Regional-scale variation of characteristics of hydrocarbon fluid inclusions and thermal conditions along the Paleozoic Laurentian continental margin in eastern Quebec, Canada

Fluid inclusions in diagenetic mineral phases and organic matter in host ro cks were studied in Paleozoic strata along the Laurentian continental margi n in eastern Quebec. The types of hydrocarbon fluid inclusions correspond b roadly with the tectonic units: the St. Lawrence Platform contains oil incl usions; the Humber Zone contains mainly methane inclusions; and the success or basin (the Gaspe Belt) contains oil inclusions in the upper succession, oil and methane in the middle succession, and methane inclusions in the low er interval. The nature of hydrocarbon fluid inclusions also corresponds to the maturation level of organic matter in the host rocks: oil inclusions o ccur in host rocks that were heated to the oil window or the condensate zon e, 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: o il inclusions correspond to relatively low thermal conditions, and methane inclusions to relatively high conditions. These observations suggest that o il reservoirs, if they exist, are more likely to occur in host rocks that h ave 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 z one rocks suggests that oil was generated and migrated after maximum burial of the host rocks, from mature source rocks either overlying or overthrust ed 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 connect ed to, a younger source rock. This study suggests that part of the St. Lawr ence Platform and the Gaspe Belt have the thermal conditions for the format ion of oil reservoirs, whereas the Humber Zone is prone to natural gas form ation.