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Kansas Geological Survey, Bulletin 202, pt. 1, originally published in 1971


Preliminary Studies of Stable Carbon Isotopes in Selected Kansas Shales

by John A. Calder and David H. Attaway

Originally published in 1971 as part of Kansas Geological Survey Bulletin 201, pt. 1, p. 19-21. This is, in general, the original text as published. The information has not been updated.

Abstract

Ratios of stable carbon isotopes were measured on chemical fractions of four Kansas shales. Content of organic carbon and relative abundance of chemical fractions were also determined. Relationships and meaning of the data are discussed.

Distribution of stable isotopes of carbon is a geochemical parameter often used to indicate environment of deposition, relationships between petroleum and source rocks, and extent of in situ alteration of organic molecules. For a recent review see Degens (1969). Samples studied in this investigation were the Sharon Springs Shale Member (Upper Cretaceous), Graneros Shale (Upper/Lower Cretaceous), and the Heumader and Heebner Shale Members (Upper Pennsylvanian). Brief descriptions of these shales are given in Zeller ( 1968).

Article

Shales were collected at the following localities in Kansas:

The samples were crushed in a jaw crusher and ground to a fine powder in an oscillating tungsten carbide mill (Siebtechnik). Powdered samples of shale were weighed and exhaustively extracted with chloroform-methanol and finally with chloroform only. Ultrasonic energy was used to aid extraction procedures. Total extracts were weighed and saponified overnight in 0.5N KOH in methanol. Non-polar fractions were extracted with hexane and subsequently chromatographed over silica gel to obtain the hydrocarbon fractions which were weighed and set aside for isotopic analysis. The remaining portions of the saponified mixtures were acidified to pH 1 with HCl in order to precipitate acidic material which was assumed to be humic. This material was filtered and also saved for isotopic analysis. Other acidic fractions were extracted from the filtrates with hexane and were esterified with BF3 in methanol. Methyl esters were extracted from esterification mixtures with hexane and chromatographed on silica gel. Non-polar materials were eluted with hexane and methyl esters of fatty acids with benzene. The esters were dried and saved for isotopic analysis.

"Kerogen" was obtained from 100 g of extracted Sharon Springs shale by first treating with HCl:HF (1:1) for three days at 60° C. The resulting insoluble matter or "kerogen" was washed in saturated boric acid solution and collected by centrifugation.

In order to generate CO2 for measurement of isotopic ratios, a portion of each chemical fraction was combusted in a closed vacuum line at 900° C in the presence of copper oxide and oxygen (Calder, 1969). Combustion gases were passed through a catalytic furnace to convert CO to CO2. Mass spectrometric measurements were made in the laboratory of Dr. P. L. Parker at the University of Texas.

Carbon isotopic data is presented in δC13 units relative to National Bureau of Standards Isotope Reference Standard No. 20. δC13 is in units of parts per thousand and is defined as

δC13 = {[(C13/C12) sample] / [(C13/C12) std]} x 1000

A negative number indicates the sample is depleted in C13 relative to the standard.

Table 1 shows that percent of total organic carbon and percent of extractable organic matter decreased with increasing age except for the Heebner shale, which deviated in other ways also. Correlation of quantity of organic matter with age may be meaningless because the content at time of deposition is unknown. Of more importance is the observation that the fraction of organic carbon which was extractable (A/B in Table 1) was not a function of age. Thus there appears no long-term trend of organic matter toward either smaller or larger molecules. It may be significant that the two shales lowest in organic carbon (Graneros and Heumader) had the highest fraction of extractable organic matter. Formation of insoluble, high molecular weight compounds may have been hindered in these shales because of lower population density of organic molecules. This hindrance would have favored larger proportions of low molecular weight, extractable molecules.

Table 1--Relative age and chemical fractionation of four shales.

  Sharon
Springs
Shale
Member
Graneros
Shale
Heumader
Shale
Member
Heebner
Shale
Member
Relative age Increasing age →
Weight extracted (g) 1016 1017 1611 1397
Total organic carbon in percent (B) 5.6 2.2 1.0 7.0
Weight of total extract (g) 4.84 2.21 1.49 7.51
Extractable organic matter in percent (A) 0.48 0.22 0.09 0.54
(A/B) x 100 8.6 10.0 9.0 7.7
Extracted fatty acids (mg) 44.8 11.9 2.9 63.9
Extracted hydrocarbon (mg) 61.1 61.5 21.1 311.5
Fatty acids of total extract in percent (Y) 0.9 0.5 0.2 0.9
Hydrocarbons of total extract in percent (X) 1.3 2.8 1.4 4.2
Ratio of % hydrocarbon to % fatty acid in extracts (X/Y) 1.5 5.6 7.0 4.7
Total organic carbon in "kerogen" (%) 15.0      

Ratios of hydrocarbons to fatty acids increased with age except for the case of the Heebner shale. This increase is consistent with the process of production of hydrocarbons from fatty acids as proposed by Kvenvolden (1967) and also consistent with the process proposed by Hoering and Abelson (1963) in which molecules with functional groups enter the kerogen matrix and subsequently produce smaller molecules by cleavage of carbon-carbon bonds.

The δC13 data in Table 2 show significant differences among the shales. Dispersion of δC13 values among fractions of the Sharon Springs shale is 2.60/00. Dispersion is 4.10/00 for the Graneros Shale and 5.00/00 for the Heumader shale. Wider dispersion of δC13 values with greater age may result from chemical processes involving cleavage of carbon-carbon bonds. Such processes would result in enrichment of C13 in high molecular weight fractions. The Heebner shale is peculiar in having a dispersion of δC13 values of only 1.20/00. This narrow dispersion may indicate past exposure to higher temperatures which can cause greater randomization of carbon isotopes. High temperatures, however, should also have accelerated the production of hydrocarbons and the destruction of fatty acids, whereas the Heebner shale exhibits a ratio of hydrocarbon to fatty acid lower than either the Graneros or Heumader shales.

Table 2--δC13 relative to NBS 20.

  Sharon
Springs
Shale
Member
Graneros
Shale
Heumader
Shale
Member
Heebner
Shale
Member
Whole shale -25.6 -23.4 -21.4 -27.6
Total extract -25.9 -26.4 -23.6 -28.8
Fatty acids -28.2 -26.3 -24.4 -28.5
Hydrocarbons -27.2 -27.5 -26.4 -28.8
"Humic" acids -26.8 -25.5 -22.7 -28.5
"Kerogen" -26.1      

Hydrocarbon fractions show more uniform δC13 values than other fractions. The δC13 of the hydrocarbon fractions varies over a 2.40/00 range, that of total organic carbon varies over a 6.20/00 range, and those of other fractions vary over an intermediate range.

Very negative δC13 values exhibited by the Heebner shale may indicate either a high content of terrestrial organic matter (Sackett, 1964), or deposition during a time of cold climate (Rogers and Koons, 1969), or both. In any case the history of the Heebner shale must be significantly different from that of the other shales studied.

The following statements summarize these preliminary data:

  1. Shales with low content of organic carbon (Graneros and Heumader) had (a) high percentages of extractable organic carbon, (b) greatest dispersion of δC13 among their organic fractions, (c) more than 5 times as much hydrocarbon as fatty acid, (d) the most positive δC13 for total organic carbon.
  2. Shales with high content of organic carbon (Sharon Springs and Heebner) had (a) low percentages of extractable organic carbon, (b) smallest dispersion of δC13 among their organic fractions, (c) less than 5 times as much hydrocarbon as fatty acid, (d) the most negative δC13 for total organic carbon.
  3. Significant trends were indicated in the Sharon Springs, Graneros and Heumader shales. These are the following: (a) larger ratio of hydrocarbons to fatty acids with greater age, (b) wider dispersion of δC13 values with greater age.
  4. Results from studies of the Heebner shale fit none of the trends established by data on the other three shales.

References

Calder, J. A., 1969, Carbon isotope effects in biochemical and geochemical systems: Unpub. Ph.D. dissertation, University of Texas at Austin.

Degens, E. T., 1969, Biogeochemistry of stable carbon isotopes; in, Organic Chemistry, Methods and Results, G. Eglinton and M. T. J. Murphy, eds.: Springer-Verlag, New York, p. 304-329.

Hoering, T. C., and Abelson, P. H., 1963, Hydrocarbons from kerogen: Carnegie Inst. Wash., D.C., Yearbook. 62, p. 229.

Kvenvolden, K. A., 1967, Normal fatty acids in sediments: Jour. Am. Oil Chemists Soc., v. 44, p. 628.

Rogers, M. A., and Koons, C. B., 1969, Organic carbon δC13 values from Quarternary marine sequences in the Gulf of Mexico: A reflection of paleotemperature changes: Trans. Gulf Coast Assoc. Geological Societies, Vol. XIX, p. 529-534.

Sackett, W. M., 1964, The depositional history and isotopic organic carbon composition of marine sediments: Marine Geology, v. 2, p. 173-185.

Zeller, D. E., (Ed.), 1968, The stratigraphic succession in Kansas: Kansas Geol. Survey, Bull. 189, 81 p. [available online]


Kansas Geological Survey, Stable Carbon Isotopes in Selected Kansas Shales
Placed on web May 7, 2009; originally published in May 1971.
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