Diagenesis and porosity evolution of the Upper Silurian-lowermost DevonianWest Point reef limestone, eastern Gaspe Belt, Quebec Appalachians

Diagenetic analysis based on cathodoluminescence petrography, cement strati graphy, carbon and oxygen stable isotope geochemistry, and fluid inclusion microthermometry was used to reconstruct the porosity history and evaluate the reservoir potential of the Upper Silurian-Lower Devonian West Point lim estone in the eastern part of the Gaspe Belt. The West Point limestone was investigated in two areas: 1) In the Chaleurs Bay Synclinorium, the limestone diagenesis of the lower and middle complexes of the Silurian West Point Formation was affected by r epeated subaerial exposure related to late Ludlovian third-order eustatic l owstands, which coincided with the Salinic block tilting that produced the Salinic unconformity. The Anse McInnis Member (middle bank complex) underwe nt freshwater dissolution, and mixed marine and freshwater cementation duri ng deposition. Concurrently, the underlying Anse a la Barbe and Gros Morbe members (lower mound and reef complex) experienced dissolution by fresh wat er percolating throughout the limestone succession. Despite this early deve lopment of karst porosity, subsequent meteoric-influenced cementation rapid ly occluded all remaining pore space in the Gros Morbe, Anse A la Barbe, an d Anse McInnis limestones. In contrast, the overlying Colline Daniel Member limestone (upper reef complex) does not show the influence of any freshwat er diagenesis. Occlusion of its primary porosity occurred during progressiv e burial and was completed under a maximum burial depth of 1.2 kin. 2) In the Northern Outcrop Belt, the diagenesis of the Devonian pinnacle re efs of the West Point Formation followed a progressive burial trend. The pr imary pores of the reef limestone were not completely occluded before the r eefs were buried at a significant depth (in some cases, to 6 km). Therefore , hydrocarbon migration in subsurface buildups before primary porosity occl usion might have created reservoirs. Moreover, the presence of gaseous hydr ocarbons in Acadian-related veins attests to a hydrocarbon source in the ar ea.