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Abstract

Geochemical analyses on cuttings and cores of more than 30 wells in the Jeanne d'Arc basin of the Grand Banks identify three formations as petro-leum source rocks of significance for regional hydrocarbon generation.

The principal source rock is the Kimmeridgian Egret Formation, a thick sequence of calcareous organic-rich shales deposited under euxinic conditions that were established with the initial breakup of an epeiric basin. Initial rifting formed the semi-silled basin geometry essential for the formation of anoxia. The kerogen is mainly amorphous- bituminous, and dinoflagellates and bacteria are the primary source materials.

Euxinic conditions and rich source rock formation were terminated with the climax of rifting in the Kimmeridgian, which flooded the basin with coarse elastics and mud. Conditions of source rock formation were extended stratigraphically at the eastern margin of the basin, but the amount of ter-rigenous material also increased. These conditions resulted in a considerably thicker, but diluted source sequence in the east.

In the late Kimmeridgian the rich source facies was terminated across the basin. Subsequent depo-sitions incorporated a mainly terrestrial kerogen, which has contributed to hydrocarbon generation in the Kimmeridgian Jeanne d'Arc Shale above the Egret Formation and the Jeanne d'Arc reservoir sands.

The formation of potential petroleum source rocks was reintroduced in the early Tertiary in a now- open marine system. This was the result of high bioproductivity in surface waters and the associated oxygen depletion of midwater zones. However, the oxygen-minimum zone was apparently not effective enough to permanently preserve the oil-prone kerogen. Although lower Tertiary sediments are high in organic carbon of predominantly marine origin, most of the labile kerogen is severely degraded by oxidation. However, prolific Tertiary oil source rock may have been preserved in isolated pockets which, upon maturation, generated the Adolphus oil in the center of the basin.

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