Pyrolysis gas chromatography-mass spectrometry to characterize organic matter and its relationship to uranium content of Appalachian Devonian black shales

Pyrolysis gas chromatography-mass spectrometry to characterize organic matter and its relationship to uranium content of Appa~a~hjanDevonian black shales JOELS.

tEVENTHAL

U.S. Geological Survey, Denver, CO 80235, U.S.A. (Receirud

14 Mny 1980: accepted

it1 revisedform

29 Januurr

1981)

Abstract---Gas ~hromatographi~ analysis of volatile prodncts formed by stepwise pyrolysis of black shales can be used to characterize the kerogen by relating it to separated, identified precursors such as Iand-derived vitrinite and marine-source TL~smunirrs.Analysis of a Tmnanites sample shows exclusively n-alkane and -alkene pyrolysis products. whereas a vitrinite sample shows a predominance of one- and two-ring substituted aromatics. For core samples from northern Tennessee and for a suite of outcrop samples from eastern Kentucky, the organic matter type and the U content (cc10.12Oppm)show variations that are related to precursor organic materials. The samples that show a high vitrinite component in their pyrolysis products are also those samples with high contents of U.

lNTRODUCTlON

BREGER and BROWN (1962, 1963) measured the hydrogen content of the kerogen for many samples as SEVERALSTUDIEShave shown that Organic matter in a means to determine the proportions of sapropelic sedimentary rocks can be characterized by pyrolysis and humic material. They show that sapropelic rich gas chromatography. (See for example GIRAUD, 1970: kerogens yield more oil. They propose that since the LEVENYHAL, 1976, 1978; SIGLEO,1978; SCRIMAet ol., uranium is highest near the inferred shoreline, the 1974.) However, only a few of these studies have uranium is from the land. They also feel that since the related the pyrolysis products to a precursor biouranium is generally correlated with the organic logical material or identified marine and terrestrial content the type of organic matter is not important in organic components. concentrating uranium. Many syngenetic and low-temperature epigenetic This study extends the earlier work on the organic mineral deposits are intimately associated with matter and uranium contents of Upper Devonian organic matter. (See for example WEDEPOHL,1964; shales of Tennessee by including samples from a GULBRANDSON,1966; TAYLOR, 1971; MAUGWAN, larger part of the Appalachian basin and by employ1976; DES~~OR~UGH, 1977; lMi30R0uGH at al.. 1979: ing new analytical methods to more clearly define the i__EvENTHAL, 1979.) In studies of such ore deposits the syngenetic ur~ium-organic matter association. The types of organic matter were sometimes recognized, overall study was related to the occurrence of gas and but the origin of the organic material and its possibly oil (CLAYPOOLand THRELKELU,1978). trace elements diverse roles in the formation of the ore deposits was (LEVENTHAI.,1979), and uranium (J. S. LEVENMAL not elucidated. and R. C. KEPFERLE,unpublished data), which all are Work done during the 1950’s by two groups directly or indirectly related to the amount (LEVENpresented evidence that uranium was concentrated by THAL and SHAW, 1980) and type of organic matter organic matter in shales (SWANSON, 1960, 1961: present and to the depositiona? environment. The BRECER and BROWN, 1962, 1963): however their uranium and organic matter in these shales is synexplanations were somewhat different. SWANXON genetic, the uranium contents range from cc IO to (1960) discussed the oil yield and uranium content. He 120 ppm and are sub-economic at this time. However, distinguished organic matter based on source: saprotwo points should be made: {1t future economic utilizpelic vs humic, the former being hydrogen rich and ation of the whole shale for minerals and energy; and derived from algae, spores etc., and latter from cellu(2) the role of black shales as a first step enrichment of lose and lignin being H poorer. He attributes the high uranium prior to epigenetic mineralization. oil yield to the former and the high uranium content to the latter. His interpretations rely on the uranium SAMPLES content and ail yield of the alga Foerstiu and wood Core samples were taken from Overton County. Tennessee (Tennessee coordinates 708,550 N. and 2,154,‘/‘50 E.: from Cdiixybn from the Devonian Chattanooga API well No. 41-133-01001) (LEVENTH.AL 1979).At this loshale which represent the sapropelic and humic prototypes, respectively. He proposes that uranium is preferentially removed from the sea water by the humic type kcragen or its precursor. 883

cation the Devonian Chattanooga Shale is at a depth of t81-207ft (55.67 m). The samples used in this study were IO-cm pieces of core (Table 1). The general stratigraphy of this area is given in COWAN and SWANSON (1961).

.I. S. LEWNTHAL

884

Table 1. Organic carbon and uranium contentsand pyrolysis peak ratios for core samples of Devonian shale from Overton County, Tennessee (see Figure 3 for M/L explanation) Sample

Depth

T-Z T-3 T-4 T-5 T-6 T-7 T-8 T-9 T-10 T-11 T-13

(ft)

Percent

182.7 184.2 186.7 189.7 192.2 194.7 197.2 199.2 199.7 200.5 205.3

C organic 6.5 7.7 12.5 11.1 13.5 10.8 4.8 10.2 7.1 6.8 8.8

ppm U 25 29 42 81 7": 41 56 4533

M/L 0.83 .81 .92, .98* 1.20 1.27 1.21 <1.05 1.01 1.0 1.06

34

.83

Vitrinite

-60

120

1.47

Taslnanites

>60


0.60

* Replicate. Outcrop samples were collected from a recent roadcut on Interstate 64 in Rowan County, Kentucky, near the Morehead interchange (PROVOet cd.,1978) by the author and R. Kepferle (USGS). The samples represent 15-cm thicknesses of rock taken after removing surface material and were collected at 5-ft (1.5-m) intervals (Table 2). Vitrinite was collected from a new roadcut north of Nashville, Tennessee, on Interstate 24 by the author and R. Kepferle. At this locality there are several l-2-cm thick vitrinite layers in which cellular structures can be observed (vitrinite identification confirmed by N. Bostick. USGS). A samfile of shale rich in the marine palynomorph genus Tasmanites was supplied by Kepferle, who obtained it from F. Ettensohn (Univ. of Kentucky). It was collected from the Devonian New Albany Shale in Bullitt County, Kentucky. EXPERIMENTAL Specimens of Tusmunirrs were hand picked from broken shale pieces or surfaces and then were cleaned ultrasonically and dried. Similarly, pieces of vitrinite were hand picked from outcrop and core material. All samples except the Tumanites specimens were ground with an agate mortar and pestle before pyrolysis. Stepwise pyrolysis-gas chromatography has been described by LEVENTHAI. (1976), and so the experimental procedure is only briefly reviewed here. One to ten mg of finely powdered sample is placed in a 2.5-cm by 2-mm quartz tube. which is then put in spiral heating coil of a Pyroprobe* pyrolysis device. The Pyroprobe is inserted in the injection port of a gas chromatograph (GC) in helium carrier gas (- 12cm3,‘min) and heated for IOsec at 250 C. The pyrolysis products then pass into a 15-m by 0.5-mm (id.) porous-layer-open-tubular (PLOT) capillary column coated with Apiezon L* on Chromosorb* R-6740 and are trapped within the first 6 cm of the column which has been immersed in liquid nitrogen. In some experiments, packed 3.2-mm (o.d.) by 2-m OV-101 or OV-17* columns were used without the liquid nitrogen trap. After the trap has warmed to room temperature, the pyrolysis products are temperature programmed at 6 C, min from 50 to 280°C. The pyrolysis procedure is repeated 450,600,750, and 9WC (and sometimes 1050 and 1200 C), and in some analyses pyrolysis products from several steps * Mention of a brand name is for identification onI4 and does not imply endorsement by the US. Geological Survey.

are combined. GC output was fed to a Columbia Scientific

Industries* Supergrator I electronic integrator and to a strip chart recorder. Mass spectrometry (MS) was done using the same gaschromatography columns and pyrolysis system in tandem with an AEI-MS-30* and DS .50* data system. Scan rate was 3 s/decade for the mass range 28-400 AMU. The ionizing voltage was 70eV with a filament emission of 8011A (ANDERSrl al.. 1978). Carbon was analyzed as follows: total was by Leco* combustion, carbonate by acid leach gasometric, and organic by difference (LEVENTHAL ef ut.. 1978). Uranium was analyzed by the delayed neutron technique (MILLARD,1976). Table 2. Organic carbon and uranium contents and pyrolysis peak ratios for outcrop samples of Devonian and Mississippian shale from Rowan County. Kentucky Sample su + 15 su + 10 su + 5 Sun base Bed Oh + 45 Oh + 40 Oh + 35 Oh + 30 Oh + 25 Oh + 20 Oh + 15 Oh + 10 Vitrinite

% c org 8.7

12.2 11.1 14.9 0.4 19.1 15.1 15.1 13.8 11.9 10.7 9.4 8.2

PPD ”

P/Q

23 72 57 39 7 17 22 33 16 22 37 22 39

n.a. 1.08 .8R .80 *.a. . 50 .49 .53

120

2.77

.5O .54 57 :66

.64

Sample name and number refer to stratigraphic unit and distance from base in feet. Su is Sunbury Shale, Bed is Bedford Shale, Oh is Ohio Shale, in ft above Three Lick Bed (PROVO et ul., 1978). See Figure 5 for P,/Q explanation; n.a., not analyzed.

88.5

Organic matter and its relationship to uranium content

0

I

2

3

4

5

6

7

‘3

9

CARBON,IN Fig. 1. Organic

carbon

in Appalachian Devonian shale samples. Solid circle5 and line of (1960): triangles show data from LEVE\ITH.AL. and GOI.DHARFR and LEVENTHAL(1979). open circles show data from this study. SWANSON

RESULTS

Organic carbon and uranium contents of the two sample suites are given in Tables 1 and 2. As previously recognized the organic carbon and uranium values show a correlation (BREGER and BROWN, 1963; SWANSON, 1960). Figure 1 adapted from SWANSON (1960) has additional new data added, however. the line is the one drawn by SWANSON (1960). The data from SWANSON (1960) are from central Tennessee, whereas the new data are from core samples from Kentucky, West Virginia. New York. Ohio and Vir-

HIGH

Fig. 2. High-temperature and a uranium-rich

(LE~ENTHALand GOLDHABER, 1978: LEVE.NTHAL. 1979). Figure 1 also shows the data for the two suites (Tables I and 2) reported on here. Several of these samples show high amounts of organic material without the accompanying high uranium contents: this anomalous relation will be discussed later.

ginia

General

vitrinite,

II

PERCENT

vs uranium

correlation represent data from (1978).

IO

Pyrograms of the core and outcrop samples show at least two types of organic matter are present. One type shows a predominance of n-alkane and -alkene pyrolysis products. the other shows a predominance

TEMPERATURE~PYROLYSIS-FINGERPRINTS

fingerprints showing n-alkane-alkene-rich shale. Column

Apieron

L. Numbers

L and M.)

Tusmurlitr,t.

refer to n-alkane

n-alkane-alkene-poor position. (See text for

886

J. S. LEVENTHAL

T-4 42 rwm

U

.

Oh+45

M T-7 76 ppm

I

U

Fig. 3. Pyrogram of selected core samples from Overton County, Tennessee, showing relative heights of n-alkanealkene peak (L) and adjacent non-n-alkane peak (M), See Table 1 for peak ratios. Column OV-101.

0.4 j IO

I 20

I 30

I 40

1 50

Fig. 5. Pytograms of selected outcrop samples from Rowan County, Kentucky, showing relative heights of peak P inon-n-alkane-alkene) and peak Q (*-alkane-alkene). See Table 2 for peak ratios. Column is OV- 17.

I 60

\ 70

I 80

L 90

I 100

I 110

t20

Fig. 4. Uranium content vs M/L peak area ratio for core samples from Overton County, Tennessee. Linear least-squares fit of data, for U = M/L m -f b, n = 10, F = 0.95. The Ewnanites-rich sample and the vitrinjterich sample were not used in the statistical fit.

70

10

=

I

20

1

30

I

U.

40

wm

I

50

i

60

I

70

I

80

90

COLUMN

50

150

TEMPERATURE,

100 IN

DEGREES

200

250 C (QV-17) of identification

Kentucky.

Fig. 7. Total ion plot from mass spectrometer analysis of pyrolysissgas chromatography vitrinite collected near Nashville. Tennessee. Numbers are n-alkane-alkene peaks, other symbols are for dominant substituted aromatic molecules.

r-l ”

20

5

cf

30

$

aw 40

50

60

80

g’

5

I

10

content vs P/Q peak area ratio for outcrop samples from Rowan County, Line is linear least squares fit for U = P/Q m + b, n = 11, r = 0.91.

lI

Fig. 6. Uranium

0

OL

Fig. 8. Down-hole plot for uranium and organic carbon core samples from Overton County, Tennessee.

URANIUM IN PARTS PER MILLION

ORGANIC CARBON(%)

for

J. S. L~VENTHAL

888

components. Pyrolysis of Tasmanifes gives n-alkanes and alkenes; vitrinite pyrolysis produces substituted aromatics. Figure 2 shows the pyrograms for Tasmanites, a uranium-rich shale, and a vitrinite; the uranium contents were ~5, 70, and 115 ppm, respectively. Pyrograms of selected core samples from Overton County, Tennessee (Fig. 3), showed a variation in types of organic material which can be related to uranium content. None of these samples shows the n-alkane series as clearly as it was shown in the upper chromatogram of Fig. 2. However, mass spectrometry of these samples identified peak L as the n-Cl3 alkane-alkene peak and the adjacent peak M as naphthalene. Considerable variations in the relative peak height and the area between these two peaks were observed. The integrator areas of L and M were tabulated and their ratio (M/L) plotted vs uranium content (Fig. 4). The linear regression is significant at the 950,; level. Thus, the qualitative difference of alkane and aromatic pyrolysis products can be quantified in this way by using two adjacent peaks and then can be related to uranium content. Figure 5 shows the pyrograms for samples from the roadcut, taken at S-ft (1.5-m) intervals. in Rowan County, Kentucky. These samples were pyrolyzed, and chromatography was done using an OV-17 column. Peak Q was identified as the n-C, alkanealkene peak. The peak area for Q was compared to adjacent peak P, which was identified as toluene. Figure 6 shows the P/Q ratio vs the ppm U for these samples. The line shows the linear regression, which is statistically significant at the 952, level. Figure 7 shows the mass spectrometer total-ion plot for a vitrinite sample run on an OV-17 column. This is the same vitrinite plotted in Fig. 4 (run on OV-17) and Fig. 2 (run on Apiezon L). The n-alkene and -alkane peaks are identified, as are the aromatics. The peaks P and M are identified as toluene and naphthalene, respectively, based on parent and fragment ions on GC-MS runs. Figures 8 and 9 (and Tables 1 and 2) show how the uranium and organic carbon contents vary vertically in the Tennessee core of aromatic

U PPM 5 15 2535455565

+ ti

Table 3. Rock-Eva1 results for four samples from Rowan County. Kentucky (see Table 2 for other data) Sample

H2 index

su + 10 su+ 5 Oh + 45 Oh f 30

02 index

408 380* 473 633

* Average of

21 30* 15 29

Kerogen

II-III II-III I-II I-II

3 analyses.

and the Kentucky outcrop. In general, a good covariante is seen between organic carbon and uranium content (BREGERand BROWN, 1962, 1963; SWANSON, 1960; LEVENTHAL and GOLDHABER, 1978; J. S. LEVENTHAL,unpublished data), and this covariance is also observed in these two suites of samples. However. there are several exceptions: samples T-4. T-13, Oh + 40, and Oh + 45 show high amounts of organic matter without similarly high amounts of uranium. Rock-Eva1 analysis (TISSOTand WELZ, 1978) was run on several samples for comparison with the pyrolysis. The results for four of the Kentucky samples are shown in Table 3. Samples Oh + 30 and ,+45 with the lowest U contents show high hydrogen indices from Rock-Eva1 data which puts them between type 1 and II kerogen. Conversely, samples Su + 5 and + 10 with the highest U contents show lower H indices and plot between type II and III kerogen. DISCUSSION AND CONCLUSIONS It was found that the uranium content in these Devonian shales can be correlated with the ratio of aromatic compounds (toluene or naphthalene) to n-alkane compounds derived from pyrolysis of the organic matter in the shales. This assoeiation is true even where the total organic carbon does not correlate with total uranium. This distinction between types of organic matter has been suggested by pre-

ORGANIC 5

C %

7 9 1113151719

su

base Bed

LOh+40 Oh+30 Oh+20 Oh+

type

lo

Fig. 9. Vertical plot of uranium and organic carbon contents in outcrop samples from Rowan County. Kentucky. Oh + 10 is 10 ft above the Upper Devonian Three Lick Bed of the Ohio Shale (PROVO d (II.. 1978); Bed is the Devonian or Mississippian Bedford Shale: Su is the Mississippian Sunbury Shale.

Organic

matter

and its relationship

(SWANSON, 1960) from analysis of pure source members but was not recognized (BREC;ER and BROWN, 1962, 1963) in shale samples themselves. Although the aromatic component is correlated with pyrolysis products from a true vitrinite sample, this component might come from a solubilized amorphous vitrinite-derived material rather than from vitrinite itself: that is, the shale may yield a large amount of vitrinite pyrolysis products but contain only a small amount of visually recognizable vitrinite. If this solubilized material was derived from vitrinite. it probably was a humic material, derived from the land and brought into the Devonian Appalachian sea by rivers (SWANSON. 1960). These same rivers could also have transported the uranium from the inland source areas. However, it is not known whether the vious

workers

organic

land-derived

organic

material

was the transport

agent

for the uranium. Samples T-13 and Oh + 45. which have high organic contents but relatively low uranium contents, are exceptions to the generally observed uranium and organic-carbon covariance. This circumstance may be explained by the distribution of organic types shown by pyrograms and M/L or P/Q peak ratios. These two samples show a predominance of rl-alkanes and low P/Q or M/L ratios and corresponding low U in spite of the high amounts of organic matter. This alkane-rich (marine) organic matter is not the type that shows a covariance with uranium. In summary. pyrolysis+gas chromatography has been used to characterize a marine and terrestrial organic component in Devonian shales. The syngenetic uranium content is related to the terrestrial organic component of the shale. Whether this is due to scavenging of the uranium by the terrestrial organic matter in rivers, the seawater or the sediment-water interface is not known. .4~,~no~cledyertlt,f1~.\ Thanks are due to R. C. MILI(.I (formerly of the Tennessee Division of Geology) for obtaining the Tennessee core samples: to D. ANDERS, U.S. Geological Survey. who made the mash spectrometer perform at the appropriate time and to T. DAWS (USGS) for Rock-Eva1 analysis. Mark LeFont asslsted in some of the pyrolysisCC work. This work is supported under interagency agreement EX-76-C-01-227X with the U.S. Department of Energy.

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to uranium

content

889

CONANT L. C. and SWANSON V. E. (1961) Chattanooga shale and related rocks of central Tennessee and nearby areas. U.S. Geol. Surr. Prof: Pap. 357. 91pp. DESBOR~UGH G. (1977) Preliminary report on certain metals of potential economic interest in thin vanadiumrich zones in the Meade Peak Member of the Phosphoria Formation in western Wyoming and eastern Idaho. L’S, Guol. Sure. Opm-File Rep. 77-341. 32 pp. DESBOR~U~;H G. (1979) Metals in Devonian kerogenous marine strata at Gibellini and Bisoni properties in southern Fish Creek Range. Eureka County, Nevada. U.S. Geol. Surr. Open-File Rep. 79-530. 31 pp, GIRAUD A. (19701 Application of pyrolysis and gas chromatography to geochemical characterization of kerogen in sedimentary rock. .4m. 4ssoc. Per. Grol. Bull. 54, 439-455. GULBRANDSOX R. A. (1966) Chemical composite of phosphorites of Phosphoria Formation. Gzocltim. Cosmochim Acta 30, 769-778. LEVE&;THAL J. S. (1976) Stepwise pyrolysis-gas chromatography of kerogen in sedimentary rocks. Chem. Grol. 18, 5 ‘0. L~VEN~HAL J. S. (1978) Sources of organic matter in Devonian black shale. Grol. Sot. .4m. Ah.vtr. Proyr. 8, 444. LEVEK~H~I. J. S. (1979) Chemical analysis and geochemical associations of Devonian black shales from Ohio. Kentucky, Virginia and Tennessee. I:.S. Grol. Surr,. Oprn-File Rep. 79-1503, 51 pp. LEL’ENTHALJ. S. and GOLDHABER M. B. (1978) New data for uranium, thorium. carbon. and sulfur in Devonian Black Shale from West Virginia, Kentucky and New York. In First Eeastern Gas Shales Symp. Morgantown Energy Res. Cent. Rep. MERC,‘SP 77./5. pp. 183~220. Nat. Tech. Inf. Serv. LE~E~‘~HAL J. S. and SHAW V. (1980) Organic matter in Appalachian Devonian black shales: 1. Measurement techmques. and II. Short range variation. .1. Scdime,tt. Prtrol 50, 77-81. LE~ENT~~AL J. S.. CROCK J.. Mou~-~~oY W.. THOMAS J.. SHA\~ V.. BKI~ZS P.. WAHLBER~; J. and MAI.COLM M. (1978) Preliminary results for a new U.S. Geological Survey Devonian Ohio Shale standard. SDO-1. I;.S. Gw)/. Surr. Open-File Rep. 78-447. I I pp. MAU(;HAN E. K. (1976) Organic carbon and selected element distribution m the phosphatic shale members of the Permian Phosphoria Formation. eastern Idaho and parts of adjacent states. I.T.S. Grol. Sure-. Open-f‘il,) Rep, 76-577. 92 pp. MILLAKI) H. T. JR (1976) Determination of uranium and thorium in USGS standard rocks by delayed neutron technique. L’.S. Gcwl. Surr Pro/. Pup X40 (cd. F. J. Flanaganl. pp. 61 66. PROVO L. J.. KEPFERL~ R. C. and POTTER P. E. (1978) Division of black Ohio Shale in eastern Kentucky. B~r/l. .4rn. .4.s.\~,<~. Pet. Grol. 62, 1703%1713. SCRIMA D. A., YEN T. F. and WARRF~ P. L. (1974) Thermal chromatography of Green River oil shale. Enrry~ Sourws 1, 321-336. SIGL~O A. C. (1978) Degraded lignm compounds identified m silicified wood 200 million years old. S~~iewc 200, 1054 1055. SWANSOF; V. E. (1960) Oil yield and uranium content of black shales. L;.S. G&. Surr. Prof. Pun. 356-A. DD. I 44. SWA~.SO\ V. E. (1961) Geology. an; geochemistry of uranium in marine black shales. L;.S. Grol. Sur[. Prof. Pup. 356-C. pp. 67 11’. TAYL~H G. H. (1971) Carbonaceous matter: a guide to the genesis and history of ores. SIX. Min. Grol. Jpn Spw. f\w 3. pp. 3X3-288. (IAGOD Vol). Trssol B. and WELTE D. (1978) Pc~trolcum Formcrtion and Occurruwc, pp. 443-446. Springer. W~DI:POHI. K. H. (1964) Untersuchungen am Kupferschiefer. &x#II,?I. C~~.\rno&im. 3l,t