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Palynology, thermal maturation, and time temperature history of three oil wells from the Beaufort-Mackenzie Basin Forman, Robert Douglas


Palynological and maturation data are combined to reconstruct the burial and thermal history of three oil wells in the Beaufort-Mackenzie Basin. From south to north, the three wells are Netserk F-40, Tarsuit A-25, and Orviiruk 0-03. Each well was examined palynologically and zoned based on species ranges of pollen, spores, fungi, and algal cysts. Using local extinction events of zonally diagnostic species to define the tops of intervals, seven informal palynozones are presented: Laevigatosporites (Pleistocene); ChenopodipoJlis (Pliocene to early Pleistocene); Ericipites (middle to late Miocene); Selenopemphix-1 (middle to late Oligocene); Integricorpus (early Oligocene); Araliaceoipollenites (late Eocene to early Oligocene); Pistillipollenites (middle Eocene). Correlations within the basin indicate that the proposed zonation may be useful for local correlations. Correlations outside the basin indicate that the palynological assemblages from the Beaufort-Mackenzie Basin may not be as isolated and endemic as first thought. A high recovery of algal cysts is attributed to less harsh maceration techniques, and confirms a significant population of cysts from a region in which they were formerly believed to be relatively scarce. The palynology does not exhibit an increase in marine influence with decreasing proximity to the basin margin. Instead it shows a consistent, strong terrestrial influence throughout each well. The large terrestrial discharge from the Mackenzie River is interpreted to have masked the effect of basin proximity on the palynology of the area. The study wells are dominated by terrestrial Type III organic matter. Recycled and terrestrial inert material often make up over 95 % of the residues. These results support a terrestrial source for the offshore oils in the Beaufort -Mackenzie Basin. There is a small but consistent presence of potential oil-generating material throughout each well (amorphous and liptinite). The liptinite is largely composed of pollen grains, spores, and leaf cuticle. Algal cysts are present but less abundant. If the observed amounts of amorphous and liptinite material continue to some depth, where the required level of thermal maturation might be reached, these sediments could act as source rocks for hydrocarbons. The rare occurrence of resinite in the study wells questions the resinite source theory for the hydrocarbons in the basin The sediments in each of the three study wells are immature to total depth, and could not be the source of Tertiary oils in the Beaufort - Mackenzie Basin. The levels of maturity in the wells, and the low maturation gradient calculated for Netserk F-40 (0.07 Ro/km), suggest that thermal maturation will only be achieved at much greater depths. This is most likely due to rapid sedimentation rates in the basin during the Tertiary. By combining the zonations from Chapter 3 with the maturation data from Chapter 4, the burial and thermal history of each study well is reconstructed. Using a modified version of Lopatin's method, paleo-geothermal gradients are calculated for each well. In each case, the gradient that best accountes for the measured maturities is 15 °C/km. The calculated gradient is approximately 1/2 to 1/3 of the present geothermal gradients for the wells. The gradient is in agreement with those previously calculated from similar basins, and is considered responsible for the failure of any of the study wells to encounter effective source rocks. Source rocks of Tertiary oils in the Beaufort-Mackenzie Basin will only exist at greater depths than those encountered in this study. Prospective targets may therefore be located adjacent to sites where vertical migration of hydrocarbons is likely, such as steeply-dipping faults.

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