Abstract

Fluid inclusions in diagenetic mineral phases and organic matter in host rocks were studied in Paleozoic strata along the Laurentian continental margin in eastern Quebec. The types of hydrocarbon fluid inclusions correspond broadly with the tectonic units: the St. Lawrence Platform contains oil inclusions; the Humber Zone contains mainly methane inclusions; and the successor basin (the Gaspé Belt) contains oil inclusions in the upper succession, oil and methane in the middle succession, and methane inclusions in the lower interval. The nature of hydrocarbon fluid inclusions also corresponds to the maturation level of organic matter in the host rocks: oil inclusions occur in host rocks that were heated to the oil window or the condensate zone, whereas methane inclusions occur in the condensate zone or dry gas zone, but mainly in the latter. The variation in the nature of hydrocarbon fluid inclusions is related to the thermal history of the successions studied: oil inclusions correspond to relatively low thermal conditions, and methane inclusions to relatively high conditions. These observations suggest that oil reservoirs, if they exist, are more likely to occur in host rocks that have not been buried beyond the condensate zone than in those that have gone through the dry gas zone. The occurrence of oil inclusions in condensate zone rocks suggests that oil was generated and migrated after maximum burial of the host rocks, from mature source rocks either overlying or overthrusted by the host rocks. However, the absence of oil inclusions in the dry gas zone rocks suggests that such a late oil generation-migration scenario may not be viable if the host rocks are heated far beyond the oil window. This is probably because these rocks are farther from, thus less likely connected to, a younger source rock. This study suggests that part of the St. Lawrence Platform and the Gaspé Belt have the thermal conditions for the formation of oil reservoirs, whereas the Humber Zone is prone to natural gas formation.

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