions of known iron content, an estimate of at least one of the volumetric content and the concentration of iron in the irradiated formation.

Claim 55

The apparatus of claim 46, wherein the means for deriving the estimate of the iron content of or the iron concentration in the irradiated formation comprises means for subtracting from said relative inelastic iron yield the relative inelastic iron yield for a borehole region of substantially the same iron content as the irradiated borehole.

Claim 56

The apparatus of claim 46, wherein the energy range of gamma rays detected is from substantially 1.0 MeV to 7.5 MeV.

Claim 59

The method of claim 49, wherein the estimate of the iron content of or the iron concentration in the irradiated formation is derived by subtracting from the relative inelastic iron yield of step (b) the relative inelastic iron yield for a borehole region of substantially the same iron content as the irradiated borehole.

Claim 60

The method of claim 49, wherein the energy range of said inelastic gamma ray energy spectrum of step (a) is from substantially 1.0 MeV to 7.5 MeV.