Hydrocarbon-Related, Diagenetic Effects Preserved in Microfossils (Foraminifera) in a Gas Chimney Penetrated by the Immiugak A-06 Well, Beaufort-Mackenzie Basin

D. H. McNeil, J. R. Dietrich, D. R. Issler, L. D. Stasiuk, N. Wilson, and J. Dixon
Geological Survey of Canada, Calgary, AB

The Gulf et al. Immiugak A-06 well, drilled in the offshore Tertiary fold belt of the west-central Beaufort Sea, penetrated 3800 metres of marine to terrestrial strata of Late Cenozoic to Early Eocene age. The well is situated on a northwest-southeast aligned, shale-cored anticline, which has been breached by vertically migrating hydrocarbons. Syndepositional growth of the diapiric anticline occurred from the Eocene to the Early Pliocene. An apparent gas chimney occurs in Oligocene-Miocene strata at the crest of the structure. The gas chimney may extend to the top of the sedimentary succession, suggesting gas migration to the seafloor. An overpressured zone occurs below 1300-1500 metres, within the Eocene Richards and Taglu sequences, based on well log interpretations.

Benthic foraminifera, both agglutinated and calcareous, from cuttings of the A-06 well show clear evidence of migrating hydrocarbons (bitumen) and other fluids (both dissolution and precipitation of silica). The microfossil analysis indicates that the main phase of hydrocarbon migration passed through Oligocene and Miocene strata immediately above the overpressured zone. Pliocene-Pleistocene foraminifera of the Iperk Sequence show little or no diagenetic alteration. Additionally, foraminifera in the shale/mud and coaly section (Richards to Taglu Sequence) in the lower part of the well show essentially normal burial diagenetic trends.

Thermal maturity, determined by Rock-Eval, vitrinite reflectance (%Ro), and FCI (foraminiferal colouration index), indicate that Taglu strata are within the early oil generation window (e.g., ~0.60%Ro; Tmax up to 436oC, and FCI up to 5.9).

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005