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Abstract

McNeil, David H., James R. Dietrich, Dale R. Issler, Stephen E. Grasby, James Dixon, and Lavern D. Stasiuk, 2010, A new method for recognizing subsurface hydrocarbon seepage and migration using altered foraminifera from a gas chimney in the Beaufort-Mackenzie Basin, in L. Wood, ed., Shale tectonics: AAPG Memoir 93, p. 197–210.

DOI:10.1306/13231315M933425

Copyright copy2010 by The American Association of Petroleum Geologists.

A New Method for Recognizing Subsurface Hydrocarbon Seepage and Migration Using Altered Foraminifera from a Gas Chimney in the Beaufort-Mackenzie Basin

David H. McNeil,1 James R. Dietrich,2 Dale R. Issler,3 Stephen E. Grasby,4 James Dixon,5 Lavern D. Stasiuk,6

1Geological Survey of Canada, Natural Resources Canada, Calgary, Alberta, Canada
2Geological Survey of Canada, Natural Resources Canada, Calgary, Alberta, Canada
3Geological Survey of Canada, Natural Resources Canada, Calgary, Alberta, Canada
4Geological Survey of Canada, Natural Resources Canada, Calgary, Alberta, Canada
5Geological Survey of Canada, Natural Resources Canada, Calgary, Alberta, Canada
6Shell Canada Limited, Calgary, Alberta, Canada

ACKNOWLEDGMENTS

The authors express their appreciation to the SEPM (Society for Sedimentary Geology) for awarding an earlier version of this work with an Honorable Mention for Best Poster at the 2005 AAPG Annual Convention. We thank the reviewers of this article, Dan Jarvie and Ron Waszczak, for their constructive comments and to Lesli Wood for encouragement to participate in the AAPG/Geological Society of Trinidad and Tobago (GSTT) Hedberg Conference ldquoMobile Shale Basins—Genesis, Evolution and Hydrocarbon Systemsrdquo and this AAPG Memoir ldquoShale Tectonics.rdquo Mark Obermajer (GSC Calgary) completed a preliminary review of the manuscript and provided many useful suggestions, including a title that did not contain more than 25 words. Microfossil samples were processed at GSC Calgary by Denise Then and picked by Judy Labadi. Krista Letourneau collected the magnetic separations from washed well cuttings and conducted the foraminiferal coloration determinations. The XRD and EDS analyses and scanning electron microscope images were conducted by Jenny Wong (GSC Calgary). Isotope analyses were supplied by Steve Taylor, University of Calgary Isotope Laboratory. This is Geological Survey of Canada contribution no. 20070325.

ABSTRACT

A new method for recognizing hydrocarbon seepage and migration in exploration wells is documented from the Immiugak A-06 exploration well that drilled through a hydrocarbon-related diagenetic zone (HRDZ). The HRDZ is seismically conspicuous as part of a gas chimney on a shale-cored anticline in the Tertiary of the Beaufort-Mackenzie Basin, Arctic Canada. The HRDZ contains classic diagenetic minerals, notably greigite (Fe3S4) and calcite with delta34S and delta13C values diagnostic of hydrocarbon-related, sulfate-reducing, microbial activity. The HRDZ also contains exceptionally preserved calcareous benthic foraminifera with conspicuous bitumen-filled chambers and agglutinated foraminifera with bitumen and diagenetic silica with bound particles. Silica was highly mobile within the seepage or migration system and was precipitated and dissolved extensively in the agglutinated foraminifera. Seismic profiles, resistivity anomalies, diagenetic minerals, and altered foraminifera all suggest that significant hydrocarbons migrated or seeped through sandy Oligocene and Miocene strata at the crest of a shale-cored anticline in response to late Miocene tectonism. Hydrocarbon-related diagenesis can be distinguished from standard burial diagenesis using the foraminiferal coloration index (FCI). Foraminiferal coloration within the HRDZ was controlled by silicification in a bitumen-rich environment. The FCI values in the HRDZ are much higher than predicted for normal burial and show abnormal variance caused by variable dissolution of foraminiferal silica. The FCI values from agglutinated foraminifera outside the HRDZ show a uniform linear trend increasing with depth. The extent of hydrocarbon-related diagenesis observed in foraminifera can be used to assess the relative magnitude of hydrocarbon seepage in the Beaufort-Mackenzie Basin and potentially other petroleum basins.

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