ABSTRACT

The thermal history and levels of organic maturation of Phanerozoic sedimentary sequences in northern Yukon and northwestern District of Mackenzie have been investigated by measurement of vitrinite reflectance (%Ro; mean random reflectance in oil) and conodont alteration index (CAI). The strata are immature to overmature with respect to the oil window, and maturation increases laterally with increasing structural complexity. Coeval strata have generally lower maturity in southern Mackenzie Delta, Peel Plateau and Eagle Plain than in the Richardson and Ogilvie mountains.

In Upper Cambrian to Lower Devonian strata, CAI values vary from 3.5 to 5 whereas vitrinite reflectance ranges from 0.2 to 3.75% Ro in Middle Devonian to Upper Cretaceous strata. Time-averaged numerical modeling of measured maturation gradients (0.10 to 0.32 log %Ro/km) suggests paleogeothermal gradients on the order of 20 to 45° C/km in southern Mackenzie Delta and Peel Plateau, from 10 to 20° C/km in central Eagle Plain and from 20 to 45° C/km adjacent the Richardson and Ogilvie mountains. Higher maturity levels in mountainous areas reflect higher maturation gradients and, in the Richardson Mountains, deeper burial due to rapid subsidence caused by the foundering of grabens within the Richardson Fault Array. Anomalously high maturation values (0.92 to 1.60% Ro) measured in Lower Cretaceous strata on the Campbell Uplift are interpreted to reflect high paleoheat flow associated with basement uplift. The low paleogeothermal gradients are considered to reflect low paleoheat flow and/or rapid Late Cretaceous sedimentation and uplift. Based on interpretation of measured maturation gradients, the thickness of eroded section in the study area varies from 0.7 to 4.7 km. Such values are consistent with rates of subsidence and uplift estimated from the ages and thicknesses of preserved section in the study area.

Interpretation of burial and thermal history indicates the degree of organic maturity (DOM) required for hydrocarbon generation for Devonian strata was attained in the Late Devonian to Early Carboniferous in western Peel Plateau but not until Late Carboniferous to Permian in Eagle Plain. Devonian strata left the oil window during the Carboniferous to Early Tertiary in most parts of the study area. Carboniferous and Permian strata entered the oil window in Late Carboniferous to Early Tertiary time in most of Eagle Plain. In western Eagle Plain, the Carboniferous sequence exited the oil window during the Late Cretaceous. Carboniferous and Permian strata are immature with respect to hydrocarbon generation in central Eagle Plain. Lower and Upper Cretaceous strata in southern Mackenzie Delta, Peel Plateau and most of Eagle Plain, for the most part, have not entered the oil window, whereas, in the northwestern Eagle Plain, Lower Cretaceous strata entered the oil window in Late Cretaceous to Early Tertiary time.

RESUME

L'evolution thermique et les niveaux de maturite de la matiere organique des sequences sedimentaires Phanerozoiques dans le nord du Yukon et le nord-ouest du district du Mackenzie ont ete etudiees a l'aide de la reflectance de la vitrinite et l'indice d'alteration des conodontes. Les strates varient entre immatures et surmatures par rapport a la fenetre de generation d'huile, et le niveau de maturite augmente lateralement de concert avec l'augmentation de la complexite structurale. Les strates contemporaines dans le sud du delta du Mackenzie, et dans les regions du plateau de Peel et de la plaine Eagle, ont en general un niveau de maturite moins eleve que celui des montagnes de Richardson et d'Ogilvie.

Traduit par Marc Charest

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