HYDROTHERMAL DOLOSTONES IN LOWER SILURIAN CARBONATES OF THE GASPÉ BELT: RELATIVE TIMING AND HYDROCARBON CHARGE

Porous dolostones of the Lower Silurian Sayabec Formation from the Gaspé Peninsula are studied. The dolomitized section is close to the Shickshock Sud fault that cuts the southern limb of the syncline. The dolostone occurs either as a highly brecciated unit or as massive stratiform replacement of peritidal carbonates. Residual bitumen is seen in the brecciated intervals as well as filling small secondary vugs and fractures within the stratiform dolostone. The dolostone consists predominantly of replacive matrix dolomite; oxygen and carbon stable isotope ratios of the matrix dolomite indicate early precipitation from little modified marine fluids (d¹⁸O_PDB=-6.3 to -7.8‰ and d¹³C_PDB=1.2 to 3.3‰). Saddle dolomite is found as pore filling cement in secondary pores and in fractures. Oxygen stable isotope ratios of the saddle dolomite (d¹⁸O_PDB=-14,5 and -15,3‰) indicate high temperature of precipitation. Dull luminescent burial calcite cement and hydrocarbon migration follows saddle dolomite. The carbonates are locally dissolved and the scalloped surfaces covered by finely laminated, bright-very dull luminescent calcites. Petrography and stable isotope ratios of the calcite (d¹⁸O_PDB=-10.1 and -11.2‰ and d¹³C_PDB=-2.3 and -6.9‰) suggest precipitation from meteoric waters.

The meteoric dissolution and oxidation event are related to a Pridolian sea level lowstand, it provides a first constraint on timing of the hydrothermal dolomitization and hydrocarbon charge of the Sayabec Formation along the northern edge of the Gaspé Belt. The Shickshock Sud fault channelled the hydrothermal fluids, which dolomitized the Sayabec Formation shortly after initial burial. Because the best potential hydrocarbon source rocks in the Gaspé Peninsula are Upper Ordovician black shales, the early formation of reservoirs is critical for the hydrocarbon potential of the Lower Silurian section. From recent regional seismic program, complex compressive structures in the Sayabec Formation in the subsurface occurred in latest Silurian – Early Devonian; efficient structural traps are added to the stratigraphic (shaly facies) and diagenetic (tight non-dolomitized limestone) seals. Some “flat spots” in the Lower Silurian section in eastern Quebec suggests the presence of hydrocarbon-filled reservoirs.