Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals
- University of Oslo, Oslo (Norway). Dept. of Geoscience
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).
- OSTI ID:
- 20939735
- Journal Information:
- AAPG Bulletin, Journal Name: AAPG Bulletin Journal Issue: 5 Vol. 91; ISSN 0149-1423; ISSN AABUD2
- Country of Publication:
- United States
- Language:
- English
Similar Records
Nature of natural gas in anomalously thick coal beds, Powder River basin, Wyoming
Origin and significance of surface occurrences of natural gas, northern Denver Basin, Colorado
Occurrence and geochemistry of natural gases, Piceance basin, northwestern Colorado
Conference
·
Fri Sep 01 00:00:00 EDT 1989
· AAPG Bulletin (American Association of Petroleum Geologists); (USA)
·
OSTI ID:5495790
Origin and significance of surface occurrences of natural gas, northern Denver Basin, Colorado
Conference
·
Wed May 01 00:00:00 EDT 1985
· Am. Assoc. Pet. Geol., Bull.; (United States)
·
OSTI ID:5101884
Occurrence and geochemistry of natural gases, Piceance basin, northwestern Colorado
Conference
·
Tue Feb 28 23:00:00 EST 1989
· AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States)
·
OSTI ID:6128402