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Title: Stable carbon isotope compositions during the thermal alteration of organic matter

Abstract

The use of the amount and carbon isotopic composition of methane as a maturation index was tested by pyrolysis of sedimentary organic carbon (kerogen) at 600 C. The parameters used to describe the maturity are CMR (CH{sub 4}-C/kerogen carbon) and the {Delta}{sup 13}C ({delta}{sup 13}C{sub CH4-} {delta}{sup 13}C{sub OC}). With increasing maturities, smaller amounts of methane are generated and there is a decrease in the fraction between methane and the parent carbon. The pyrolysis of Bakken shale samples, with varying maturities, show high correlation coefficients between the CMR and {Delta}{sup 13}C vs. the atomic H/C ratios (r = +0.91 and {minus}0.89 respectively) which indicates that each of these parameters, independently, can be used as a maturity index. The Bakken shale pyrolysis experiments also show that methane generated from the most thermally altered samples is up to 2% heavier than the parent carbon. In addition, methane-CO{sub 2} exchange experiments, at 600 C, show a shift toward heavier methane values after heating of CH{sub 4} and CO{sub 2} for 504 hrs. The isotopic composition of methane formed under high temperature regimes may be determined by exchange reactions if any CO{sub 2} is present. For these reasons, it becomes difficult to use carbonmore » isotope compositions of methane to distinguish between thermogenic and mantle methane without any other supporting evidence. The effect of metagenesis on the isotopic composition of organic carbon was determined for a suite kerogen samples from the Cape Verde Rise, DSDP Leg 41, Site 386. With increasing maturities, the {delta}{sup 13}C-OC values are heavier due to a loss of lighter carbon in the form of methane. This is shown by a decrease in the carbon mole ratio, with increasing maturities.« less

Authors:
Publication Date:
Research Org.:
University of South Florida, St. Petersburg, FL (USA)
OSTI Identifier:
6762723
Resource Type:
Miscellaneous
Resource Relation:
Other Information: Thesis (Ph. D.)
Country of Publication:
United States
Language:
English
Subject:
02 PETROLEUM; 04 OIL SHALES AND TAR SANDS; KEROGEN; MATURATION; PYROLYSIS; METHANE; ISOTOPE RATIO; OIL SHALES; BENCH-SCALE EXPERIMENTS; CARBON 13; CARBON DIOXIDE; CARBON ISOTOPES; HEAT TREATMENTS; MEASURING METHODS; ORGANIC MATTER; ALKANES; BITUMINOUS MATERIALS; CARBON COMPOUNDS; CARBON OXIDES; CARBONACEOUS MATERIALS; CHALCOGENIDES; CHEMICAL REACTIONS; DECOMPOSITION; ENERGY SOURCES; EVEN-ODD NUCLEI; FOSSIL FUELS; FUELS; HYDROCARBONS; ISOTOPES; LIGHT NUCLEI; MATERIALS; NUCLEI; ORGANIC COMPOUNDS; OXIDES; OXYGEN COMPOUNDS; STABLE ISOTOPES; THERMOCHEMICAL PROCESSES; 020200* - Petroleum- Reserves, Geology, & Exploration; 040200 - Oil Shales & Tar Sands- Reserves, Geology, & Exploration; 023000 - Petroleum- Properties & Composition; 040500 - Oil Shales & Tar Sands- Properties & Composition

Citation Formats

Conkright, M E. Stable carbon isotope compositions during the thermal alteration of organic matter. United States: N. p., 1989. Web.
Conkright, M E. Stable carbon isotope compositions during the thermal alteration of organic matter. United States.
Conkright, M E. 1989. "Stable carbon isotope compositions during the thermal alteration of organic matter". United States.
@article{osti_6762723,
title = {Stable carbon isotope compositions during the thermal alteration of organic matter},
author = {Conkright, M E},
abstractNote = {The use of the amount and carbon isotopic composition of methane as a maturation index was tested by pyrolysis of sedimentary organic carbon (kerogen) at 600 C. The parameters used to describe the maturity are CMR (CH{sub 4}-C/kerogen carbon) and the {Delta}{sup 13}C ({delta}{sup 13}C{sub CH4-} {delta}{sup 13}C{sub OC}). With increasing maturities, smaller amounts of methane are generated and there is a decrease in the fraction between methane and the parent carbon. The pyrolysis of Bakken shale samples, with varying maturities, show high correlation coefficients between the CMR and {Delta}{sup 13}C vs. the atomic H/C ratios (r = +0.91 and {minus}0.89 respectively) which indicates that each of these parameters, independently, can be used as a maturity index. The Bakken shale pyrolysis experiments also show that methane generated from the most thermally altered samples is up to 2% heavier than the parent carbon. In addition, methane-CO{sub 2} exchange experiments, at 600 C, show a shift toward heavier methane values after heating of CH{sub 4} and CO{sub 2} for 504 hrs. The isotopic composition of methane formed under high temperature regimes may be determined by exchange reactions if any CO{sub 2} is present. For these reasons, it becomes difficult to use carbon isotope compositions of methane to distinguish between thermogenic and mantle methane without any other supporting evidence. The effect of metagenesis on the isotopic composition of organic carbon was determined for a suite kerogen samples from the Cape Verde Rise, DSDP Leg 41, Site 386. With increasing maturities, the {delta}{sup 13}C-OC values are heavier due to a loss of lighter carbon in the form of methane. This is shown by a decrease in the carbon mole ratio, with increasing maturities.},
doi = {},
url = {https://www.osti.gov/biblio/6762723}, journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 1989},
month = {Sun Jan 01 00:00:00 EST 1989}
}

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