THE HORN RIVER - DUVERNAY SHALE SYSTEM, WESTERN CANADA: HYDROTHERMAL PETROLEUM RESERVOIRS

Devonian shale units from Western Canada contain enormous petroleum resources. These include the Horn River shale of northeast British Columbia and the Duvernay shale of Alberta. We consider these as portions of a larger system that extends to the Canol shale of the Northwest Territories. The shale basins are bordered by platform carbonates that contain contemporaneous high temperature dolomite. Dolomite formed from hydrothermal fluids that moved upward along deep seated faults along the platform margin. These faults also extent into the adjoining basins. This begs the question of the influence of these hydrothermal fluids on the basinal shale units.

The Horn River and Duvernay shale are black organic carbon-rich deposits within thicker successions of lighter colored shale with lower organic carbon content. We interpret the organic carbon-rich shale to be a result of preservation of organic matter in stratified water columns. Previous studies of fluid from dolomite on the platform margins surrounding these shale units show high temperature, but also high salinity fluids (> 20 weight percent NaCl). These fluids are more dense than seawater and hence tend to pool on the bottom resulting in stratification whic leads to anoxic conditions and preservation of organic matter. Similar anoxic black shale units are commonly recognized in association with SEDEX deposits.

Previous fluid inclusion studies of the dolomite on the platform margins show a strong temperature gradient, from average temperatures near 180 C in the Slave Point dolomite on the margin of the Horn River basin to the north to near 100 C in dolomite in proximity to the Duvernay shale to the south. Both the Horn River and Duvernay shale contain quartz cements, trace elements and hydrothermal minerals commonly associated with SEDEX deposits such as barite, sphalerite, celsian and hyalophane . The Horn River shale contains higher amounts of these constituents as well as authigenic monazite.