The Lower Silurian Sayabec Formation in northern Gaspe: carbonate diagenesis and reservoir potential

The Lower Silurian Sayabec Formation represents a peritidal -dominated carb onate ramp that developed at the northern edge of the post-Taconian Gaspe s uccessor basin. In the Late Silurian, during the Salinic disturbance, the S ayabec ramp was subaerially exposed locally. This could have lead to the fo rmation of economically significant secondary dissolution porosity. A detai led diagenetic study of the Sayabec Formation was carried out at selected l ocalities along the Northern Outcrop Belt in the Gaspe Peninsula, where the Salinic unconformity and hydrothermal alteration of the carbonate facies h ave been documented. The diagenetic history consists of initial minor marine diagenesis (marine cements in boundstones and neptunian dykes) followed by pervasive burial di agenesis that resulted in the emplacement of various pore- and fracture-fil ling calcite cements, due to the mixing of basinal brines and hydrothermal fluids. Late Silurian tectonic exhumation of the lithified carbonate ramp i s recorded locally in meteoric-cement-filled fractures that were dissolutio n-enhanced after early burial. The significance of this event in generating porosity was relatively minor. Preserved porosity is observed where limestone facies and calcite cements w ere completely replaced by hydrothermal saddle dolomite. However, the porou s dolostone is of geographically limited extent. The hydrothermal event is mostly recorded in high-temperature calcite cements that occlude burial fra ctures.