|Résumé||(disponible en anglais seulement)|
The Sverdrup Basin is one of the major geological structures in the Canadian Arctic Archipelago. It covers the area of 350,000 km2 and contains over 8,000 m of
Mesozoic sedimentary sequences. These strata were explored for hydrocarbons between 1969 and 1985. A total of 124 petroleum exploratory wells were drilled resulting in a discovery of 19 petroleum fields (Chen et al., 2000) containing predominantly
natural gas; only few accumulations of crude oil were found. The oils have strong genetic affinity and were classified as belonging to a single oil family (Curiale, 1992) with the Triassic Schei Point Group proposed as their only source interval
(Powell, 1978). A detailed biomarker analysis fully confirmed a positive oil-source correlation between the oils and Schei Point organic extracts, also suggesting that, as there was no significant variations in the composition of the source organic
facies, rather than individual units, the entire Schei Point Group should be considered as the oil source (Brooks et al, 1992).
In this study, organic matter from several Mesozoic stratigraphic assemblages, including, amongst others, the Triassic
Murray Harbour, Roche Point and Hoyle Bay, the Jurassic Heiberg, Jameson Bay and Ringnes, and the Cretaceous Isachsen and Kanguk formations, were extracted from mostly well cuttings and less frequently cores, and geochemically assessed. Although the
extract yields are typically low, often below 100 mg HC/g TOC, about 25% of the extracted samples have hydrocarbon (HC) yields greater than 50 mg HC/g TOC and therefore could be considered as good petroleum source rocks (Powell, 1978). Almost 80% of
the higher HC-yielding samples are of Triassic age (predominantly the Hoyle Bay Formation) with the remaining 20% being Jurassic (mostly the Jameson Bay Formation). While in the Triassic samples these values are almost certainly resulting from
excellent petroleum source potential of the Schei Point interval, most of the Jurassic extracts also contain higher proportion of hydrocarbons probably implying some hydrocarbon staining.
Biomarker analyses indicate that most Cretaceous and some
Jurassic extracts are thermally immature. Their saturate fraction gas chromatograms (sfgc) often show broad n-alkane profiles with odd to even predominance, high pristane (pr) and phytane (ph) with high pr/ph ratios (commonly >2.0) and sometimes a
pronounced terpane/sterane baseline hump. Triassic extracts appear more mature showing smooth distributions of n-alkanes with relative amounts decreasing with increasing carbon number, occasionally superimposed over a broader but low baseline hump,
and pr/ph ratios typically within 0.5-2.0 range. The distributions of terpane and sterane biomarkers are typical for marine-derived organic matter. Some Schei Point extracts display biomarker characteristics not observed in the crude oil, such as
noticeable presence of C24 tetracyclic terpane or C35 hopane prominence, perhaps suggesting occurrences of more variable organic facies within this stratigraphic interval.
Overall, terpane and sterane signatures display thermal maturity trends
similar to sfgc, with low maturity extracts occurring within the Jurassic-Cretaceous and mature extracts in the Triassic strata. Notable exceptions to this general trend include samples from Balaena D-58, North Sabine H-49 and Dome Bay P-36. The
Balaena field contains significant oil accumulation in the Lower Cretaceous Isachsen and, somewhat smaller, in the Lower Jurassic Heiberg reservoirs, which are separated by about 1.5 km of sedimentary strata. The occurrence of stacked reservoirs in
this field was interpreted as resulting from hydrocarbon leakage from the primary Heiberg reservoir, and led to a hypothesis that such process might have resulted in a loss of significant quantities of hydrocarbons (Waylett and Embry, 1992). The
compositions of extracts recovered from the intermediate interval provide geochemical evidence for the oil upward migration. While the saturate biomarker signatures are still immature several hundred meters below the upper reservoir within the
lowermost Isachsen-Deer Bay-Ringnes interval, samples extracted from about 200 m under that reservoir display mature biomarker distributions indicating presence of migrated hydrocarbons. Similar observation was made at North Sabine H-49 where
biomarkers in extracts from even younger stratigraphic interval - the Upper Cretaceous Kanguk strata, not only appear more mature compared with the underlying Hassel Formation, but also have sterane distributions that matches that of the Triassic
Schei Point strata where in this well minor oil was present. Interestingly, the Jurassic Sandy Point extract from the Dome Bay P-36 also displays more mature biomarker signature compared with the underlying Jameson Bay Formation. This well, however,
is in the area of extensive Cretaceous volcanism, where the sedimentary strata are intruded with numerous sills and dykes. Several sills occur within the McConnell Island Formation that directly overlies the Sandy Point interval in Dome Bay P-36, and
hence, it is likely that the reversed biomarker maturity pattern in this well is due to localized thermal anomaly associated with these volcanic intrusions.