CONTRASTING STYLES OF DOLOMITIZATION BETWEEN THE LOWER ORDOVICIAN ROMAINE AND MIDDLE-UPPER ORDOVICIAN MINGAN (“TRENTON”) FORMATIONS IN THE SUBSURFACE OF ANTICOSTI ISLAND: IMPLICATIONS FOR HYDROTHERMAL RESERVOIR DEVELOPMENT

Hydrothermal dolomitization played a key role in the development of hydrocarbon reservoirs for the Ordovician platform carbonates at the continental margin of Laurentia. The previous studies of the St. Lawrence platform in the Quebec Reentrant (southern Quebec and Anticosti Island) did not record unequivocal evidence for hydrothermal alteration of the carbonates. From the recent documentation of hydrothermal fluid circulation in the oil-reservoir host Lower Ordovician St. George Group in Newfoundland, industry-supported studies of the Ordovician succession from Anticosti Island was carried out. The Lower Ordovician Romaine Formation is a peritidal-dominated succession with significant marine incursion. The formation recorded multiple events of dolomitization and a late hydrocarbon migration. A late hydrothermal event created significant porosities imperfectly filled by high temperature saddle dolomite (average Th: 120°C) derived from a highly saline fluid (24 wt% NaClequi). A lower temperature, hydrocarbon inclusion-rich calcite (average Th: 70°C) later filled part of the open fractures. Even if locally highly porous, unfractured zones of the Romaine have low permeabilities. The Middle-Upper Ordovician Mingan Formation (formerly Trenton Formation) has received less attention. It is predominantly open marine limestones, dolomitization can locally be significant. Three dolomitization events are documented from the Chaloupe well (northeast Anticosti). All three events generated replacement dolomites and only pore-filling calcites were observed in the available side-wall cores. The latest event of replacement dolomite is characterized by high temperature (average Th: 105°C) and highly saline (24 wt% NaClequi) fluid inclusions. It has been assumed that the hydrothermal fluids used the Romaine aquifer for circulation and breached into the Mingan at the erosional edge of the Lower Ordovician platform on northern Anticosti. However, circulation of these fluids along some of the major extensional faults likely altered the Middle-Upper Ordovician Mingan Formation. As they move upwards, the fluids were slightly colder but were able to generate secondary porosity as suggested by some significant gas shows from the Trenton interval in southern Anticosti cores.