Abstract

The Queen Charlotte Basin comprises a diverse succession of syn-rift and post-rift marine and nonmarine clastics up to 7000 m thick that includes organic rich mudrocks and coals of highly varying hydrocarbon potential and maturity. The abundance of organic matter and to a lesser extent source rock quality in the offshore wells varies markedly and closely mimics the gross stratigraphy. The total organic carbon content (TOC) varies systematically in mudrocks from less than 0.5 per cent at the base of the syn-rift succession to a maximum of about 30 per cent at the base of the post-rift strata and then in turn declines in abundance through the post-rift succession. The strata have low hydrogen indices (mainly less than <200 mg/g Corg) and the kerogen types, based on cross-plots of the hydrogen and oxygen indices, are mainly Type III (HI <300 mg HC/g Corg). The total extractable bitumen ranges from 90 to 9000 ppm and the pristane/phytane ratio from 1 to 11. Petrographically, the organic matter is predominantly vitrinite. The depth to the top of the oil window determined by vitrinite reflectance and Tmax from Rock-Eval pyrolyses closely mimics the stratigraphy and depth of burial, and varies from 500 m on Graham Island to more than 3000 m in the Harlequin D-86 well in Queen Charlotte Sound. The depth to the base of the oil window and thickness of strata within the oil window varies throughout the basin in response to varying depth of burial and paleoheat flow. The stratigraphic variations in maturation are consistent with basin formation and subsequent heat flow history predicted by an extensional McKenzie-type tectonic model with a pre-rifting equilibrium heat flow of about 30 to 50 mW/m 2 (which is close to the published range of equilibrium heat flow for surrounding areas). The modelled crustal stretching factors (beta ) range from 1.4 to 2.2, which compare favorably with estimates based on seismic, gravity and plate motion analyses. The calculated current heat flows at the offshore wells range from 50 to 110 mW/m 2 and average 77 mW/m 2 . Based on the calculated thermal history of the strata and measured kinetic factors of kerogen, the onset of oil generation at the base of the syn-rift succession is estimated at between 16 and 27 Ma (average 23 Ma). In most offshore wells the basal syn-rift deposits reached over maturity (reaction extent > or =100 per cent) between 9 and 16 Ma. In some locations the base of the post-rift succession is immature whereas in others it is within the oil window. Overall the tested stratigraphic succession in the Queen Charlotte Basin has poor liquid hydrocarbon potential as conventionally determined by Rock-Eval pyrolyses and total extractable bitumen. Because of the great volume of carbonaceous mudstones and discrete coal seams, and the thick succession of mature strata, the Queen Charlotte Basin has major gas and condensate resource potential. Locally, better oil source rocks occur, such as in the Sockeye B-10 well. It is premature to consider the Queen Charlotte Basin as a solely gas prone basin because of the few number of exploration wells drilled to date.

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