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Title: Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals

Abstract

This study was initiated to investigate if coals on the Norwegian offshore continental shelf (NOCS) expel petroleum and in which form. The results revealed that equally isotopically light methane (C{sub 1}) was released from fluid inclusions in sandstones and from adjacent coal (-60.9 to -72.7 parts per thousand). The analyzed samples were collected from cored northern North Sea and mid-Norwegian shelf wells in the depth interval 3924-5095 m. The vitrinite reflectance (R{sub o}) values of the coals range between 0.53 and 1.12%, with most values between 0.8 and 1.0%. The light C{sub 1} isotope values released both from the coals and from the fluid inclusions in the adjacent sandstones suggest that the origin of the gas is the coal, and that no isotope fractionation occurs during release of the gas in nature. Traditional isotope interpretation schemes suggest the C{sub 1} to have a biogenic origin, whereas recently published data also show the possibility for an early mature thermogenic origin. The isotope values represent averages of the total gas released from all the individual disintegrated fluid inclusions in each sample. These did not form simultaneously, but during multiple events potentially covering several million years. We speculate that significant volumes of isotopicallymore » light C{sub 1} have been expelled from the analyzed coals over time. The expelled isotopically light C{sub 1}, may mix with mature thermogenically produced gas and skew the overall methane isotope values of gas accumulations toward lighter values, thus explaining the isotopically lighter-than-expected gas accumulations on the NOCS (e.g., Troll, Frigg, and Draugen fields).« less

Authors:
;  [1]
  1. University of Oslo, Oslo (Norway). Dept. of Geoscience
Publication Date:
OSTI Identifier:
20939735
Resource Type:
Journal Article
Resource Relation:
Journal Name: AAPG Bulletin; Journal Volume: 91; Journal Issue: 5
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; 03 NATURAL GAS; SANDSTONES; COAL; NORWAY; CONTINENTAL SHELF; PETROLEUM; METHANE; INCLUSIONS; COAL SEAMS; CARBON ISOTOPES; ORIGIN

Citation Formats

Ohm, S.E., and Karlsen, D.A. Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals. United States: N. p., 2007. Web. doi:10.1306/11280606021.
Ohm, S.E., & Karlsen, D.A. Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals. United States. doi:10.1306/11280606021.
Ohm, S.E., and Karlsen, D.A. Tue . "Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals". United States. doi:10.1306/11280606021.
@article{osti_20939735,
title = {Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals},
author = {Ohm, S.E. and Karlsen, D.A.},
abstractNote = {This study was initiated to investigate if coals on the Norwegian offshore continental shelf (NOCS) expel petroleum and in which form. The results revealed that equally isotopically light methane (C{sub 1}) was released from fluid inclusions in sandstones and from adjacent coal (-60.9 to -72.7 parts per thousand). The analyzed samples were collected from cored northern North Sea and mid-Norwegian shelf wells in the depth interval 3924-5095 m. The vitrinite reflectance (R{sub o}) values of the coals range between 0.53 and 1.12%, with most values between 0.8 and 1.0%. The light C{sub 1} isotope values released both from the coals and from the fluid inclusions in the adjacent sandstones suggest that the origin of the gas is the coal, and that no isotope fractionation occurs during release of the gas in nature. Traditional isotope interpretation schemes suggest the C{sub 1} to have a biogenic origin, whereas recently published data also show the possibility for an early mature thermogenic origin. The isotope values represent averages of the total gas released from all the individual disintegrated fluid inclusions in each sample. These did not form simultaneously, but during multiple events potentially covering several million years. We speculate that significant volumes of isotopically light C{sub 1} have been expelled from the analyzed coals over time. The expelled isotopically light C{sub 1}, may mix with mature thermogenically produced gas and skew the overall methane isotope values of gas accumulations toward lighter values, thus explaining the isotopically lighter-than-expected gas accumulations on the NOCS (e.g., Troll, Frigg, and Draugen fields).},
doi = {10.1306/11280606021},
journal = {AAPG Bulletin},
number = 5,
volume = 91,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}