DATA REPORT: ONSHORE GEOCHEMICAL PROCESSING, LEG …€¦ · DATA REPORT: ONSHORE GEOCHEMICAL PROCESSING, ... plains the data processing techniques, ... The well-log data from …

Rea, D.K., Basov, I.A., Scholl, D.W., and Allan, J.F. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 145

46. DATA REPORT: ONSHORE GEOCHEMICAL PROCESSING, LEG 1451

James F. Bristow,2 Cristina Broglia,3 Peter B. deMenocal,3 and Peter K. Harvey2

ABSTRACT

Geochemical well logs were obtained in the igneous and sedimentary sequences drilled at Sites 883 and 884 of Ocean DrillingProgram Leg 145. Corrections have been applied to the logs to account for variations in borehole size, borehole fluid composition,downhole temperature, and logging speed, to derive the concentration of the elements Th and U, and the dry weight oxidepercentages of the elements K and Al.

INTRODUCTION

Seven sites were drilled during Leg 145. Sites 883 and 884, situatedon the Detroit Seamount in the northwest Pacific, were logged with thegeochemical logging tool. These two sites represent the shallowest anddeepest end members of a three-site depth transect down the slopes ofthe Detroit Seamount. The main objective was to define the nature andvariability of the North Pacific Ocean deep waters.

Geochemical logs provide continuous in situ measurements of themajor rock-forming elements in the formation at intervals of 0.1524m (6 in.). They provide a large database of geochemical informationunaffected by two main problems that inhibit core-based studies:incomplete or preferential core recovery and core disturbance.

This report describes the basic principles of the tool string, ex-plains the data processing techniques, and presents the processedelemental and oxide data from the geochemical logs obtained at Sites883 and 884.

GEOCHEMICAL TOOL STRING

The geochemical logging tool string (GLT) consists of four sepa-rate logging tools: the natural gamma-ray spectrometry tool (NGT),the compensated neutron tool (CNT), the aluminum activation claytool (AACT), and the gamma-ray spectrometry tool (GST). A sche-matic drawing of the GLT, which was run in Holes 883F and 884Eduring Leg 145, is shown in Figure 1. These four tools use threeseparate modes of gamma-ray spectroscopy for a comprehensiveelemental analysis of the formation. The NGT is located at the top ofthe tool string so that it can measure the naturally occurring radionu-clides, thorium (Th), uranium (U), and potassium (K), before theformation is irradiated by the nuclear sources contained in the lowertools (Fig. 1). The CNT, located below the NGT, carries a low-energycalifornium (252Cf) neutron source to activate the Al atoms in theformation. The AACT, a modified NGT, is located below the 252Cfsource, measuring the activated gamma rays in the formation. Bycombining the AACT measurement with the previous NGT measure-ment, the background radiation is subtracted out and a reading offormation Al is obtained (Scott and Smith, 1973). The gamma-rayspectrometry tool, at the base of the string, carries a pulsed neutrongenerator to induce prompt-capture gamma-ray reactions in the bore-hole and formation and an Nal(Tl) scintillation detector to measurethe energy spectrum of gamma rays generated by the prompt-neutron-

1 Rea, D.K., Basov, LA., Scholl, D.W., and Allan, J.F. (Eds.), 1995. Proc. ODP, Sci.Results, 145: College Station, TX (Ocean Drilling Program).

Borehole Research, Department of Geology, University of Leicester, Leicester LEI7RH, United Kingdom.

3 Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia Uni-versity, Palisades, NY 10964, U.S.A.

capture reactions. As each of the elements in the formation is charac-terized by a unique spectral signature, it is possible to derive thecontribution (or yield) of each of the major elements silicon (Si), iron(Fe), calcium (Ca), titanium (Ti), sulfur (S), gadolinium (Gd), andpotassium (K) from the measured spectrum and, in turn, to estimatethe relative abundance of each in the formation when combined withthe elemental concentrations from the NGT and AACT. The GST alsomeasures the hydrogen (H) and chlorine (Cl) in the borehole andformation, although these elements are not used for determining therock geochemistry.

The only major rock-forming elements not measured by the geo-chemical tool string are magnesium (Mg) and sodium (Na); the neu-tron-capture cross sections of these elements are too small relative totheir typical abundances for the GLT to detect. A rough estimate of Mg+ Na can be made in some instances by using the photoelectric factor(PEF), measured by the lithodensity tool. Further explanation of thistechnique is found in Hertzog et al. (1989). This calculation could notbe implemented on data from this leg due to the invalid GST data.

DATA REDUCTION

The well-log data from the Schlumberger tools are transmitteddigitally up a wireline and are recorded and processed on the JOIDESResolution in the Schlumberger Cyber Service Unit (CSU). The re-sults from the CSU are made available as "field logs" for initial, ship-board interpretation. Subsequent reprocessing is necessary to correctthe data for the effects of fluids added to the well, logging speed, anddrillpipe interference. Processing of the spectrometry data is requiredto transform the relative elemental yields into oxide weight fractions.

The processing is performed with a set of log-interpretation pro-grams written by Schlumberger but slightly modified to account forthe lithologies and hole conditions encountered in ODP holes. Thesteps are summarized below:

Step 1: Reconstruction of Relative Elemental Yields fromRecorded Spectral Data

This first processing step compares the measured spectra from thegamma-ray spectrometry tool with a series of "standard" spectrato determine the relative contribution (or yield) of each element.These "standards" approximate the spectrum of each element. Usinga weighted, least-squares inversion method, the relative elementalyields are calculated at each depth level.

Six elemental standards (Si, Fe, Ca, S, Cl, and H) are used to pro-duce the shipboard yields, but three additional standards (Ti, Gd, andK) can be included in the post-cruise processing to improve the fit ofthe spectral standards to the measured spectra (Grau and Schweitzer,1989). Although Ti, Gd, and K often appear in the formation in verylow concentrations, they can make a large contribution to the mea-

677

DATA REPORT

TCCB

NGT

CNT-G

AACT

D

DD

DD

D

GST

ü

Telemetry cartridge

Natural gamma-ray tool: measures naturally radioactiveelements thorium, uranium, and potassium.

Dual-porosity compensated neutron tool: Measuresneutron porosity in the thermal and epithermal energy ranges.

Thermal detectors

Californium 252 source

Epithermal detectors

Aluminum activation clay tool: Measures aluminumactivation and natural count rates as it passes the formationactivated by Californium 252.

Gamma-ray spectrometry tool: Measures concentrationof calcium, silicon, iron, sulfur, gadolinium, titanium,hydrogen, chlorine, and formation capture cross-section.

Boron sleeve - 3.75 in.

Neutron accelerator

Figure 1. Schematic drawing of the Schlumberger geochemical logging tool string used in the Ocean Drilling Program.

sured spectra because they have large neutron-capture cross sections.For example, the capture cross section of Gd is 49,000 barns, whereasthat of Si is 0.16 barns (Hertzog et al., 1989). Therefore, including Gdis necessary when calculating the best fit of the standard spectra to themeasured spectrum.

During the spectral processing of the GST data from Holes 883Fand 884E, it became apparent that there were serious data calibrationerrors caused by problems during the log acquisition phase. A hard-ware malfunction in the telecommunications cartridge (Fig. 1) hadprevented the proper spectral calibration of the tool during loggingoperations at the two sites. The spectral inversion routine was run onthe data, but unfortunately, due to the calibration errors, the data areinvalid and are not presented. The data from the NGT and AACT arenot similarly affected, and the processing of these data are describedin the following.

Step 2: Depth-shifting

Geochemical processing involves the integration of data from thedifferent tool strings; consequently, it is important that all the data are

depth-correlated to one reference logging run. The NGT, run on eachof the logging tool strings, provides a spectral gamma-ray curve withwhich to correlate each of the logging runs. A reference run is chosenon the basis of constant, low cable tension and high cable speed (toolsrun at faster speeds are less likely to stick and are less susceptible todata degradation caused by ship heave). The depth-shifting procedureinvolves picking a number of reference points based on similar logcharacter and then invoking a program that expands and compressesthe matching logging run to fit the reference logging run. The geo-chemical tool string was chosen as the reference run in Holes 883Fand 884E.

Step 3: Calculation of Total Radioactivity and Th, U, and KConcentrations

The third processing routine calculates the total natural gamma-rayradiation in the formation, as well as concentrations of Th, U, and K,using the counts in five spectral windows from the NGT (Lock andHoyer, 1971). This routine resembles shipboard processing; however,

678

DATA REPORT

the results are improved during post-cruise processing by includingcorrections for hole-size changes and temperature variations. A Kal-man filtering (Ruckebusch, 1983) is used in the CSU processing at seato minimize the statistical uncertainties in the logs, which can other-wise create erroneous negative values and anticorrelations (especiallybetween Th and U). An alpha filter has been introduced more recentlyand is now recommended by Schlumberger for shore-based process-ing. This filter strongly smoothes the raw spectral counts but keeps thetotal gamma-ray curve unsmoothed before calculating out the Th, U,and K (C. Flaum, pers. comm., 1988). The outputs of this program areK (wet wt%), U (ppm), and Th (ppm), as well as total gamma-ray andcomputed gamma-ray (total gamma ray minus U contribution).

Step 4: Calculation of Al Concentration

The fourth processing routine calculates the concentration of Al inthe formation using four energy windows recorded on the AACT.During this step, corrections are made for natural radioactivity, bore-hole-fluid neutron-capture cross section, formation neutron-capturecross section, formation slowing-down length, and borehole size.

Porosity and density logs are needed as inputs into this routine toconvert the wet weight percentages of K and Al curves to dry weightpercentages. Porosity logs from the neutron porosity tool and derivedfrom resistivity and density logs were compared to core measure-ments to determine the best core-log correspondence. The calculatedresistivity-porosity was found to be best in Hole 883F, and a spliceddensity- and resistivity-porosity was best for Hole 884E.

A correction also is made for Si interference with Al; the 252Cfsource activates the Si, producing the aluminum isotope, 28A1,(Hertzog et al, 1989). The program uses the Si yield from the GST todetermine the Si background correction. The program outputs dryweight percentages of Al and K.

The output weight percent concentrations of Al and K are nor-mally input into an oxide closure model with the other GST-derivedelements. As no useful GST data were obtained in Holes 883F and884E, the processing moves straight on to the final step.

Table 1. Oxide factors used in normalizing elements to 100% and con-verting elements to oxides.

Element Oxide

Conversionfactor

KAl

K2OA12O3

1.2051.889

as shown in Table 1. The resulting oxides are presented in Figures 2and 3.

REFERENCES*

Grau, J.A., and Schweitzer, J.S., 1989. Elemental concentrations from thermalneutron capture gamma-ray spectra in geological formations. Nucl. Geo-phys., 3:1-9.

Hertzog, R.C., Colson, J.L., Seeman, B., O'Brien, M.S., Scott, H.D., McKeon,D.C., Wraight, P.D., Grau, J.A., Ellis, D.V., Schweitzer, J.S., and Herron,M.M., 1989. Geochemical logging with spectrometry tools. SPE Form.Eval, 4:153-162.

Lock, G.A., and Hoyer, W.A., 1971. Natural gamma-ray spectral logging. LogAnalyst, 12:3-9.

Ruckebusch, G., 1983. A Kalman filtering approach to natural gamma rayspectroscopy in well logging. IEEE Trans. Autom. Control, AC-28:372-380.

Scott, H.D., and Smith, M.P., 1973. The aluminum activation log. Log Analyst,14:3-12.

* Abbreviations for names of organizations and publications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).

Step 5: Calculation of Oxide Percentages

The final routine converts the elemental weight percentages intooxide percentages by multiplying each by its associated oxide factor,

Date of initial receipt: 27 January 1994Date of acceptance: 31 October 1994Ms 145SR-147

679

DATA REPORT

Hole 883F: Geochemical Log Summary

OC IUIU CD

Q E CAPTURE CROSSSECTION POTASSIUM ALUMINUM8 E5

ir Q w [20 capture units 40 |o wt. % 2.5 I0 wt. % 10

13X1

14X1

15X1

16X1

17X1

18X1

19X|

20X

1H

2H

3H

4H

5H

6 H |

7H

8H

9H

10H|

12X1

100-

150-

- 5 0

- 100

DRILL PIPEOPEN HOLE

-150

Figure 2. Processed oxides from the NGT and AACT for Hole 883F.

680

Hole 883F: Geochemical Log Summary (continued)

, If ?s>- —I DC —I DCDC I•U O LU Q

LU S E § I CAPTURE CROSS I 3DC o cü< SECTION POTASSIUM ALUMINUM £<ü DC Q w I 2 0 capture units 4 0 T 0 vvt. % 2.5 l o wt % 101 l o w

1 9 H I > ^ • **

I 200- ^ <J ^ -200

2 3 H • j)r * ‰ - ^ '

2 6 H I " , I ** ^

30H• _ \ ' ^ " ^ •

32x1 < r <f~~* C ^I 300- t T *• -^ -300

3 3 X ^ 1 ^ ^ β ^ S i .

36X < ^ » ^^».

Figure 2 (continued).

681

DATA REPORT


> - - J QCQC LU O

, , > I ß CAPTURE CROSSLU O \— LU "

S O Q. <LU

SECTION POTASSIUM ALUMINUMO ir Q <n 120̂ capture units 40T0 2.5 10 wt. % 10

37XF

38X1

39X1

40X1

41X1

42X1

43X1

44X1

45X1

46X

47X

48H|

49X1

50X1

1X1

52X

53X1

54X1

3 5 0 -

400 -

4 5 0 -

5 0 0 -

-350

-400

-450

-500


682


DATA REPORT

I p CAPTURE CROSSSECTIONQ. < I Oto I IWIN I

QCO H ö capture units 4 0 |oPOTASSIUM ALUMINUM

2.5 10 wt. % 10

IIQ- <LLJ LU

550-

54X1

55X1

56X

57X1

58X

59X1

60X1

61X1

62X1

63X1

64X

65X1

66X1

67X1

6 8 X • I 6 5 0 -

69X1

70X1

71X1

6 0 0 -

<>

- 5 5 0

- 6 0 0

- 6 5 0


683

DATA REPORT


> i j CAPTURE CROSSQC 8 C L < I SECTIONü CC Q w 120 capture units 4 0 1 0 wt. % 2.5 10 **-% 10

POTASSIUM ALUMINUM

71X1

72>

73>

74)

75)

76>

77X|

78x1

79x1

80X

81X

82X1

83X1

84X

85x1

86x1

87X[

88x|

700-

750 -

800-

^8fiα. <

- 7 0 0

- 7 5 0

- 8 0 0


DATA REPORT

Hole 884E: Geochemical Log Summary

£ LUUJ m> i

£ ü ECAPTURE CROSS

SECTIONcc Q co I 20 capture units 40 | 0

POTASSIUMwt. %

ALUMINUM3 1 0 wt. % 30

α. <LU LUQ CO

13XF

14X1

15X1

16X1

17X1

18X1

19X1

20X

1H

2H

3H

—

4H

SH

6 H |

7H

8H

9H

10H{

12X

100-

150-

DRILLPIPEOPEN HOLE

- 5 0

-100

- 150

Figure 3. Processed oxides from the NGT and AACT for Hole 884E.

685

DATA REPORT

Hole 884E: Geochemical Log Summary (continued)

>- —i

g O hECAPTURE CROSS

g 8 CL< I SECTIONo DC Q w [20 capture unit

POTASSIUM ALUMINUM

capture units 40 I 0 wt. % 3 | 0 wt. % 30

§1

Q. <LU LUQ C/3

21X1

22X1

23X1

24X1

25X1

26X1

27X1

28X

29XF

30X1

31X1

32×l

33X1

34X1

35X1

36X1

37X1

38X1

2 0 0 -

250-

3 0 0 -

- 2 0 0

- 2 5 0

-300


686


C UIQ LU S

> X 3 CAPTURE CROSS X 3g Q £ < I SECTION I POTASSIUM I ALUMINUM I I £ £ü a. Q w I20 capture units 40 | o wt. % 3T0 **•o/o 301 l o w

3βxM ~~^ \ ~ «EUT

39X• I 3 5 0 _ ^ " > \ _ 3 5 0

• \ 'I" <>

42X• J ! ^ » ^

43x1 ^ C. V

I 400- X ^ ^ -400

46X• S T ^

48x1 j " [ >

49x1 4^ ^ ^

I I 450- j ' ^ ^ > ^ -450

50x1 ^ i ^ ^

53x1 '. " . * p

54X• • I 4 < >

I 500- « » < £ ~500

I 1 Mil I Mil II k\ IIIIIIFigure 3 (continued).

687

DATA REPORT <


SJcc LU ö> X 3 CAPTURE CROSS

SECTIONS O O. < SECTION ILU LU LU I , I

ü a. Q to I 20 capture units 40 10

POTASSIUM ALUMINUMwt. % 3 10 *vt. % 30

α. <LU LUQ V)

56X1

57X1

58X1

59X1

60X1

61X1

62X1

63X1

64X1

65X1

66X1

67X1

68X1

69X1

70X1

71X1

72X1

73X1

5 5 0 ^

6 0 0 -

650-

- 5 5 0

- 6 0 0

-650


Documents

DATA REPORT: ONSHORE GEOCHEMICAL PROCESSING, LEG …€¦ · DATA REPORT: ONSHORE GEOCHEMICAL PROCESSING, ... plains the data processing techniques, ... The well-log data from …