Stable carbon isotope compositions during the thermal alteration of organic matter

Thesis/Dissertation · Sun Jan 01 04:00:00 EST 1989

OSTI ID:6762723

Conkright, M E

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.

• Research Organization: University of South Florida, St. Petersburg, FL (USA)
• OSTI ID: 6762723
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
020200* -- Petroleum-- Reserves
Geology
& Exploration
023000 -- Petroleum-- Properties & Composition
04 OIL SHALES AND TAR SANDS
040200 -- Oil Shales & Tar Sands-- Reserves
Geology
& Exploration
040500 -- Oil Shales & Tar Sands-- Properties & Composition
ALKANES
BENCH-SCALE EXPERIMENTS
BITUMINOUS MATERIALS
CARBON 13
CARBON COMPOUNDS
CARBON DIOXIDE
CARBON ISOTOPES
CARBON OXIDES
CARBONACEOUS MATERIALS
CHALCOGENIDES
CHEMICAL REACTIONS
DECOMPOSITION
ENERGY SOURCES
EVEN-ODD NUCLEI
FOSSIL FUELS
FUELS
HEAT TREATMENTS
HYDROCARBONS
ISOTOPE RATIO
ISOTOPES
KEROGEN
LIGHT NUCLEI
MATERIALS
MATURATION
MEASURING METHODS
METHANE
NUCLEI
OIL SHALES
ORGANIC COMPOUNDS
ORGANIC MATTER
OXIDES
OXYGEN COMPOUNDS
PYROLYSIS
STABLE ISOTOPES
THERMOCHEMICAL PROCESSES