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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 δ34S and δ13C 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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