Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.
 

Hydrocarbon charge systems on the Arctic shelf were evaluated using 450 cores from five geologic provinces at different states of exploration maturity: Hanna Trough (Chukchi), western Beaufort rift margin, Camden Bay Basin, Demarcation Basin, and Canadian Beaufort-Mackenzie Basin. Each core was screened for seeping hydrocarbons, and any samples with high hydrocarbon content were also characterized by biomarker content and isotope-mass spectrometry for oil-source correlation.

The Chukchi and western Beaufort areas are potentially charged from the North Slope upper Paleozoic-Mesozoic source rocks. Timing of generation and migration was mainly due to Cretaceous burial. Hence, present-day seepage is likely to be limited. Passive seepage was anticipated and minimal evidence of seepage was detected in only 2% of the cores. However, poor core penetration and recovery means these areas were essentially not evaluated by this study. In contrast, deeper site-specific samples collected from shallow rotary boreholes in an earlier survey detected thermogenic liquid hydrocarbons below 10 m. These can be correlated with reservoired oils tested in several exploration wells. This result shows how critical it is to use more site-specific deep coring acquisition techniques here, such as shallow rotary boring or jet-coring. The three Tertiary depocenters farther east have deeply buried the upper Paleozoic-Mesozoic source rocks to postmature levels. No correlation between the seeps and subsurface oils could be made. Fluorescence was due to reworked early Cretaceous source rocks eroded from outcrop and redeposited unoxidized in the Arctic climate. This result shows the importance of ground truthing seeps identified from screening techniques, which can identify hydrocarbons of variable origin.

The absence of significant evidence for a seep in 450 near-surface cores suggests that seepage is not reaching the seabed. Shallow seismic evidence shows that this may be due to a hydrate (or even relict permafrost in the near-shore) barrier preventing leakage to the sediment-water interface. Future seepage detection should focus on obtaining 10-20 m penetration by jet-core or rotary boring acquisition techniques to get through this barrier and also reach pre-Holocene sediments where reworked HRZ should not be such a problem.